Peripheral Nervous System (1977)

[Music] All of us have to learn to use the nervous system with which we are born. The actions of a newborn baby are automatic. But already there are patterns in its behavior which can help us understand the nervous system and how it works. put something into its hand and it grasps. Looking at this in slow motion, we can imagine sensory receptors in the skin being stimulated, messages being sent through the nervous system to the muscles to move the fingers. The action is automatic and we call it a reflex reaction. There is a stimulus and a response. Reflex reactions can be learned. The sucking reflex is an example. It takes about 3 days for a baby to learn to suck successfully. Even though this baby hasn't yet learned to walk, it seems that some of the reflexes are already present as he tries to take steps. People of your age behave quite differently because the brain has a store of experience and varied responses. Reflex reactions still play an important part in your lives, but most responses are the result of conscious control. The control which your nervous system gives you is remarkable both for its flexibility and complexity. Much of what you do is a response to information from the world around coming in through your senses. Sight, hearing, touch. When you touch something, receptors in the skin are stimulated. This is a microraph of a receptor of touch in the skin. It is a special ending to a nerve cell which runs back to the spinal cord. Individual nerve cells are called neurons. They are long and thin. This particular one runs from the receptor to the spinal cord. It is called a sensory neuron. The spinal cord contains many millions of neurons that connect to and from the brain. This microraph shows neurons in the brain. They formed complicated patterns. Of course, you use these cells to think, learn, and to control your bodies. Information from the brain is sent to the body by special neurons, the motor neurons. Motor neurons connect the spinal cord to the muscles. Here are the endings of motor nerves in a muscle. This system of receptors, sensory nerves, spinal cord, brain motor nerves and muscles is the basic working circuit in the nervous system. By means of it, a stimulus gives rise to a response. Reflexes have even shorter pathways as we shall see here. In this model, we can follow the neuron from a sensory nerve ending in the hand to the spinal cord. On the way, it becomes bundled together with other neurons to form nerves. The spinal cord leads to the brain and from there motor neurons run down the spinal cord and so to the muscles. The nerves that serve the senses and the muscles form the peripheral nervous system. The brain and spinal cord are the central nervous system in which the brain is the control center of the body and the spinal cord an extension of it with some controlling functions. Another part of the nervous system looks after the internal organs. It is the autonomic nervous system. As you look at the nervous system, remember it is composed of nerve cells, the neurons, which are arranged in working units that repeat again and again throughout the body to build up the complex system. It is the most intricate and wonderful system known to man, and it is inside him. On this microraph, we indicate a single neuron. It is long and thin and has the task of carrying signals through the body. At one end is the cell body, headquarters of the neuron. It contains the nucleus and other structures. Other fibers arise from the cell body. They are dendrites and they carry signals from other axons to the cell body. Through the dendrites, neurons make contact with each other. These signals travel away from the nucleus along a tube called the axon which is filled with fluid. Here is a pressure receptor in human skin. Imagine that we have stimulated it and that signals are being sent along the sensory neuron to the spinal cord. This trace on an oscilloscope shows what happens when we push an electrode into an axon which is signaling. the trace goes negative and we have a peak which shows the passage of a nerve signal. However, there is also a rhythmic up and down motion due to the animals breathing and we will learn more if we look at the event in diagram. Here is the experimental setup with the electrode being pushed into the nerve until it touches an axon. As it does so, the trace shows a marked change in potential. The outside of the axon is positive. Then as the electrode enters the nerve, we intercept an impulse which appears as a blip on the screen showing that the polarity has reversed and the inside of the axon has become positive. The blip on the left shows the stimulation of an axon. Then there is a regular succession of impulses in which the interior of the axon becomes positive with respect to the outside. This diagram shows what is happening. The axon is stimulated and an impulse travels along it. As the meter shows, there is a change in potential. The resting potential between impulses is 90 m with the outside of the axon positive. When the impulse passes, the polarity reverses to give the action potential of 110 m. Normally the outside of the axon is positive and the inside negative. In an impulse the polarity reverses and the inside of the axon becomes positive for a moment before returning to negative after the impulse has passed. Impulses travel like waves along the axon. In and around the axon are ions of chlorine and of sodium outside the axon and of chlorine and potassium inside. The number of sodium ions makes the outside positive. When the axon is stimulated, sodium ions flood in and the polarity reverses. The impulse moves forward as sodium moves in. Behind the impulse, the sodium ions are pumped out again. Until this is complete, the axon cannot transmit another impulse. It is in the so-called refractory period. The impulses travel along the axon, each a wave of positive potential separated by the refractory period. So far we have seen how a