Structure of the nervous system and neurones

Homeostasis and response • The human nervous system

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Name the three main types of neurone.

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Sensory neurone, relay neurone and motor neurone.

Key concepts

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Definition of the central nervous system (CNS)

The central nervous system comprises the brain and spinal cord and acts as the primary coordinator of nervous activity. The CNS receives electrical impulses from receptors, processes information and sends out signals to effectors to produce responses . The brain performs higher functions such as memory, consciousness and decision-making, while the spinal cord transmits information and supports rapid reflex pathways that avoid the brain when speed prevents damage fileciteturn0file10.

Organisation and structural adaptations

The nervous system divides into the CNS and the peripheral network of nerves made from bundles of neurones; nerves spread throughout the body to connect receptors and effectors to the CNS fileciteturn0file7. Long axons and insulating myelin sheaths speed electrical impulses across distances; branched dendrites receive signals from neighbouring cells so that each neurone connects into a flexible network adapted for fast, directed transmission .

Stimulus → receptor → coordinator → effector → response pathway

A stimulus is a detectable change in the internal or external environment that activates a receptor. Receptors generate electrical impulses that travel along sensory neurones to the coordinator (CNS), where the information is processed and a suitable command is formed. Motor neurones carry the command to effectors (muscles or glands), which produce the observable response such as muscle contraction or hormone secretion . The linear sequence ensures rapid detection and targeted action through matched receptor and effector types.

Neurone structure and transmission of electrical impulses

Neurones contain a cell body, dendrites, an axon and sometimes a myelin sheath. Dendrites receive chemical signals that initiate electrical impulses; the axon conducts the impulse away from the cell body at high speed; myelin insulation increases conduction velocity so that distant effectors respond quickly . At synapses the electrical impulse triggers release of chemical neurotransmitters from the axon terminal; these chemicals diffuse across the synaptic gap, bind to receptors on the next neurone and initiate a new electrical impulse, enabling directional, one-way signal transfer and rapid stopping by enzymatic breakdown of neurotransmitters .

Reflex arc structure and function

A reflex arc routes an impulse via a sensory neurone to a relay neurone in the spinal cord and then immediately back along a motor neurone to an effector, bypassing conscious processing in the brain to save time . The sensory neurone detects the stimulus and transmits the impulse toward the CNS; the synapses between sensory, relay and motor neurones enable rapid chemical transmission and integration; the motor neurone transmits the command to an effector such as a muscle, producing a fast, protective response that reduces tissue damage fileciteturn0file6.

Coordination of effectors by the CNS

The CNS interprets incoming impulse patterns and coordinates precise responses by routing signals to appropriate motor neurones and endocrine glands. Motor outputs cause muscles to contract in controlled patterns for movement; glandular outputs trigger hormone secretion into the blood, producing slower but longer-lasting effects. The CNS selects between immediate reflex outputs and deliberate, processed outputs to balance speed and control fileciteturn0file6.

Key notes

Important points to keep in mind

CNS equals brain plus spinal cord; only relay neurones lie inside the CNS .

Pathway sequence: stimulus → receptor → coordinator (CNS) → effector → response; motor neurones move impulses to effectors .

Sensory neurones send impulses to the CNS; motor neurones send impulses away to muscles or glands .

Myelin sheath increases conduction speed; long axons transmit signals across the body rapidly .

Synapses use chemical neurotransmitters for one-way signal transfer; enzymes break down neurotransmitters to stop continuous firing .

Reflex arcs produce automatic, rapid protection by routing impulses through the spinal cord to effectors without initial brain involvement .

Brain regions specialise: cerebrum for consciousness and memory, cerebellum for muscle coordination, medulla for automatic life functions .

Effectors produce movement (muscles) or chemical release (glands); the CNS chooses reflex versus processed routes to balance speed and control fileciteturn0file6.