2. Nervous Physiology
The nervous system allows the animal to quickly detect,
communicate and co-ordinate information about its
external and internal environment so it can make efficient
appropriate responses for survival and/or reproduction
The two major parts of nervous system are:
Central nervous system (CNS)- brain and spinal cord
Peripheral nervous system (PNS)- cranial nerves, spinal
nerves and ganglia
2
3. Neurons are the basic unit of the nervous system. They carry
information or impulses as electrical signals from one place to
another in the body
Structure of Neurons
A neuron consists of three main parts:
A. Cell body- the largest part, contains the nucleus and much of
the cytoplasm, where most of the metabolic activity of the cell
including the generation of ATP and synthesis of protein occurs
B. Dendrites- short branch extensions spreading out from the cell
body
3
4. Dendrites receive stimulus (action potentials) and carry impulses
from the environment or from other neurons and carry them
toward the cell body
C. Axon- a long fiber that carries impulses away from the cell body
and ends in a series of small swellings called axon terminals at
which the neuron may make contact with the dendrites of
another neuron, with a receptor/an effector
Each neuron has only one axon or nerve fiber
Axons of most neurons are covered with a lipid layer known as
the myelin sheath which insulates and speeds up transmission of
action potentials through the axon
4
6. In peripheral nervous system, myelin is produced by
Schwann cells & in CNS by Oligodendrocytes which surround
the axon
Gaps (nodes) in myelin sheath along the length of axon are
nodes of Ranvier and they allow impulses to travel faster
than if they travelled along the entire length of neuron
This structure reflects functional subdivision of neurons into
receiving, integrating and transmitting compartments
Neurons and some other excitable cells in mammals send
messages electrochemically, i.e. chemicals in the body (are
electrically-charged) cause an electrical signal
6
7. Resting Membrane Potentials
Cell membrane of all cells is permeable only to certain ions
(e.g., K+), these forces create a potential difference across
their plasma membrane i.e. the potential or chemical charge
inside of the cell is different to that of the solution outside of
the cell. This potential difference is referred to as the resting
membrane potential
[Potential means a separation of charge. In this case the
separation is across the membrane. Resting means that no
current is flowing across the membrane.]
7
8. The RMP of a cell is the electrochemical state (membrane
permeability) of a cell (neuron) at rest, at which there is
no net movement of a particular ion across the cell
membrane
It is the potential that would be maintained if there were
no action potentials, synaptic potentials, or other active
changes in the membrane potential
For most animal cells, potassium ions (K+) are the most
important for the resting potential
8
10. Value of RMP
Value of the RMP varies from cell to cell; depending on
the cell type & ranges from -20 mV to -100 mV
E.g. it is -70 mV in a typical neuron, -90 mV in a typical
skeletal muscle cell, and around -50 mV in most other
mammalian cells
In most cells, the RMP is negative (i.e., inside of the cell is
negative with respect the outside, which serves as the
reference)
E.g. the RMP of a neuron has the inside of the neuron 70mV
more negative than the extracellular space
10
11. Changes in the resting membrane potential is the basis of
electrical signaling in cells.
In non-excitable cells, such as epithelial cells and adipose
cells, the resting membrane potential does not change
appreciably over time.
In excitable cells (such as neurons, muscle cells, some
endocrine cells, and some other cells in the body), however,
upon stimulation of the cell, the membrane potential can
change dramatically for short periods of time (milliseconds).
11
12. Therefore, in excitable cells the MP is not always at the
RMP in which deviations away from it are extremely
important to the physiological function of these cells.
All cell membranes produce electrical signals by ion
movements, but trans-membrane potential is particularly
important to neurons, b/c rapid changes in MP of neurons
bring about nervous impulse, w/c is the basis of neuronal
signaling.
Changes in the MP bring about contraction in muscle cells
and release of hormones in endocrine cells
12
13. Importance of RMP
Cells’ ability to fire an action potential is due to their ability to
maintain cellular RMP at approximately –70 mV (for a neuron)
The basic signaling properties of neurons are determined by
changes in the RMP which is the basis of cell to cell
communication
How are RMPs determined/created?
