Mammals_Concept_5

The central nervous system (the brain and spinal cord) processes and coordinates all the information coming in from sense organs and the outgoing commands to muscles. The brain is a mass of nervous tissue in which complex functions such as memory, intelligence, learning and emotion occur. The spinal cord is a long nerve that connects the brain to the rest of the body.

The peripheral nervous system is all of the nervous tissue outside the brain and the spinal cord It is responsible for sending information to, and receiving it from, the rest of the body Instructions to move, which are sent out through the motor nerves, can either be voluntary (as when moving muscles to stand up) or involuntary (as when muscles control the flow of food in the digestive system). The part of the nervous system that controls involuntary movements is also known as autonomic.

The autonomic nervous system is further divided into the sympathetic and parasympathetic nervous systems. The first of these produces the 'fight or flight' response: it increases alertness and stimulates the body, preparing it for dangers. The second produces the 'rest and repose' response: it conserves energy for normal bodily functions such as digestion.

Transmission and communication

A nerve cell or neuron receives and passes on information in the form of electrical pulses. Figure 9 shows how a sensory neuron works. Small protrusions from the ceil, known as dendrites, receive stimulation from a sense organ (see Concept 7) or another neuron. When stimulated, the neuron undergoes a slight change in its chemical content, resulting in the production of a small electrical impulse. This impulse travels along the axon, a sort of long tail that can be up to \ m long. When it reaches the end of the axon, chemical transmitters are released which travel across a gap, called a synapse, to the dendrite of the next neuron — and so the signal passes on to the appropriate section of the brain for processing. Motor neurons, which convey instructions from the brain to muscles or other organs, have a slightly different structure, but they work in a similar way.

Reflex reactions

Sometimes a stimulus is such that the information doesn't even need to get to the brain before action is taken. The difference between 'very warm' and 'hot' may be marginal — but reactions to it will be significantly different If a very warm object is picked up, the brain will make a conscious decision to put it down before the heat causes damage. With a hot object, the signals about the heat only get as far as the spinal cord before messages are sent to the muscles to drop the object and move away from it. A message will also be sent to the brain to let it know what has happened Reflex reactions of this kind — very fast and completely automatic. have obvious survival value.

Links to learning

Almost as rapid as reflex reactions are the 'learned responses' that we make to familiar stimuli — for example, a driver stopping at a red light. This phenomenon can be linked to issues surrounding early learning, When a new skill is being learned, considerable effort and concentration is needed. Writing needs initially to be thought about and focused on — but once the skill has been learned, it can be carried out ‘automatically'. I no linger have to think where particular letters are on the keyboard of my computer (except when I start to think about it, and then I really have to think about it). I just think of a word, my fingers move and the word appears on the screen — but such a response takes time and practice to become embedded.

Why you need to know these facts

Many of our everyday movements and reactions appear to be quite automatic Some of them, such as the heartbeat, are truly automatic, Even when you make a conscious decision to do something, for example to scratch behind your left ear with your left index finger, an incredibly complex series of events has to take place to ensure that the correct muscles are operated in the correct order with the correct intensity until the response comes from the skin that the itch has been scratched. Beginning to appreciate this greatly undervalued aspect of our body control systems is an important factor in developing children's understanding of their own worth.

Vocabulary

Brain — the organ that receives and analyses sensory input, and controls actions.

Involuntary — nervous messages that are sent automatically (for example, to control the heartbeat),

Reflex — an involuntary reaction in response to a particular stimulus.

Spinal cord — a long nerve that is the main conduit for messages and signals to and from the brain.

Voluntary — nervous messages involving conscious actions such as walking,

Amazing facts

Some mammals have deeply embedded responses to certain danger signals', such as the smell of a lion or the sound of a wolf,

Messages pass along nerve cells (neurons) at up to 400km/h.

Questions

Why do you jump when you hear a sudden, loud noise? The sound is received by the ear; like any other sound — but the high volume overloads the nerves that send the impulses to the brain, causing a message to be sent directly to the coordinating section of the spinal cord, which sends messages to muscles telling them to move, So before the brain has even registered the sound, the body is beginning to move in preparation for running away. It only works for sudden sounds, because the reflex reaction wears off if the sound continues. Continuous loud noises can be blacked out' by the brain, and pushed in to the background because the body adjusts to them.

Why do doctors test your reflexes?

This is a simple way to test the efficiency of the nerve signals travelling along the spinal cord.

Teaching ideas

Knee-jerk reaction (testing, observing)

Ask a child to sit cross-legged on a chair (with one knee over another). Gently tap just under the knee with the edge of a ruler — the leg should jerk upwards. Safety note: all taps MUST be applied gently! This happens because the tendon below the knee has been dented and so lengthened where it was hit. The body's automatic response is to keep the leg in position by keeping the tendon the same length. In this case, it is 'fooled' into shortening the tendon and bringing the [eg up. Now try comparing the speed of the reflex with that of the response to the command 'Lift'. When the message has to go via the brain, which has to understand the message and act on it, the reaction will take longer.

Ruler drop (testing, observing)

Hold the bottom end of a 30cm ruler just above the open finger and thumb grasp of a child, Release the ruler. How far does the ruler drop before it is grasped? Does the reaction get faster with practice? Is the reaction faster if the stimulus (of the ruler being released) is visual, aural or tactile — that is, if you watch the ruler, hear someone say 'Now!' (while your eyes are closed) or feel the ruler drop?

Report abuse