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Chapter 24

chapter 24

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Biology 1225
Michael Butler

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Chapter 24- Neural Control and the Senses (25) Overview Neurons are the basic units of communication in the nervous system. Neurons detect and integrate information about external and internal conditions. The direction of flow of a signal in a neuron is as follows, a signal is received by dendrites, then transmitted to and integrated in the cell body, where a response is initiated that results in the transmission of an action potential along the axon onwards to a synapse, which then chemically transmits the response to an effector such as another neuron or to a muscle cell etc. Thus neurons act on muscles and glands in ways that produce appropriate responses. Action potentials are the means by which messages travel along neurons through the nervous system, but chemical signals are required to bridge the gaps between neurons. The nervous system includes the brain, spinal cord, and many nerves. The brain and the spinal cord make up the central nervous system. Paired nerves that thread through the rest of the body make up the peripheral nervous system. The brain is complex in both structure and function. In its three main regions - the forebrain, the midbrain, and the hindbrain - are the centers that receive, integrate, store, and respond to information Responsibilities Be sure to understand the organization of the nervous system and be able to distinguish between the different types of neurons in the central nervous system. Be sure to understand the operation of a neuron and how it communicates with other neurons. A common mis-perception is that an action potential can vary in intensity depending on the strength of the stimulus that induces it, this is FALSE! an action potential is "all or nothing" once initiated, it cannot be stopped and remains at the same intensity while it is being propagated along the length of the axon and to the synapse. None of the material on the brain and the senses covered on pages 480 to 488 will be tested in this coursNeurons, Nerves, and the Nervous System Nerves respond to a stimulus by generating an electrical signal - an action potential. The functional units of the nervous system are cells called neurons. All neurons have a cell body; filament-like and often branching dendrites conduct signals to the cell body, which then integrates the stimuli and initiates the response that is transmitted onwards by a long cell extension called the axon. The terminal end of an axon is also called the synaptic terminal, and a single axon can have many branches that form more than one synaptic terminal, in some cases there are many synaptic ends to an axon, each of which can communicate with a different neuron. There are three types of neuron. Sensory - which relay their response to a stimulus to the brain or spinal cord, where interneurons integrate information with other signals and send their response to effector neurons - such as motor neurons that cause glands and muscles to act. Neurons in the brain are physically supported and nourished by support cells called neuroglia (glia for short), some of which also form the insulating membrane sheath that is wrapped around many long axons in nerves. All cells in our body (i.e., not just neurons) need to control the numbers of sodium and potassium ions they contain, in order to perform their specific functions. For that reason, all cells, be they muscle, nerve, skin - whatever - have channel proteins in their membranes that allow sodium, potassium, chloride and other ions to move back and forth by non ATP requiring passive transport, (diffusion) in response to their concentration gradients. All cells also have sodium and potassium “pumps” - these are transport proteins that move sodium and potassium ions by active transport, across a cell membrane against their concentration gradient, and require ATP to do so. These pumps use ATP derived energy to transport ions from where they are in low numbers to where they are in high numbers. A given cell type, nerve, muscle etc, will have a characteristic distribution of sodium and potassium ions across the cell membrane that has been produced by the ATP fueled action of the Na-K pumps acting in concert with the movement of Na and K ions allowed by the passive diffuser channel proteins. Just as with any other type of cell, this process of active adjustment of Na and K levels occurs in the neuron, across its axonal membrane, to achieve the correct balance of ions inside and out. In neurons this process of sodium and potassium transport through diffuser channel proteins and through sodium-potassium pumps, acts in conjunction with large negatively charged proteins in the cytoplasm (that are too big to cross the cell membrane) to cause the interior of the neuron - the cytoplasm - to have a negative electrical charge with respect to the exterior (the cytoplasm happens to be at about -70 mv). The particular distribution of sodium and potassium ions maintained across the membrane of the axon by the two types of transport proteins is such that there is a high potassium concentration inside the axon, and a high sodium concentration outside the axon (in relative terms). In this condition, in the absence of a signal (the action potential from the cell body of the neuron), the electrical charge across the axonal membrane is said to be at resting potential (-70 mv) and it will stay at this potential in the absence of a suitable stimulus to change. In this resting condition, with a marked difference of electrical charge across the membrane, the axon is said to be in a polarized state. How is it that a neuron can generate an electrical signal that passes down an axon and acts to transmit a message on to another neuron or to a muscle? What is special about the neuron is a unique kind of transport protein spanning its cell membrane - what is called a voltage gated channel protein. This voltage gated channel protein does not require ATP for its operation, it is a passive transport protein, but it has the unique property of only acting (opening the gate) when it receives a suitable electrical signal. When it has received such a signal, the gate will open and allow ions to stream in or out. Once the signal is ended the gate "swings shut". One such voltage gated channel protein works with sodium, another with potassium. Which way the ions diffuse when the gate opens, is solely determined by the concentration gradient, because the gated channel protein does not undertake active transport. Imagine that a neuron has received electrical stimuli from other neurons into its dendrites region, and that the cell body region of the neuron has integrated these stimuli and “decided”, in response, to generate an action potential. The action potential is a sudden “all or nothing” electrical signal that causes a reversal of the ion distribution across the cell membrane, at the beginning region of the axon, which will then propagate down the length of the axon - this is what is called a wave of depolarization. The signal is called a wave of depolarization because the “negative inside - positive outside” state of the resting potential across the axonal membrane is suddenly reversed, and this state propagates down the axon. This reversal of charge across the membrane is the electrical signal. During this wave of depolarization a given (very tiny) region of the axon receives the voltage signal of the action potential - this voltage causes the voltage gated protein channels to open in that tiny region. When the gates open in that region, they allow sodium ions to flood in and potassium ions to flood out in a manner that suddenly causes the axon cytoplasm to become positive with respect to outside the axon, about +50 mv instead of -70 mv when the neuron is in its resting state. Now this tiny region of the axon has reversed its electrical voltage state (i.e., it depolarized) and this reversal acts as the voltage signal to cause the voltage gated channel proteins in the next small region down the axon to open and for that region to depolarize, and so on - this wave of depolarization propagates down the length of the axon to the synaptic terminal and so delivers its electrical signal. To be more precise, when a patch of an axon receives the action potential, it reacts first by the voltage gated sodium channels opening, so that the sodium ions flood in and reverse the potential across the membrane, then, as the signal has passed on to the next patch of the axon membrane, the voltage gated potassium channels open to release potassium ions to the exterior, which starts the re-establishment of the resting membrane potential ready for the next impulse. Very soon after the electrical signal has passed on, the voltage gated channel proteins close, and the sodium-potassium pumps go into action to restore the ion distribution to what it was during the resting state, re-establishing the resting electrical potential and readying the neuron for its next response. In that short (thousandths of a second) time when the resting potential is being re- established, the axonal membrane is said to in a refractory state - it cannot respond to another action potential, and it will not allow the wave of depolarization to travel backwards. The synapse Neurons transmit their electrical signal down axons away from the cell body, and that signal will be passed on to other neurons, or to special contact points on muscles or glands. The signal does NOT (as a rule) pass from one neuron to another or to a muscle or gland electrically - by direct electrical contact with the receiving structure, instead, it passes on as a chemical message. There is a definite but tiny gap between neurons, at the end of axons and the start of dendrites, or between the ends of axons and muscle or gland cells. This tiny gap is called a synapse. When the wave of depolarization hits the end of the axon at the synapse (the presynaptic membrane) a set of processes come into play which cause the pre-synaptic m
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