Electricity (flow of current) requires 2 things: charged particles-
neurons use ions (charged particle) & separation of charge-
created by neural membrane
13
14. Potential difference occurs at the level of the cell membrane
because biological membranes can act to allow separation of
electrical charge, by separating solutions in two compartments
by the very short-distance, non-conducting, hydrophobic core of
the membrane (=3nm)
Charge separation across the membrane leads to an electric
field across the membrane which rise to the measured
membrane potential
Neurons (and other cells) make use of several ions to create
electric currents- Na+, Cl-, K+ and proteins (negatively
charged)
14
15. The RMP is created by the concentrations of the ions in the
fluids on both sides of the cell membrane and the ion transport
proteins that are in the cell membrane. How?
For determination of membrane potentials, the two most
important types of membrane ion transport proteins are ion
channels and ion pumps
Ion channels- ion channel proteins create paths across cell
membranes through which ions can pass and they have
selectivity for certain ions, thus, there are potassium, chloride
and sodium-selective ion channels
15
16. Different cells and even different parts of one cell
(dendrites, cell bodies, nodes of Ranvier) have different
amounts of various ion transport proteins. Typically, the
amount of certain K+ channels is most important for
control of the RMP
Can be: non-gated channels which are always open. E. g.
plasma membrane has many more K+ non-gated channels
than Na+ non-gated channels, thus membrane
permeability to K+ is higher
16
17. Gated channels- open or close in response to stimuli such as
voltage, chemicals, mechanical pressure…
Ion pumps- some of them like Na+/K+ ATPase are
electrogenic, i.e. they produce charge imbalance across the cell
membrane and can also contribute to the membrane potential
17
18. MPs are established primarily by three factors which act on
ions:
1) Concentration of ions on the inside and outside of the cell,
and their asymmetric distribution across the membrane to
form a concentration gradient (Na+, K+)
2) Selective permeability of cell membrane to those ions (i.e.,
ion conductance or electrical force) through specific ion
channels (K+ channels and Na+ channels) and
3) Activity of electrogenic pumps (Na+/K+- ATPase and Ca++
transport pumps)
18
19. Principles to create the RMP:
Concentration gradient moves ions from high to low
concentration (Na+ intracellularly and K+ extracellularly)
Electrical force moves ions with same charge away from
each other and ions with opposite charge towards each
other. E. g. Na+ & K+ driven to proteins (A-)
Na+/K+ pump moves Na+ to the EC space. It can't get
back in b/c the membrane is impermeable to Na+
The membrane is permeable to K+ (it flows either way)
Large negatively charged proteins (A-) are stuck inside the
neuron 19
21. Action potentials
Membrane potentials are used to convey signals
Generally, there are two types of signals:
Graded potentials (short distance signals)- are short lived
local changes in membrane potential. They can be either
depolarizations or hyperpolarizations
Current flow decreases with the distance traveled
Depolarization- a reduction in membrane potential. Here,
the inside of the cell becomes less negative (closer to zero)
than the resting potential
21
22. On this occasion, membrane potential can reverse and
become greater than zero. Generally, this increases the
probability of producing an impulse
Hyperpolarization- membrane potential increases
(becomes more negative) than the resting potential. This
decreases the probability of producing an impulse
Action potentials/impulse/fire/spike (long distance signals)-
are found in cells with excitable membranes such as neurons
and muscle cells
Are how electrical messages are transmitted within a neuron
22
23. During an action potential, the cell membrane becomes more
permeable to Na+, which increases sodium entry into the
cell through sodium channels. Ca++ diffuses into the cell
through calcium channels
There is:
1. Resting phase: all Na+ and K+ gates are closed
2. Depolarizing phase: Na+ gates open
3. Repolarizing phase: Na+ gates closing, K+ gates opening
4. Undershoot (hyperpolarization): K+ gates still open, Na+
gates closed, Na+ inactivation gate is opening
23
24. There is a complete reversal of membrane potential with a
total change of 100 mV (from -70 mV to 30 mV). This
happens in a few milliseconds and, unlike graded potentials,
does not decrease over distance
In response to the appropriate stimulus, the cell membrane
of a nerve cell goes through a sequence of depolarization
from its rest state followed by repolarization to that rest
state. In the sequence, it actually reverses its normal polarity
for a brief period before re-establishing the rest potential
24
26. The action potential sequence is essential for neural
communication. The simplest action in response to
thought requires many such action potentials for its
communication and performance
The process involves several steps:
