1. In lecture today: CHAPTER 7
Structure and function of neurons and other cells in the nervous system.
Electrical activity in axons.
Action potential
Conduction of nerve impulses
Synaptic transmission & neurotransmitters
Dr. Hameed Al-Sarraf
Dept. Physiology

2. Nervous System The nervous system is divided into:
1- Central Nervous System (CNS)
Brain and Spinal cord
2- Peripheral Nervous System (PNS)
Cranial and Spinal nerves

3. Neurons and Glial Cells
There are two types of cells in the nervous system:
1- Neurons: are the functional units of the nervous system.
2- Supporting cells: help functioning of the neurons

4. Neuron
In general the structure of neuron is divided into:
1- cell body - contains nucleus and is the nutritional center of neuron.
groups of cell bodies in the CNS = neuclei
groups of cell bodies in the PNS = ganglia
2- dendrites- transmit electrical signals to the cell body.
3- axon- longer than dendrite and transmits signals away from cell body.

5. Neuron
Substances synthesized in the cell body are transported through the axon by two
mechanisms:
1- axoplasmic flow- wave like contraction that pushes cytoplasm away from the cell body.
2- axonal transport- more rapid and can occur in both directions.

6. Neurons

7. Classification of Neurons
Neurons maybe classified according to their function:
1- Sensory or afferent neurons
2- Motor or efferent neurons
3- Interneurons or association neurons
OR according to their structure:
1- Pseudounipolar-
2- Bipolar-
3- Multipolar-
Sensory
Motor

9. Supporting Cells Extracellular Fluid Cerebrospinal Fluid
There are 6 types of supporting cells in the nervous system:
1- Schwann cells- form myelin sheath in PNS
2- Oligodendrocytes- form myelin sheath in CNS
3- Microglia- phagocytic cells in CNS
4- Astrocytes- regulate ECF of CNS
5- Ependymal cells- separate CNS from CSF
6- Satellite cells- support cell bodies in PNS
Extracellular Fluid
Cerebrospinal Fluid

10. Schwann Cells
Some axons in the PNS are surrounded by a myelin sheath formed by Schwann cells, this type of axons are called myelinated.
The insulation in the myelinated axons is by successive wrapping of the cell membrane of Schwann cells around the axon.
Unmyelinated is the type of axons that do not have myelin insulation.
Each Schwann cell wraps only a mm of length of axon leaving gaps (node of Ranviour) of exposed axon between adjacent Schwann cells.

11. Myelination

12. Myelination in the CNS
The process of forming a myelin sheath around axons in the CNS is carried out by oligodendrocytes. These cells form myelin sheath around several axons in the CNS.

13. Astrocytes
The most abundant type of supporting cells in the CNS are astrocytes.
Astrocytes support the function of neurons in two ways:
1- They regulate extracellular K+ concentration around neurons.
2- the also regulate extracellular neurotransmitter concentration.
Glutamic acid g-aminobutyric acid (GABA)
Astrocytes also interact with brain capillaries. It is thought that
they help in the formation of blood-brain barrier.

14. Electrical Activity in Axons
All cells in the body show potential difference between their inside and outside.
The inside is negatively charged compared to the outside.
Extracellular
Intracellular
- 65 mV
This potential difference between inside and outside is called resting membrane potential (r.m.p.).
In excitable cells (nerve and muscle) the r.m.p. can change in response to stimulation.

15. Recording Membrane Potential
Depolarization
Hyperpolarization

16. Action Potential
Depolarization
Repolarization

17. Ion Gating in Axons
Channels in the cell membrane which regulate movement of ions in and out
of the cell are called ion gates.
Some channels have no gates and are always open.
Channels for Na and K are specific.
K channels are:
a) gated
b) non-gated
All Na channels are gated.

18. Membrane potential - 70 mV
Membrane potential is produced by the action of Na/K pump, which transports
3Na from inside to outside and at the same time it moves 2K+ from outside to
inside of the cell. _ _ _
protein K+ _
Resting axon is very impermeable to Na+
but permeable to K + .
The result is difference between concentrations
of Na+ and K+ inside and outside of the cells
which produces a potential difference across the
membrane.
Na+
2K+ K+
3Na+
Na+ 142 mM
K+ 4 mM
14 mM
140 mM
- 70 mV

19. Ionic Movements During Action potential
1) r.p.m.
2) depolarization
3) repolarization + -
Na+ - K+ +

20. Undershoot Repolarization Depolarization r.p.m. = Na+ = K+
= Na+ Channel
= K+ Channel

21. Ionic Movements During Action potential

22. Conduction of Impulses
Electrical signals in the nerve axon are
generated and transmitted in the form of
action potential.
A nerve signal maybe initiated by a
stimulus which can be:
chemical, electrical, physical,
or change in temperature.

23. Characteristics of Nerve Impulse & Conduction
Threshold:
Is the minimum stimulus required for generation of action potential.
Refractory Period:
During the time of action potential area of axon which the action
potential is occuring can not produce another action potential.
All-or None Law:
The amplitude of the action potential is constant regardless of the strength of stimulus.
Q- How the nerve will transmit different strength of stimuli?

24. Types of Nerve Fibers 1- Unmyelinated Nerve Fibers:
most of the axon is exposed and without myelin covering. Transmits signals slower than
myelinated fibers, e.g. used in slow response such as control of blood vessels.

25. 2- Myelinated Nerve Fibers (saltatory conduction):
Their axon is covered with myelin sheath, it transmits nerve signals very fast the larger the
diameter of axon and myelin sheath around it the faster nerve conduction, e.g. fast sensory
Signals from skin to brain or spinal cord.

26. Synapse
Synapse is the functional connection between a neuron and a second cell.
Synapse can occur between:
Neurons (neuron-neuron)
A neuron and an effector cell in muscle/gland
Two types of synapses:
- Electrical synapse
- Chemical synapse

27. Electrical Synapse
Gap Junction

28. Electrical Synapse

29. Chemical Synapse

30. Neurotransmitter released
Axon
Chemical Synapse
Action potential
Ca2+
Ca2+
Synaptic Vesicle
Ca2+
Ca 2+
Activates calmodulin
Protein kinase
(inactive)
Protein kinase
(active)
Fusion & exocytosis
Ca2+
Ca2+
Neurotransmitter released

31. Neuromuscular Junction

32. Neurotransmitters
Chemical synapses transfer information from neuron to the next cell
across the synapse by chemical compounds called neurotransmitters.
There are many chemicals which act as neurotransmitters in the nervous
system, here is a list of some of the most abundant ones:
1- Amines, e.g. Acetylcholine (Ach)
2- Cathecolamines, e.g Norepinephrine and Epinephrine
3- Amino acids, e.g Glutamic acid, and GABA
4- Polypeptides, e.g Glucagone, insulin
5- Nitric oxide