BASIC STRUCTURE OF THE BRAIN – ITS PARTS AND FUNCTIONS

The brain which incorporates about 86 million neurons is an important component of our nervous system.

The nervous system controls, coordinates and regulates all voluntary and involuntary functions of body by making use of electrical and chemical signals after sensing variations in external and internal environment. Nerve cells called neurons transmit the information they receive to other neurons or to effector organs.

The part of the nervous system which occupies the central axis of the body is called the central nervous system. It is made up of the brain (enclosed in cranial cavity) and the spinal cord (enclosed in vertebral canal).

The part of nervous system which lies outside the central nervous system makes up the peripheral nervous system. It comprises of cranial nerves and spinal nerves.

The grey matter contains nerve cells and proximal portions of their processes which are unmyelinated.

The white matter contains myelinated nerve fibers which gives it whitish appearance.

THE BRAIN

The brain is enclosed in the bony structure called cranium or skull. It weighs about 1.5 kg in adults. It is divided into forebrain, midbrain and hindbrain. Twelve pairs of cranial nerves are attached to the brain. The brain also contains four ventricles ( 2 lateral ventricles, third ventricle and fourth ventricle).

Brain can be subdivided into forebrain, midbrain and hindbrain. Forebrain consists of telencephalon and diencephalon. Midbrain connects pons to cerebrum. Hindbrain is made up of pons, medulla oblongata and cerebellum.

Ventricles of brain are cavities filled with cerebrospinal fluid. The lateral ventricles are roughly C shaped, one in each cerebral hemisphere. Third ventricle is the cavity of diencephalon. Fourth ventricle is the cavity of hindbrain.

Twelve pairs of cranial nerves are attached to the brain. Following are attachments of cranial nerves:

• Cranial nerve I – Ventral surface of frontal lobe.
• Cranial nerve II – To diencephalon.
• Cranial nerves III and IV – To midbrain.
• Cranial nerves V, VI, VII, VIII – To pons.
• Cranial nerves IX, X, XI, XII – To medulla.

FORE BRAIN

The forebrain consists of telencephalon and diencephalon.

TELENCEPHALON

The telencephalon comprises of the two cerebral hemispheres (cerebrum) and their interconnections.

The cerebrum is the largest part of brain and is made of right and left cerebral hemispheres. The cerebral hemispheres are interconnected by bundles of nerve fibers known as commissures. The corpus callosum is the largest commissure.

Each cerebral hemisphere consists of –

• An outer layer of grey matter, the cerebral cortex which is about 3mm thick and is highly folded to increase surface area for neurons.
• Central core of white matter.
• Basal nuclei embedded in the white matter.
• A cavity known as lateral ventricle.

Elevations of the convoluted cerebral cortex are called gyri (singular – gyrus) and the intervening depressions are called sulci (singular – sulcus).

Each cerebral hemisphere has four lobes, namely frontal, parietal, temporal and occipital lobes. Frontal lobe lies in front of central sulcus and parietal lobe behind central sulcus. The temporal lobe is situated below lateral sulcus. The occipital lobe lies behind an imaginary line joining joining the upper end of parieto-occipital sulcus with the pre-occipital notch.

The frontal lobe possesses motor functions. The parietal, temporal and occipital lobes possess sensory functions.

In frontal lobe : precentral gyrus situated in front of central sulcus contains the motor area, which initiates and controls the movements of the opposite half of the body.

In parietal lobe : postcentral gyrus lying behind the central sulcus is the general sensory area (somesthetic area), which receives touch, pain, temperature and proprioceptive sensations from the opposite half of the body.

In occipital lobe : visual area is located on the medial surface adjoining the calcarine sulcus.

In temporal lobe : auditory area concerned with hearing is situated below lateral sulcus in the superior temporal gyrus.

BASAL NUCLEI

Basal nuclei are masses of grey matter embedded in the white matter of each cerebral hemisphere.

PARTS – Corpus striatum ( formed by caudate nucleus and lentiform nucleus), claustrum and amygdaloid nucleus.

* Lentiform nucleus has globus pallidus medially and putamen laterally.

FUNCTION – The corpus striatum together with subthalamic nucleus and substantia nigra play an important role in the control of movement. Its main role is to facilitate behaviour and movements as well as to inhibit unwanted movement and behaviour.

DIENCEPHALON

It is the part of forebrain which surrounds the third ventricle. Each half of diencephalon consists of four parts – thalamus, hypothalamus, epithalamus and subthalamus.

THALAMUS

This is an oval mass of grey matter in the upper part of the lateral wall of third ventricle. It consists of several nuclei. Some of these nuclei are relay stations for all sensory pathways except olfaction and project to various sensory areas of cerebral cortex. Some nuclei have motor functions. Few other thalamic nuclei are related to emotion and complex mental processes.

HYPOTHALAMUS

It forms the lower part of the lateral wall and floor of third ventricle. It regulates autonomic functions, body temperature, appetite, thirst, sleep-wake cycle. It also plays important role in neuroendocrine system. The pituitary gland is connected to the hypothalamus by infundibulum (pituitary stalk).

