The brain is the central information processing organ of our body and acts as the ‘command and control system’. It is well protected by the skull, and inside the skull it is covered by the cranial meninges — an outer dura mater, a very thin middle arachnoid, and an inner pia mater in contact with the brain tissue.

The brain is divided into three major parts.

(i) Forebrain — consists of cerebrum, thalamus and hypothalamus.

• Cerebrum forms the major part of the human brain. A deep cleft divides it longitudinally into the left and right cerebral hemispheres, connected by a tract of nerve fibres called the corpus callosum. The layer of cells covering the hemisphere is the cerebral cortex, thrown into prominent folds and referred to as grey matter because the neuron cell bodies are concentrated there. The cortex contains motor areas, sensory areas and large association areas responsible for intersensory associations, memory and communication. The tracts inside, whose fibres are covered with the myelin sheath, form the opaque white matter.
• Thalamus — the structure around which the cerebrum wraps; a major coordinating centre for sensory and motor signalling.
• Hypothalamus — lies at the base of the thalamus; contains centres controlling body temperature and the urge for eating and drinking, and groups of neurosecretory cells secreting hypothalamic hormones.
• Limbic system — the inner parts of the cerebral hemispheres with associated deep structures like the amygdala and hippocampus; with the hypothalamus it regulates sexual behaviour, the expression of emotional reactions and motivation.

(ii) Midbrain — located between the thalamus/hypothalamus of the forebrain and the pons of the hindbrain. A canal called the cerebral aqueduct passes through it, and its dorsal portion consists mainly of four round swellings called corpora quadrigemina. It receives and integrates visual, tactile and auditory inputs.

(iii) Hindbrain — comprises pons, cerebellum and medulla.

• Pons consists of fibre tracts that interconnect different regions of the brain.
• Cerebellum has a very convoluted surface to provide additional space for many more neurons, and integrates information received from the semicircular canals of the ear and the auditory system.
• Medulla is connected to the spinal cord and contains centres which control respiration, cardiovascular reflexes and gastric secretions.

Midbrain, pons and medulla oblongata together constitute the brain stem, which forms the connections between the brain and the spinal cord.

(a) CNS and PNS

(b) Resting potential and action potential

(a) Polarisation. When a neuron is not conducting any impulse, different types of ion channels on the neural membrane make it comparatively more permeable to K⁺, nearly impermeable to Na⁺, and impermeable to the negatively charged proteins present in the axoplasm. Consequently the axoplasm contains a high concentration of K⁺ and negatively charged proteins and a low concentration of Na⁺, while the fluid outside has a low K⁺ and a high Na⁺ concentration, forming a concentration gradient. These gradients are maintained by the active transport of ions by the sodium-potassium pump, which transports 3 Na⁺ outwards for 2 K⁺ into the cell. As a result the outer surface of the axonal membrane possesses a positive charge while its inner surface becomes negatively charged — the membrane is polarised, and this potential difference is the resting potential.

(b) Depolarisation. When a stimulus is applied at a site on the polarised membrane, the membrane at that site becomes freely permeable to Na⁺. This leads to a rapid influx of Na⁺, followed by the reversal of the polarity at that site: the outer surface becomes negatively charged and the inner side becomes positively charged. The polarity is thus reversed and the membrane is said to be depolarised. The electrical potential difference across the plasma membrane at this site is the action potential, i.e. the nerve impulse.

(c) Transmission across a chemical synapse. At a chemical synapse the membranes of the pre- and post-synaptic neurons are separated by a fluid-filled space called the synaptic cleft, and chemicals called neurotransmitters carry the signal across. The axon terminals contain vesicles filled with these neurotransmitters. When an impulse (action potential) arrives at the axon terminal, it stimulates the movement of the synaptic vesicles towards the membrane, where they fuse with the plasma membrane and release their neurotransmitters into the synaptic cleft. The released neurotransmitters bind to their specific receptors present on the post-synaptic membrane. This binding opens ion channels, allowing the entry of ions which can generate a new potential in the post-synaptic neuron. The new potential developed may be either excitatory or inhibitory.

(a) Neuron — label the cell body (with nucleus and Nissl’s granules), dendrites, axon, Schwann cells forming the myelin sheath, nodes of Ranvier, and the synaptic knobs at the branched distal end.

(b) Brain — a sagittal section, labelling the cerebrum (with cerebral cortex and corpus callosum), thalamus, hypothalamus, midbrain, pons, cerebellum and medulla oblongata.

(a) Neural coordination. Coordination is the process through which two or more organs interact and complement the functions of one another. The neural system coordinates and integrates functions as well as the metabolic and homeostatic activities of all the organs, providing an organised network of point-to-point connections for a quick coordination. During physical exercise, for instance, the muscles, lungs, heart, blood vessels and kidneys are coordinated together; when the exercise stops, their activities gradually return to normal. The neural system works jointly with the endocrine system, which provides chemical integration through hormones.

