Fundamentals of the Nervous System and Nervous Tissue Chapter 12.
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Transcript of Fundamentals of the Nervous System and Nervous Tissue Chapter 12.
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Fundamentals of the Nervous System and
Nervous Tissue
Chapter 12
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Introduction The nervous system is the master controlling and communicating system of the body
It is responsible for all behavior
Along with the endocrine system it is responsible for regulating and maintaining body homeostasis
Cells of the nervous system communicate by means of electrical signals
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Nervous System Functions
The nervous system has three overlapping functions
Gathering of sensory input Integration or interpretation of sensory input Causation of a response or motor output
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Introduction Sensory input
The nervous system has millions of sensory receptors to monitor both internal and external change
Integration It processes and interprets the sensory input and makes decisions about what should be done at each moment
Motor output Causes a response by activating effector organs (muscles and glands)
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Organization There is only one, highly integrated nervous system
Basic divisions of the nervous system
Central Nervous Systems
Peripheral Nervous System
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Organization In order to discuss the nervous in smaller portions, for convenience the nervous system is divided into two parts The central nervous system
•Brain and spinal cord•Integrative and control centers
The peripheral nervous system•Spinal and cranial nerves•Communication lines between the CNS and the rest of the body
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Organization of the Nervous System
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Organization The peripheral nervous system has two fundamental subdivisions Sensory (afferent) division
•Somatic and visceral sensory nerve fibers
•Consists of nerve fibers carrying impulses to the central nervous system
Motor (efferent) division•Motor nerve fibers•Conducts impulses from the CNS to effectors– (glands and muscles)
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Organization of the Nervous System
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Organization The motor division of the peripheral nervous system has two main subdivisions The Somatic motor
•Voluntary motor•Conducts impulses from the CNS to skeletal muscle
The Visceral motor•Involuntary motor•Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands
•Equivalent to the autonomic nervous system (ANS)
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Organization of the Nervous System
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Peripheral Nervous System
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Organization of the Nervous System
Somatic sensory The sensory receptors that are spread widely throughout the outer tube of the body
These include the many senses experienced on the skin and in the body wall, such as touch, pain, pressure, vibration and temperature
Proprioception provides feedback from the stretch of the muscles, tendons and joint capsules - your “body sense”
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Organization of the Nervous System
Somatic sensory The special somatic senses are receptors are more localized and specialized
The special senses include; sight, hearing, balance, smell and taste.
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Organization of the Nervous System
Visceral sensory The general visceral senses include stretch, pain, and temperature which can be felt widely in the digestive and urinary tracts, reproductive organs, and other viscera
Sensations such as hunger and nausea are also general visceral sensations
The chemical senses such as taste and smell are considered by some as special visceral senses
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Organization of the Nervous System
Somatic motor The general somatic motor is part of the PNS that stimulates contraction of the skeletal muscle in the body
Also referred to as voluntary nervous system
Skeletal muscles are widely distributed throughout the body, and therefore there is no special somatic motor category
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Organization of the Nervous System
Visceral motor The general visceral motor part of the PNS regulates the contraction of smooth and cardiac muscle and secretion by the body’s many glands
General visceral motor neurons make up the autonomic nervous (ANS) which controls the function of the visceral organs
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Organization of the Nervous System
Visceral motor Because we generally have no voluntary control over such activities as the pumping of the heart and movement of food through the digestive tract
The ANS is also called the involuntary nervous system
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Organization of ANS The autonomic nervous system has two principle subdivisions Sympathetic division
•Mobilizes body systems during emergency situations
Parasympathetic division•Conserves energy•Promotes non-emergency functions
The two subdivisions bring about opposite effects on the same visceral organs
What one subdivision stimulates, the other inhibits
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Nervous Tissue The nervous system consists mostly of nervous tissue whose cells are densely packed and tightly intertwined
Nervous tissue is made up just two main types of cells Neurons - the excitable cells that transmit electrical signals
Neuroglia - nonexcitable supporting cells that surround and wrap the neurons
Both cell types develop from the same embryonic tissues: neural tube and crest
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The Neuron The human body contains many billions of neurons which are the basic structural units of the nervous system
Neurons are highly specialized cells that conduct electrical signals from one part of the body to another
These signals are transmitted along the plasma membrane in the form of nerve impulses or action potentials
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Neurons Neurons are the structural units of the nervous system
Neurons are highly specialized cells that conduct messages in the form of nerve impulses from one part of the body to another
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The Neuron Special characteristics of neurons They have extreme longevity. Neurons can and must function over a lifetime
They do not divide•As fetal neurons assume their roles as communication links in the nervous system, they lose their ability to undergo mitosis
