ANS 331: Nervous System
I. Structure and Function
1. Function
A. Homeostasis
B. Body Control
C. Learning and Thinking
2. Structure
A. Neuron (Nerve Cell)
a. Basic anatomical and physiological unit of
nervous system
b. Anatomy
i. Cell body
--Contains nucleus and organelles
ii. Dendrite
--Conducts impulses toward cell body
and axon
iii. Axon
--Conducts impulses away from cell
body and dendrites
--Also called the nerve fiber
--Much of the time covered by a
myelin sheath, separated by
gaps (Nodes of Ranvier)
c. Groupings
i. Nucleus--a group of neuron cell
bodies within the brain or spinal
chord (Central Nervous System, CNS)
ii. Ganglion--a group of nerve cell
bodies outside the brain or spinal
chord (Peripheral Nervous System,
PNS)
iii. Tract or Bundle--a bundle of neuron
fibers in the CNS
iv. Nerve--a bundle of neuron fibers
in the PNS
B. Synapse
a. Space between neurons
b. Impulses carried by chemicals
--Neurotransmitters
C. Myelin Sheaths
a. Myelin
i. White lipid (sphingomyelin) that
forms a sheath
around nerve fibers
ii. Electrical insulator
b. Made by Schwann cells in P.S.
i. Neuroglia (C.T.)
ii. Cytoplasm wraps around nerve fiber
many times and outer layer
(neurilemma) contains the cell
nucleus
iii. Nodes of Ranvier separate individual
sheaths (Schwann Cells)
--Exposed to extracellular fluid
(uninsulated)
--Site of depolarization
c. Myelin sheath made by oligodendricytes
in CNS
i. Many nerve fibers are not myelinated
--Gray Matter
ii. Myelinated nerves fibers
--White Matter
iii. Cytoplasmic processes from
oligodendricytes serve many nerve
fibers
iv. Also considered neuroglia
II. Organizations of the Nervous System
1. Described either anatomically or functionally
2. Anatomical Locations
A. Central Nervous System
a. Brain
b. Spinal Chord
B. Peripheral Nervous System
a. Cranial Nerves
b. Spinal Nerves
3. Directions of Impulses
A. Afferent Nerves
a. Conduct nervous impulses toward the CNS
b. Sensory Nerves
B. Efferent Nerves
a. Conduct nervous impulses away from CNS
b. Motor Nerves
4. Functional
A. Somatic Nervous System
a. Conscious or Voluntary Control
b. Efferent signals to Skeletal Muscle
c. Afferent signals from Muscle, Skin, Eye, and Ear
B. Autonomic Nervous System
a. Unconscious or Involuntary Control
b. Efferent signals to Smooth and Cardiac Muscle,
and Endocrine Glands
c. Afferent signals from receptors all over the body.
III. The Nerve Impulses and Transmission
1. Mechanisms of Transmission
A. Resting Membrane Potential
a. "Potential" is the relative electric
charge between two points
b. Neuron
i. Outside more positive than inside
of cell
ii. Due to Na+-K+ ATPase Pump and K+
leak channels
c. Na+-K+ Pump
i. Pumps 3 Na+ out
ii. Pumps 2 K+ in
d. K+ leak channels
i. Ion channel (specific)
ii. Let K+ reach equilibrium
e. Na+ is relatively impermeable to membrane
f. Causes a negative resting potential
of 70 mV
B. Depolarization
a. Chemical or Physical Stimulation
b. Makes membrane permeable to Na+
c. Voltage-gated Na+ Channels open
i. Inactivated when equilibrium is
reached (+30 mV)
d. Inside of cell more positive than outside
C. Repolarization
a. Voltage-gated K+ channels are activated
b. Activation is delayed behind Na+ voltage
channel
--Stimulated at same time but slower
kinetics
b. K+ flows outward (lower to higher concn)
c. Brings membrane back to resting potential
D. Nerve Impulse
a. Positive charge will flow to Negative
charge within a microregion of the neuron
membrane
b. Current will be released just beyond
site of depolarization
i. Causes another depolarization
ii. Causes direction of propagation
E. Action Potential
a. Changes in the resting membrane potential that are
