The Human Nervous System

Central and peripheral nervous systems

The nervous system is divided into two parts. The central nervous system (CNS) consists of the brain and spinal cord, and is where information is processed and responses are coordinated.

The peripheral nervous system (PNS) consists of all the cranial and spinal nerves that branch out from the CNS to every part of the body, carrying sensory information in from receptors and carrying instructions out to effectors.

Main regions of the brain

Reflex action and the reflex arc

Because the impulse travels through the spinal cord rather than the brain, a reflex action happens automatically and very quickly, protecting the body from harm before pain is even consciously felt.

1. A receptor detects a stimulus, such as heat from a flame.
2. A sensory neurone carries the impulse from the receptor to the spinal cord.
3. A relay neurone in the spinal cord passes the impulse directly to a motor neurone, without waiting for the brain to consciously process it.
4. The motor neurone carries the impulse to an effector, usually a muscle.
5. The effector responds immediately, for example by contracting to pull the hand away from the flame.

How it is examined

Exam questions on this standard often give a labelled diagram of the brain or of a reflex arc and ask you to identify the parts or state their functions, or to put the events of a reflex action, such as touching a hot object, in the correct order. You may also be asked to explain why a reflex action does not require conscious thought, unlike a voluntary response.

Worked exam-style question

Question. A person accidentally touches the sharp point of a pin with a fingertip and pulls their hand away almost instantly, before feeling any pain. (a) Name the type of neural response shown, and identify the region of the central nervous system chiefly responsible for coordinating it.

(b) List, in the correct order, the components of the pathway that carries the impulse from the receptor in the finger to the effector that withdraws the hand. (c) Explain why the hand is withdrawn before the person consciously feels pain.

(d) State one region of the brain that becomes involved only afterwards, when the person becomes aware of what happened, and give its main function.

Model answer. (a) This is a reflex action, and it is chiefly coordinated by the spinal cord rather than the brain. (b) Receptor (in the skin) → sensory neurone → relay neurone (in the spinal cord) → motor neurone → effector (a muscle in the arm).

(c) The impulse travels only as far as a relay neurone in the spinal cord, which passes it directly to a motor neurone without waiting for the brain to consciously process it, so the muscle contracts before the pain is perceived. (d) The cerebrum becomes involved afterwards, since it is responsible for conscious thought and the interpretation of sensory information, which is why the person only registers the pain a moment later.

Practice question

Try this. A patient with damage to one region of the brain, but with an otherwise healthy nervous system, struggles to walk in a straight line and has difficulty picking up a cup without overshooting or spilling it, even though the patient can still feel sensations normally and can still move their limbs. Name the brain region most likely affected, and explain the symptoms in terms of its normal function.

Exam tip

Key terms

These glossary terms connect directly to this standard:

• Stimulus, any change in the environment that a receptor detects and that triggers a response.
• Reflex action, a fast, automatic, involuntary response to a stimulus, coordinated mainly by the spinal cord.
• Neuron, a nerve cell adapted to carry electrical impulses along the pathways of the nervous system.
• Hormone, a chemical messenger that coordinates slower, longer-lasting responses, unlike the fast electrical signals of the nervous system.

Source:SRC-DSKP-EN

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