1. A stimulus is received by the dendrites of a nerve cell.
This causes the Na+ channels to open. If the opening is
sufficient to drive the interior potential from -70 mV up
to -55 mV, the process continues
26
27. 2. Having reached the action threshold, more Na+ channels
(sometimes called voltage-gated channels) open. The Na+
influx drives the interior of the cell membrane up to about
+30 mV. The process to this point is called depolarization
3. The Na+ channels close and the K+ channels open. Since
the K+ channels are much slower to open, the
depolarization has time to be completed. Having both Na+
and K+ channels open at the same time would drive the
system toward neutrality and prevent the creation of the
action potential
27
28. 4. With the K+ channels open, the membrane begins to repolarize
back toward its rest potential
5. The repolarization typically overshoots the rest potential to
about -90 mV & called hyperpolarization which seems to be
counterproductive, but it is actually important in the
transmission of information.
It prevents the neuron from receiving another stimulus during
this time (stage) triggering another action potential in the
opposite direction, or at least raises the threshold for any new
stimulus. In other words, it assures that the signal is
proceeding in one direction
28
29. 6. After hyperpolarization, the Na+/K+ pump eventually brings
the membrane back to its resting state of -70 mV
AP is an all/none phenomenon & dependent upon strength
& duration of stimulus. Once initiated all APs are alike
Not all local depolarizations produce action potentials. The
depolarization must reach threshold levels if the axon is to
“fire”. This is due to an exchange of Na+ and K+ ions
across the cell membrane and occurs when the outward
current carried by K+ is exactly equal to the inward current
of Na+. Usually, this is seen when membrane has a
depolarization change of 15-20 mV
29
30. Refractory periods
o Absolute refractory period- the period from the opening of
the voltage gated Na+ channels to the closing of the sodium
inactivation gates
No new action potential can be generated during this time
o Relative refractory period- sodium gates are closed and most
have returned to their resting state
Potassium gates are open and repolarization is occurring
Conduction Velocities (CV): CV of neurons vary greatly and are
usually associated with the axon’s anatomical function
30
31. Where speed is essential (postural reflexes), fast conduction
neurons exist. Slow conducting neurons are generally found in
areas where speed is not essential, such as in the gut, glands, &
blood vessels
Conduction velocity generally depends on two factors:
1. Axon diameter- the larger the axon diameter, the faster the
conduction velocities
2. Myelination- the presence of a myelin sheath greatly increases
the rate of impulse propagation because myelin acts as an
insulator to prevent almost all leakage of charge from the axon
to EC space
31
32. Synapses and synaptic transmission
Incoming signals enter to neuron through synapses
located mostly on neuronal dendrites, but also on cell
body
For different types of neurons, there may be only a few
hundred or as many as 200,000 such synaptic connections
from input fibers. Conversely, the output signal travels by
way of a single axon leaving the neuron
Synapses- a junction where axon or some other portion
of one cell (presynaptic cell) terminates on dendrites,
soma/axon of another neuron (post synaptic cell)
32
33. If the target cell is another neuron, the swelling of axon
terminal is called a bouton, and the specialized contact is
called a synapse. If the target is a muscle fiber, the
bouton is often called a motor endplate and the synapse is
