International A Level Biology / The nervous system and neurones
From stimulus to response revision notes
Edexcel International A Level Biology (YBI11), chapter 23: The nervous system and neurones, specification point 8.2. Free lesson.
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A nervous response follows a pathway from a detected stimulus to an action by a muscle or gland. Knowing each step helps you explain how the body responds, including how the brain perceives pain.
The pathway from stimulus to response
A stimulus is a change inside or outside the body. A receptor detects it and starts nerve impulses. These travel to the central nervous system (CNS), which is the brain and spinal cord. Relay neurones in the CNS pass the impulses on, then motor neurones carry them to an effector. The effector’s action is the response. Every nervous response follows the same route, whatever the stimulus. Neurones meet at synapses, tiny gaps between neurones. An impulse crosses a synapse between one neurone and the next.
What receptors and effectors do
The receptor and the effector have opposite roles. A receptor detects a stimulus and converts its energy into nerve impulses. An effector receives impulses and turns them into an action. Receptors connect to sensory neurones; motor neurones carry impulses to effectors. Examples of receptors include rod cells in the retina and pain receptors in the skin. Each receptor type responds to a particular kind of stimulus. For example, rods respond to light, not touch. Effectors are muscles, such as the biceps, or glands, such as the salivary glands.
The same pathway with different effectors
The route stays the same in different situations, but the receptor and effector can change. When you step on something sharp, pain receptors detect pressure or damage in the skin. When food reaches your tongue, taste receptors detect chemicals. The CNS coordinates each response, and a different effector acts. Stepping on something sharp: relay neurones in the spinal cord pass impulses on; leg muscles contract, lifting the foot away. Food on the tongue: relay neurones in the brain pass impulses on; salivary glands secrete saliva into the mouth. When describing a response, name both the…
How capsaicin leads to feeling pain
The burning feeling from a chilli is a response to capsaicin, a chemical stimulus. Capsaicin binds to receptor proteins on sensory neurone endings in the mouth. These proteins are ion channels. When they open, positive ions enter and start impulses. The brain perceives the pain, so the explanation must follow the impulses all the way there. Impulses travel along sensory neurones to the CNS. Relay neurones carry the impulses on to the brain, where pain is perceived. Do not jump straight from capsaicin binding to feeling pain. Include the named neurones and the brain’s role.
Explaining a nervous response
A complete answer follows the route in order. Start with what the receptor detects, name the neurones that carry the impulses, and finish with a named effector and its action. For a sprinter hearing the starting gun, receptors in the ear detect the sound and start impulses in sensory neurones. The impulses travel to the brain. Relay neurones pass them to motor neurones, which carry them to the leg muscles. The muscles contract, so the sprinter starts running.