Responding to stimuli: the nervous system and reflexes
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Explainer Video
Summary
Responding to stimuli: the nervous system and reflexes
In a nutshell
Responding to stimuli in the environment is a way organisms can protect themselves from danger and increase their chance of survival. These responses are automatic as they occur without a conscious decision.
Stimulus and response
Receptors detect stimuli, these are any changes in the external or internal environment. Receptors can be cells or proteins on cell surface membranes, they are specific to one type of stimulus.
Effectors are cells that coordinate the response to the stimulus. Receptors are able to communicate with effectors using the nervous system and/or the hormonal system.
Stimulus Receptor Effector Response
Nervous system
The nervous systems sends information around the body via electrical impulses. It is made up of cells called neurones, of which there are three types.
Neurone | Description |
| Sensory neurones | These transmit electrical impulses from receptors to the central nervous system (CNS). |
| Motor neurones | These transmit electrical impulses from the CNS to effectors. |
| Relay neurones | These transmit electrical impulses between sensory and motor neurones. |
Note: Nervous communication is localised, rapid and short-lived. This is because electrical impulses cause neurotransmitters to be secreted onto their target directly. Neurotransmitters are then removed quickly and this whole process occurs very rapidly.
Structure of the nervous system
The nervous system is made up of the CNS and the peripheral nervous system. The peripheral nervous system can be further divided into other systems.
| ||||||||||||
System | Description |
| Central nervous system | The CNS is made up of the brain and the spinal cord. |
| Peripheral nervous system | The peripheral nervous system is made of up neurones that connect the CNS to the rest of the body. |
| Somatic nervous system | The somatic nervous system controls conscious activities. |
| Autonomic nervous system | The autonomic nervous system controls unconscious activities. |
| Sympathetic nervous system | The sympathetic nervous system prepares the body for stress. It is responsible for the 'fight or flight' response. |
| Parasympathetic nervous system | The parasympathetic nervous system calms the body down. |
Reflexes
Definition
Reflexes are rapid, automatic responses to a stimulus. As there is no time spent on thinking about a response, reflexes are very fast.
Reflex arc
A reflex arc is the pathway of neurones in a reflex that link receptors to effectors.
Example
| 1. | Thermoreceptors in the skin detect the stimulus (heat from a flame). |
| 2. | The sensory neurone carries electrical impulses to the relay neurone. |
| 3. | The relay neurone connects the sensory neurone and the motor neurone. |
| 4. | The motor neurone sends electrical impulses to the bicep, which is the effector. |
| 5. | The bicep contracts to move the hand away from the flame. This prevents it from being burnt. |
FAQs - Frequently Asked Questions
What do relay neurones do?
Relay neurones transmit electrical impulses between sensory and motor neurones.
What are reflexes?
Reflexes are rapid, automatic responses to a stimulus. As there is no time spent on thinking about a response, reflexes are very fast.
What are effectors?
Effectors are cells that coordinate the response to the stimulus.