•Cells cannot be replaced if destroyed - Some limited exceptions do exist in the CNS as neural stem cells have been identified
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The Neuron Special characteristics of neurons They have an exceptionally high metabolic rate requiring continuous and abundant supplies of oxygen and glucose
Neurons cannot survive for more than a few minutes without oxygen
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The Neuron Neurons are typically large, complex cells
Neurons vary in their structure but they all have two fundamental components Neuron cell body One or more processes
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The Cell Body The cell body of the neuron is also called a soma
The cell bodies of different neurons vary widely in size (from 5 to 140 m in diameter)
However, all consist of a single nucleus surrounded by cytoplasm
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The Cell Body Typically large, complex cells, neurons have the following structures Cell body
•Nuclei•Chromatophilic (Nissl) bodies•Neurofibrils•Axon hillock
Cell processes•Dendrites•Axon•Myelin sheath or neurilemma
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The Cell Body Cell Body
Nuclei Chromatophilic (Nissl) bodies
Neurofibrils Axon hillock
Neuron Processes
Dendrites Axons Myelin sheaths Axon terminals
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The Cell Body In all but the smallest neurons, the nucleus is spherical and clear and contains a nucleolus near its center
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The Cell Body The cytoplasm contains all the usual cellular organelles with the exception of centrioles (not needed in amitotic cells) as well as Nissl bodies
These cellular organelles continually renew the membranes of the cell
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The Cell Body Neurofibrils are bundles of intermediate filaments that run in a network between chromatophilic bodies
These filaments keep the cell from being pulled apart when it is subjected to tensile forces
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The Cell Body The cell body is the focal point for the outgrowth of the neuron processes during embryonic development
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The Cell Body In most neurons, the plasma membrane of the cell body acts as a receptive surface that receives signals from other neurons
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The Cell Body Most neuron cell bodies are located within the CNS
However, clusters of cell bodies called ganglia (singular ganglion) lie along the nerves in the PNS
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Neuron Processes Bundles of neuron processes in the CNS are called tracts
Bundles of neuron processes in the PNS are called nerves
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Neuron Processes Armlike processes extend from the cell bodies of all neurons
There are two types of processes
Dendrites Axons
Motor neuron
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Neuron Processes
The cell processes of neurons are described here using a motor neuron
Motor neurons represent a typical neuron, but sensory neurons differ from the typical pattern
Motor neuron
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Dendrites Dendrites are short, tapering, diffusely branching extensions from the cell body
Motor neurons have hundreds of dendrites clustering close to the cell body
Dendrites function as receptive cites providing an enlarged area for the reception of signals from other neurons
By definition, dendrites conduct electrical signals toward the cell body
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Dendrites Dendritic spines represent areas of close contact with other neurons
These electrical signals are not nerve impulses but are short distance signals call graded potentials
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Axons Each neuron has only one axon
The axon arises from the cone shaped axon hillock
It narrows to form a slender process that stays uniform in diameter the rest of its length
Length varies; short or absent to 3 feet in length
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Axons Each axon is called a nerve fiber
Axons are impulse generators and conductors that transmit nerve impulses away from the cell body
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Axons Chromatophilic bodies (Nissl) and the Golgi apparatus are absent from the axon and the axon hillock
Axons lack ribosomes and all organelles involved in protein synthesis, so they must receive their proteins from the cell body
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Axons Neurofilaments, actin microfilaments, and microtubules are especially evident in axons, where they provide structural strength
These cytoskeletal elements also aid in the transport of substances to and from the cell body as the axonal cytoplasm is recycled and renewed
The movement of substances along axons is called axonal transport
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Axons The axon of some neurons are short, but in others it can be extremely long
Motor neurons in the CNS have axons that must reach to the musculature that they control that might be 3-4 feet away
Any long axon is called a nerve fiber and travels in a group of fibers composing a nerve
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Axons Although axons branch far less frequently than dendrites, occasional branches do occur along their length
These branches, called axon collaterals, extend from the axon at almost right angles
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Axons Axons branch profusely at its terminus
Ten thousand of these terminal branches per neuron is not unusual
These branches end in knobs called axon terminals or boutons
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Axon A nerve impulse is typically generated at the axon’s initial segment and is conducted along the axon to the axon terminals, where it causes the release of chemicals called neurotransmitters into the extracellular space
The neurotransmitters excite or inhibit the neurons or target organs with which the axon is in close contact
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Axon Axon diameter varies considerably among the different neurons of the body
Axons with larger diameters conduct impulses faster than those with smaller diameters
Neurons follow the law of physics: The resistance to the passage of an electrical current decreases as the diameter of any “cable” increases
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Synapses The site at which neurons communicate is called a synapse
Most synapses in the nervous system transmit information through chemical messengers
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Synapses Some neurons in certain areas of the CNS transmit signals electrically through gap junctions
