The nervous system: organisation, neurones and nerve impulses

Learning objectives

By the end of this lesson, you should be able to:

• explain why organisms need coordination and control
• distinguish the central and peripheral nervous systems
• identify sensory, relay and motor neurones
• describe the structure of a typical neurone and relate it to its function
• explain how a nerve impulse travels through a reflex arc
• distinguish a reflex action from a voluntary response

Why coordination is necessary

An organism is exposed to changes inside and outside its body. A change that can be detected and may produce a response is called a stimulus. Examples include bright light, heat, sound, pressure, injury and a change in blood glucose concentration.

Coordination is the organisation of activities in different parts of the body so that they work together. Control involves regulating these activities and responses. In humans, the nervous system provides rapid communication using electrical impulses along neurones, while the endocrine system uses hormones carried in the blood. The two systems interact to maintain a stable internal environment.

A typical nervous response follows this sequence:

stimulus → receptor → sensory neurone → central nervous system → motor neurone → effector → response

A receptor detects the stimulus. The central nervous system (CNS) processes information. An effector, such as a muscle or gland, carries out the response.

Divisions of the human nervous system

The nervous system is divided structurally into two main parts.

The brain is the major control centre. It receives sensory information, interprets it and coordinates many responses. The spinal cord carries impulses between the brain and the body and also coordinates many rapid reflex actions.

The PNS includes nerves that carry information towards the CNS and nerves that carry instructions away from it. Functionally, it includes:

• Sensory (afferent) pathways: carry impulses from receptors towards the CNS.
• Motor (efferent) pathways: carry impulses from the CNS to effectors.
• Somatic nervous system: controls skeletal muscles and carries sensory information from the body; it is involved in voluntary movement and somatic reflexes.
• Autonomic nervous system: regulates involuntary activities of structures such as cardiac muscle, smooth muscle and glands.

The autonomic system includes sympathetic and parasympathetic divisions. They often have opposing effects on the same organ. For example, sympathetic activity generally increases heart rate during an emergency, while parasympathetic activity helps reduce it during rest.

The neurone: a specialised nerve cell

A neurone is a specialised cell adapted to receive and transmit nerve impulses. A typical neurone has the following parts:

• Cell body (cyton): contains the nucleus and most of the cytoplasm; it maintains the cell.
• Dendrites: short, branched extensions that receive signals from receptors or other neurones and carry them towards the cell body.
• Axon: a long fibre that carries impulses away from the cell body.
• Myelin sheath: an insulating layer around many axons that helps impulses travel faster.
• Nodes of Ranvier: gaps between myelin segments where the axon membrane is exposed; they support rapid conduction.
• Axon terminals: branched endings that communicate with another neurone, muscle or gland.

Not every neurone has the same shape. The arrangement of the cell body, dendrites and axon varies with function.

Types of neurones

A sensory neurone commonly has its cell body along the fibre, outside the spinal cord. A motor neurone has its cell body within the CNS and a long axon extending towards an effector. Relay neurones are found within the CNS.

How impulses pass between neurones

A small gap called a synapse separates the terminal of one neurone from the next cell. An impulse reaching the axon terminal causes chemical messenger substances called neurotransmitters to be released. They diffuse across the synaptic gap and bind to receptors on the next cell. This can initiate a new electrical impulse if the stimulation is sufficient.

Synapses help ensure that signals are transmitted in an organised direction and allow the nervous system to modify or integrate information. A synapse is not simply a continuous joining of two neurones: there is a narrow gap between them.

Reflex actions and the reflex arc

A reflex action is a rapid, automatic response to a stimulus that occurs without deliberate conscious control at the time of the response. Pulling the hand away from a hot object is a familiar example. Reflexes help protect the body from injury and support functions such as posture.

The pathway followed by impulses during a reflex is called a reflex arc.

1. A receptor in the skin detects a harmful stimulus, such as intense heat.
2. A sensory neurone carries an impulse towards the spinal cord.
3. In the CNS, the impulse passes across synapses, commonly through a relay neurone.
4. A motor neurone carries an impulse away from the CNS.
5. The motor neurone stimulates an effector, such as a skeletal muscle.
6. The muscle contracts and withdraws the hand.

The brain is informed about the stimulus, but the rapid withdrawal can be coordinated by the spinal cord before the person consciously experiences the pain. This reduces delay; it does not mean the brain is permanently excluded from the response.

Reflex action compared with voluntary action

A reflex is not necessarily a simple or unlearned movement in every context, but the protective withdrawal reflex is an example of an innate spinal reflex.

WASSCE examination focus

• State the direction of an impulse when identifying a sensory or motor neurone.
• Do not confuse a nerve with a neurone. A nerve is a bundle of many nerve fibres in the PNS; a neurone is an individual nerve cell.
• In a reflex arc, show the receptor, sensory neurone, CNS/relay neurone, motor neurone and effector in the correct order.
• A receptor detects a stimulus; an effector produces the response.
• The spinal cord is part of the CNS, not the PNS.

Quick check

1. Name the two structural divisions of the human nervous system.
2. What is the function of a relay neurone?
3. Distinguish a nerve from a neurone.
4. Put these in the correct order: motor neurone, receptor, effector, sensory neurone, CNS.
5. Explain why withdrawing a hand from a hot object is a reflex action.

Answers

1. The central nervous system and peripheral nervous system.
2. It connects neurones within the CNS and helps transmit or integrate information.
3. A neurone is an individual specialised nerve cell; a nerve is a bundle of nerve fibres in the peripheral nervous system.
4. Receptor → sensory neurone → CNS → motor neurone → effector.
5. It is rapid and automatic, and the withdrawal is coordinated through a reflex pathway without a deliberate decision at the time.