EPITHALAMUS

Has small nuclei, tracts and the pineal gland, which secretes melatonin to regulate circadian rhythm. The habenulae is involved in reward and aversion behaviours.

SUBTHALAMUS

Contains subthalamic nucleus. It is involved in motor control pathways.

MID BRAIN

The midbrain or mesencephalon is that part of the brain that connects the pons with the diencephalon. It is connected to the cerebellum by the superior cerebellar peduncles. A narrow cavity called the cerebral aqueduct of sylvius runs through the midbrain and connects the third and fourth ventricles.

FUNCTION – Midbrain regulates visual and auditory reflexes, controls pupil dilation and eye movement and has a role in motor coordination, sleep cycle, pain modulation and alertness.

PARTS – Midbrain has two parts : cerebral peduncles in front of the aqueduct and tectum behind it.

CEREBRAL PEDUNCLES – Each cerebral peduncle has three parts – anterior is crus cerebri, middle part is substantia nigra and posteriorly there is tegmentum. The upper part of tegmentum has the red nucleus on either side.

TECTUM – Tectum lies behind the cerebral aqueduct and has four small round elevations, two superior colliculi and two inferior colliculi. Superior colliculi are concerned with vision and inferior colliculi with auditory function.

HIND BRAIN

PONS

The pons is situated between the medulla and midbrain. The middle cerebellar peduncle connects pons to cerebellum.

Pons has two parts – A dorsal tegmental part which contains sensory tracts, motor tracts and nuclei of some cranial nerves. The basilar part lies ventrally and it connects cortex of cerebral hemisphere with opposite cerebellar hemisphere for synchronization of voluntary movements.

FUNCTIONS –

• Regulates breathing
• Coordinates sleep – wake cycle
• Regulates REM (rapid eye movement) sleep
• Controls facial expressions and movements
• Maintains balance, equilibrium and posture
• Plays role in sense of touch and position
• Coordinates smooth movements of voluntary muscles

MEDULLA OBLONGATA

Medulla oblongata connects the spinal cord with the pons.

Th e anterior part of medulla has an anterior median fissure. On each side of the fissure, there is a swelling called pyramid produced by the corticospinal tract. Posterolateral to pyramid is olive, an oval elevation produced by inferior olivary nucleus. The posterior surface of medulla in its lower part has two small elevations, gracile tubercle medially and cuneate tubercle laterally caused by nucleus gracilis and nucleus cuneatus respectively.

Inferior cerebellar peduncle connects medulla with the cerebellum.

FUNCTIONS –

• Regulates heart rate and force of contraction
• Controls diameter of blood vessel
• Controls autonomic reflexes such as swallowing, coughing, sneezing, vomiting and hiccuping

*Midbrain, pons and medulla oblongata together form the brainstem.

CEREBELLUM

The cerebellum is the largest part of the hindbrain. It is located in the posterior cranial fossa behind the pons and medulla oblongata. The cerebellum is divided into right and left cerebellar hemispheres by the vermis. The surface of cerebellum is composed of thin sheets called folia. Transverse fissures lie in between folia.

The cerebellum consists of an outer layer of grey matter, known as the cerebellar cortex and a central core of white matter. Four pairs of cerebellar nuclei are embedded in the white matter. Cerebellar nuclei are fastigial, globose, emboliform and dentate from medial to lateral. Dentate is the largest among them.

FUNCTIONS –

• Maintenance of equilibrium.
• Maintenance of muscle tone for posture and locomotion.
• Control of fine voluntary movements.

SPINAL CORD

The lower tubular portion of the central nervous system is the spinal cord. It is located in the upper two-thirds of the vertebral canal.

EXTENT – Begins at upper border of first cervical vertebra and ends at lower border of first lumbar vertebra.

The lower tapering end of spinal cord is called conus medullaris. Thirty one pairs of spinal nerves are attached to the spinal cord. Each of them is attached to spinal cord by a ventral (anterior or motor) and a dorsal (posterior or sensory) root. The dorsal roots possess a ganglion called dorsal root ganglion.

The spinal cord consists of grey matter and white matter. The grey matter forms ‘H’ shaped figure in transverse section. Grey matter is divided into a ventral (anterior) horn, a dorsal (posterior) horn and an intermediate zone in between. The white matter is divided into anterior, lateral and posterior funiculi.

FUNCTIONS –

• The dorsal horn receives sensations from trunk and limbs and contains ascending pathways, which carry sensory impulses to the brain.
• The ventral horn contains motor neurons which innervate skeletal muscles.
• The white matter consists of ascending and descending nerve fiber tracts. The ascending tracts carry sensations from trunk, limbs and viscera to the brain. The descending tracts control anterior and lateral horn of spinal cord which are concerned with muscular contractions and autonomic responses respectively.
• Mediates spinal reflexes.