referred to as a neuromuscular junction
Synapses mediate information transfer from one neuron to
another neuron or to an effector cell
33
36. Types of synapses
Axodendritic- synapses between the axon of one neuron and
the dendrite of another
Axosomatic- synapses between the axon of one neuron and the
soma of another
Other types of synapses include: axoaxonic (axon to axon),
dendrodendritic (dendrite to dendrite) and dendrosomatic
(dendrites to soma)
Synapses have synaptic cleft (the space between the axon
terminal and sarcolemma) and synaptic knobs (presynaptic
terminal)
36
37. Presynaptic neuron conducts impulse towards synapse
while postsynaptic neuron transmits impulses away from
the synapse
Presynaptic nerve terminal contains mitochondria which
provides ATP and numerous vesicles that contain signal
molecules or neurotransmitter (Ach in NMJ, different
neurotransmitters like GABA in synapses)
Vesicles fuse with presynaptic membrane → action
potential reach the nerve terminal → depolarization of the
membrane → voltage gated Ca++ channels opened →
Ca++ flow into the nerve terminal (Ca+ influx) from ECF
37
39. → causes synaptic vesicles to empty their transmitter
content into synaptic cleft by exocytosis → the transmitter
molecules diffuse across synaptic cleft and bind to receptors
on ion channels in post synaptic membrane → binding opens
ligand-gated ion channels → Na+ and K+ ions influx →
depolarization will occur & elicit action potential in muscle
cell → contraction of muscle cell
Depolarization of postsynaptic membrane lasts as long as
the neurotransmitters (Ach) remain bound to their receptors
39
40. Synaptic cleft contains acetylcholinesterase enzyme which
hydrolyzes Ach into acetate and choline (taken by the
transport proteins of presynaptic membrane into nerve
terminal for Ach synthesis)
Synapse differ from neuromuscular junctions in that:
Individual neurons receive synaptic input from many
other neurons
Signal transmission is either excitatory or inhibitory
Different neurotransmitters involve
40
41. Functional types of synapses
A. Chemical synapse
Almost all synapses used for signal transmission in the CNS are
chemical synapses. i.e. first neuron secretes a chemical
substance (neurotransmitter) at the synapse to act on receptor
on the next neuron to excite it, inhibit or modify its sensitivity
B. Electrical Synapses
Membranes of the pre- and post-synaptic neurons come close
together and gap junctions forms → low membrane borders
which allow passage of ions
41
42. Are less common than chemical synapses
Correspond to gap junctions found in other cell types
C. Conjoint synapse: both electrical and chemical
Action of the transmitter substance on post-synaptic neuron፡
At the synapse, the membrane of post-synaptic neuron
contains large number of receptor proteins.
Binding of the neurotransmitter to its receptor will result in
inhibition or excitation of the postsynaptic membrane
depending on the type of the neurotransmitter i.e. excitatory
or inhibitory
42
43. These receptors have two components
1. Binding site that face the cleft to bind the neurotransmitter
2. Ionophore: passes all the way through the membrane to
the interior. It is of two types
o Ion channels
Cation channels: Na+ (most common), K+, Ca++
Opening of Na+ channels → MP in positive direction
toward threshold level of excitation → (+) neuron
Anion channels: Cl¯ (mainly)
43
44. Opening of Cl¯ channels → diffusion of negative
charges into the membrane → ↓ MP making it more
negative → away from threshold level → (-) neuron
o 2nd messenger system in the post-synaptic membrane: is
important where prolonged post-synaptic changes are
needed to stay for days, months or years (memory).
