Nervous system controls the functions of human body. It is composed of specialized nerve cells called neurons. Each neuron has a body, a long process called axon and dendrites.

Nervous system correlates sensory stimuli and coordinates the efferent impulses so that effector organs work harmoniously. In addition, the nervous system has the ability to store sensory information received in past times and can integrate this information with other nervous impulses and channel into common efferent pathways.

Main divisions of nervous system
Structurally nervous system has two main parts:
1. Central nervous system. It consists of brain and spinal cord
2. Peripheral nervous system. It consists of cranial nerves, spinal nerves and ganglia.

Functionally nervous system is also divided into two parts:
1. Somatic nervous system
It is that part of nervous system which is concerned with the innervation of voluntary structures of the body. For example skeletal muscles.
2. Autonomic nervous system
Autonomic nervous system is the part of nervous system concerned with the innervation of involuntary structures of the body. For example heart, smooth muscle, and glands. It is distributed throughout the central and peripheral nervous systems.

Central nervous system has gray and white matter.
Gray matter consists of neurons embedded in neruoglia cells (supporting cells of nervous tissue).
White matter consists of nerve fibers embedded in neuroglia.

Central nervous system
Central nervous system has two main parts:
Spinal cord

The two parts are continuous with each others.
Brain is placed in skull and spinal cord is present in vertebral column.

Both brain and spinal cord are surrounded by three meninges and suspended in fluid called cerebrospinal fluid.
The meninges of brain and spinal cord are
dura mater,
arachnoid mater,
pia mater,
These meninges of brain and spinal cord are continuous with each other.

The brain lies in the cranial cavity and is continuous with the spinal cord through the foramen magnum.

It is divided into three main parts. They are
forebrain or prosencephalon
midbrain or mesencephalon
hindbrain or rhombencephalon

Prosencephalon is further subdivided into
telencephalon or cerebrum.
diencephalon (between brain).

Mesencephalon is not further suddivided.

Rhombencephalon is further subdivided into
1. metencephalon. It consists of pons, and cerebellum.
2. myelencephalon. It is medulla oblongata.

Cerebrum is the largest part of brain. It consists of two cerebral hemispheres, which. The cerebral hemispheres are separated by a deep cleft called longitudinal fissure, into which projects falx cerebri. The two (right and left) cerebral hemispheres are connected through corpus callosum. Cerebrum lies in the cranial cavity. From before backward it lies superior to anterior cranial fossa, middle cranial fossa and tentorium cerebelli. Each cerebral hemisphere contains a cavity called lateral ventricle. Lateral ventricles communicate with third ventricle through interventricular foramina.

Diencephalon is the central part of forebrain. It is almost completely hidden. It consists of a dorsal thalamus and a ventral hypothalamus. The cavity of diencephalons is third ventricle.
Thalamus is a large egg-shaped mass of gray matter that lies on either side of the third ventricle. The anterior end of thalamus forms the posterior boundary of interventricular foramen, the opining between the third and lateral ventricles.
Hypothalamus forms the lower pat of the lateral wall and floor of the third ventricle.

Midbrain or mesencephalon is continuous superiorly to forebrain and inferiorly to hindbrain. The narrow cavity of the midbrain is the cerebral aqueduct, which connects third and fourth ventricles.

Pons is continuous superiorly to midbrain and inferiorly to medulla oblongata. It is situated anterior to cerebellum. It is connected posteriorly to the two cerebellar hemispheres through middle cerebellar peduncles.

Medulla oblongata is conical in shape. It is continuous superiorly to pons and inferiorly to spinal cord. It is connected posteriorly to the two cerebellar hemespheres through inferior cerebellar peduncles.

Brainstem consists of midbrain, pons and medulla oblongata. So brainstem is that part of brain that remains after the cerebral and cerebellar hemispheres are removed.

Cerebellum is situated posterior to brainstem. It lies within the posterior cranial fossa. It consist of two cerebellar hemispheres, connected by a median portion called vermis. Cerebellum is connected anteriorly to midbrain through superior cerebellar peduncles, to pons through middle cerebellar peduncles, and to medulla through inferior cerebellar peduncles.

Hindbrain consists of pons, medulla oblongata and cerebellum. The cavity of the hindbrain is fourth ventricle. Fourth ventricle is connected superiorly to third ventricle through cerebral aqueduct, and inferiorly it is continuous with central canal of spinal cord.

Spinal cord
Spinal cord lies in vertebral column. It begins at foramen magnum and terminates inferiorly at the level of the lower border of the first lumbar vertebra. It is continuous superiorly with medulla oblongata and tapers off inferiorly into the conus medullaris. Filum terminale (a prolongation of pia mater) descends from the apex of conus medullaris to the back of coccyx.