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Synapses Because signals pass across most synapses in one direction only, synapses determine the direction of information flow through the nervous system
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Synapses The neuron that conducts signals toward a synapse is called the presynaptic neuron; the neuron that transmits signals away from the synapse is called the postsynaptic neuron
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Synapses Most neurons in the CNS function as both presynaptic (information sending) and postsynaptic (information receiving) neurons, getting information from some neurons and dispatching it to others
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Synapses There are two main types of synapses
Most synapses occur between the axon terminals of one neuron and the dendrites of another neuron
These are called axondendritic synapses
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Synapses Many synapses also occur between axons and neuron cell bodies
These synapses are called axosomatic synapses
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Synapses Synapses are elaborate cell junctions
This section shows axodendritic synapses because its structure is representative of both types of synapses
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Synapses Structurally synapses are elaborate cell junctions
At the typical axodendritic synapse the presynaptic axon terminal contain synaptic vesicles
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Synapses Synaptic vesicles are membrane bound sacs filled with molecular neurotransmitters
These molecules transmit signals across the synapse
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Synapses Mitochondria are abundant in the axon terminal as the secretion of neurotransmitters requires a great deal of energy
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Synapses At the synapse, the plasma membranes of the two neurons are separated by a synaptic cleft
On the under surfaces of the opposing cell membranes are dense materials; the pre- and post- synaptic densities
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Synapses When an impulse travels along the axon of the presynaptic neuron, it signals the synaptic vesicles to fuse with the presynaptic membrane at the presynaptic density
The fused area then ruptures releasing neurotransmitter molecules to diffuse across the synaptic cleft and bind to the postsynaptic membrane at the post synaptic density
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Synapse The binding of the two membranes changes the membrane charge on the postsynaptic neuron, influencing the membrane’s ability to generate a nerve impulse
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Signals Carried by Neurons
Neurons carry information via electrical signals called nerve impulses, or action potentials
Signals are relayed from neuron to neuron via chemical neurotransmitters
In essence an impulse is a reversal of electrical charge that travels rapidly along the neuronal membrane
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Signals Carried by Neurons
In a resting (un-stimulated) neuron, the membrane is polarized which means that the inner cytoplasmic side is negatively charged with respect to its outer, extracellular side
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Signals Carried by Neurons
In addition, the concentration of potassium ions (K+) is higher inside the neuron and the concentration of sodium ions (Na+) is higher outside the neuron
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Signals Carried by Neurons
When a neuron is stimulated the permeability of the plasma membrane changes at the site of the stimulus, allowing Na+ ions to rush in.
As a result, the inner face of the membrane becomes less negative or depolarized
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Signals Carried by Neurons
If the stimulus initiating the depolarization is strong enough, the membrane at the axon’s initial segment is depolarized, so that it is positively charged inside the axon and negatively charged outside
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Signals Carried by Neurons
Once begun, this depolarization occurs all along the axon length
It is this wave of charge reversal that constitutes the nerve impulse
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Signals Carried by Neurons
The impulse travels rapidly down the entire length of the axon without decreasing in strength
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Signals Carried by Neurons
After the impulse has passed the membrane repolarizes itself
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Signals Carried by Neurons
Neurons in the body receive stimuli either directly from the environment or from signals received at synapses
In signals received at synapses neurotransmitters released by presynaptic neurons alter the permeability of the postsynaptic membrane to certain ions
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Signals Carried by Neurons
Synapses that result in an influx of positive ions into the postsynaptic neuron depolarize the neuron’s membrane and bring the neuron closer to impulse generation
These synapses are called excitatory synapses because they stimulate the postsynaptic neuron
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Signals Carried by Neurons
Other synapses increase membrane polarization, making the external surface of the postsynaptic cell even more positive than it was
This makes the postsynaptic cell less likely to generate a nerve impulse
These types of synapses are called inhibitory synapses because they reduce the ability of the postsynaptic neuron to generate a nerve impulse
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Signals Carried by Neurons
Thousands of excitatory and inhibitory synapses act on every neuron, competing to determine whether or not that neuron will generate an impulse
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Classification of Neurons
Neurons can be classified structurally or functionally
Neurons are grouped structurally according to the number of processes that extend from the cell body
By this classification there are three types of neurons; Multipolar Bipolar Unipolar
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Classification of Neurons Multipolar - many processes extend from cell body, all dendrites except one axon
Bipolar - Two processes extend from cell, one a fused dendrite, the other an axon
Unipolar - One process extends from the cell body and forms the peripheral and central process of the axon
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Classification of Neurons
Multipolar neurons usually have a single axon and many dendrites
This type of neuron constitutes 99% of the neurons in the body