Effects: intracellular enzymes activation, gene
transcription, etc…
44
46. Synaptic properties
1. One-way conduction
Synapses generally permit conduction of impulses in one-way
i.e. from pre-synaptic to post-synaptic neuron
2. Synaptic delay- is the minimum time required for transmission
across the synapse (0.5 ms)
This time is taken by: discharge of transmitter substance by
pre-synaptic terminal, diffusion of transmitter to post-synaptic
membrane, action of transmitter on its receptor, action of
transmitter to ↑ membrane permeability, increased diffusion of
Na+ to ↑ post-synaptic potential
46
47. 3. Synaptic inhibition
A. Direct inhibition: occurs when an inhibitory neuron (releasing
inhibitory substance) acts on a post-synaptic neuron leading
to → its hyperpolarization due to opening of Cl¯ and/or K+
channels
B. Indirect inhibition (Pre-synaptic inhibition): happens when an
inhibitory synaptic knob lie directly on the termination of a
pre-synaptic excitatory fiber
The inhibitory synaptic knob release a transmitter which
inhibits the release of excitatory transmitter from the pre-
synaptic fiber
47
48. C. Reciprocal inhibition
Inhibition of antagonist activity is initiated in the spindle in
the agonist muscle. Impulses pass directly to the motor
neurons supplying the same muscle and via branches to
inhibitory inter-neurons that end on motor neurons of
antagonist muscle
D. Inhibitory interneuron (Renshaw cells)
Negative feedback inhibitory interneuron of a spinal motor
neuron
48
49. 4. Summation
• Graded potentials (EPSPs and IPSPs) are summed to
either depolarize or hyperpolarize a postsynaptic neuron
a. Spatial summation: when EPSP occurs in more than one
synaptic knob at the same time
b. Temporal summation: if EPSPs in a pre-synaptic knob are
successively repeated without significant delay so the
effect of the previous stimulus is summated to the next
49
51. 5. Convergence and divergence
Convergence: when many pre-synaptic neurons converge on
any single post-synaptic neuron.
Divergence: axons of pre-synaptic neurons divide into many
branches that diverge to end on many post-synaptic neurons
6. Fatigue: is due to exhaustion of neurotransmitter
If the pre synaptic neurons are continuously stimulated,
there may be an exhaustion of the neurotransmitter resulting
in stoppage of synaptic transmission
51
52. Factors affecting synaptic transmission:
o Alkalosis: greatly increases neuronal excitability
E.g. a rise in arterial blood pH from 7.4 to 7.8 - 8.0 often
causes cerebral epileptic seizures b/c of increased
excitability of some/all of the cerebral neurons
o Acidosis: greatly depresses neuronal activity
A fall in pH from 7.4 to below 7.0 usually causes a
comatose state. E.g. in very severe diabetic or uremic
acidosis, coma virtually always develops
52
53. o Drugs: are known to increase and decrease the excitability
of neurons
E.g. Caffeine found in coffee & tea, increases neuronal
excitability, by reducing the threshold for excitation of
neurons
Strychnine: increase excitability of neurons in by inhibiting
the action of some inhibitory transmitter substances
(glycine in the spinal cord)
o Hypoxia: causes depression of neurons
53
54. NEUROTRANSMITTERS
• Are the brain chemicals that communicate information
throughout
• They relay signals between nerve cells, our brain and body
• Stress, poor diet, neurotoxins, genetic predisposition, drugs
(prescription and recreational), alcohol and caffeine usage
can cause their levels to be out of optimal range
• There are two kinds of neurotransmitters
Excitatory neurotransmitters- are not necessarily exciting –
they are what stimulate the brain like catecholamines
54
55. Inhibitory neurotransmitters- are those that calm the brain and
help create balance. They balance mood and are easily
depleted when the excitatory neurotransmitters are overactive.
