Nervous system

it around the body. This is called a double circulation.

The functions of interaction are those that permit a living being to receive information from the exterior or from its interior and respond to the information with the most adequate response in order to assure its survival.

All of the functions of relation are determined by the stimulus, the receptor that captures the stimulus and transmits the information to the efector, that executes the most adequate response.

In large multi-cellular organisms like humans, a complex system is required to co-ordinate the necessary responses to the hundreds of changes in the surroundings that we detect. The nervous system enables humans to react to their surroundings and to co-ordinate their behaviour. The nervous system consists of receptors, nerve cells, and the central nervous system or CNS for short. Receptors detect changes in the environment, called stimuli. Nerve cells, called neurones, transmit messages to and from the CNS.

Cells called receptors are located in our sense organs, which are the eyes, ears, nose, tongue, and skin. Receptors detect changes in our surroundings or our interior. These changes are called stimuli. These changes can be temperature, light, sound, the concentration of a certain substance, etc.

The nervous system allows us to respond to the stimuli. The principles of a response to a stimulus can be summarised in a key sequence. In this sequence, the stimulus is the change we feel, for example, a change in temperature, a sound, or a sight. The receptor is the specific receptor cell sensitive to one type of stimulus, for example receptors in the eye are sensitive to the stimulus light. The co-ordinator is the brain or spinal cord, which processes the information and makes a decision. The effector is a muscle or a gland. Sometimes there is more than one effector involved in a response. The response is the contraction of muscle tissue causing movement or the production of a secretion by the gland. If more than one effector is involved there will be more than one response.

When someone with a fear of spiders sees a spider, the stimulus is the sight of the spider. The receptor is

The The The The Nervous systemNervous systemNervous systemNervous system

The interaction function

in the eye and the co-ordinator is the brain. The effectors are the leg muscles, the jaw muscles, and the adrenal glands. The responses are running away, opening the mouth to scream, and the production of adrenaline. Sensory neurones carry messages as electrical impulses from the receptor to the co-ordinator. Motor neurones carry messages as electrical impulses from the co-ordinator to the effectors.

1. See if you can match the sense organs to the stimuli they respond to.

2. The following table shows the steps involved in the response pathway. If the stimulus is the hot surface of an oven, can you complete the table using the list below? Drag the steps to the correct boxes in the table.

1. Define stimulus, efector and receptor.

_______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Lesson Review

2. Explain the differences between glands and muscles in relation to the response. ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

3. Look at the conceptual map carefully and fill in the blanks with the words in the box

below:

Stimuli

responses (2)

nervous impulses

effectors (glands)

nervous system

endocrine system

receptors

hormones

effectors (muscles)

messages to

Each nerve is a bundle of many neurones lying alongside each other. Each neurone sends electrical impulses from one place to another in the body. Some neurones are very long cells, like the ones that reach from the bottom of your back to the tip of your toes. Others are short, like the neurones in your brain.

Neurons are cells which have a strange and variable shape. They have three different parts: the body of the cell, the axon and the dendrites.

- The body of the cell is the thickest part of the neuron. The nucleus and the majority of the organelles are in the body of the cell.

- The axon is a large prolongation which comes out of the body of the cell and it branches out at the end. The axon is normally protected by Schwann cells. These cells protect and isolate the axon.

- The dendrites are small branching prolongations of the body of the cell.

There are three types of neurone: sensory, relay, and motor neurones.

The sensory neurone transmits impulses from a receptor into the CNS. They can be over a metre long. The long axon is a special adaptation of the basic animal cell, which ensures very fast transmission of an impulse. The myelin sheath acts as an insulator and it also speeds up the transmission of impulses along the axon.

Relay neurones are found in the CNS and, as the name implies,

pass messages from one neurone to another, just as runners pass a baton in a relay race. A relay neurone is much shorter than a sensory or motor neurone.

A motor neurone transmits impulses away from the CNS to a muscle or a gland, which then brings about a response to the original stimulus.

Between different neurones, there are small gaps called synapses. The synapse ensures that an impulse can only travel one way. The impulse is transmitted across the synapse by a chemical called a neurotransmitter. As an impulse reaches a synapse it causes the

The neuron

secretion of a neurotransmitter, which diffuses across the synapse and attaches to the next neurone. This causes an electrical impulse to be generated in the next neurone. When the impulse reaches the effector, the muscle contracts. If the effector is a gland, it responds by secreting a chemical.

1. This diagram of a neurone is incomplete. Drag the name of each part of the diagram to the appropriate text box in the diagram.

2. Can you match these parts of the nervous system with their descriptions? Drag the name of each part to its corresponding description.

Unit Review

Reflex Actions

Some responses to stimuli are automatic and happen very fast, for example blinking or pulling away from a hot object, or withdrawing your finger from a hot object. These are called reflex actions and often protect the body from danger. They take place very rapidly, before you have time to make a decision. This is because very few neurones are involved in the pathway. This arrangement of neurones (three neurons: a sensory neurone, relay neurone and motor neurone) is called a reflex arc. You register what has happened after it has taken place.

The sensory neurone transmits the nerve impulse into the back of the spinal cord. The nerve impulse is then transmitted through a small relay neurone to the motor neurone. It then travels along the motor neurone to the arm muscle, causing the arm to move. In a reflex response, the part of the CNS which is closest to the site of the receptor will be used to co-ordinate the response. The coordinator in this example is the spinal cord. For the change in pupil size of the eye, the brain is the coordinator.

For example, in the knee jerk reflex:

stimulus → receptor → sensory neurone → coordinator → motor neurone → effector → response

tap below knee → in leg → sensory neurone → spinal cord → motor neurone → muscle in thigh → leg jerks straight

Voluntary Actions

Voluntary actions involve the conscious part of the brain and are brought about when a decision or choice is made. For example you can choose to get up and walk away from the computer at this moment, just because you want to! The response of moving away will be brought about in a similar way to the reflex responses, where an impulse travels from the co-ordinator, to an effecter, to cause a response. In the case of deciding to stand up and walk somewhere, the co-coordinator is the CNS. The effectors are the leg muscle cells, and the response is to move away. The difference between reflex responses and voluntary actions is that a voluntary response has to be learned and requires thought. The co-ordination is taking place within your brain in response to a thought of your own.

1. Write the correct order for a reflex response using the following words: Response, efector, receptor, coordinator, stimulus.

Voluntary and reflex actions

Lesson Review

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