Neural Control and Coordination

In this chapter: organisation of the human nervous system; structure and types of neurons; myelinated and non-myelinated fibres; generation and conduction of the nerve impulse; electrical and chemical synapses; the brain: forebrain, midbrain, hindbrain. (Eye, ear and the reflex arc are not in the current rationalised syllabus.)

The nervous system

Lower invertebrates like Hydra have a network of neurons. Insects have a brain, several ganglia and neural tissue. Vertebrates have a much more developed system.

• Central nervous system (CNS): brain and spinal cord; the site of information processing and control.
• Peripheral nervous system (PNS): all nerves associated with the CNS (cranial and spinal nerves). Nerve fibres are afferent (carry impulses from tissues and organs to the CNS) or efferent (carry impulses from the CNS to peripheral tissues and organs).
• The PNS has a somatic division (CNS to skeletal muscles) and an autonomic division (CNS to involuntary organs and smooth muscles). The autonomic division is further divided into sympathetic and parasympathetic.
• The visceral nervous system is part of the PNS: nerves, fibres, ganglia and plexuses carrying impulses between the CNS and the viscera.

The neuron

• Cell body: cytoplasm with typical organelles and Nissl's granules.
• Dendrites: short, repeatedly branched fibres that also contain Nissl's granules; they carry impulses towards the cell body.
• Axon: a long fibre whose branched end terminates in bulb-like synaptic knobs containing synaptic vesicles with neurotransmitters. Axons carry impulses away from the cell body to a synapse or neuromuscular junction.

Myelinated fibres are enveloped by Schwann cells that form a myelin sheath; the gaps between sheaths are nodes of Ranvier. They are found in spinal and cranial nerves. Unmyelinated fibres are enclosed by a Schwann cell that does not form a myelin sheath; they are common in the autonomic and somatic nervous systems.

Generation of a nerve impulse

Resting potential

• At rest, the axonal membrane is more permeable to K⁺, nearly impermeable to Na⁺, and impermeable to the negatively charged proteins inside.
• So the axoplasm has high K⁺ and negative proteins and low Na⁺; the fluid outside has low K⁺ and high Na⁺.
• These gradients are maintained by the sodium-potassium pump, which moves 3 Na⁺ out for every 2 K⁺ in.
• The outer surface is positive and the inner surface negative: the membrane is polarised. The potential difference across it is the resting potential.

Action potential

1. A stimulus at a site (A) makes the membrane freely permeable to Na⁺. Na⁺ rushes in.
2. Polarity reverses: the outer surface becomes negative and the inner positive. The membrane is depolarised. This potential difference is the action potential, the nerve impulse.
3. At the next site (B), the membrane is still positive outside. Current flows on the inner surface from A to B and on the outer surface from B to A, completing a circuit. Site B depolarises, and so the impulse is conducted along the axon.
4. The rise in Na⁺ permeability is extremely short-lived. It is quickly followed by a rise in K⁺ permeability; K⁺ diffuses out, restoring the resting potential (repolarisation). The fibre is ready for the next stimulus.

Common trap: depolarisation is caused by Na⁺ influx; repolarisation by K⁺ efflux. The Na-K pump does not create the action potential; it maintains the resting gradients.

Transmission across a synapse

A synapse is formed by the membranes of a pre-synaptic and a post-synaptic neuron, with or without a gap (synaptic cleft).

At a chemical synapse: the impulse reaches the axon terminal and moves synaptic vesicles towards the membrane; they fuse with it and release neurotransmitters into the cleft; these bind to specific receptors on the post-synaptic membrane, opening ion channels; the ions entering create a new potential in the post-synaptic neuron, which may be excitatory or inhibitory.

The human brain

The brain is protected by the skull and covered by three cranial meninges: the outer dura mater, a very thin middle arachnoid, and the inner pia mater (in contact with brain tissue).

Forebrain: cerebrum, thalamus, hypothalamus

• Cerebrum: divided by a deep cleft into left and right cerebral hemispheres, connected by a tract of nerve fibres, the corpus callosum. The outer layer, the cerebral cortex, is folded and appears grey (neuron cell bodies): the grey matter. It has motor areas, sensory areas and large association areas (neither clearly sensory nor motor) responsible for intersensory associations, memory and communication. The inner part, made of myelinated fibre tracts, is the white matter.
• Thalamus: wrapped by the cerebrum; the major coordinating centre for sensory and motor signalling.
• Hypothalamus: at the base of the thalamus; contains centres controlling body temperature and the urge for eating and drinking; its neurosecretory cells secrete hypothalamic hormones.
• Limbic system: the inner parts of the cerebral hemispheres with deep structures like the amygdala and hippocampus. With the hypothalamus, it regulates sexual behaviour, expression of emotions (excitement, pleasure, rage, fear) and motivation.

Midbrain

The midbrain lies between the thalamus/hypothalamus and the pons. A canal, the cerebral aqueduct, passes through it. Its dorsal part has four round swellings, the corpora quadrigemina.

Hindbrain: pons, cerebellum, medulla

• Pons: fibre tracts interconnecting different regions of the brain.
• Cerebellum: very convoluted surface, providing space for many more neurons.
• Medulla oblongata: connected to the spinal cord; contains centres controlling respiration, cardiovascular reflexes and gastric secretions.

Midbrain + pons + medulla = brain stem, which connects the brain and the spinal cord.

NEET focus

• Divisions of the nervous system; afferent vs efferent.
• Neuron parts; multipolar, bipolar, unipolar with locations; nodes of Ranvier.
• Ion distribution at rest; Na-K pump ratio; sequence of depolarisation and repolarisation.
• Chemical vs electrical synapse.
• Meninges order; functions of brain parts; brain stem components.

Practice questions