Nervous System
description
Transcript of Nervous System
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Nervous System
• Master control and communication system
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Nervous System: Functions
• Three overlapping functions• Sensory receptors monitor changes inside and
outside the body• Change – a stimulus
• Gathered information – sensory input
• CNS Processes and interprets sensory input• Makes decisions – integration
• Dictates a response by activating effector organs• Response – motor output
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Basic Divisions of the Nervous System: CNS
• Central nervous system (CNS)• Brain and spinal cord
• Integrating and command center
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Basic Divisions of the Nervous System: PNS
• Peripheral nervous system (PNS)• Outside the CNS• Nerves extending
from brain and spinal cord• Cranial nerves• Spinal nerves
• Link all regions of the body to the CNS
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Sensory Input and Motor Output
• Sensory signals picked up by sensory receptors• Carried by afferent nerve fibers of PNS to the CNS
• Motor signals are carried away from the CNS • Carried by efferent nerve fibers of PNS to effectors
• Innervate muscles and glands
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Sensory Input and Motor Output
• Divided according to region they serve• Somatic body region
• Visceral body region
• Results in four main subdivisions• Somatic sensory
• Visceral sensory
• Somatic motor
• Visceral motor
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Somatic Sensory
• Somatic sensory • General somatic senses – receptors are widely
spread • Touch, pain, vibration, pressure, and temperature
• Proprioceptive senses – detect stretch in tendons and muscle
• Body sense – position and movement of body in space
• Special somatic senses • Hearing, balance, vision, and smell
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Visceral Sensory
• Visceral sensory• General visceral senses – stretch, pain,
temperature, nausea, and hunger• Widely felt in digestive and urinary tracts,
reproductive organs
• Special visceral senses – taste
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Somatic Motor
• Somatic motor• General somatic motor – signals contraction of
skeletal muscles• Under voluntary control
• Often called “voluntary nervous system”
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Visceral Motor
• Visceral motor• Regulates the contraction of smooth and cardiac
muscle and gland secretion
• Makes up autonomic nervous system
• Controls function of visceral organs
• Often called “involuntary nervous system”
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Peripheral Nervous System Summary
Figure 12.3
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Types of Sensory and Motor Information
Figure 12.3
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Types of Sensory and Motor Information
Figure 12.3
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Nervous Tissue
• Cells are densely packed and intertwined • Two main cell types
• Neurons – transmit electrical signals
• Support cells (neuroglial cells) – nonexcitable
• Surround and wrap neurons
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
The Neuron
• The human body contains billions of neurons• Basic structural unit of the nervous system
• Specialized cells conduct electrical impulses along the plasma membrane
• Graded potentials
• Action potentials
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
The Neuron: Special Characteristics
• Longevity – can live and function for a lifetime
• Do not divide – fetal neurons lose their ability to undergo mitosis; neural stem cells are an exception
• High metabolic rate – require abundant oxygen and glucose
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Neuron Structure
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
The Cell Body or Soma (also called Perikaryon)
• Size varies from 5–140µm
• Contains nucleus, organelles plus other structures• Chromatophilic bodies (Nissl bodies)
• Clusters of rough ER and free ribosomes
• Stain darkly and renew membranes of the cell
• Neurofibrils – bundles of intermediate filaments
• Form a network between chromatophilic bodies
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Nissl Body Staining
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
The Cell Body
• Most neuronal cell bodies• Located within the CNS (clustered in nuclei)
• Protected by bones of the skull and vertebral column
• Ganglia – clusters of cell bodies in PNS
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Cell Body Structure
Figure 12.4
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Neuron Processes: Dendrites
• Dendrites • Extensively branching from
the cell body
• Transmit electrical signals (graded potentials) toward the cell body
• Chromatophilic bodies – only extend into the basal part of dendrites
• Function as receptive sites
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Dendritic Spines
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Neuron Processes: Axons
• Axons (nerve fibers)• Neuron has only one, but it can
branch
• Impulse generator and conductor
• Transmits action potentials away from the cell body
• Chromatophilic bodies absent
• No protein synthesis in axon
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Neuron Processes: Axons
• Axons• Neurofilaments, actin
microfilaments, and microtubules• Provide strength along
length of axon
• Aid in the transport of substances to and from the cell body
• Axonal transport
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Neuron Processes
Neuron Structure
• Axons• Branches along length are
infrequent• Axon collaterals
• Multiple branches at end of axon• Terminal branches (telodendria)
• End in knobs called axon terminals (also called end bulbs or boutons)
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Neuron Processes: Action Potentials
• Nerve impulse (action potential)• Generated at the initial segment of the
axon
• Conducted along the axon
• Releases neurotransmitters at axon terminals
• Neurotransmitters – excite or inhibit neurons
• Neuron receives and sends signals
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Synapses
• Site at which neurons communicate