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Classification of Neurons
Multipolar neurons have more than two processes
Most common type in humans
Major neuron of the CNS
Some neurons lack an axon
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Classification of Neurons
Bipolar neurons have two processes that extend from opposite sides of the cell body
This rare type of neuron occurs in the special sensory organs
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Classification of Neurons
Bipolar neurons are found only in special sense organs where they function as receptor cells
Examples include those found in the retina of the eye, inner ear, and epithelium of the olfactory mucosa
They are primarily sensory neurons
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Classification of Neurons
Unipolar neurons have a short, single process that emerges from the cell body and divides like a “T” into two long branches
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Classification of Neurons
Unipolar neurons have a single process that emerges from the cell body
The central process (axon) is more proximal to the CNS and the peripheral is closer to the PNS
Unipolar neurons are chiefly found in the ganglia of the peripheral nervous system
Function primarily as sensory neurons
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Functional Classification
The functional classification scheme groups neurons according to the direction in which the nerve impulse travels relative to the CNS
Based on this criterion there are three types of neurons Sensory neurons Motor neurons Interneurons
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Functional Classification
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Sensory Neurons
These afferent neurons make up the sensory division of the PNS
They transmit impulses toward the CNS from sensory receptors in the PNS
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Sensory Neurons
Sensory neurons have their cell bodies in ganglia outside of the CNS
The single (unipolar) process is divided into the central process and the peripherial process
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Sensory Neuron The central process is clearly an axon because it carries a nerve impulse and carries that impulse away from the cell body which meet the criteria which define an axon
The peripheral by contrast carries nerve impulses toward the cell body which suggests that it is a dendrite
However, the basic convention is that the central process and the peripheral process are parts of a unipolar neuron
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Motor Neurons
Neurons that carry impulses away from the CNS to effector organs (muscles and glands) are called motor or efferent neurons
Upper motor neurons are in the brain
Lower motor neurons are in PNS
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Motor Neurons
Motor neurons are multipolar and their cell bodies are located in the CNS (except autonomic)
Motor neurons form junctions with effector cells, signaling muscle to contract or glands to secrete
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Interneuron or Association Neurons These neurons
lie between the motor and sensory neurons
Form complex neural pathways
Confined to CNS Make up 99.98% of the neurons of the body and are the principle neuron of the CNS
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Interneuron Neurons
Almost all interneurons are multipolar Interneurons show great diversity in the size and branching patterns of their processes
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Interneurons The Pyramidal cell is the large neuron found in the primary motor cortex of the cerebrum
The Purkinje cell is from the cerebellum
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Supporting Cells All neurons associate closely with non-nervous supporting cells called neuroglia Support cells of the CNS
•Astrocytes•Microglial•Ependymal•Oligodendrocyte
Support cells of the PNS•Schwann cells•Satellite cells
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Supporting Cells While each support cell has a unique specific function, in general these cells provide a supportive scaffolding for neurons
In addition, they all cover nonsynaptic parts of the neurons thereby insulating the neurons and keeping the electrical activities of adjacent neurons from interfering with each other
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Supporting Cells in the CNS
Like neurons, glial cells have branching processes and a central cell body
Neuroglia can be distinguished from neurons by their much smaller size and darker staining nuclei
They outnumber neurons in the CNS by a ratio of 10 to 1
Make up half of the mass of the brain
Unlike neurons, glial cells divide throughout one’s lifetime
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Astrocytes Star shaped Most abundant type of glial cell
Radiating projections cling to neurons and capillaries, bracing the neurons to their blood supply
Astrocytes play a role in exchanges of ions between capillaries and neurons
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Astrocytes Astrocytes take up and release ions to control the environment around neurons
Concentrations of ions must be kept within narrow limits for nerve impulses to be generated & conducted
Astrocytes recapture and recycle potassium ions and released neuro- transmitters
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Astrocytes Astrocytes contact both the neuron and the capillary in order to sense when the neuron are highly active and releasing large amounts of neurotransmitters (glutamate)
Astrocytes then extract blood sugar from the capillaries they contact to obtain the energy they need to fuel the process of glutamate uptake
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Astrocytes Astrocytes also are involved with synapse formation in developing neural tissue, produce molecules necessary for neural growth (brain-derived trophic factor BDTF) and propagate calcium signals that may be involved in memory
Understanding the role of these abundant glial cells in neural functioning is an area of ongoing research
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Microglial Smallest and least abundant type of neuroglial cell
The elongated cells have relatively long “thorny” processes
They are phagocytes, the macrophages of the CNS
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Microglial Microglial derive from blood cells and migrate to the CNS during embryonic and fetal development
Microglial engulf invading microogranisms and injured or dead neurons
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Microglial When invading micro- organisms are present or damaged neurons have died, the micro- glial transforms into a special type of macro- phage that protects the CNS by phagocytizing the microorganisms or neuronal debris
