Cerebrum: Structure and Function

• The Cerebrum is the largest part of the brain, occupying the anterior and middle portion of the brain.
• A deep cleft, the longitudinal cerebral fissure, divides it into right and left cerebral hemispheres.
• Each hemisphere contains one lateral ventricle, which produces and circulates cerebrospinal fluid
• The cerebrospinal fluid cushions and protects the brain and removes toxins and wastes.
• The hemispheres are connected deep within the brain by the corpus callosum, a mass of white matter with myelinated nerve fibres.
• The cerebrum's peripheral part is composed of nerve cell bodies, or grey matter, forming the cerebral cortex.
• The grey matter is located on the outer surface with the white matter beneath it, opposite the arrangement in the spinal cord.
• The cerebral cortex shows many infoldings or furrows called convolutions
• Convolutions greatly increase the surface area of the cerebrum.
• Each hemisphere is divided into lobes named after the cranial bones they lie under: Frontal, Parietal, Temporal, Occipital.
• Deep sulci (fissures) define the boundaries of the lobes.

Interior of the Cerebrum

• Lobes are connected by nerve fibre masses, or tracts, that make up the brain's white matter.
• The internal capsule is an area with projection fibres that connect the cerebral cortex with grey matter and the spinal cord.
• The internal capsule lies deep within the brain between the basal nuclei (ganglia) and the thalamus.
• Many nerve impulses to and from the cerebral cortex are carried by the internal capsule.

Functions of the Cerebrum

• Mental activities such as memory, intelligence, thinking, reasoning, moral sense, and learning.
• Sensory perception, including pain, temperature, touch, sight, hearing, taste, and smell.
• Initiation and control of skeletal (voluntary) muscle contraction.

Other areas of the Cerebrum

• Deep within the cerebral hemispheres are masses of grey matter: Basal nuclei, Thalamus, and Hypothalamus.
• Basal nuclei are areas of grey matter connecting to the cerebral cortex and thalamus, and are involved in initiating muscle tone in slow, coordinated activities
• Inadequate or absent control of the basal nuclei results in jerky, clumsy, and uncoordinated movements.
• The thalamus consists of two masses of nerve cells and fibres within the cerebral hemispheres, below the corpus callosum.
• Sensory input from the skin, viscera, and special sense organs is transmitted to the thalamus before being redistributed to the cerebrum.
• The hypothalamus is composed of groups of nerve cells below and in front of the thalamus, immediately above the pituitary gland.
• The hypothalamus is linked to the posterior pituitary lobe by nerve fibres and to the anterior lobe by a complex system of blood vessels to controls hormone output from both lobes.

Hypothalamus Functions Include Control of

• The autonomic nervous system
• Appetite, thirst, and water balance
• Body temperature and circadian rhythms
• Emotional reactions, such as pleasure, fear, rage, and sexual behaviour

Brain Stem

• The brain stem is a vital structure connecting the cerebrum to the spinal cord, controlling basic life-sustaining functions and reflexes.
• The brainstem consists of three parts: Midbrain, Pons, and Medulla Oblongata.

Midbrain

• Located around the cerebral aqueduct between the cerebrum and the pons.
• It consists of cell bodies and nerve fibres connecting the cerebrum with lower brain parts and the spinal cord.
• The cell bodies act as relay stations for ascending and descending nerve fibres.

Pons

• Situated in front of the cerebellum, below the midbrain and above the medulla oblongata.
• Consists mainly of nerve fibres forming a bridge between the two hemispheres of the cerebellum and fibres passing between the brain and the spinal cord.
• There are groups of cells within the pons which act as relay stations and some of these are associated with the cranial nerves.
• Pons has grey matter lying deeply and white matter lying on the surface.

Medulla Oblongata

• The medulla oblongata connects the brainstem and the spinal cord.
• It extends from the pons and is continuous with the spinal cord.
• It is about 2.5 cm long, with white matter on the outer aspect and grey matter centrally.
• Vital centres, consisting of groups of cells associated with autonomic reflex activity, lie in its deeper structure.

Medulla Oblongata Vital Centers

• Cardiac centre.
• Respiratory centre.
• Vasomotor centre.
• Reflex centres of vomiting, coughing, sneezing, and swallowing.
• The cardiovascular centre controls the rate and force of cardiac contraction.
• Sympathetic stimulation increases the rate and force of the heartbeat, and parasympathetic stimulation has the opposite effect.
• The respiratory centre controls the rate and depth of respiration, stimulating contraction of the diaphragm and intercostal muscles to initiate inspiration.
• The respiratory centre is stimulated by excess carbon dioxide, deficiency of oxygen, and nerve impulses from chemoreceptors in the carotid bodies.
• The vasomotor centre controls the diameter of blood vessels, especially small arteries and arterioles, through the autonomic nervous system, and stimulation may cause constriction or dilatation.
• Sources of stimulation of the vasomotor centre include body temperature and emotions.
• Reflex centres are stimulated by irritating substances in the stomach or respiratory tract, initiating reflex actions to expel the irritant.

Cerebellum

• Situated behind the pons and immediately below the posterior portion of the cerebrum.
• Ovoid in shape with two hemispheres, separated by the vermis.
• Grey matter forms the surface of the cerebellum, and the white matter lies deeply.

Cerebellum Functions

• Coordination of voluntary muscular movement, posture, and balance.
• The cerebellum controls and coordinates the movements of various muscle groups, ensuring smooth, even, and precise actions.
• The cerebellum coordinates activities associated with balance and equilibrium, with sensory input from the muscles, joints, eyes, and ears.
• Impulses from the cerebellum influence the contraction of skeletal muscle to maintain balance and posture.
• Damage to the cerebellum results in clumsy, uncoordinated muscular movement, staggering gait, and inability to carry out smooth, steady, precise movements.