The nervous system and the reflex arc

In a nutshell

Nervous coordination is important to make sure that living things can respond to their environment. The nervous system contains central components and peripheral components, and is made up of receptor cells and neurones. Most information is processed at the brain, however dangerous stimuli are processed at the spinal cord to produce a quick automatic reflex response to the stimulus.

The nervous system

Organisation of the nervous system

The nervous system can be split into two parts: the central nervous system (CNS) and peripheral nervous system (PNS). The CNS is made up of cells in the brain and spinal cord. The PNS is made up of cells found elsewhere in the body that transmit information to the CNS.

Biology; Cells and control; KS4 Year 10; The nervous system and the reflex arc — 1. Central nervous system, 2. Peripheral nervous system.

Cells in the nervous system

The nervous system has several different types of cells.

Receptor cells in the PNS detect things in the environment called stimuli.

Examples

Heat and pressure are examples of stimuli.

Information in the nervous system is carried by neurones as electrical impulses. Neurones are simply nerve cells. Neurones have a specific structure that makes them suited to their function. They have a cell body which contains the nucleus and dendrites that help them receive information from other neurones. They also have an axon which the information can travel along across long distances in the body. The axon ends in terminals, which pass the information onto the next neurone.

Biology; Cells and control; KS4 Year 10; The nervous system and the reflex arc — 1. Nucleus, 2. Dendrites, 3. Cell body, 4. Axon.

Sensory neurones in the PNS connect the receptor cell to the spinal cord. Intermediate neurones are found exclusively in the CNS. Motor neurones carry information from the CNS back to the rest of the body to co-ordinate a response to the stimulus.

The effectors carry out the response. The response may be contraction of muscles, or it may be release of hormones. The effectors are not cells that are specifically part of the nervous system; they are simply cells that the nervous system transmits information to in order to produce an action.

Synapses

Information in the form of electrical impulses is tricky to pass along directly between neurones. This is because there are small gaps between neurones called synapses. These synapses can be overcome by converting the electrical impulses into chemical signals called neurotransmitters, which can diffuse across the synapse.

Nervous coordination

Neuronal pathway

The sequence for nervous coordination is as follows.

1.	Receptor cells detect stimuli from the environment. This information is converted into an electrical impulse.
2.	Sensory neurones carry the electrical impulse from the receptor cells to the spinal cord.
3.	The electrical impulse is processed in the CNS.
4.	From the intermediate neurone or the neurones from the brain, the electrical impulse is passed to the motor neurone.
5.	The motor neurone carries the impulse to the effectors.
6.	The effectors carry out a response to the stimulus.

Most sensory information in the CNS is processed in the brain, although some information is processed directly in the spinal cord.

Transmission across a synapse

In order to share electrical information about a stimulus from neurone to neurone, it must cross synapses between neurones. This is tricky as synapses cannot carry electricity.

Information is transmitted across a synapse as follows.

1.	Electrical impulse is carried to the axon terminal and converted into chemical signals called neurotransmitters.
2.	Neurotransmitters are packaged into vesicles.
3.	The vesicles release neurotransmitter into the synapse and neurotransmitters diffuse across the synapse to the dendrites of the next neurone.
4.	Neurotransmitters bind to receptors at the dendrites, which triggers an electrical impulse in this neurone.
5.	Neurotransmitter diffuses back into the first neurone for re-use.

Biology; Cells and control; KS4 Year 10; The nervous system and the reflex arc

The reflex arc

A reflex is a quick automatic reaction to a dangerous stimulus. It does not involve the brain, as this would take too long to process and act upon. For an immediate response, the information from sensory neurones is processed by an intermediate neurone in the spinal cord. We are born with these reflexes; we do not need to learn them. Reflexes can include withdrawing your hand from a hot object, blinking when something goes into your eye, or your heart rate increasing when you experience something exciting.

The reflex arc is as follows.

1.	A receptor cell detect a harmful stimulus, in this a case the stimulus is a hot flame. This information is converted into an electrical impulse.
2.	A sensory neurone carries the electrical impulse from the receptor cell to an intermediate neurone in the spinal cord. This intermediate neurone starts and ends within the spinal cord and its only function is to relay information from sensory neurones to motor neurones.
3.	The electrical impulse is sent from the intermediate neurone to the motor neurone within the spinal cord.
4.	The motor neurone carries the electrical impulse from the spinal cord to the effector (this could be a muscle or a hormone-secreting gland).
5.	The effector carries out a response to the stimulus.

Biology; Cells and control; KS4 Year 10; The nervous system and the reflex arc