Chapter 18: Neural Control and Coordination | Summary

The human neural system is divided into two parts: (i) the Central Neural System (CNS), which includes the brain and spinal cord and serves as the site of information processing and control, and (ii) the Peripheral Neural System (PNS), which comprises all the nerves of the body associated with the CNS. PNS nerve fibres are classified as afferent fibres, which transmit impulses from tissues to the CNS, and efferent fibres, which carry regulatory impulses from the CNS to peripheral tissues.

The PNS is further divided into the somatic neural system, which relays impulses from the CNS to skeletal muscles, and the autonomic neural system, which transmits impulses to involuntary organs and smooth muscles. The autonomic neural system is classified into the sympathetic and parasympathetic neural systems. The visceral nervous system is a part of the PNS that connects the CNS with visceral organs.

A neuron is a microscopic structure with three major parts: the cell body, dendrites, and axon. The cell body contains cytoplasm with typical organelles and granular bodies called Nissl's granules. Dendrites are short, repeatedly branching fibres that also contain Nissl's granules and transmit impulses towards the cell body. The axon is a long fibre whose distal end is branched, with each branch terminating in a synaptic knob that contains synaptic vesicles filled with neurotransmitters. Axons transmit impulses away from the cell body.

Based on the number of axons and dendrites, neurons are classified as multipolar (one axon, two or more dendrites; found in cerebral cortex), bipolar (one axon, one dendrite; found in retina), and unipolar (cell body with one axon; found in embryonic stage). Axons are also classified as myelinated, which are enveloped by Schwann cells forming a myelin sheath with gaps called nodes of Ranvier and found in spinal and cranial nerves, and non-myelinated, which are enclosed by Schwann cells without myelin sheath and found in autonomous and somatic neural systems.

At rest, the axonal membrane is more permeable to K+ and nearly impermeable to Na+. The axoplasm has high K+ and negatively charged proteins but low Na+, while the extracellular fluid has high Na+ and low K+. The sodium-potassium pump actively transports 3 Na+ outward and 2 K+ inward, making the outer surface positively charged and the inner surface negatively charged. This electrical potential difference across the resting membrane is the resting potential.

When a stimulus is applied, the membrane at that site becomes freely permeable to Na+, causing rapid Na+ influx and reversal of polarity (depolarisation). This electrical potential difference is the action potential, also termed the nerve impulse. The depolarisation propagates sequentially along the axon as current flows from depolarised to polarised sites. The short-lived Na+ permeability is followed by increased K+ permeability, and K+ diffuses out, restoring the resting potential (repolarisation) so the fibre becomes responsive again.

A nerve impulse is transmitted between neurons through junctions called synapses, formed by the membranes of a pre-synaptic and a post-synaptic neuron. Two types of synapses exist: electrical synapses, where pre- and post-synaptic membranes are in very close proximity allowing direct electrical current flow, and chemical synapses, where the membranes are separated by a fluid-filled synaptic cleft. Electrical synapses allow faster transmission but are rare in humans.

At chemical synapses, the arrival of an action potential at the axon terminal stimulates synaptic vesicles to fuse with the pre-synaptic membrane and release neurotransmitters into the synaptic cleft. These neurotransmitters bind to specific receptors on the post-synaptic membrane, opening ion channels that generate a new potential in the post-synaptic neuron. This new potential may be excitatory or inhibitory depending on the type of neurotransmitter and receptor.

The brain is the central information processing organ and acts as the command and control system. It is protected by the skull and covered by cranial meninges consisting of three layers: the outer dura mater, the thin middle arachnoid, and the inner pia mater in contact with brain tissue. The brain is divided into three major parts: forebrain, midbrain, and hindbrain.

The forebrain consists of cerebrum, thalamus, and hypothalamus. The cerebrum is the largest part, longitudinally divided into two cerebral hemispheres connected by the corpus callosum. The cerebral cortex (grey matter) contains motor areas, sensory areas, and association areas responsible for memory and communication. The white matter lies beneath the cortex. The thalamus coordinates sensory and motor signalling. The hypothalamus controls body temperature, hunger, thirst, and secretes hypothalamic hormones. The limbic system, including amygdala and hippocampus, regulates sexual behaviour, emotional reactions, and motivation. The midbrain contains the cerebral aqueduct and corpora quadrigemina. The hindbrain comprises pons (interconnects brain regions), cerebellum (coordinates movement and balance), and medulla oblongata (controls respiration, cardiovascular reflexes, and gastric secretions). The brainstem consists of midbrain, pons, and medulla.

Chapter 18 establishes that neural coordination is achieved through a highly organised network of specialised neurons that generate and transmit electrochemical impulses. The resting potential, maintained by ion gradients and the sodium-potassium pump, is the foundation for action potential generation upon stimulation. Synaptic transmission, especially at chemical synapses via neurotransmitters, ensures directional impulse flow between neurons. The human brain, divided into forebrain, midbrain, and hindbrain, integrates sensory information and controls both voluntary and involuntary functions.

For CBSE board examinations, this chapter is highly important as questions are regularly asked on neuron structure, types of axons, resting and action potential, synaptic transmission, and the functions of various brain parts. Students should be able to draw and label diagrams of a neuron and the human brain, compare CNS and PNS, and explain the molecular basis of nerve impulse generation and conduction. Mastery of key terms like depolarisation, repolarisation, and neurotransmitters is essential for high scores.