Nervous systemDr. Abdul Qadeer

MBBS; FCPS; FICS

Introduction Nervous system is divided into:1. Central nervous system: consists of brain &

spinal cord2. Peripheral nervous system: consists of

cranial & spinal nerves3. Somatic nervous system4. Autonomic nervous system

Cell types of nervous systemNeurons: excitable cellsNeuroglia : non-excitable /

supporting cells e.g. neuroglia and ependymal cells in CNS, and Schwann cells in PNS

Neuron It is nerve cell along with all its processesIt consists of:

- cell body or soma or perikaryon- cell processes: two types 1. axon 2.

dendritesDendrites are afferent processes, and usually

multiple in numberAxon is an efferent process, and usually single

in numberTelodendria or terminals are the branches of an

axonSomata (cell bodies) form grey matter and

nuclei in CNSAxons form tracts in CNS & nerves in PNS

Neuron

Types of neuronsAccording to the number of processes:1. Unipolar2. Psudo-unipolar3. Bipolar4. Multipolar According to the length of axon:1. Golgi type I neuron; with a long axon2. Golgi type II neuron (microneuron); with

short or no axon

Neuroglia Non-excitable supporting cells of nervous

systemThese include:1. Neuroglial cells: found in parenchyma of

CNS2. Ependymal cells: lining the internal cavities3. Capsular or satellite cells: surrounding

neurons of the sensory and autonomic ganglia

4. Schwann cells: forming sheaths of axons of peripheral nerves

NeurogliaNeuroglial cells found in the parenchyma of

CNS are:1. Macroglia : e.g. astrocytes,

oligodendrocytes, glioblasts2. Microglia (mesodermal origin)

Astrocytes (Star-shaped)

Oligodendrocytes (fewer cell processes)

Glioblasts Are stem cells, which can differentiate into

macroglial cells

Microglial cellsSmallest of glial cellsPhagocytic activityMesenchymal origin

Functions of glial/ependymal cells

1. Provide mechanical support to neurons2. 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 substances6. Myelination of tracts7. Exchange materials between brain & CSF

CNSConsists of:1. Brain2. Spinal cord

PNSCranial nerves: 12 pairsSpinal nerves: 31 pairs as under8 cervical12 thoracic5 lumbar5 sacral1 coccygealNote that there are 8 cervical nerves and 7

cervical vertebrae, while 1 coccygeal nerve & 4 coccygeal vertebrae fused.

A typical spinal nerve

A typical spinal nerveConnected to the spinal cord by two roots,

anterior root and posterior rootAnterior roots are efferent i.e. carry nerve

impulses away from CNSThe 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

A typical spinal nervePosterior 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

A typical spinal nerveAt 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

A typical spinal nerveThe 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

A spinal nerveSpinal 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

Nerve plexusesAt 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.

Reflex arcA 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.

Reflex arcA reflex arc is made up of1. A receptor e.g. skin2. A sensory or afferent neuron3. A motor or efferent neuron4. An effector organ e.g. muscle

Reflex actionAn involuntary motor response of the body is

called a reflex actionMonosynaptic reflex = stretch reflex e.g.

tendon jerk (knee jerk)Polysynaptic reflex = withdrawal reflex,

response to a pain stimulus