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 $\rarr$ Receptor $\rarr$ Effector $\rarr$ 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.

Peripheral nervous system $\swarrow \searrow$ Somatic nervous system Autonomic nervous system
	$\swarrow \searrow$
	Sympathetic nervous system	Parasympathetic nervous system

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

Biology; Stimuli; KS5 Year 12; Responding to stimuli: the nervous system and reflexes

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.