(b) Forebrain. The forebrain consists of the cerebrum, thalamus and hypothalamus. The cerebrum is the major part of the human brain, divided longitudinally into two hemispheres connected by the corpus callosum; its covering layer, the cerebral cortex, is the grey matter and carries motor, sensory and association areas, with the myelinated tracts of the white matter beneath. The thalamus is a major coordinating centre for sensory and motor signalling. The hypothalamus, at the base of the thalamus, controls body temperature and the urge for eating and drinking, and its neurosecretory cells secrete hypothalamic hormones. The inner parts of the cerebral hemispheres with deep structures such as the amygdala and hippocampus form the limbic system, which with the hypothalamus regulates sexual behaviour, emotional reactions and motivation.

(c) Midbrain. The midbrain is located between the thalamus/hypothalamus of the forebrain and the pons of the hindbrain. A canal called the cerebral aqueduct passes through it. Its dorsal portion consists mainly of four round swellings (lobes) called corpora quadrigemina. The midbrain receives and integrates visual, tactile and auditory inputs.

(d) Hindbrain. The hindbrain comprises pons, cerebellum and medulla (medulla oblongata). Pons consists of fibre tracts that interconnect different regions of the brain. The cerebellum has a very convoluted surface to provide additional space for many more neurons, and integrates information received from the semicircular canals of the ear and the auditory system. The medulla is connected to the spinal cord and contains centres controlling respiration, cardiovascular reflexes and gastric secretions.

(e) Synapse. A synapse is the junction through which a nerve impulse is transmitted from one neuron to another. It is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron, which may or may not be separated by a gap called the synaptic cleft. There are two types: electrical synapses, where the membranes are in very close proximity and current flows directly, which is always faster but rare in our system; and chemical synapses, where a fluid-filled synaptic cleft separates the membranes and neurotransmitters carry the signal, generating a new potential in the post-synaptic neuron which may be either excitatory or inhibitory.

A nerve impulse is transmitted from one neuron to another through junctions called synapses, and the mechanism differs between the two types of synapse.

At an electrical synapse the membranes of the pre- and post-synaptic neurons are in very close proximity. Electrical current can flow directly from one neuron into the other across these synapses, so transmission is very similar to impulse conduction along a single axon. It is always faster than chemical transmission, but electrical synapses are rare in our system.

At a chemical synapse the membranes of the pre- and post-synaptic neurons are separated by a fluid-filled space called the synaptic cleft, and chemicals called neurotransmitters are involved. The axon terminals contain vesicles filled with these neurotransmitters. The steps are:

1. An impulse (action potential) arrives at the axon terminal.
2. It stimulates the movement of the synaptic vesicles towards the membrane.
3. The vesicles fuse with the plasma membrane and release their neurotransmitters into the synaptic cleft.
4. The released neurotransmitters bind to their specific receptors present on the post-synaptic membrane.
5. This binding opens ion channels, allowing the entry of ions which can generate a new potential in the post-synaptic neuron.
6. The new potential developed may be either excitatory or inhibitory.

The capacity to produce an inhibitory as well as an excitatory potential is the functional advantage of chemical transmission, and is what makes integration — rather than mere relay — possible.

Before the stimulus. When the neuron is resting, the axonal membrane is nearly impermeable to Na⁺. The sodium-potassium pump, transporting 3 Na⁺ outwards for 2 K⁺ into the cell, keeps the concentration of Na⁺ low inside the axoplasm and high in the fluid outside the axon. Sodium is therefore held in a steep concentration gradient across a membrane it cannot cross, and the outer surface of the membrane is positive while the inner surface is negative.

On stimulation. When a stimulus is applied at a site on the polarised membrane, that site becomes freely permeable to Na⁺. Sodium now moves inwards driven by both its concentration gradient (high outside, low inside) and the electrical gradient (negative inside). This produces a rapid influx of Na⁺.

The consequence. The inward rush of positive charge reverses the polarity at that site — the outer surface becomes negatively charged and the inner side becomes positively charged. The membrane is thus depolarised, and the electrical potential difference across the plasma membrane at this site is the action potential, which is in fact termed the nerve impulse.

Its brevity matters. The rise in the stimulus-induced permeability to Na⁺ is extremely short-lived. It is quickly followed by a rise in permeability to K⁺, and within a fraction of a second K⁺ diffuses outside and restores the resting potential, making the fibre once more responsive to further stimulation.

In summary: Na⁺ supplies the inward positive charge whose sudden entry constitutes the depolarisation. Without the stored sodium gradient there would be no influx, and hence no action potential at all.

(a) Myelinated and non-myelinated axons

(b) Dendrites and axons

(c) Thalamus and Hypothalamus

(d) Cerebrum and Cerebellum

(a) The cerebrum. The cerebrum forms the major part of the human brain and is its most developed part. Its cortex is thrown into prominent folds, greatly increasing the surface available, and it carries the large association areas responsible for complex functions like intersensory associations, memory and communication — which is what makes the human brain distinctive.

(b) The hypothalamus. Among the functions of the brain is the control of the circadian (24-hour) rhythms of our body, and it is the hypothalamus — the part of the forebrain lying at the base of the thalamus, which also controls body temperature and the urge for eating and drinking — that acts as the master clock.

(a) Afferent and efferent neurons

(b) Conduction in myelinated and unmyelinated nerve fibres

(c) Cranial nerves and spinal nerves

Note that both cranial and spinal nerves belong to the peripheral neural system and both carry afferent as well as efferent fibres; they are distinguished by where they arise and what they supply.