propagated along the cell (nerve impulse)
b. Happen when membrane potential changes
8 to 12 mV (more positive)--Threshold
c. Nerve cannot fire until repolarization
is nearly complete
--Refractory Period
d. When one action potential is initiated
in a nerve fiber the entire fiber will
"fire"
--All or None Principle
e. All impulses are the same strength
--No weak impulses
F. Saltatory Conduction
a. In myelinated fibers action potentials
occur from one Node of Ranvier to the next
b. Impulse "leaps"--Saltatory conduction
c. The voltage-gate channels are concentrated
in the Nodes
d. Two functions:
i. Efficiency--Less energy needed to
repolarize
ii. Speed--Impulse is accelerated
G. Transmission Velocity
a. Larger the fiber and thicker the myelin
sheath the faster the impulse
b. Fastest = 100 m/s
Slowest = 0.5 m/s
c. Most impulses = 2500 impulses/s
(Large myelinated fibers)
Least impulses = 250 impulses/s
(Small unmyelinated fibers)
IV. The Synapse
1. Synaptic Cleft
A. Space between neurons
a. Presynaptic
--Terminal End Bulb of Axon
b. Postsynaptic
--Neurotransmitter Receptors on Dendrites
B. Synaptic Transmission
a. Perpetuation of nerve impulse
from neuron to neuron
C. Impulses carried by chemicals
a. Neurotransmitters
2. Neurotransmitters
A. Features
a. Chemicals
--Amino acids, amines, to NO
b. Released from Presynaptic Terminal Bulb
i. Stored in Vesicles
ii. Released by exocytosis following
impulse
c. Diffuses across synaptic cleft
d. Opens Ligand (Chemical)-gated Na+ Channels
--Also called Transmitter-gated
e. Causes change in membrane potential
--If change is enough it will cause an
action potential
B. Peripheral Neurotransmitters
a. Acetylcholine
--Cranial, Spinal, Parasympathetic Nerves,
and Preganglionic Sympathetic Nerves
b. Norepinephrine (Noradrenaline)
--Postganglionic Sympathetic Nerves
c. Sympathetic--Adrenergic System
Parasympathetic--Cholinergic System
d. Acetylcholine and Norepinephrine are
excitatory
C. Central Neurotransmitters
a. Excitatory and Inhibitory
i. Excitatory stimulate Ligand-gated
Na+ channels
ii. Inhibitory stimulate Ligand-gated
Cl- channels
--Keep inside of cell negative
--Inhibit depolarization
b. Excitatory Neurotransmitters
--Acetylcholine, Norepinephrine, Glutamate,
Serotonin
c. Inhibitory Neurotransmitters
--GABA, Glycine
d. Final Common Pathway
i. Multiple axons will branch to
individual
postsynaptic neuron
ii. Both Excitatory and Inhibitory
Neurotransmitters will act on that
synapse
iii. If the Excitatory override
Inhibitory enough to reach threshold
potential the neuron will fire
D. Stopping and Recycling Neurotransmitters
a. Neurotransmitters must be broken down
after receptor binding so effect can be controlled
i. Over stimulation
ii. Over inhibition
a. Acetylcholinesterase
i. Found on Post-synaptic Neuron Membrane
ii. Breaks down Acetylcholine
iii. Components reabsorbed and recycled by postsynaptic neuron
b. MAO and COMT
i. Breaks down Norepinephrine
V. Anatomy of the Nervous System
1. The Brain
A. Cerebrum (Cerebral Hemispheres)
a. Cortex (Gray)
i. High Area
1. Gyrus
ii. Low Area or Groove
1. Fissure--Deep Groove
2. Sulcus--Shallow Groove
iii. Acquired late in vertebrate
evolution
iv. Higher Order Functions:
1. Consciousness/Awareness
2. Association/Intelligence
3. Learning
vii. Possesses Motor Areas (Movement)
1. Contralateral control
2. Size of motor area directly
related to number and
complexity of skeletal
muscle movements
vi. Contains Sensory Areas
1. Somesthetic
2. Visual
3. Auditory
4. Olfactory
b. Medullary Substance (White)
i. Myelinated nerve fibers beneath
the cerebral cortex