E.g. serotonin, GABA…
Fate of a neurotransmitter
After a transmitter substance is released at a synapse, it must
be removed by:-
Diffusion out of synaptic cleft into surrounding fluid
Enzymatic destruction e.g. Ach esterase for Ach
Active transport back into pre-synaptic terminal itself e.g. nor-
epinephrine
55
56. Organization of the Nervous System
The basic structural and functional unit of the nervous
system is the nerve cell or NEURON
Neurons come in all sizes and shapes, but the basic
functions of all neurons are more or less similar: they
receive (and integrate) inputs, and relay their output, in
the form of an action potential, to some other target cell
The NS also contains cells which are not neurons and
which do not DIRECTLY participate in the task of sending
and receiving electrical signals. These supporting cells are
called GLIA
56
57. Ganglion: are clusters of cell bodies in the periphery
Nerve: a bundle of axons traveling together in the
periphery. If the nerve contains sensory axons only, it is
called a sensory nerve. If it contains motor axons (going
to muscles) only, it is called a motor nerve. Virtually all
nerves in the body contain both sensory and motor axons
and are therefore called mixed nerves
A connective tissue envelope wrapping individual axons is
endoneurium, wrapping bundles or fascicles of axons is
perineurium and the nerve as a whole is enveloped by
epineurium
57
58. The nervous system consists of two major subdivisions:
1. CNS- consists of the brain housed entirely within cranial
cavity and spinal cord housed within vertebral canal
2. PNS- consists of cranial and spinal nerves to connect
neurons receiving sensory information, receptors which
relay sensory input to the CNS and the muscles to be
controlled
58
59. THE BRAIN
Consists of many parts that function as an integrated whole
The major parts are the medulla, pons and midbrain
(collectively called the brain stem), the cerebellum, the
hypothalamus, the thalamus, and the cerebrum
Ventricles
Are four cavities within the brain: two lateral ventricles, the
third ventricle, and the fourth ventricle. Each ventricle
contains a capillary network called a choroid plexus, which
forms cerebrospinal fluid (CSF) from blood plasma
59
62. Medulla oblongata
Extends from the spinal cord to the pons and is anterior to
the cerebellum
Contains cardiac centers that regulate heart rate, vasomotor
centers that regulate the diameter of blood vessels and,
thereby, blood pressure, and respiratory centers that
regulate breathing
Pons
Bulges anteriorly from the upper part of the medulla
Are important relay station b/n cerebral & cerebellar cortex
62
63. Midbrain
Extends from the pons to the hypothalamus and encloses
the cerebral aqueduct, a tunnel that connects the third
and fourth ventricles
Has large bundles of nerve fibers connecting the spinal
cord and brainstem to the cerebral hemispheres
Cerebellum
Is separated from the medulla and pons by the fourth
ventricle and is inferior to the occipital lobes of the
cerebrum
63
64. Critical to accurate timing and execution of movements; it
acts to smooth and coordinate muscle activity
Hypothalamus
Located superior to the pituitary gland and inferior to the
thalamus. It is a small area of the brain with many diverse
functions:
1. Production of releasing hormones (also called releasing
factors) that stimulate the secretion of hormones by the
anterior pituitary gland
64
65. 2. Regulation of body temperature by promoting responses
such as sweating in a warm environment or shivering in a
cold environment
3. Regulation of food intake
4. Integration of the functioning of the autonomic nervous
system
5. Stimulation of visceral responses during emotional
situations
6. Regulation of body rhythms such as secretion of hormones,
sleep cycles, changes in mood, or mental alertness
65
66. Thalamus
Is superior to the hypothalamus and inferior to the
cerebrum
Many of the functions of the thalamus are concerned with
sensation in which it integrates the impulses from the
cutaneous receptors and from the cerebellum
Parts of the thalamus are also involved in alertness and
awareness and others contribute to memory
66
67. Cerebrum
Is the largest part of brain which consists of two
hemispheres separated by longitudinal fissure at the base
of which there is corpus callosum that connects the right
and left hemispheres and enables each of them to know
the activity of the other
The surface of the cerebrum is gray matter called the
cerebral cortex consisting of cell bodies of neurons, which
carry out the many functions of the cerebrum
Cerebral cortex has folds which are called convolutions/gyri
and the grooves b/n them are fissures or sulci
67
68. The folding permits the presence of millions more neurons in
the cerebral cortex enabling humans to read, speak, do long