Spinal cord is cylindrical in cross section. It has anterior median fissure on ventral surface and posterior median sulcus on dorsal surface. In spinal cord gray mater is inside and white mater is outside. Gray mater is H-shaped and surrounds central canal. White mater surrounds gray mater and is divided into anterior, lateral, and posterior white columns.

There are two fusiform enlargements in spinal cord.
Cervical enlargement. It in cervical region. Here spinal cord gives origin to brachial plexus.
Lumbar enlargement. It is in lower thoracic and lumber regions. Here spinal cord gives origin to lumbosacral plexus.

Spinal cord has segments. Along the entire length of the spinal cord are attached 31 pairs of spinal nerves. Each spinal nerve is attached to one segment of spinal cord. Each spinal nerve has two roots.
Anterior or motor root
Posterior or sensory root

Each root is attached to the spinal cord by a series of rootlets, which extend the whole length of the corresponding segment of the cord. Each posterior nerve root has a posterior root ganglion.

Peripheral nervous system
Peripheral nervous system consists of
Cranial nerves and their associated ganglia
Spinal nerves and their associated ganglia

Cranial and spinal nerves are grayish-white cords made up of bundles of nerve fibers supported by connective tissue.

The connective tissue forms three successive coverings.
1. Endoneurium. It is a delicate sheath of connective tissue around each nerve fiber.
2. Perineurium. It is a sheath of connective tissue around each bundle of nerve fiber.
3. Epineurium. It is in a sheath of dense connective tissue around the nerve.

Nerve fibers transmit nerve impulses.
Nerve impulse is a massage either from central nervous system to the various structures of the body or from these structures to central nervous system.

The fibers carrying impulses from central nervous system various organs and structures of the body are called efferent fibers. The efferent fibers that pass to the muscles to make them contract are given the name motor nerve fibers.

The fibers carrying impulses to central nervous system are afferent fibers. Because these fibers are concerned with conveying information about sensations of touch, pain, temperature, and vibration, they are called sensory fibers.

In addition to the impulses which they carry, nerve fibres also transmit substances in both directions in the nerve cell process. Thus there is a flow of materials to and from the nerve cells which give rise to these processes.

Nerves are classified into two categories.
1. Cranial nerves
2. Spinal nerves

Cranial nerves
There are 12 pairs of cranial nerves. They are attached to brain. They leave brain and emerge from the skull or cranium through foramina in the skull. They are
Olfactory nerve or first cranial nerve
Optic nerve or second cranial nerve
Occulomotor nerve or third cranial nerve
Trochlear nerve or fourth cranial nerve
Trigeminal nerve or fifth cranial nerve (V)
Abducent nerve or sixth cranial nerve
Facial nerve or seventh cranial nerve (VII)
Vestibulocochlear nerve or eighth cranial nerve (VIII)
Grossopharyngeal nerve or ninth cranial nerve (IX)
Vagus nerve or tenth cranial nerve (X)
Accessory nerve or eleventh cranial nerve
Hypoglossal nerve or twelfth cranial nerve

Cranial ganglia are found along the course of following cranial nerves
Trigeminal nerve
Facial nerve
Vestibulocochlear nerve
Glossopharyngeal nerve
Vagus nerve
These cranial ganglia are called sensory ganglia of these nerves.

Spinal nerves
There are 31pairs of spinal nerves. They are attached to spinal cord. They leave spinal cord and pass through intervertebral foramina in vertebral column. Spinal nerves are named according to the regions of vertebral column with which they are associated. There are
1. 8 cervical nerves (first cervical, second cervical, third cervical etc.)
2. 12 thoracic nerves (first thoracic, fourth thoracic, tenth thoracic etc.)
3. 5 lumber nerves (first lumbar, second lumbar, fifth lumbar etc.)
4. 5 sacral nerves (first sacral, third sacral, fourth sacral etc.)
5. 1 coccygeal nerve (the coccygeal nerve)

Please note two very important points
There are 7 cervical vertebrae in vertebral column and there are 8 cervical nerves arising from spinal cord.
There are 4 coccygeal vertebrae and there is only 1 coccygeal nerve.

All the spinal nerves emerge caudal to the corresponding vertebrae except cervical nerves. The first seven cervical nerves emerge cranial to the corresponding vertebrae while the eighth emerges between the seventh cervical and first thoracic vertebrae.

Each spinal nerve is attached to spinal cord by two roots. They are
Anterior or ventral or motor root
Posterior or dorsal or sensory root

Main parts of a typical spinal nerve.

A typical spinal nerve is formed by the union of anterior and posterior roots attached to the sides of the spinal cord within the vertebral canal.

