The nervous system (CCEA)

The human body has two communication systems that help us to respond to changes:

1. nervous system
2. hormonal system

Comparisons:

They both:

• send messages around our body

• enable the body to respond to stimuli

Differences:

The brain and spinal cord form the central nervous system (CNS).

The CNS controls and coordinates responses between receptor and effectors.

• Stimuli are changes in our environment that we respond to and are detected by receptors.

• Different receptors are sensitive to different stimuli (eg receptors in the eye are sensitive to light).

• A nerve cell/neurone carries information in the form of nerve impulses from the receptor to thecoordinator.

• The coordinator – brain or spinal cord – determines whether or not to respond to the stimulus.

• A neurone carries information from the coordinator to an effector.

• An effector is a muscle or gland that can bring about a response.

Our eyes are specialised sense organs that contain receptors sensitive to light.

There are many parts that work together to produce a focused image on the retina under different light conditions.

As light passes through the cornea some refraction takes place. More refraction occurs when the light passes through the lens.

The thickness of the lens can be adjusted in order to focus light on the retina.

When looking at a distant object the lens is thin as little adjustment is needed to focus light onto the retina.

When looking at nearby objects the lens is thick to provide additional refraction to focus light on the retina.

The ciliary muscle is a circular ring of muscle around the lens, connected by suspensory ligaments.

These work together to allow our eyes to focus on objects at different distances.

Too little or too much light can damage the light sensitive receptor cells in the retina.

In dim light, the pupil becomes larger to allow as much light as possible to enter.

In bright light, the pupil becomes smaller to restrict the amount of light entering.

Receptors and effectors are connected to the central nervous system by neurones.

A neurone’s function is to transmit electrical impulses across the nervous system quickly.

A neurone is adapted for this function in the following ways:

• The cell body contains the cytoplasm and nucleus (the control centre of the cell).

• The axon is a long extension of the cytoplasm (can be up to 1m). This means nerve impulses can be transmitted to the extremities by one cell.

• The myelin sheath is a fatty layer that surrounds the axon. The sheath acts as an insulator and speeds up nerve impulses.

• The branched ends of the axon and the smaller branches coming from the cell body allow the neurone to make connections with many other neurones.

A synapse is the gap between two neurones.

It functions as a junction because it links two neurones, allowing messages to pass between them and it also controls the direction of messages.

• When an impulse reaches the end of the axon, a transmitter chemical is released.

• This chemical diffuses across the gap.

• When in high enough concentration it triggers an electrical impulse in the next neurone, allowing the signal to continue.

Voluntary actions require conscious thought and take more time due to thinking.

Reflex actions happen quickly without thinking, protecting the body from harm, for example, pulling your hand away from a hot surface.

A reflex action is fast because it follows a short pathway with only three neurones and two synapses:

• Sensory neurone: carries impulse from the receptor to the spinal cord (coordinator).

• Association or relay neurone: connects the sensory neurone to the motor neurone.

• Motor neurone: sends impulses from the spinal cord to an effector (muscle or gland) to cause a response.