Important because cells of the immune system can enter CNS
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Ependymal CNS tissue originates in the embryo as a hollow neural tube and retains a central cavity throughout life
Form a simple epithelium that lines the central cavity of the spinal cord and brain
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Ependymal Forms a fairly permeable barrier between cerebrospinal fluid of those cavities and the cells of the CNS
Ependymal cells bear cilia that helps circulate the cerebrospinal fluid
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Oligodendro- cytes
Fewer branches than astrocytes
Cells wrap their cytoplasmic extensions tightly around the thicker neurons in the CNS
Produce insulating coverings called myelin sheaths
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Neuroglia in the PNS There are two supporting cells in the PNS Satellite cells Schwann cells
These cells are similar in type and differ mainly in location
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Satellite Cells
Somewhat flattened satellite cells surround cell bodies within ganglia
Thought to play some role in controlling the chemical environment of neurons with which they are associated, but function is largely unknown
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Schwann Cells
Surround and form myelin sheaths around the larger nerve fibers in PNS
Similar to the oligodendrocytes of CNS Schwann cells are vital to peripheral nerve fiber regeneration
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Myelin Sheaths Myelin sheaths are produced by oligo dendrocytes in the CNS and Schwann cells in the PNS
Myelin sheaths are segmented structures, each composed of the lipoprotein myelin and surround the thicker axons of the body
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Myelin Sheaths
Each segment of myelin consists of a plasma membrane of a supporting cell rolled in concentric layers around the axon
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Myelin Sheaths
Myelin sheaths form an insulating layer that… Prevents the leakage of electrical current from the axon
Increases the speed of impulse conduction
Makes impulse propagation more energy efficient
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Myelin Sheaths in PNS Myelin sheaths in the PNS are formed by Schwann cells
Myelin sheaths develop during the fetal period and continue to develop during the first year of postnatal life
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Myelin Sheaths in the PNS
In forming, the cells indent to receive the axon and then wrap themselves around the axon repeatedly in a jellyroll fashion
Initially loose, the wrapping eventually squeeze the cytoplasm outward between cell membrane layers
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Myelin Sheaths in the PNS
When the process is complete many concentric layers of Schwann cell plasma membrane wrap the axon in tightly packed coil of membranes
The nucleus and most of the cytoplasm of the Schwann cell end up just external to the myelin layers
This external material is called the neurilemma
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Myelin Sheaths - PNS
Because the adjacent Schwann cells along a myelinated axon do not touch one another, there are gaps in the myelin sheath
These gaps called the Nodes of Ranvier, occur at regular intervals about 1mm apart
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Myelin Sheaths - PNS
In myelinated axons, nerve impulses do not travel along the myelin-covered regions of the axonal membrane, but instead jumps from the membrane of one Node of Ranvier to the next greatly increasing impulse conduction
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Myelin Sheaths in the PNS
Only thick, rapidly conducting axons are sheathed in myelin
Thin, slowly conducting axons lack a myelin sheath and are called unmyelinated axons
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Myelin Sheaths in the PNS
In unmyelinated axons the Schwann cells surround the axons but do not wrap around them in concentric layers of membrane
A single Schwann cell can partly enclose 15 or more unmyelinated axons with each in a separate tubular recess on the surface of the cell
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Myelin Sheath Myelin increases the speed of transmission of nerve impulses
Myelinated axons transmit nerve impulses rapidly; 150 meters/second
Unmyelinated axons transmit quite slowly; 1 meter/second
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Myelin Sheaths in the PNS
Unmyelinated axons are found in portions of the autonomic nervous system as well as in some sensory fibers
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Myelin Sheaths of the PNS
Electron micrograph of an unmyelinated axon
Note the tubular tunnels that separate the axons
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Myelin Sheaths in the CNS
Oligodendrocytes form the myelin sheaths in the brain and spinal cord
Each oligodendrocyte has multiple processes that coil around several different axons
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Myelin Sheaths - PNS The nucleus
of the cell and most of the cytoplasm end up just external to the myelin layers
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Myelin Processes - PNS Myelin sheaths are associated only with axons and their collaterals as these are impulse conducting fibers and need insulation
Dendrites which carry only graded potentials are always unmyelinated
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Myelin Sheaths - PNS When the wrapping process is complete many concentric layers wrap the axon
Plasma membranes of myelinating cells have less protein which makes them good electrical insulators
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Myelin Sheaths - PNS Because the adjacent Schwann cells do not touch one another there are gaps in the myelin sheath
These gaps, called nodes of Ranvier, occur at regular intervals about 1 mm apart
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Myelin Sheaths - PNS Since the axon is only exposed at these nodes nerve impulses are forced to jump from one node to the next which greatly increases the rate of impulse conduction
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Myelin Sheaths - PNS Schwann cells that surround but do not coil around peripheral fibers are considered unmyelinated
A single Schwann cell can partly enclose 15 or more axons
Each ends occupying a separate tubular recess
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CNS Axons Oligodendrocytes form the CNS myelin sheaths
In contast to Schwann cells, oligodendrocytes can form the sheaths of as many as 60 processes at one time
Nodes are spaced more widely than in PNS
Axons can be myelinated or unmyelinated