division, write poetry, songs…
The cerebral cortex is divided into lobes
Frontal Lobes
Have motor areas that generate the impulses for voluntary
movement
The left motor area controls movement on the right side of
the body, and the right motor area controls the left side of
the body
68
69. Anterior to the motor areas are the premotor areas, which
are concerned with learned motor skills that require a
sequence of movements
The parts of the frontal lobes just behind the eyes are the
prefrontal or orbitofrontal cortex which is concerned with
things such as keeping emotional responses appropriate to
the situation, realizing that there are standards of
behavior (laws or rules of a game) and following them,
and anticipating and planning for the future
69
70. Parietal Lobes
Have general sensory areas which receive impulses from
receptors in the skin and feel and interpret the cutaneous
sensations. They also receive impulses from stretch
receptors in muscles for conscious muscle sense
The left area is for the right side of the body and vice
versa
The taste areas, which overlap the parietal and temporal
lobes, receive impulses from taste buds on the tongue and
elsewhere in the oral cavity
70
71. Temporal Lobes
Have olfactory areas which receive impulses from receptors in
the nasal cavities for the sense of smell
The auditory areas receive impulses from receptors in the inner
ear for hearing
Occipital Lobes
Have visual areas to which impulses from the retinas of the
eyes travel along the optic nerves
Other parts of the occipital lobes are concerned with spatial
relationships; things such as judging distance and seeing in
three dimensions…
71
72. The cerebral cortex has the characteristic of neural plasticity,
the ability to adapt to changing needs, to recruit different
neurons for certain functions, as may occur during childhood
or recovery from a stroke
Association Areas- many parts of the cerebral cortex which
are not concerned with movement or a particular sensation
and give us the ability to reason and use logic, learning and
memory (involve the hippocampus of the temporal lobe)
Basal ganglia- are paired masses of gray matter within the
white matter of the cerebral hemispheres
72
73. Meninges and Cerebrospinal Fluid
The connective tissue membranes that cover the brain and
spinal cord are called Meninges
The thick outermost layer, made of fibrous connective tissue,
is the dura mater which lines the skull and vertebral canal
The middle arachnoid membrane (arachnids are spiders) is
made of web-like strands of connective tissue
The innermost pia is a very thin membrane on the surface of
the spinal cord and brain
73
74. Between the arachnoid and the pia mater is the
subarachnoid space, which contains cerebrospinal fluid
(CSF), the tissue fluid of the central nervous system
Cerebrospinal fluid (CSF)
Formed by the choroid plexus from blood plasma and
circulates in and around the CNS
It flows from the lateral and third ventricles through the
fourth ventricle, then to the central canal of the spinal
cord, and to the spinal and cranial subarachnoid spaces
74
75. From the cranial subarachnoid space, cerebrospinal fluid is
reabsorbed through arachnoid villi into the blood in cranial
venous sinuses (large veins within the double-layered cranial
dura mater)
The CSF becomes blood plasma again, and the rate of
reabsorption normally equals the rate of production
Functions of CSF:
Bring nutrients to CNS neurons and to remove waste
products to the blood as the fluid is reabsorbed
Act as a cushion for the central nervous system
75
76. THE SPINAL CORD
Transmits impulses to and from the brain and is the integrating
center for the spinal cord reflexes
Extends from the foramen magnum of the occipital bone to the
disc between the first and second lumbar vertebrae
Has internal gray matter consisting of cell bodies of motor
neurons and interneurons; and external white matter made of
myelinated axons and dendrites of interneurons
Has ascending & descending tracts carrying sensory impulses to
brain & motor impulses away from brain, respectively; & central
canal (contains CSF and is continuous with brain cavities)
76
78. Peripheral Nervious System/PNS
Is a collection of neurons and their processes which relay
information from the periphery to the CNS, in which case
they are afferent or sensory; or from the CNS to the
periphery, in which case they are efferent or motor
Cranial Nerves
Are the nerves which connect the brain with the periphery
(mainly in the head and neck)
There are 12 pairs of cranial nerves which leave the brain
on its underside then exit the cranial cavity by a series of
holes in the base of the skull, called foramina 78
79. Many of them do carry impulses for functions involving the
head. Some, however, have more far-reaching destinations
