Fundamentals of the Nervous
System
Three functions of the nervous system
1- sensory (afferent) input: sensory
receptors that work with the NS.
2- integration: processing and interpreting
of data from the sensory input
3. motor (efferent) output: a response
initiated by integration.
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Organization:
 CNS: (central nervous system)
- brain
- spinal cord
 (PNS) Peripheral Nervous System
- cranial nerves (12 pairs)
- spinal nerves (31 pairs)
Communication lines between CNS and the
rest of body
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Divisions:
- Sensory (afferent) division
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Somatic (skin, muscle, joints) and visceral
(organs) sensory neurons
Conducts impulses from receptors to the CNS
- Motor (efferent) division
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Motor neurons
Conducts impulses from the CNS to effectors
(muscles and glands)
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Motor Divisions:
Somatic Nervous System
- Voluntary
- Conducts impulses from CNS to skeletal
muscles
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Autonomic Nervous System
- Involuntary
- Conducts impulses from CNS to cardiac
muscles, smooth muscles, and glands.
- Divisions:
- Sympathetic
- Parasympathetic
Two types of cells in the nervous system.
1- Neurons (nerve cells)
2- Glia: support, nourish and protect
neurons. “glue”
Glia: (neuroglia)
PNS:
1- Schwann cells: cells produce a white fatty
substance called myelin around the large nerve
fibers of the PNS. Also called neurolemmocytes.
2- Satellite cells: surround neuron cell bodies and
may help regulate chemical environment.
Myelin Sheath and
Neurilemma
CNS:
Astrocytes:
- connecting neurons to blood supply
- projections with bulbous ends that cling to
neurons and capillaries
- ½ of neural volume
- BBB
Microglia:
- small and remain stationary. When disease or
inflammation is present they become mobile and
phagocytize microbes. (macrophages)
Oligodendrocytes:
- Line up along the thicker neuron fibers
- Produce myelin sheath around axons
Ependymal cells:
- line central cavities of brain and spinal
cord, creating a barrier between CNS
cavities and tissues surrounding cavities
- cilia circulates the cerebrospinal fluid
Neurons (nerve cells):
- Conducts messages in form of nerve impulses
- Has longevity
- Amitotic
- Have a high metabolic rate
 Functionally composed of:
- A receptive (input) region
- A conducting component (generates and
propagates an action potential)
- A secretory (output) component
- neurotransmitter
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Neurons are composed of
- Cell body
- Processes
Cell body: (perikaryon or soma)
- Most neuron cell bodies located within CNS
- Clusters of cell bodies in CNS are called
nuclei
- Few/clusters of cell bodies in PNS are
called ganglia
- Rough ER called Nissl bodies.
Processes
- Cellular processes are called either tracts (in
CNS) or nerves (in PNS)
- Dendrites - have large surface area to receive
chemical signals as well as conduct electrical
signals (graded potentials)
- Axons - single in each neuron, transmit graded
potential away from cell body to axonal terminal
(generates action potential)
- Axon hillock arises from the cell body into the
axon (graded potential)
- Profuse branching at end of axon called terminal
branches or telodendria. At the tips there are
bulbous structures storing neurotransmitters.
Substances travel continuously up and down
the axon.
Anterograde: substances move from cell
body to terminal axon
Retrograde: substances move from terminal
axon to cell body.
Plasma membrane of axon is axolemma.
- Electrochemical signals transmitted with
the aid by myelin sheath (protein-lipoid)
which insulates nerve fibers (long axons)
and increases the transmission
- Nodes of Ranvier also aid in the
transmission of nerve impulses.
- saltitory conduction
- Myelinated processes form the white
matter of nervous tissue and unmyelinated
processes form the gray matter of nervous
tissue.
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In the spinal cord, gray matter forms the
H- shape in the center while white matter
surrounds it.
In the brain, gray matter forms the thin
outer cortex with white matter filling the
inside
Classification of neurons:
Structural
Functional
Structural:
Neurons are classified as to how many processes
extend from their cell body
Multipolar: three or more processes extend from
cell body
- posses single axon
- most common type of neuron in the CNS
Bipolar: two processes (axon and dendrite) extend
from opposite sides of neuron.