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CNS Axons Regions of the brain containing dense collections of myelinated fibers are referred to as white matter and are primarily fiber tracts
Gray matter contains mostly nerve cell bodies and unmyelinated fibers
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Graded Potential In humans, natural stimuli are not applied directly to axons, but to dendrites and the cell body which constitute the receptive zone of the neuron
When the membrane of this receptive zone is stimulated it does not undergo a polarity reversal
Instead it undergoes a local depolarization in which the inner surface of the membrane merely becomes less negative
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Graded Potential This local depolarization is called a graded potential which spreads from the receptive zone to the axon hillock (trigger zone) decreasing in strength as it travels
If this depolarizing signal is strong enough when it reaches the initial segment of the axon, it acts as the trigger that initiates an action potential in the axon
Signals from the receptive zone determine if the axon will fire an impulse
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Synaptic Potential Most neurons in the body do not receive stimuli directly from the environment but are stimulated only by signals received at synapses from other neurons
Synaptic input influences impulse generation through either excitatory or inhibitory synapses
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Synaptic Potential In excitatory synapses, neurotransmitters released by presynaptic neurons alter the permeability of the postsysnaptic membrane to certain ions, this depolarizes the postsynapatic membrane and drives the postsynaptic neuron toward impulse generation
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Synaptic Potential Inhibitory synapses cause the external surface of the postsynaptic membrane to become even more positive, thereby reducing the ability of the postsynaptic neuron to generate an action potential
Thousands of excitatory and inhibitory synapses act on every neuron, competing to determine whether or not that neuron will generate an impulse
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Neural Integration The organization of the nervous system is hierarchical
The parts of the system must be integrated into a smoothly functioning whole
Neuronal pools represent some of the basic patterns of communication with other parts of the nervous system
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Neuronal Pools Note: The illustrations presented are a gross oversimplification of an actual neuron pool
Most neuron pools consist of thousands of neurons and include inhibitory as well as excitatory neurons
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Neuronal Pools Neuronal pools are functional groups of neurons that process and integrate incoming information from other sources and transmit it forward
One incoming presynaptic fiber synapses withSeveral different neurons in the pool. WhenIncoming fiber is excited it will excite somePostsynaptic neurons and facilitate others.
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Neuronal Pools Neurons most likely to generate impulses are those most closely associated with the incoming fiber because they receive the bulk of the synaptic contacts
These neurons are in the discharge zone
Discharge Zone
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Neuronal Pools Neurons farther away from the center are not excited to threshold by the incoming fiber, but are facilitated and can easily brought to threshold by stimuli from another source
The periphery of the pool is the facilitated zone
Facilitatedzone
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Types of Circuits Individual neurons in a neuron pool send and receive information and synaptic contacts may cause either excitation or inhibition
The patterns of synaptic connections in neuronal pools are called circuits and they determine the functional capabilities of each type of circuit
There are four basic types of circuits Diverging, converging, reverberating, and parallel discharge circuits
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Diverging Circuits In diverging circuits one incoming fiber triggers responses in ever-increasing numbers of neurons farther and farther along in the circuit
Diverging circuits are often called amplifying circuits because they amplify the response
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Diverging Circuits These circuits are common in both sensory and motor systems
Input from a single receptor may be relayed up the spinal cord to several different brain regions
Impulses from the brain can activate a hundred neurons and thousands of muscle fibers
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Converging Circuits The pattern of converging circuits is opposite to that of diverging circuits
Common in both motor and sensory pathways
In these circuits, the pool receives inputs from several presynaptic neurons, and the circuit as a whole has a funneling or concentrating effect
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Converging Circuits Incoming stimuli may converge from many different areas or from the same source, which results in strong stimulation or inhibition
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Reverberating (oscillating) Circuits
In reverberating circuits the incoming signal travels through a chain of neurons, each of which makes collateral synapses with neurons in the previous part of the pathway
As a result of this positive feedback, the impulses reverberates through the circuit again and again
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Reverberating (oscillating) Circuits
Reverberating circuits give a continuous signal until one neuron in the circuit is inhibited and fails to fire
These circuits are involved in the control of rhythmic activities such as the sleep-wake cycle and breathing
The circuits may oscillate for seconds, hours, or years
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Parallel After-Discharge Circuits The incoming fiber
stimulates several neurons arranged in parallel arrays that eventually stimulate a common output cell
Impulses reach the output cell at different times, creating a burst of impulses called an after discharge that may last 15 ms after initial input ends
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Parallel After-Discharge Circuits
This circuit has no positive feedback and once all the neurons have fired, circuit activity ends
These circuit may be involved with complex problem solving activities
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Patterns of Neural Processing
Processing of inputs in the various circuits is both serial and parallel
In serial processing, the input travels along a single pathway to a specific destination
