Mammalian nervous system

Biology · Coordination and control. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

The nervous system coordinates and regulates the body. The central nervous system (CNS) is the brain and spinal cord. The peripheral nervous system (PNS) is all the nerves outside them. Messages travel along neurones as electrical impulses.

A reflex action is a rapid, automatic response to a stimulus. It follows a reflex arc: a receptor detects the stimulus, a sensory neurone carries the impulse to the CNS, a relay neurone passes it on, and a motor neurone carries it to an effector, which is a muscle or a gland.

A synapse is the junction between two neurones. The impulse cannot cross the gap, so a chemical called a neurotransmitter carries the message across by diffusion.

Rules to remember

CNS = brain + spinal cord; PNS = nerves outside the brain and spinal cord.
Reflex arc: stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response.
Sensory neurone: receptor to CNS. Relay neurone: inside the CNS. Motor neurone: CNS to effector.
Effectors are muscles (contract) and glands (secrete).
At a synapse: an impulse arrives, vesicles release neurotransmitter into the synaptic gap, it diffuses across and binds to receptor proteins, and an impulse starts in the next neurone.
Synapses make impulses travel in one direction only, because vesicles are on one side and receptor proteins on the other.

Common mistake

Do not write that the electrical impulse jumps across the synapse. The impulse stops at the end of the first neurone, and a chemical (neurotransmitter) diffuses across the gap.

Worked example

A person steps on a sharp stone with a bare foot and the leg lifts at once. Which neurone carries the impulse into the spinal cord, which carries it out, and what is the effector?

Pain receptors in the skin of the foot detect the stimulus.
A sensory neurone carries the impulse from the receptor into the spinal cord.
A relay neurone in the spinal cord passes the impulse to a motor neurone.
The motor neurone carries the impulse out to a leg muscle, which contracts.

Answer: In: sensory neurone. Out: motor neurone. Effector: the leg muscle that lifts the foot.

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The Nervous System

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