Summer PBP Program:
Structure and Functions –
Physiology of the Nerve
Detron M. Brown, MPH
Cells of the Nervous System
 There are two types of cells:
 Neurons
 Glia
Neuron
 These are known as “excitable” cells
 They are responsible for conducting the “signal” or
impulse that make the nervous system function.
 The human brain contains approximately about 100
billion or 10 % of all nerve cells in the body.
Structure of Neuron
Cell Body
 The largest part of the nerve.
 Its has the majority of the of the major organelles.
 The ROUGH ER is responsible for making the
ribosomes that form the nissl bodies. These provide
the protein molecules needed for the transmission
of nerve signals and are useful in maintaining and
regenerating nerve fibers.
Dendrites
 The branched part of the cell.
 Receive the stimuli that initiate the nerve signals.
 The electrical signal received is conducted toward
the cell body/and or axon of the neuron.
Axon
 Single process
 Stems from the tapered portion of the cell body at
location called the axon hillock
 Axons conduct impulses AWAY from the cell body
Glial cells
 100 billion neurons
 10x more glial cells
 Glial cells
 Support neurons (literally, provide physical support,
as well as nutrients)
 Cover neurons with myelin
 Clean up debris
 “Housewives”
Size of Axon and Its effect on
Function
 Some can be a meter long and some can be as short
as a couple millimeters.
 The larger the diameter the faster the impulse.
 Axons can be myelinated or not. Only axons can
have myelin sheath.
Neuron Classification
Structural
Functional
Functional organization of neurons
 Conductile component/action potential/ :

Signal is electrical

action potentials are all or none

All action potentials have same amplitude and duration

information in the signal is represented by frequency and duration
 Out put component :

signal chemical transmitter

total number of action potential determine how much
neurotransmitter should be released
Myelin
 Insulates axons and facilitates speed of action potential
transmission
 Arranged in concentric bimolecular layers
 Has a composition similar to plasma membranes
 Schwan cells form myelin of peripheral nerves and
Oligodendrocyts that of central nerves
 Schwan cells express their myelin gene in response to contact with
axon while Oligodendrocytes depend also on the presence of
Astrocytes
Myelin Sheath
 Fatty material made by glial cells
 Insulates the axon
 Allows for rapid movement of
electrical impulses along axon
 Nodes of Ranvier: gaps in myelin sheath
where action potentials are transmitted
 Multiple sclerosis is a breakdown of
myelin sheath
 Speed of neural impulse Ranges from 2 –
200+ mph
Ion channels
Conduct ions at fast rate
Selective for specific ions
Ion channels are proteins that span the cell membrane
Flux of ions through the ion channel is passive
Opening and closing of ion channel involves conformational
change
Open and close in response to specific stimulus
Voltage –gated
Ligand –gated
Mechanically –gated
Gap-junction channels
The binding of exogenous ligands /toxins, poisons and drugs/can
make channels open or close
Ion channels are composed of several subunits
 Channels are also important targets of diseases
 myasthenia gravis
 hyperkalemic periodic paralysis
Figure 1: Ion
Channels
Synaptic transmission
 The average neuron makes 100000 connections
 Two basic forms of transmission
 Chemical
 Electrical
 Electrical transmissions are
 Short lasting
 Only excitatory
 Do not induce long lasting postsynaptic changes
 Gap junction channels
 Bidirectional transmission
Synaptic Transmission
 Chemical transmissions are
 Variable signaling: inhibitory or excitatory
 Produce complex behavior
 Longer lasting / delay in transmission
 Amplify signals
 Modify post synaptic receptors both functionally and
anatomically
 Ionotropic receptors :conformational change that opens the
channels on binding transmitter
 Metabotropic receptors: act by altering intracellular
metabolic reaction
Chemical transmitters



Classical

Acetylcholine

Cathecolamines

Glutamates

GABA

Serotonine

histamine
Peptides

Substance p

Enkephaline

Endorphine

Prolactin, oxytocine, vasopresin
Soluble gases

NO
Cellular basis of connectionist approach
 Principle of dynamic polarization : electrical signals within a
nerve flow only in one direction
 Principle of connectional specificity : nerve cells do not connect
indiscriminately with one another to from a network
Cont.
 Chemical transmitter
 The type of neurotransmitter they use
 The type of Receptors they have
 Myelin content
 Location in the nervous system –central/peripheral
 These differences and others may account for different patterns of
disease
Neurotransmitters
 chemical messengers that traverse the
synaptic gaps between neurons
 when released by the sending neuron,
neurotransmitters travel across the
synapse and bind to receptor sites on the
receiving neuron, thereby influencing
whether it will generate a neural
impulse
Neurotransmitters (>60)
 Acetylcholine (ACh)
 1st substance identified as NT
 Links motor neurons and muscles (contract or relax)
 e.g. curare vs black widow spider
 Also involved in memory, learning, sleep, dreaming
(acetylcholine movie)
 Endorphins (the brain’s own morphine)
 1973 injected rats with morphine
 Bound like NTs
 Brain had receptors for exogenous substance?
 Brain must produce its own morphine
 Released during pain and discomfort
Reference
1. http://up.edu.ps/ocw/repositories/academic/up
/bs/dent/MGDC1205/012009/data/Physiology_I
_Week_4.ppt
2. Human Anatomy and Physiology . Marib and
Hoehn