In parallel processing, the input travels along several different pathways to be integrated in different CNS regions
Each pattern has its advantages The brain derives its power from its ability to process in parallel
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Serial Processing In serial processing the whole system works in a predictable all-or-nothing manner
One neurons stimulates the next in sequence, producing a specific, anticipated response
Reflexes are examples of serial processing but there are others
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Parallel Processing In parallel processing inputs are segregated into many different pathways
Information delivered by each pathway is dealt with simultaneously by different parts of neural circuitry
During parallel processing several aspects of the stimulus are processed Barking dog
The same stimulus can hold common or unique meaning to different individuals
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Parallel Processing Parallel processing is not repetitious because the circuits do different things with more information
Each parallel path is decoded in relation to all the others to produce a total picture of the stimulus
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Parallel Processing Even simple reflex arcs do not operate in complete isolation
As an arc moves through an association neuron this activates parallel processing of the same input at higher brain levels
The reflex arc may cause you to pull away from a negative stimulus while parallel processing of the stimulus initiates problem solving about what need to be done
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Parallel Processing Parallel processing is extremely important for higher level mental functioning
An integrated look at the whole problem allows for faster processing
Parallel processing allows you to store a large amount of information in a small volume
This allows logic systems to work much faster
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Reflexes Reflexes are rapid, automatic responses to stimuli, in which a particular stimulus always causes the same motor response
Reflex activity is stereotyped and dependable
Some your are born with and some you acquire as a consequence of interacting with your environment
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Reflex Arcs Reflex arcs are simple chains of neurons that explain our simplest, reflective behaviors and determine the basic structural plan of the nervous system
Reflex arcs are responsible for reflexes, which are defined as rapid, automatic motor responses to stimuli
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Reflex Arcs Reflexes that involve the contraction of skeletal muscle are referred to as somatic reflexes
Reflexes that involve the contraction of smooth muscle, cardiac muscle, or glands are referred to as visceral reflexes
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Serial Processing: A Reflex Arc
Reflexes occurs over neural pathways called reflex arcs that contain five essential components
Receptor Sensory neuron CNS integration center Motor neuron Effector
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Reflex Arcs The receptor, sensory neuron, motor neuron, and effector are all relatively straightforward components
When considering the integration center associated with reflex arcs, it is important to understand that the number of synapses involved can vary
The simplest reflex arcs involve only one synapse in the CNS while others involve multiple synapses and interneurons
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Reflex Arcs
At the top is a reflex arc, at the left is a monosynaptic reflex and on the right is a poly synaptic reflex
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Reflex Arcs
The monosynaptic reflex has only one synapse and no interneuron, while the polysynaptic has multiple synapses and an interneuron
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Reflex Arcs - Monosynaptic
This is the simple knee-jerk reflex
The impact of the hammer on the patellar tendon stretches the quadriceps muscles
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Reflex Arcs - Monosynaptic
Stretching activates a sensory neuron that directly activates a motor neuron in the spinal cord, which then signals the quadriceps muscle to contract
This contraction counteracts the original stretching caused by the hammer
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Reflex Arcs - Monosynaptic
Many skeletal muscles of the body can be activated by monosynaptic stretch reflexes
These reflexes help maintain equilibrium and upright posture
In these postural muscles, sensory neurons sense the stretching of muscles that occurs when the body begins to sway
Motor neurons activate muscles that adjust the body’s position to prevent a fall
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Reflex Arcs - Monosynaptic
Because stretch reflexes contain just one synapse monosynaptic reflexes are the fastest of all reflexes
They are used in the body to maintain balance and equilibrium where speed of adjustment is essential to keep from falling
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Reflex Arcs - Polysynaptic
Polysynaptic reflexes are the more common reflexes in the body
In these reflexes, one or more interneurons are part of a reflex pathway between the sensory and motor neurons
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Reflex Arcs - Polysynaptic
Most of the simple reflex arcs in the body contain a single interneuron and therefore have a total of three neurons
Since there are two synapses joining the three neurons they are referred to as polysynaptic
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Reflex Arcs - Polysynaptic
Withdrawal reflexes by which we pull away from danger are three-neuron reflexes
Pricking a finger with a tack initiates an impulse in the sensory neuron, which activates the interneuron in the CNS
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Reflex Arcs - Polysynaptic
The interneuron signals the motor neuron to contract the muscle that withdraws the hand from the negative stimulus
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Reflex Arcs - Polysynaptic
The three neuron reflex arc are of special importance in the science of neuroanatomy
Three neuron reflex arcs reveal the fundamental design of the entire nervous system
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Design of the Nervous System
Three neuron reflex arcs from the basis of the structural plan of the nervous system
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Design of the Nervous System
Note that the cell bodies of the sensory neurons lie outside the CNS in sensory ganglia and that their central processes enter the dorsal aspect of the cord
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Design of the Nervous System