stimulus is turned into impulses and how they are transmitted along the axon. At the end of the axon, the impulses are passed on to another neuron. This microraph shows the axons of sensory neurons making contact with the cell body of another neuron in the spinal cord. These contacts have a special character because the neurons never touch each other. They always separated by their membranes between which is a narrow space called a syninnapse. The axon thickens at a syninnapse into a bulb, the synaptic bulb. The axon terminates in the synaptic bulb adjacent to the membrane of the next cell body. In between is the gap or synaptic cleft. It's about 25 nanometers wide. In the bulb are packets of a chemical transmitter substance. When impulses arrive at the syninnapse, they cause some of the packets to burst and spill transmitter substance into the synaptic clft. The transmitter diffuses across the syninnapse and stimulates the next neuron. Then the transmitter substance is taken up into the packets again. Until this happens, the syninnapse cannot transmit another impulse. So the impulses cross the syninnapse and run through the next cell body and out along the axon. In the spinal cord, sensory neurons make contact with other neurons. Others lead to the brain. In this way, the signals are relayed through the body. The signals leave the cord on a motor neuron. The axon of the motor neuron ends in a motor end plate, a special synapse between the axon and a muscle where a chemical transmitter produces a response. So we have a basic pattern of stimulus and response as shown in this simple reflex reaction. It may seem that the nervous system works instantaneously with no time lag between stimulus and response. But that cannot be so because the impulses have to travel over quite appreciable distances. In this experiment, we're going to time the transmission of nerve impulses as they run through the body. A number of electrodes have been taped to Joe's body, and Sue is measuring the length of the nerve pathways between each electrode and the brain. Kim is recording the results. All the electrodes are connected to an electronic timer. The timer starts as contact is made to the electrode. Joe stops the counter with a switch as she feels the touch of the contact against her body. In fact, the length of nerve is that between the electrode and the brain and the brain and the muscles of the arm. Watch the experiment for a moment and note the results. As the pathway gets longer, the reaction time increases. Here's the time for the shoulder. Now the chest, the hip, and the ankle. If we take all Kim's results and work out the speed that impulses travel through the body, we find that the impulses are traveling at about 10 m/s. As a check, we asked a nerve physiologist to perform a similar experiment on a nerve from the leg of a frog. He moves the stimulus nearer to the electrodes that take the impulse from the nerve. By displaying the traces that result on a persistence oscilloscope, we see that as the distance to the electrodes is reduced, the transmission time also falls. However, here we are measuring in millimeters and microsconds. The speed worked out at about 20 m/s, twice Kim's result. Obviously, the impulses had to cross a number of synapses as they traveled through Joe's body, and it was this that caused delay. This trace shows the result of strengthening the stimulus. Notice the three peaks, which indicate three sets of nerves, each transmitting impulses at different speeds, and each with different sensitivities. So far we have seen neurons like these with bare axons. But if you look again at this microraph, you will see other axons which are enclosed in cells which are folded around the nerve fiber. The schwan cells membrane is a substance called myelin which is an insulator and this forms a sheath inside the cells along the axon. Between the cells are gaps where the axon is bare. These are the nodes of rvier seen here in an electron microraph. We saw how impulses travel along unminated axons. Now we add the myelin sheath. This encloses the axon except for the nodes and stops leakage of current from the axon. Currents flow between the nodes and the impulse jumps from node to node with a marked increase in speed. In fact, currents flow to nodes far ahead of the impulse which jumps to the node where the current is just strong enough to depolarize the membrane which may be several nodes further on. The simplest nervous activities concern the reflex reactions. These are seen clearly in babies where the brain has only crude control of the body. The reflexes have survival value. They help us do things like obtaining food and avoiding danger. In a reflex, the response is always the same and it happens rapidly. The brain is not necessarily informed or involved. In a reflex, the sensory receptor is stimulated and the sensory neuron fires. In the spinal cord, the signal passes to an interneuron and then to a motor neuron that leads to the appropriate muscle. The neurons involved form a system called a reflex arc. There are many reflex arcs in the spinal cord. Often they are linked together by neurons that run up and down the chord. The fibers that link the reflex arcs allow the reaction to spread to other sets of muscles. There are also neurons that link the reflex arcs to the brain. In the brain are centers which sort the signals before they enter the consciousness. The signals pass to the highest levels where memory and learning are called upon and a decision is made. The decision is passed to the motor nerves and so to the muscles. All that can be said in summary is that the nervous system is complex. We know about the overall structure of the system, the behavior of individual neurons and how they transmit impulses. We believe we understand synapses and the way neurons are combined into reflex arcs, but we do not understand the workings of the brain. And to most of the questions you will ask, the answer is we don't yet know. [Music]