Describe the organization of the nervous system. Explain the three basic functions of the nervous...
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Transcript of Describe the organization of the nervous system. Explain the three basic functions of the nervous...
Chapter 9: Nervous TissueLearning Objectives
Describe the organization of the nervous system. Explain the three basic functions of the nervous
system. Contrast the histological characteristics and the
functions of neuroglia and neurons. Distinguish between gray matter and white matter. Describe how a nerve impulse is generated and
conducted. Explain the events of synaptic transmission and
the type of neurotransmitters used.
SUBDIVISIONS OF PNSSomatic Nervous System (SNS) Sensory neurons conveys information from somatic receptors
in head, body wall, and limbs, and from receptors for special senses of vision, hearing, taste, and smell to CNS
Motor neurons conduct impulses from CNS to skeletal muscles only; can be consciously controlled, voluntary actions
Autonomic nervous system (ANS) Sensory neurons convey information from autonomic sensory
receptors, located in visceral organs (stomach, lungs) to CNS Motor neurons conduct nerve impulses from CNS to smooth
muscles, cardiac muscles and glands; cannot be consciously controlled, involuntary actions
Two divisions of ANS are sympathetic division and parasympathetic division; these divisions usually perform opposite functions
“Fight-or-flight” responses emergency actions (sympathetic) “Rest-and-digest” activities GI tract activities
(parasympathetic)
Enteric Nervous System (ENS)
nnGet a copy of checkpoint questions 1-3, complete them and hand them in…
b
Get a copy of checkpoint questions 1-3, complete them and hand them in…
Functions of the Nervous System
Sensory functions _ (AFFECTORS) Sensory receptors_ detect stimuli inside and outside the body. - Sensory or afferent neurons carry information from cranial to spinal nerves into brain and spinal cord or visa versa
Integrative functions _(LIKE A CONTROL CENTER)
Process sensory information by analyzing and storing some of it and by making decisions for appropriate responses - Interneurons; have short axons that connect with neurons in brain, spinal cord, and ganglion; are majority neurons in the body
Functions…continued Motor functions _(EFFECTORS)
Respond the integrative decisions
- Motor of efferent neurons carry information from brain
toward spinal cord or out of brain to spinal cord into cranial or spinal nerves
HISTOLOGY OF NERVOUS TISSUE
Two types of cells Neurons
unique functions of the nervous system; sensing, thinking, remembering, controlling muscle activity, and regulating glandular secretions
Neuroglia support, nourish, and protect neurons and maintain
homeostasis in the intestinal fluid that baths neurons
Neurons
Three parts: Cell Body
nucleus surrounded by cytoplasm; includes RER, lysosomes, mitochondria, Golgi
synthesizes cellular molecules needed for a neuron’s operation
Processes or extensions: Dendrites (“little trees”)
multiple per single axon combined with cell body receiving and input parts of a neuron
short, tapering, and highly branched, tree-branch array emerging from cell body
Axons_ conducts nerve impulses toward another neuron, muscle fiber, or
gland cell long, thin, cylindrical projection that joins cell body at a cone-shaped
elevation
Classification of Neurons
STRUCTURAL CLASSIFICATION Multipolar neurons: have several dendrites and
one axon; most in brain and spinal cord Bipolar neurons: have one main dendrite and
one axon; retina of the eye, inner ear, olfactory area of brain
Unipolar neurons: dendrites and one axon fused together forming a continuous process that emerges from cell body; begin in embryo as bipolar neurons; most function as sensory receptors for touch, pressure, pain, or thermal stimuli. Cell bodies of most of this type located in ganglia of spinal and cranial nerves.