ii. Association fibers
--Connect different parts of cortex
iii. Commissural fibers
--Connect two hemispheres of cerebrum
--Corpus Callosum
iv. Projection fibers
--Connect Cortex with other parts
of the brain and spinal
chord
c. Basal Ganglia
i. Controls basic movement
--Walking, eating, fighting, sex
--Doesn't control sophisticated
movement in mammals
ii. Well developed in birds
--Controls all movements
B. Cerebellum
a. Makes adjustments to motor signals from
the cerebrum
b. Receives signals from
i. Tactile & Proprioreception
ii. Equilibrium apparatus of inner ear
iii. Visual cortex
iv. Motor cortex
c. Ipsilateral control
C. Interbrain (Diencephalon)
a. Pituitary
i. Endocrine Gland
b. Hypothalamus
i. Endocrine Gland
ii. Integration of functions of the
autonomic nervous system
(ANS)
-–Rage and Anger
iii. Homeostasis
1. Temperature regulation
2. Hunger and Thirst
c. Thalamus
i. Relay center from body to Cerebral
Cortex
ii. Relay center of impulses within
the brain
d. Epithalamus
i. Olfactory correlation center
ii. Pineal gland (Produces Melatonin)
1. Seasonal Breeding
2. Daily Rhythms
D. Brain Stem
a. Midbrain
i. Visual Reflex Center
ii. Auditory Reflex Center
iii. Nuclei (2 Cranial Nerves) and
fiber tracts
b. Pons and Medulla Oblongata
i. Contain many ascending and
descending tracts
ii. Nuclei for rest of cranial nerves
iii. Postural reflexes
iv. Other reflex centers
--heart rate
--vasomotor tone
--respiration
--motor & secretory activity of
digestive tract
2. Spinal Chord
A. Caudal continuation of the medulla
B. Segmented with vertebral segments
a. Each segment gives rise to a pair of
spinal nerves
C. Centrally located Gray Matter
a. "Gray H"
b. Cell bodies and processes
D. Peripheral White Matter
a. Contains sensory and motor tracts
E. Narrows as you move caudally
a. Terminal end--Cauda equina
3. The Meninges and Cerebral Spinal Fluid
A. Meninges of the Brain
a. Coverings of Brain (and spinal chord)
i. Dura Mater
ii. Arachnoidea
iii. Pia Mater
b. Subarachnoid Space contains Cerebral Spinal Fluid
c. Pia Mater lines follows all fissures and grooves
into the brain
i. Lies between brain and blood vessels
B. Meninges of the Spinal Chord
a. Same make-up
B. Epidural Space
i. Fatty area outside Dura Mater
ii. Innervated by Spinal Nerve Projections
(Roots)
iii. Used in local anesthesia
C. Ventricles of the Brain
a. Four Cavities in the Brain
i. Two lateral right and left (1st and 2nd)
ii. 3rd Ventricle-- Surrounds Interbrain
iii. 4th Ventricle--Lies beneath Cerebellum
--connects subarachnoid space through
foramina
b. Contain Choroid Plexus
i. Tufts of Capillaries
ii. Secrete Cerebral Spinal Fluid
D. Cerebral Spinal Fluid
a. Circulation
i. Ventricles to Subarachnoid Space
to Venous Blood
ii. Pressure Driven
b. Function
i. Derived from Blood Plasma
--Thin and Watery
--No cells except for a few lymphocytes
ii. Principle Function--Brain Cushion
iii. Some Lymphatic Function
4. Nerves (PNS)
A. Spinal Nerves
a. A left/right pair is derived from between
every vertebra, except the coccygeal
i. Cervical (one extra at cranial end)
ii. Thoracic
iii. Lumbar
iv. Sacral
v. Coccygeal
b. Organization
i. Dorsal Root
--Afferent impulses (Sensory)
ii. Dorsal Root Ganglion
--Afferent neuron cell bodies
iii. Ventral Root (Motor)
--Efferent impulses
iv. Roots join to form mixed nerve
--Afferent and Efferent pathways
c. Spinal nerves supply innervations to areas
dorsal and ventral to transverse process of
vertebra
d. Appendages innervated by ventral branches
of several spinal nerves
--Join to become plexuses
B. Cranial Nerves
a. 12 pairs
b. Usually innervate structures in head and
neck
