0610 · 14.1Coordination and response

Nervous system organisation

0:00Imagine you touch a hot pan by accident. Your hand jerks away before you have even had time to think "that's hot". The pain arrives a moment later. So what moved your hand? It was not a decision. Something in your body detected the heat, sent a message, and made your muscles pull your hand back, all in a fraction of a second. That is the nervous system at work.

0:18By the end of this video you will be able to describe how the nervous system is organised, recognise the three types of neurone, describe a reflex arc and a reflex action, and explain exactly what happens at a synapse, the tiny gap between two neurones. First, why does the body need a nervous system at all? Your body is made of trillions of cells in many different organs, and they all have to work together.

0:40When something changes, like a sudden noise, a bright light or a hot surface, the right parts of the body have to respond, and respond at the right time. Making all those parts work together is called coordination. The syllabus describes the role of the nervous system as the coordination and regulation of body functions. Coordination means making different parts of the body work together. Regulation means keeping things under control, for example, keeping your breathing and your heart rate at the right level for what you are doing.

1:08The nervous system carries its messages as electrical impulses, and those impulses travel along cells called neurones. A neurone is a nerve cell. It is long and thin, a bit like a wire, so it can carry an impulse a long way through the body very quickly. The mammalian nervous system has two parts. The first is the central nervous system, or CNS. It is made of the brain and the spinal cord.

1:30The spinal cord is the thick bundle of neurones that runs down inside your backbone. The CNS is where information is processed, and where decisions about how to respond are made. The second part is the peripheral nervous system, or PNS. It is made of all the nerves outside the brain and spinal cord. These nerves reach every part of the body, carrying impulses into the CNS from the sense organs, and back out from the CNS to the muscles and glands.

1:56Be careful with two words here. A neurone is one single cell. A nerve is a bundle of many neurones wrapped together, like a cable made of lots of wires.

Types of neurone

2:05There are three types of neurone you need to know: sensory neurones, relay neurones and motor neurones. The easiest way to remember them is by the job each one does in the chain. A sensory neurone carries impulses from a receptor to the central nervous system. A receptor is a cell that detects a stimulus, such as heat, light or pressure. A relay neurone is found inside the central nervous system, and it passes impulses from the sensory neurone to the motor neurone.

2:30And a motor neurone carries impulses from the central nervous system to an effector. An effector is the part of the body that responds, which is a muscle or a gland. In an exam you may be asked to identify each type in a diagram, so here is what to look for. The key is where the cell body is. The cell body is the round part of the neurone that contains the nucleus.

2:50In a sensory neurone, the cell body sits on a short branch off to the side, part of the way along the fibre, not at either end. In a motor neurone, the cell body is at one end, in the central nervous system, with lots of short branches coming off it. From the cell body, one long fibre runs all the way out to the effector, where it ends in small branches on the muscle.

3:10A relay neurone is short, and it has many short branches. It sits entirely inside the brain or spinal cord, where it links other neurones together. So in a diagram: cell body out on a side branch means sensory, cell body at the end with a long fibre means motor, and short with lots of branches means relay.

Reflex arcs and actions

3:28Now let's put the three neurones together to explain the hot pan. The pathway an impulse takes in a reflex is called a reflex arc. The syllabus describes it in terms of five parts: the receptor, the sensory neurone, the relay neurone, the motor neurone and the effector. Here is the sequence. The stimulus is the heat of the pan. Receptors in the skin of your fingers detect the heat, and they start impulses in a sensory neurone.

3:53The sensory neurone carries the impulses to the spinal cord. In the spinal cord, the impulses pass to a relay neurone, and then to a motor neurone. The motor neurone carries the impulses out to the effector, which is a muscle in your arm. The muscle contracts, and that is the response: your hand pulls away from the pan. Notice what is missing. The impulses do not have to go to the brain and wait for a decision.

4:15The relay neurone in the spinal cord connects the sensory neurone straight to the motor neurone. That is why the response is so fast. The brain does get told what happened, a moment later, and that is when you feel the pain. By then your hand has already moved. This kind of response is called a reflex action. The syllabus describes a reflex action as a means of automatically and rapidly integrating and coordinating stimuli with the responses of effectors, which are muscles and glands.

4:41Let's unpack that. Automatically means you do not have to think about it, and you cannot easily stop it. Rapidly means it happens very fast. And integrating and coordinating stimuli with responses means the nervous system links a particular stimulus to the right response by the right effector. The stimulus is heat, so the response is pulling the hand away. Other examples are blinking when something flies towards your eye, and your salivary glands releasing saliva when food touches your tongue.

5:06In that last one, the effector is a gland rather than a muscle. Reflexes protect the body. They deal with danger faster than a thought ever could.