• Signals pass across synapse in one direction
• Presynaptic neuron• Conducts signal toward a synapse
• Postsynaptic neuron• Transmits electrical activity away from a synapse
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Two Neurons Communicating at a Synapse
Figure 12.6
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Structure of a Synapses
Figure 12.8a, b
PLAYPLAY Synapse
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Signals Carried by Neurons: Resting Membrane Potential
• Plasma membranes of neurons conduct electrical signals
• Resting neuron – membrane is polarized
• Inner, cytoplasmic side is negatively charged
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Changes in Membrane Potential
• Signals occur as changes in membrane potential
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Directional Signals
• Stimulation of the neuron depolarization
• Inhibition of the neuron hyperpolarization
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Action Potentials
Figure 12.9a, b
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Action Potentials on Axons
• Strong depolarizing stimulus applied to the axon hillock triggers• Action potential
• Membrane becomes positive internally
• Action potential travels the length of the axon
• Membrane repolarizes itself
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Action Potentials on Axons
Figure 12.9c–e
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Graded Potentials on Dendrites and the Cell Body
• Natural stimuli applied to dendrites and the cell body• Receptive zone of the neuron
• Membrane stimulation causes local depolarization• A graded potential – inner surface becomes less
negative• Depolarization spreads from receptive zone to the
axon hillock• Acts as the trigger that initiates an action potential
in the axon
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Synaptic Potentials
• Excitatory synapses• Neurotransmitters alter the permeability of the
postsynaptic membrane
• Leads to an inflow of positive ions • Depolarizes the postsynaptic membrane
• Drives the postsynaptic neuron toward impulse generation
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Synaptic Potentials
• Inhibitory synapses• The external surface of the postsynaptic membrane
becomes more positive• Reduces the ability of the postsynaptic neuron to
generate an action potential
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Classification of Neurons
• Structural Classification
• Functional Classification
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Structural Classification of Neurons
Classification based on number of processes• Multipolar
• Bipolar
• Unipolar (pseudounipolar)
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Multipolar Neurons
Figure 12.10a–c
Possess more than two processes
Numerous dendrites and one axon
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Bipolar Neurons
Figure 12.10a–c
Possess two processes Rare neurons – found in some special sensory organs
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Unipolar (Pseudounipolar) Neurons
Figure 12.10a–c
Possess one single processStart as bipolar neurons during development
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Afferent neurons
• Afferent (sensory) neurons – transmit impulses toward the CNS• Virtually all are pseudounipolar neurons (some true
bipolar)
• Cell bodies in ganglia outside the CNS• Short, single process divides into
• The central process – runs centrally into the CNS
• The peripheral process – extends peripherally to the receptors
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Afferent Neurons
Sensory receptorsAxon terminals
Periphery CNS
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Efferent Neurons
• Efferent (motor) neurons • Carry impulses away from the CNS to effector
organs
• Most efferent neurons are multipolar
• Cell bodies are within the CNS
• Form junctions with effector cells
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Interneurons
• Interneurons (association neurons) – most are multipolar • Lie between afferent and efferent neurons
• Confined to the CNS
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Neurons Classified by Function
Figure 12.11
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Variety of Interneurons
• Purkinje cell, stellate cell, granule cell, and basket cell• Located in the cerebellum
• Pyramidal cell – located in the cerebral cortex
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Variety of Interneurons
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Glial Cells (Supporting Cells)
• Six types of glial cells• Four in the CNS
• Two in the PNS
• Provide supportive functions for neurons
• Cover nonsynaptic regions of the neurons
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Supporting Cells (Neuroglial Cells) in the CNS
• Neuroglia – usually only refers to supporting cells in the CNS, but can be used for PNS• Glial cells have branching processes and a central
cell body
• Outnumber neurons 10 to 1
• Make up half the mass of the brain
• Can divide throughout life
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Types of Glial Cells in the CNS
• Astrocytes
• Microglia
• Ependymal Cells
• Oligodendrocytes
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Astrocytes
• Astrocytes – most abundant glial cell type• Take up and release ions to control the environment
around neurons
• Recapture and recycle neurotransmitters
• Involved with synapse formation in developing neural tissue
• Produce molecules necessary for neural growth (BDTF)
• Propagate calcium signals that may be involved in memory
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Astrocytes
Figure 12.12a
Necessary for development and maintenance of theblood brain barrier
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
• Microglia – smallest and least abundant
• Phagocytes – the macrophages of the CNS
• Engulf invading microorganisms and dead neurons
• Derived from blood cells called monocytes
Microglia
Figure 12.12b
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Ependymal Cells
• Ependymal cells• Line the central cavity of the spinal cord and brain
• Bear cilia – help circulate the cerebrospinal fluid
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Oligodendrocytes