Anterior or ventral root consists of bundles of efferent nerve fibers. It is formed by axons of spinal neurons occupying anterior and lateral gray columns. Their cells of origin lie in the anterior gray horn of the spinal cord. In thoracic region lateral horn also share.
Each root arises as a series of 2-3 regular rows of rootlets attached to anterolateral surface of the spinal cord. These nerves carry motor (efferent) fibers passing on to skeletal muscles.

Posterior or dorsal root consists of bundles of sensory or afferent nerve fibers. The cell bodies of the nerve fibers in dorsal root are situated in posterior root ganglion. Each of these ganglion cells sends one process into the spinal nerve (peripheral process) and another into the spinal cord through the dorsal root (central process).
Posterior or dorsal nerve root is also attached to the posterolateral surface of the spinal cord by a series of rootlets.
The rootlets of adjacent dorsal are often connected by oblique filaments.

A spinal ganglion is present along the course of each posterior root. They are called posterior root ganglia. They are fusiform structures. These spinal ganglia are also called sensory ganglia of spinal nerves.
Each ganglion is composed of large groups of sensory neurons. It is oval and reddish, and the size corresponds to that of the root of spinal nerve.
Immediately lateral to the dorsal root ganglion, the ventral and dorsal roots unite to form a mixed spinal nerve which emerges through the intervertebral foramen.

Anterior and posterior roots unite to form the trunk of a spinal nerve at the level of their respective intervertebral foramina. The trunk is short. Here the motor and sensory fibers become mixed together, so that a spinal nerve is made up of a mixture of motor and sensory fibers.

The nerve passes out via intervertebral foramina. After emerging from the intervertebral foramen, each spinal nerve gives of a recumbent meningeal branch and divides into an anterior or ventral ramus and a posterior or dorsal ramus. Each ramus contains both efferent and afferent fibers.

Posterior or dorsal ramus is smaller. It passes posteriorly around the vertebral column into the muscles on the back of the vertebral column. Here it divides into lateral and medial branches that supply the muscles, and one of them sends a branch to the overlying skin. These cutaneous branches of the dorsal rami form a row of nerves on each side of the midline of the back.

Remember that first cervical and coccygeal dorsal rami do not divide into medial and lateral branches. In cervical dorsal rami medial branches give branches to skin. Remember that lower three cervical rami do not supply skin.

The medial branches of upper six thoracic dorsal rami supply skin while lateral branches of lower six thoracic dorsal rami supply skin.

Regarding lumbar dorsal rami upper three give of cutaneous nerves through their lateral branches.

Sacral dorsal rami also supply skin through lateral branches.

A typical spinal nerve.

Anterior or ventral ramus is larger. It runs laterally and anteriorly to supply the muscles and skin over the antero-lateral body wall, and all the muscles and skin of the limbs.

Anterior rami in thoracic region run along the lower border of corresponding ribs. They form eleven intercostal nerves and one subcostal nerve (twelfth nerve). Each of these ventral rami supplies the strip of muscle in which it lies.
Each intercostal nerve gives off a collateral branch, which follows the inferior border of the same intermuscular space. The collateral branch rejoins the main trunk before it is distributed as anterior cutaneous nerve.
Each intercostal nerve also gives off a lateral cutaneous branch, which accompanies the main trunk for a short distance. Lateral cutaneous branch pierces the intercostal muscles obliquely and divides into anterior and posterior branches. The anterior branch runs forward while posterior branch runs posteriorly. Both supply the overlying skin.

Ventral rami of the cervical, lumber, sacral, and coccygeal nerves join in complicate manner to form nerve plexuses.
Anterior rami in cervical region join one another at the roots of upper limbs to form cervical and brachial plexuses. Brachial plexus supplies most of the nerves to upper limb.

Anterior rami in lumbar and sacral regions similarly form lumbar and sacral plexuses at the roots of lower limbs. These plexuses are mainly concerned with the nerve supply of the lower limb.

The cutaneous branches of ventral and dorsal rami supply a strip of skin from anterior median line to posterior median line. This strip of skin supplied by a single spinal nerve is called a dermatome. It is interesting to note that no area of skin is supplied solely by a single spinal nerve because adjacent dermatomes overlap. The total mass of muscle supplied by a single spinal nerve is called a myotome. Muscles receive afferent as well as efferent nerve fibers.

Because of the disproportionate, growth in length of the vertebral column during development, compared with that of the spinal cord, the length of the roots increase progressively from above downward.
In the upper cervical region the spinal nerve roots are short and run almost horizontally, but the roots of the lumber and sacral nerves below the level of spinal cord (lower border of the first lumbar vertebra in the adult) form a vertical leash of spinal nerves called cauda equina. It is present around filum terminale.