Synapses and neurotransmitters

5:15There is one more thing we need to look at in the reflex arc. When an impulse passes from the sensory neurone to the relay neurone, or from the relay neurone to the motor neurone, the two neurones do not actually touch. There is a tiny gap between them. The junction between two neurones is called a synapse. Let's zoom in on a synapse and look at its structure. On one side is the end of the first neurone.

5:38Inside it are lots of tiny bubbles called vesicles. Each vesicle contains chemicals called neurotransmitter molecules. A neurotransmitter is a chemical that carries the message across the synapse. Between the two neurones is a very narrow space called the synaptic gap. And on the membrane of the next neurone, facing the gap, are receptor proteins. Each receptor protein has a shape that fits the neurotransmitter molecule, a bit like a lock that only one key fits.

6:04So the structure of a synapse has three key parts: vesicles containing neurotransmitter molecules, the synaptic gap, and receptor proteins on the next neurone. Now, what actually happens at a synapse? The syllabus describes it as four events, and they happen in order. One. An impulse arrives at the end of the first neurone. The impulse stimulates the release of neurotransmitter molecules from the vesicles into the synaptic gap. Two.

6:30The neurotransmitter molecules diffuse across the gap. Diffusion is the net movement of molecules from where they are more concentrated to where they are less concentrated. There are lots of neurotransmitter molecules on the side where they were released, and very few on the other side, so they spread across the gap. Because the gap is so narrow, this takes a tiny fraction of a second. Three. The neurotransmitter molecules bind with the receptor proteins on the next neurone.

6:55Each molecule fits into a receptor, like a key into a lock. Four. An impulse is then stimulated in the next neurone, and it travels on along that neurone. So the message has crossed the gap. Notice how the message changes form. It travels along the first neurone as an electrical impulse. It crosses the gap as a chemical, the neurotransmitter. And then it becomes an electrical impulse again in the next neurone.

7:19Synapses do one more important job. They ensure that impulses travel in one direction only. Why? Look at where the parts are. The vesicles of neurotransmitter are only on one side of the synapse, at the end of the first neurone. The receptor proteins are only on the other side, on the next neurone. So neurotransmitter can only be released from the first side, and it can only be detected on the second side.

7:41The message can only cross one way. That matters, because it keeps the nervous system organised. Impulses from a receptor always go towards the central nervous system, and impulses from the central nervous system always go out to the effectors. Without synapses, impulses could spread in both directions, and the messages would get mixed up.

Exam answers and recap

8:00Let's see how this can be tested. A typical question shows a reflex arc and asks you to describe what happens when someone steps on a pin. Build the answer as a chain, and name every part in order. The stimulus is the pin, and receptors in the skin of the foot detect it. Impulses travel along a sensory neurone to the spinal cord. They pass across a synapse to a relay neurone, and across another synapse to a motor neurone.

8:22The motor neurone carries the impulses to the effector, a muscle in the leg. The muscle contracts, and the foot is lifted away from the pin. Receptor, sensory neurone, relay neurone, motor neurone, effector. Five parts, in that order. Another common question asks you to describe how an impulse crosses a synapse. Here, use the four events in order, and use the key words. The impulse causes the release of neurotransmitter from vesicles into the synaptic gap.

8:49The neurotransmitter diffuses across the gap. It binds with receptor proteins on the next neurone. And that stimulates an impulse in the next neurone. Now some common mistakes. First, saying that the electrical impulse jumps across the synapse. No, it does not. The gap is crossed by a chemical, the neurotransmitter, which diffuses across it. Second, saying that the neurotransmitter is pumped or carried across the gap. It moves by diffusion.

9:15Use that word. Third, saying that the brain decides the response in a reflex. In a simple reflex arc like the hot pan, the connection is made by the relay neurone in the spinal cord. That is what makes it automatic and rapid. Fourth, mixing up nerve and neurone. A neurone is a single cell. A nerve is a bundle of many neurones. And fifth, forgetting that effectors can be glands as well as muscles.

9:40Let's recap. Electrical impulses travel along neurones. The mammalian nervous system is made of the central nervous system, the brain and spinal cord, and the peripheral nervous system, the nerves outside them. Its role is the coordination and regulation of body functions. Sensory neurones carry impulses from receptors to the central nervous system. Relay neurones pass them on inside it. Motor neurones carry them out to effectors, which are muscles and glands.

10:08In a reflex arc, the impulse goes from receptor to sensory neurone, to relay neurone, to motor neurone, to effector. A reflex action is automatic and rapid, and it links a stimulus to the response of an effector. A synapse is a junction between two neurones. It has vesicles containing neurotransmitter, a synaptic gap, and receptor proteins. An impulse releases neurotransmitter into the gap, the neurotransmitter diffuses across and binds with receptor proteins, and an impulse starts in the next neurone.

10:37And because the vesicles are on one side and the receptors are on the other, impulses travel in one direction only. So the next time you pull your hand away from something hot before you even feel it, you will know exactly which cells did it, and how the message crossed every gap on the way.