- rare in adult but may be found in retina and
olfactory mucosa (special senses).
Unipolar: one process extending from cell body
and forms central and peripheral processes
- Central process associated with secretory region
- Peripheral process associated with sensory
region (receptor).
- Found in ganglia of PNS where they function as
sensory neurons
Functional Classification
of Neurons
Functional:
Sensory (afferent) neurons - transmit
impulses from sensory receptors toward
CNS
Unipolar neurons - skin or internal organs to
CNS for interpretation
Bipolar neurons - special sense organs,
retina
Motor (efferent) neurons - carry impulses
away from CNS to organs
Multipolar neurons - cell body located within
CNS and neurons form neuromuscular
junctions with effector (muscle or gland)
cells
Association neurons (interneurons) transmit impulses within CNS (usually
sensory to motor); found in CNS only;
mostly multipolar and 99% of neurons in
body. Also called connecting neurons.
Neurophysiology/Electrophysiology:
Resting Membrane Potential (RMP)
Positive charge on the outside
Negative charge on the inside
These electrical charges are in the form of
potential energy which is measured in
millivolts (-70mV in the plasma membrane)
The (-) in -70 mV refers to the inside of the
membrane being more negative than the
outside
This membrane is said to be polarized
The energy in the resting membrane is
likened to energy stored in a battery.
Flow of electrical charge is called current.
In the membrane it flows by way of ions
instead of electrons (electricity)
In a resting membrane (-70 mV) three Na+
ions are pumped out for every two K+
pumped in. This creates a more negative
situation on the inside:
Sodium/Potassium Pump
Ion channels:
Leakage
Gated
Leakage channels are always open.
Gated channels open and close in response to a
stimulus.
Stimulus:
Voltage
Chemical
Mechanical pressure
Light
Gated Channels
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Voltage gated ion channels will respond to the
change in membrane potential (voltage).
Excitability.
Chemically gated ion channels respond to the
presence of a specific chemical stimulus such as
hormone, neurotransmitter, Ca+
The stimulus will result in a graded potential
that will either cause the neuron to fire
(depolarization) or not (hyperpolarization).
Impulse traveling over a long distance (axon) is
called an action potential
Depolarization: Reduction in membrane potential.
When the membranes becomes less negative on
the inside (moves closer to 0). Increases the
probability the nerve will fire.
i.e.: -70 to -65 mV
-70 to +30 mV
Hyperpolarization: Increase in membrane
potential. When the membrane becomes more
negative on the inside. Decreases the probability
the nerve will fire.
i.e.: -70 to -90 mV.
Depolarization:
Voltage gated Na+ channels open and Na+
rushes in. -70 mV to 0mV to +30 mV.
Action Potential initiated. (all or none)
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Repolarization:
Na+ channels close, K+ channels open and K+ is
pumped out. Restores electrical conditions not
original ionic distribution of resting state. It is
the activation of the sodium-potassium pump
that reestablishes ionic distribution
+30mV to -70m.
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Synapse:
Junction between nerve cells and effector cells.
Axodendritic:
Presynaptic neuron: impulse firing away
from cell body towards the synapse
(sender)
Postsynaptic neuron: impulse away from
synapse. (receiver)
Chemical synapses uses a substance called
a neurotransmitter to get information
across the synaptic cleft.
The Synapse
1- Calcium channels open in the presynaptic
axonal terminal.
Impulse reached axonal terminal causing a rush of
Ca+ in from the extracellular fluid
2- Ca+ causes the synaptic vesicles filled with
neurotransmitter to migrate and fuse with
axonal membrane (exocytosis)
3- Ca+ is pumped out of terminal end
4- Neurotransmitter binds to postsynaptic
receptors.
5- Ion channels open in postsynaptic membrane
causing changes in membrane potential (action
potential ?)
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Classes of Neurotransmitters:
Acetylcholine: excitatory (skeletal muscle)
Biogenic amines
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Amino acids
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Dopamine, Norepinephrine, and epinephrine (feel
good catecholamines)
Serotonin: mood, sleep, appetite & anger (inhibitory)
Histamine
GABA (gamma-aminobutyric acid) (inhibitory)
Glutamate: excitatory
Peptides
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Endorphins and enkephalins: inhibitory (opioids)