c. Exception is Vagus Nerve (X)
--Pharynx and larynx
--Visceral structures of thorax and abdomen
d. Mixed, motor, or sensory
C. Autonomic Nerves
a. Innervate Smooth muscle, Cardiac muscle and
Glands
b. Divisions
i. Sympathetic
ii. Parasympathetic
iii. Generally have opposite responses
iv. Each consists of two neurons
--preganglionic, postganglionic
c. Sympathetic
i. Usually involved in "Fight, Fright,
Flight" response
ii. Originate from thoracic and lumbar
segments
iii. Short preganglionic,
Long postganglionic
iv. Ganglionic connections form paired
nerve trunk that is parallel to the
spinal chord--Sympathetic trunk
d. Parasympathetic
i. Usually involved in tranquil or
restful situations
ii. Originate from brain and sacral
segments
iii. Long preganglionic,
Short postganglionic
iv. Brain originators follow cranial
nerves
--III, VII, IX, X
v. Sacral originators follow pelvic
spinal nerves
e. Autonomic Neurotransmitter Receptors
i. Sympathetic
1. Norepinephrine
2. Adrenergic Receptors
a. Alpha 1
--Blood vessels
b. Beta 1
--Heart
c. Beta 2
--Bronchioles
ii. Parasympathetic
1. Acetylcholine
2. Cholenergic Receptors
a. Nicotinic
--Muscles
--Also found in Spinal, Cranial nerves
and Sympathetic Nerves
b. Muscarinic
--Organs and Tissues
f. Autonomic Reflex
i. Afferent/Efferent Mechanisms
ii. Impulses do not reach conscious
level
iii. Examples
--Blood Pressure
--Heart Rate
--Digestive and Urinary Activity
IV. Reflexes
1. Definition
A. An automatic or unconscious response of an
effector organ
B. Involves a chain of at least 2 neurons
a. Afferent, sensory, or receptor neuron
b. Efferent, motor, or effector neuron
C. Usually connector or inter-neurons are involved
2. Spinal Reflex
A. Simplest reflex
a. Dorsal Root--Afferent
b. Ventral Root--Efferent
c. Example: Knee jerk stretch reflex
3. Somatic and Visceral Reflexes
A. Somatic
a. Involve skeletal muscle
i. Stretch Reflex
--Knee Jerk
ii. Withdrawl Reflex
--Withdrawl from pain before being aware
iii. Cross Extender Reflex
--Posteral Reflex
B. Visceral
a. Involve smooth muscle, cardiac muscle,
or glands
i. Corneal reflex
--Blink if conscious
ii. Pupillary Light Reflex
--Brain Function
4. Reflex Centers
A. Located throughout the CNS
a. Simplest--Spinal Chord
b. More Complex--Brain
i. Medulla Oblongata
--Heart rate, vasodiameter,
respiration, swallowing,
vomiting, coughing,
and sneezing
ii. Cerebellum
--Locomotion and posture
iii. Hypothalamus
--Temperature regulation
iv. Midbrain
--Visual and auditory reflex
5. Postural Reflexes and Reactions
A. Function
a. Maintain an upright position
b. Reflex--Unconscious
c. Reaction--Involves Cerebral Cortex
B. Types
a. Standing Reflex
--Compensate and resist displacement
b. Attitudinal Reflex
--Displacement of one part followed by
other parts
c. Righting Reflex
--Cat always landing on feet
d. Hopping Reflex
--Balance with a hop or leg rigidity if
other leg can't be used
VI. Central Nervous System Metabolism
1. Metabolism
A. Energy from CHOs, primarily glucose
a. Insulin not required
--Simple Diffusion of Glucose
B. Very high oxygen need--20% of whole body
a. Gray needs 3-4X more than White Matter
2. Blood-Brain Barrier
A. Many substance in blood can't enter cells of CNS
B. Tight Junctions rather than Slit Pores in
Endothelium
C. Astrocytes
a. Glial Cell
b. Lie between CNS cells and Endothelium
c. Selective to the Materials they transport
D. Choroid Plexus Cells
a. Also Selective
3. Blood Requirements
A. Higher brain can't go more than 5-10 minutes
without blood
B. Medulla--cardiovascular and respiratory can
go longer
C. Babies can go longer without oxygen than Adults
~~~~~Revised 9/11/03~~~~~ TAW