The impulses for the senses of smell, taste, sight, hearing,
and equilibrium are all carried by cranial nerves to their
respective sensory areas in the brain
Some cranial nerves are almost purely sensory; such as
those which mediate smell, vision, and hearing
Others are almost purely motor, such as those which move
the eyes and tongue. Others are mixed
79
81. Spinal Nerves
Are the nerves linking the spinal cord and the periphery,
and they are responsible for sensory and motor
innervations of the body outside of the head and neck
The spinal nerves exit the vertebral canal by way of
spaces between adjacent vertebrae, known as
intervertebral foramina
Are 31 pairs emerging from the spinal cord
81
82. Are named according to their respective vertebrae: 8
cervical pairs, 12 thoracic pairs, 5 lumbar pairs, 5 sacral
pairs, and 1 very small coccygeal pair
The cervical nerves supply the back of the head, neck,
shoulders, arms, and diaphragm (the phrenic nerves)
The first thoracic nerve also contributes to nerves in the
arms. The remaining thoracic nerves supply the trunk of
the body
82
83. The lumbar and sacral nerves supply the hips, pelvic cavity,
and legs. They hang below the end of the spinal cord and
called Cauda equina
Each spinal nerve has two roots, which are neurons
entering or leaving the spinal cord
a) Dorsal root- is made of sensory neurons that carry
impulses into the spinal cord. It has an enlarged part that
contains cell bodies of sensory neurons- dorsal root
ganglion
b) Ventral root- is made of axons of motor neurons carrying
impulses from spinal cord to muscles or glands
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84. The Autonomic Nervous System
Is actually part of the PNS in that it consists of motor
portions of some cranial and spinal nerves
Made up by visceral motor neurons to smooth muscle,
cardiac muscle, and glands- visceral effectors
The autonomic nerve pathway from the CNS to a visceral
effector consists of two motor neurons that synapse in a
ganglion outside the CNS
The first neuron is called preganglionic neuron, from the
CNS to ganglion (cell bodies of postganglionic neurons)
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85. The second neuron is called the postganglionic neuron, from
the ganglion to the visceral effector
ANS has two divisions which function in opposition to each
other
1. Sympathetic (Thoracolumbar) Division/SNS
Their cell bodies are in the thoracic segments and some of the
lumbar segments of the spinal cord
Their axons extend to the sympathetic ganglia, most of which
are located outside the spinal column
Within the ganglia are the synapses between preganglionic and
postganglionic neurons
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86. Postganglionic axons then go to the visceral effectors
One preganglionic neuron often synapses with many
postganglionic neurons to many effectors i.e brings about
widespread responses in many organs
SNS is dominant in stressful situations, which include anger,
fear, or anxiety, as well as exercise. E.g. increasing heart
rate, vasodilation, bronchodilation during exercise
2. Parasympathetic (Craniosacral) Division/PSNS
The cell bodies of PNS neurons are in the brain stem and the
sacral segments of the spinal cord
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87. Their axons are in cranial nerve pairs 3, 7, 9, and 10 and in
some sacral nerves and extend to the parasympathetic
ganglia
These ganglia are very close to or actually in the visceral
effector and contain the postganglionic cell bodies, with very
short axons to the cells of the effector
In the parasympathetic division, one preganglionic neuron
synapses with just a few postganglionic neurons to only one
effector i.e. very localized (one organ) responses are
possible
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88. PSNS dominates in relaxed (non-stress) situations to promote
normal functioning of several organ systems. E.g. digestion,
defecation and urination
When an organ receives both sympathetic and parasympathetic
impulses, the responses are opposites
For the somatic portion of the nervous system, there are two
major types of nerve cells which connect the spinal cord to the
periphery
These are:
Primary sensory neurons (afferent neurons)- relay input from
the periphery to the spinal cord & sit in dorsal horn
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89. Have peripheral processes in the skin or muscles and
their central processes enter the spinal cord where they
make synapses with other neurons
Spinal cord motor neurons (efferent neurons)- convey
motor outflow from the spinal cord to the periphery and
sit in the ventral horn
Axons of spinal cord motor neurons pass to the
periphery to innervate striated muscle
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