• Oligodendrocytes – have few branches• Wrap their cell processes around axons in CNS
• Produce myelin sheaths
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.13
Supporting Cells in the PNS
• Satellite cells – surround neuron cell bodies within ganglia
• Schwann cells (neurolemmocytes) – surround axons in the PNS• Form myelin sheath around axons of the PNS
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Myelin Sheaths
• Segmented structures composed of the lipoprotein myelin
• Surround thicker axons
• Form an insulating layer • Prevent leakage of electrical current
• Increase the speed of impulse conduction
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Myelin Sheaths in the PNS
• Formed by Schwann cells
• Develop during fetal period and in the first year of postnatal life
• Schwann cells wrap in concentric layers around the axon• Cover the axon in a tightly packed coil of
membranes
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Myelin Sheaths in the PNS
• Nodes of Ranvier – gaps along axon
• Allow current exchange across axon membrane
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Myelin Sheaths in the PNS
• Thick axons are myelinated• Fast conduction velocity
• Thin axons are unmyelinated• Slow conduction velocity
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Myelin Sheaths in the PNS
Figure 12.14a
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Myelin Sheaths in the PNS – myelinated axon
Figure 12.15b
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Myelin Sheaths in the PNS – unmyelinated axons
Figure 12.15b
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Myelin Sheaths in the CNS
• Oligodendrocytes form the myelin sheaths in the CNS• Have multiple processes
• Coil around several different axons
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Oligodendrocytes
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Nerves
• Nerves – cordlike organs in the PNS
• Consists of numerous axons wrapped in connective tissue
• Axon is surrounded by Schwann cells
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Nerves
• Endoneurium – layer of delicate connective tissue surrounding the axon
• Nerve fascicles – groups of axons bound into bundles
• Perineurium – connective tissue wrapping surrounding a nerve fascicle
• Epineurium – whole nerve is surrounded by tough fibrous sheath
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Simplified Design of the Nervous System
• Sensory neurons – located dorsally• Cell bodies outside the CNS in sensory ganglia
• Central processes enter dorsal aspect of the spinal cord
• Motor neurons – located ventrally • Axons exit the ventral aspect of the spinal cord
• Interneurons – located centrally • Provide communication between sensory and
motor neurons and between levels of the CNS
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Example of Neuronal Organization: Reflexes
• Reflex arcs – simple neural pathways• Responsible for reflexes
• Rapid, autonomic motor responses
• Can be visceral or somatic
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Five Essential Components to the Reflex Arc
• Receptor – detects the stimulus
• Afferent (sensory neuron) – transmits impulses to the CNS
• Integration center – consists of one or more synapses in the CNS
• Efferent (motor neuron) – conducts impulses from integration center to an effector
• Effector – muscle or gland cell• Responds to efferent impulses
• Contraction or secretion
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Example of the Five Components to the Reflex Arc
Figure 12.17
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Reflex Classification
• Monosynaptic or polysynaptic
• Spinal or cranial
• Somatic or autonomic
• Innate or learned
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Types of Reflexes: Number of Classes
• Monosynaptic reflex – simplest of all reflexes• Just one synapse
• The fastest of all reflexes
• Example – knee-jerk reflex
• Polysynaptic reflex – more common type of reflex• Most have a single interneuron between the
sensory and motor neuron
• Example – withdrawal reflexes
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Monosynaptic Reflex
Figure 12.18a, b
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Polysynaptic Reflex
Figure 12.18a, b
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Spinal vs Cranial Reflexes
• Spinal = spinal cord integration center• Ex. Knee-jerk reflex
• Cranial = brain as integration center• Ex. Pupillary light reflex
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Somatic vs Autonomic Reflexes
• Somatic = motor neurons to skeletal muscles• Ex. Knee-jerk reflex
• Autonomic = autonomic neurons to smooth muscle and glands• Ex. Pupillary light reflex
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Innate vs Learned Reflexes
• Innate = born-with• Knee-jerk reflex, pupillary reflex
• Learned = develops based on experiences• Pavlov’s dogs salivation in response to bell
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Gray versus White Matter in the Central Nervous System
• Gray matter
• Cell bodies
• Dendrites
• Synapses
•White matter•Axons (myelin)
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Gray Matter in the Spinal Cord
• Gray matter in the spinal cord
• H-shaped (butterfly) region – surrounds central cavity
• Dorsal half contains cell bodies of interneurons
• Ventral half contains cell bodies of motor neurons
• Cell bodies are clustered in the gray matter
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
White Matter in the Spinal Cord
• White matter in the spinal cord
• Located externally to the gray matter
• Contains no neuronal cell bodies, but millions of axons
• Myelin sheath – white color
• Consists of axons running between different parts of the CNS
• Tracts – bundles of axons traveling to similar destinations
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Gray Matter in Brain
• Cortex and nuclei
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
White Matter in Brain
• Pathways, tracts and commissures
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings
Disorders of the Nervous System
• Multiple sclerosis – common cause of neural disability• Varies widely in intensity among those affected
• Cause is incompletely understood
• An autoimmune disease • Immune system attacks the myelin around axons in
the CNS