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CHAPTER 8 NERVOUS SYSTEM BASIC STRUCTURE AND FUNCTION (8.2 (A.…
CHAPTER 8 NERVOUS SYSTEM BASIC STRUCTURE AND FUNCTION
8.1
function of nervous system=coordinate all body systems
accomplished by transmission of (electrochemical) signals from body parts to the brain and back to the body parts
A. THE ORGANS OF THE NERVOUS SYSTEM DIVIDED IN 2 GROUPS
Central Nervous System (CNS)=brain & spinal cord
Peripheral Nervous System(PNS)=nerves that extend from brain(cranial nerves) and spinal cord(spinal nerves)
B. GENERAL FUNCTIONS OF NERVOUS SYSTEM(3 fold functions)
Sensory Input Function
a. PNS:
b. Sensory receptors(located at the ends of peripheral neurons) detect changes(stimulated) occuring in surroundings
c. once stimulated, sensory receptors transmit sensory impulse to CNS
d. sensory impulse carried on a sensory neuron
Integrative Function
a. CNS(brain and/or spinal cord)
b. involves interpretation of an incoming sensory impulse(decision made concerning whats going to happen next based on sensory impulse)
Motor Function
c. integration occurs in
_
d. a motor impulse begins...
a. PNS
b. involves response of a body part
c. Motor impulses carried from CNS to responsive body part called effectors
d. a motor impulse is carried on a motor neuron
e. Effectors=2 types:
muscles(that contract)
glands(that secrete a hormone)
8.2
A. Neuron=structural & functional unit of nervous system; nerve cell
Neuron Structure
each neuron composed of a cell body & many extensions & many extensions from cell body called neuron processes or nerve fibers
a. Cell Body=central portion of neuron contains usual organelles except centrioles
identify with nucleus, prominent nucleolus and many Nissl bodies=RER
b. Neuron Processes/Nerve Fibers=extensions from cell body 2 types:
Dendrites:
many per neuron
short & branched
receptive portion of neuron
carry impulses toward cell body
Axons
one per neuron
long thin process
carry impulses away from cell body
4.terminations of axon branch=axonal terminals; synaptic knobs
Axons in PNS
a. Large axons are surrounded by a myelin sheath produced by many layers of Shwann cells(neuroglial cell)
"myelinated nerve fiber"
myelin=lipoprotein;
interruptions in the myelin sheath between Shwann cells=Nodes of Ranvier
b. Small axons don't have a myelin sheath
"unmyelinated nerve fibers" all axons(in PNS) are associated with Shwann cells
Axons in CNS (brain & spinal cord)
a. Myelin is produced by an oligodendrocyte rather than Shwann Cells
b. A bundle of myelinated nerve fibers="White Matter"
PNS=nerve
CNS=tract/column
c. contrast to CNS "Gray Matter"=bundle of cell bodies(or unmyelinated nerve fibers)
PNS=ganglion
CNS=nucleus
Regeneration of Nerve Fibers
Cell body injury=death of neuron
Damage to axon may allow for regeneration
B. Neuroglial Cells=accessory cells of nervous system form supporting network for neurons "nerve glue"
PNS=Shwann Cells produces myelin and Satellite Cells nourish neurons
CNS=4 types-provide bulk of brain and spinal cord tissue
a. Oligodendrocyte
looks like eyeball function=produces myelin
b. Astrocyte star shaped function=nourishes neurons
c. Microglia looks like spider function=phagocytosis
d. Ependymal Cell epithelial-like layer function=lines spaces in CNS
brain=ventricles
spinal cord=central canal
Classification of Neurons
A. Functional Classification
Sensory Neurons
a. PNS
b. Afferent neurons
c. Carry sensory impulses from sensory receptors to CNS
d. Input information to CNS
e. Location of receptors=skin & sense organs
Somatic receptors
External receptors(touch, pressure, temp, sight, smell, hearing
Proprioceptors(position & movement
Visceral receptors(digestive, respiratory, cardiovascular, urinary, reproductive, taste, deep pressure & pain
Interneurons(association)
a. CNS
b. Efferent neurons
c. Carry motor impulses away from CNS and to effectors
d. output information from CNS
e. Effectors=muscles & glands
Somatic motor neurons(skeletal muscles)
Visceral motor neurons(muscle & glands
Sympathetic(adrenergic)
Parasympathetic cholinergic
B. Structural Classification
Multipolar Neurons(most common neuron in CNS)
a. many extensions
b. Many dendrites lead toward cell body, one axon leads away from cell body
Bipolar Neurons
a. two extensions
b. one fused dendrite leads toward cell body, one axon leads away from cell
Unipolar Neurons(most PNS sensory neurons)
a. One process from cell body
b. Forms central and peripheral processes
c. Only distal ends are dendrites
8.3
A. Resting Nerve Cells
a resting neuron's cell membrane is said to be polarized=electrically charged(the charge inside the cell is different than the charge outside)
a potential difference(PD) exists across this resting cell membrane
DEF:Potential Difference(PD)=the difference in electrical charge between 2 points(across a cell membrane)
The resting membrane potential(RMP) of a neuron is results from the distribution of ions across the cell membrane
a. K+;high inside
b. NA+;high outside
c. CL-;high outside
d. Negatively charged proteins or Anions;high inside these ion contractions are maintained by active transport mechanisms(mainly the Na+K+-ATPase pump;chapter 3
the RMP of a nerve cell is measured to be -70mV(millivolts) (inside or outside)
as long as the RMP in a nerve cell is undisturbed it remains polarized. In order for a nerve impulse to be started or propagated in a nerve cell, this resting potential must be disturbed
B. Membrane Potentials
The RMP of -70 mV can be disrupted or changed in one of two directions:
more negative=hyperpolarization
less negative(towards zero)=depolarization
a. The cell membrane of a neuron must be depolarized(approximately -55mV) in order for certain ion channels to open and start a nerve impulse
C. Action Potential
When the resting membrane potential(RMP) of a neuron is depolarized to -55mV threshold potential is reached
a. the threshold potential for neuron is -55mV
b.threshold stimulus=+15mV
When threshold potential is reached, the rapid opening of Na+ channels results in rapid depolarization(even reversal of membrane potential(MP) to +30mV)
a. This event is called action potential
b. action potential represents start of nerve impulse on a neuron
Then K+ channels open(while Na+ channels close) and repolarization occurs=recovery of the RMP to -70mV
This all occurs very quickly=1/1000 second
D. SUMMARY:
RMP of neuron=-70 mV +15 mV stimulus (threshold stimulus)
MP of neuron falls to -55mV=Threshold Potential Na+ channels open(rapid depolarization) Action Potential(nerve impulse) is produced
MP of neuron reaches +30mV=reversal of MP K+ channels open(repolarization) Na+ channels close
RMP of neuron returns to -70 mV
Action Potential represents the start of a nerve impulse in one small portion of the neurons membrane
V. DEF: Nerve Impulse(NI)=the propagation of action potentials(AP) along a nerve fiber(the entire length of the neuron)
A.DEF:Nerve Impulse(NI)=the propagation of action potentials(AP) along a nerve fiber;(the entire length of the neuron)
1.NI is an electrical impulse
2.NI is similar to a row of domino falls, the entire row will fall)
2 components of efferent(motor) pathway-somatic and automatic