In the CNS the cell bodies of most interneurons lie dorsal to those of the motor neurons and the long axons exit the ventral aspect of the spinal cord
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Design of the Nervous System
The nerves of the PNS consist of the motor axons plus the long peripheral process of the sensory neurons
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Design of the Nervous System
These motor and sensory nerve fibers extend throughout the body to reach the peripheral effectors and receptors
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Design of the Nervous System
Even though reflex arcs determine its basic organization, the human nervous system is obviously more complex than a series of simple reflex arcs
To appreciate its complexity, we must expand our conception of interneurons
Interneurons include not only the inter- mediate neurons of reflex arcs, but also all the neurons that are entirely confined within the CNS
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Design of the Nervous System
The complexity of the CNS arises from the organization of the vast numbers of interneurons in the spinal cord and brain into complex neural circuits that process information
The complexity of the CNS results from long chains of interneurons that are interposed between each sensory and motor neuron
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Design of the Nervous System
Although tremendously oversimplified, the infor-mation depicted is a useful way to conceptualize the organization of neurons in the CNS
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Design of the Nervous System
The CNS has distinct regions of gray and white matter that reflect the arrangement of its neurons
The gray matter is a gray colored zone that surrounds the hollow cavity of the CNS
It is H-shaped in the spinal cord, where its dorsal half contains cell bodies of interneurons and its ventral half contains cell bodies of motor neurons
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Design of the Nervous System
Gray matter is a site where neuron cell bodies are clustered
Specifically, gray matter is a mixture of neuron cell bodies, dendrites, and short unmyelinated axons
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Design of the Nervous System
White matter which contains no neuron cell bodies but millions of axons
Its white color comes from the myelin sheaths around many of the axons
Most of these axons ascend from the spinal cord to the brain or descend from the brain to the spinal cord, allowing these two regions of the CNS to communicate with each other
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Design of the Nervous System
White matter consists of axons running between different parts of the CNS
Within the white matter, axons traveling to similar destinations form axon bundles called tracts
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Nervous Tissue Development
During the embryonic period, which spans 8 weeks, the embryo goes from zygote to blastocyst, to two layer embryo, to three layer embryo
The embryo upon reaching three layers begins to form the neural tube from which will differentiate the brain and spinal cord
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Nervous Tissue Development
The nervous system develops from the dorsal section of the ectoderm, which invaginates to form the neural tube and the neural crest
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Nervous System Development
The walls of the neural tube begin as a layer of neuroepithelial cells become the CNS
These cells divide, migrate externally, and become neuroblasts (future neurons) which never again divide
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Nervous System Development
These cells divide, migrate externally, and become neuroblasts (future neurons) which never again divide
They cluster as future interneurons and motor neurons
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Nervous System Development
Just external to the neuroepithelium, the neuroblasts cluster into alar and basal plates
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Nervous System Development
Dorsally, the neurons of the alar plate become interneurons
Ventrally, the neuroblasts of the basal plate become motor neurons and sprout axons that grow out to the effector organs
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Nervous System Development
Axons that sprout from the young interneurons form the white matter by growing outward the length of the CNS
These events occur in both the spinal cord and the brain
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Nervous System Development
Most of the events described take place in the second month of development, but neurons continue to form rapidly until the about the sixth month
At the sixth month neuron formation slows markedly, although it may continue at a reduced rate into childhood
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Nervous System Development
Just before neuron formation slows, the neuroepithelium begins to produce astrocytes and oligiodendrocytes
The earliest of these glial cells extend outward from the neuroepithelium and provide pathways along which young neurons migrate to reach their final destination
As the division of its cells slows, the neuroepithelium becomes the ependymal layer
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Nervous System Development
Sensory neurons do not arise from the neural tube but from the neural crest
This explains why the cell bodies of the sensory neurons lie outside the CNS
Sensory neurons also stop dividing during the fetal period
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Nervous System Development
Sensory neurons cell bodies develop outside the CNS in the neural crest
Sensory neurons also stop dividing during the fetal period
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Nervous System Development
Neuroscientists are actively investigating how forming neurons “hook up” with each other during development
As the growing axons elongate at growth cones, they are attached by chemical signals from other neurons called neurotrophins
At the same time, the receiving dendites send out thin, extensions to reach the approaching axons to form synapses
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Nervous System Development
Which synaptic connections are made, and which persist, are determined by two factors; The amount of neurotrophin initially received
The degree to which a synapse is used after being established
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Nervous System Development
Neurons that make “bad” connections are signaled to die via apoptosis
Of the neurons formed during the embryonic period, about two-thirds die before birth
This initial overproduction of neurons ensures that all necessary neural connections will be made and that mistaken connections will be eliminated