Basic concepts about neuron and the nervous system

1. Dr. Abdul Qadeer
MBBS; FCPS; FICS

2. Introduction
 Nervous system is divided into:
1. Central nervous system: consists of brain & spinal
cord
2. Peripheral nervous system: consists of cranial &
spinal nerves
3. Somatic nervous system
4. Autonomic nervous system

3. Cell types of nervous system
Neurons: excitable cells
Neuroglia : non-excitable / supporting cells
e.g. neuroglia and ependymal cells in CNS,
and Schwann cells in PNS

4. Neuron
 It is nerve cell along with all its processes
 It consists of:
- cell body or soma or perikaryon
- cell processes: two types 1. axon 2. dendrites
 Dendrites are afferent processes, and usually multiple in
number
 Axon is an efferent process, and usually single in number
 Telodendria or terminals are the branches of an axon
 Somata (cell bodies) form grey matter and nuclei in CNS
 Axons form tracts in CNS & nerves in PNS

5. Neuron

6. Types of neurons
 According to the number of processes:
1. Unipolar
2. Psudo-unipolar
3. Bipolar
4. Multipolar
 According to the length of axon:
1. Golgi type I neuron; with a long axon
2. Golgi type II neuron (microneuron); with short or no
axon

9. Neuroglia
 Non-excitable supporting cells of nervous system
 These include:
1. Neuroglial cells: found in parenchyma of CNS
2. Ependymal cells: lining the internal cavities
3. Capsular or satellite cells: surrounding neurons of
the sensory and autonomic ganglia
4. Schwann cells: forming sheaths of axons of
peripheral nerves

10. Neuroglia
 Neuroglial cells found in the parenchyma of CNS are:
1. Macroglia : e.g. astrocytes, oligodendrocytes,
glioblasts
2. Microglia (mesodermal origin)

11. Astrocytes (Star-shaped)

12. Oligodendrocytes (fewer cell
processes)

13. Glioblasts
 Are stem cells, which can differentiate into macroglial
cells

14. Microglial cells
 Smallest of glial cells
 Phagocytic activity
 Mesenchymal origin

15. Functions of glial/ependymal cells
1. Provide mechanical support to neurons
2. Non-excitable cells, hence act as insulators and
prevent neuronal impulses from spreading in
unwanted directions
3. Phagocytosis, hence remove foreign material and cell
debris
4. Can repair the damaged areas of nervous system
5. Store neurotransmitter chemical substances
6. Myelination of tracts
7. Exchange materials between brain & CSF

16. CNS
 Consists of:
1. Brain
2. Spinal cord

17. PNS
 Cranial nerves: 12 pairs
 Spinal nerves: 31 pairs as under
 8 cervical
 12 thoracic
 5 lumbar
 5 sacral
 1 coccygeal
 Note that there are 8 cervical nerves and 7 cervical
vertebrae, while 1 coccygeal nerve & 4 coccygeal
vertebrae fused.

19. A typical spinal nerve

20. A typical spinal nerve
 Connected to the spinal cord by two roots, anterior
root and posterior root
 Anterior roots are efferent i.e. carry nerve impulses
away from CNS
 The efferent fibers that go to skeletal muscles and
cause them to contract are called Motor fibers.
 Their cells of origin lie in the anterior gray horn of the
spinal cord

21. A typical spinal nerve
 Posterior roots are afferent and connected to the
spinal cord with nerve fibers which carry impulses to
the CNS
 These fibers convey information about sensation of
touch, pain, temperature and vibrations, they are
called sensory fibers.
 Cell bodies of sensory fibers are situated in a swelling
on the posterior root, hence called the posterior root
ganglion

24. A typical spinal nerve
 At each inter-vertebral foramen the anterior and
posterior roots unite to form a spinal nerve
 Here the motor and sensory fibers become mixed
together
 On emerging from the foramen, the spinal nerve
divides into two rami, anterior and posterior.
 Anterior ramus is large while posterior ramus is
smaller

25. A typical spinal nerve
 The posterior ramus passes posteriorly around the
vertebral column to supply the muscles and skin of the
back
 Anterior ramus continues anteriorly to supply:
1. the muscles and skin over the anterolateral body wall
2. All the muscles and skin of the limbs

26. A spinal nerve
 Spinal nerves also give rise to a small meningeal
branch that supplies the vertebrae and coverings of the
spinal cord (the meninges)
 Thoracic spinal nerves also have branches called rami
communicantes, that are associated with the
sympathetic part of autonomic nervous system

28. Nerve plexuses
 At the root of the limbs, the anterior rami join one
another to form nerve plexuses e.g. cervical plexus,
brachial plexus, lumbar plexus & sacral plexus.

29. Reflex arc
 A reflex arc is a neural pathway that controls an
action reflex. In higher animals, most sensory neurons
do not pass directly into the brain, but synapse in
the spinal cord. This characteristic allows reflex
actions to occur relatively quickly by activating spinal
motor neurons without the delay of routing signals
through the brain, although the brain will receive
sensory input while the reflex action occurs.

30. Reflex arc
 A reflex arc is made up of
1. A receptor e.g. skin
2. A sensory or afferent neuron
3. A motor or efferent neuron
4. An effector organ e.g. muscle

32. Reflex action
 An involuntary motor response of the body is called a
reflex action
 Monosynaptic reflex = stretch reflex e.g. tendon jerk
(knee jerk)
 Polysynaptic reflex = withdrawal reflex, response to a
pain stimulus