Nervous System

The basic purpose of the nervous system is to co-ordinate all activities of the body. It responds and adapts to change. The nervous system works like a two way communicating system: impulses are carried from the tissues to the nerve centres, and from the nerve centres messages are carried back to the desired tissue.

Organization of the Nervous System

The nervous system is organized as follows:

• Central Nervous System (CNS)
  • Brain: forebrain, midbrain, hindbrain
  • Spinal cord
• Peripheral Nervous System (PNS)
  • Cranial nerves (12 pairs)
  • Spinal nerves (31 pairs, given elsewhere in the source as 32 pairs; kept as stated in the source at each point)
  • Autonomic nervous system (somatic and autonomic divisions)
    • Sympathetic (active under stress)
    • Parasympathetic (active under normal conditions)

Central Nervous System (CNS)

• It consists of the brain and spinal cord, having billions of interacting neurons, which acts like a central telephone exchange.
• It coordinates, integrates and controls various nervous activities of the body.
• It is covered with three membranous protective layers called meninges.

Meninges

From outside to inside, the meninges (with the CSF-filled spaces between them) are arranged as: skull bone, then dura mater, subdural space (containing CSF), arachnoid mater, subarachnoid space (containing CSF), pia mater, and then the brain itself.

1. Dura mater (dura = hard): forms the outermost covering of the brain, located just beneath the cranial cavity. It is a very hard, tough, and the thickest membranous layer, made up of fibrous connective tissue.
2. Arachnoid mater (arachnoid = weblike): located between the dura mater and pia mater. It is a thin, delicate, transparent, almost fine web-like structure.
3. Pia mater (pia = tender): the innermost layer. It is thin, tender, very delicate and highly vascular. This layer remains closely attached to the surface of the brain.

The space between the dura mater and the arachnoid mater is known as the subdural space, and the space between the arachnoid mater and pia mater is called the subarachnoid space. All these spaces are filled with cerebrospinal fluid (CSF).

Cerebrospinal fluid (CSF)

CSF is a clear, watery fluid containing glucose, enzymes and salts, which surrounds the brain as well as the spinal cord. It is also present in the ventricles and the central canal of the spinal cord. It is about 90-150 ml in an adult.

Functions of CSF:

1. It protects the brain and spinal cord.
2. It forms a spongy cushion between the cranium and the brain, thus acting as a shock absorbent.
3. It keeps the brain and spinal cord moistened.
4. It provides essential nutrients and oxygen to the brain tissue.
5. It removes unwanted excretory waste products from the brain.
6. It maintains a constant pressure inside the cranium in spite of fluctuations in the volume and pressure of blood in the cranial vessels.

White matter and grey matter

The CNS has two distinct regions: white matter and grey matter.

• White matter: looks white in colour due to the presence of fatty myelin sheath around the nerve fibres. In the spinal cord, white matter forms the outer layer while grey matter forms the central core.
• Grey matter: looks grey in colour. It consists of cell bodies, dendrites and synapses of neurons. In the brain, grey matter is situated on the surface while white matter is located deeper.

Brain

The brain is the master organ of the body, which controls and coordinates almost all the systems of the body. It is the anterior, upper part of the CNS and is made up of soft nervous tissue called neurons (about 100 billion). The brain is located and protected inside the bony skull cavity, known as the cranium or cranial cavity. The brain weighs about 1300 to 1400 g in an average adult. The brain remains covered by the meninges.

Structure of the brain

The human brain is divisible into three main parts:

1. Forebrain (Prosencephalon): further divided into the olfactory lobes, cerebrum and diencephalon.
2. Midbrain (Mesencephalon): consists of the optic lobes (optic quadrigemina) and the cerebral peduncles (crura cerebri).
3. Hindbrain (Rhombencephalon): consists of the cerebellum, pons Varolii and medulla oblongata.

I. Forebrain

a) Olfactory lobes

There is a pair of olfactory lobes. Each lobe is a solid, bulb-like structure with no ventricle inside it. Each lobe is connected by a stalk called the olfactory tract to the olfactory region in the temporal lobe. This is present just above the optic chiasma. It helps in the detection of smell.

b) Cerebrum (cerebral hemispheres)

• It is the largest part of the brain. It consists of two cerebral hemispheres (right and left) joined together by a broad, curved, thick band of nerve fibres called the corpus callosum.
• Each cerebral hemisphere is divided into four lobes, frontal, parietal, temporal and occipital, by three deep and wide fissures.
• The surface area of each hemisphere is increased by numerous folds called gyri. The gyri are separated by depressions called sulci.
• The cerebral hemisphere has an outer region of densely packed nerve cells called the cerebral cortex, which forms the grey matter of the brain; the inner medulla is white matter.
• The central sulcus separates the frontal lobe from the parietal lobe. The lateral sulcus divides the frontal lobe from the temporal lobe.
• The parieto-occipital sulcus separates the parietal lobe from the occipital lobe.
• The cerebral hemispheres have hollow cavities known as the ventricles of the cerebrum.

Functional areas of the cerebrum:

1. Frontal lobe: has the motor area, which controls the voluntary movement of skeletal muscles, and areas for learning.
2. Parietal lobe: has the sensory area for the perception of stimuli of heat, cold, light, touch and pain, and also for reading, writing and mathematical calculation.
3. Temporal lobe: the primary auditory cortex receives stimuli of hearing; the olfactory cortex is for smell, and the hippocampal cortex is for long-term memory.
4. Occipital lobe: the visual cortex receives images detected by the eye and interprets them (visualization).

c) Diencephalon

It is the part of the forebrain located between the cerebrum and the midbrain, just below the corpus callosum. Its main parts are the epithalamus, thalamus and hypothalamus.

• Epithalamus: consists of a narrow band of nerve tissue in the roof of the third ventricle. Attached to it is a highly vascular area, the anterior choroid plexus, which secretes CSF.
• Thalamus: a region present at the centre of the forebrain, composed mostly of grey matter. It lies in the cerebral medulla and acts as a relay centre, collecting stimuli and passing them on to the cerebral hemispheres. It receives impulses of pain, temperature, touch, etc.
• Hypothalamus: located below the thalamus in the floor of the third ventricle (diocoel). The pituitary is attached to the hypothalamus by a stalk, the infundibulum. A pair of small rounded structures lie behind the infundibulum, called the mammillary bodies.
  • The hypothalamus acts as a connection between the nervous and endocrine systems.
  • It controls thirst, hunger, temperature regulation, sleep, salivation, sweating, blood pressure, etc.
  • It secretes neurohormones, also called releasing hormones, that stimulate the pituitary gland.