Classification of Neurons
FUNCTIONAL CLASSIFICATION Sensory or afferent neurons: once sensory receptor
activated, these form an action potential in their axon that is conveyed into the CNS through spinal and cranial nerves contain sensory receptors at their distal ends or are located
just after sensory receptors that are separate cells; most unipolar in structure
Motor or efferent neurons: convey action potential away from CNS to effectors (muscles and glands) in PNS through cranial and spinal nerves Most are multipolar in structure
Interneurons or association neurons: integrate incoming sensory information from sensory neurons and then elicit a motor response by activating appropriate motor neurons Located within CNS between sensory and motor neurons; most
multipolar in structure
Neuroglia
Smaller than neurons 5-50 times more numerous
”glue” that holds nervous tissue together do not generate or conduct nerve impulses can multiply and divide in mature nervous
system in case of injury or disease multiply to fill in
spaces formerly occupied by neurons Gliomas: brain tumors derived from glia
called gliomas; very malignant and grow rapidly
Myelination Myelin sheath, many-layered covering composed of lipids
and protein, surround the axons of most of our neurons. Two Functions:
(1) insulates the axon (2) increases the speed of nerve impulse conductionThe amount of myelin increases from birth to maturity
Collections of Nervous Tissue
Clusters of Neuronal Cell Bodies Ganglion: cluster of neuronal cell bodies
located in PNS Nucleus: cluster of neuronal cell bodies in CNSBundles of Axons Nerve: bundle of axons located in PNS; cranial
nerves connect brain to periphery and spinal nerves connect spinal cord to periphery
Tract: bundle of axons located in CNS; tracts interconnect neurons in spinal cord and brain
Gray and White Matter White matter: myelinated and unmyelinated axons of
many neurons which is white in color; also has blood vessels
Gray matter: neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia white is grayish pink in color; also has blood vessels
ACTION POTENTIALS AKA nerve impulses Two features of plasma membrane needed for action potentials
in muscle fibers and in neurons existence of resting membrane potential presence of specific types of ion channels
Membrane potential difference in the amount of electrical charge inside and outside plasma membrane.
membrane that has potential is polarized Resting membrane potential voltage difference between the
inside and outside of a plasma membrane when not responding to a stimulus, in muscle fibers and neurons voltage created by flow of ions
Ion Channels Two types of ion channels:
Leakage channels allow small but steady stream of ions to leak across the membrane
Gated channels open and close on command
Voltage-gated channels are used to generate and conduct action potentials; open in response to a change in membrane potential
Generation of Action Potential Action potential (AP) or impulse generates rapidly occurring
events that decrease and increase the membrane potential and eventually restore it to its resting state
Ability of muscle fibers and neurons to convert stimuli into action potential is called electrical excitability.
Stimulus in cell’s environment changes resting membrane potential; if stimulus causes cell to depolarize to a critical level; called a threshold (about -55mV) then an action potential arises
Two main phases: Depolarizing phase- rapidly occurring events that decrease
and eventually reverse polarization of membrane, makes inside more positive than outside; Na+ ions move into cell
Repolarizing phase- membrane polarization is restored to resting state; Na+ ions move back out cell restoring charge to original state
Stimulus
Depolarizing phase Repolarizing
phase Reversal of polarization
Threshold
6After-hyperpolarizing phase
7.Resting membrane potential
Conduction of Nerve Impulses Also called propagation
Way cells communicate information from one part of body to another
Nerve impulses travel from where they arise, usually axon hillock, along axon to axon terminal
Positive feedback process
Continuous & Saltatory Conduction
Continuous conduction Step-by-step process; impulses travel a short
distance in 10 milliseconds Occurs in unmyelinated axons (muscle fibers)
Have smallest diameter Saltatory conduction
Impulses leap from one node of Ranvier to the next
Occurs in myelinated axons Have largest diameter
Synaptic Transmission How synapses neurons communicate with
other neurons or with effectors through a series of events neuron sending the signal is called the presynaptic
neuron neuron receiving the signal called the postsynaptic
neuron A synaptic cleft separates the presynaptic and
postsynaptic neurons
NeurotransmittersDifferent neurotransmitters are found in synaptic vesicles. These different neurotransmitters have different effects.