8. 4. nervous system. Unit 8: Pharmacological principles of drug actions

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1 8. 4 The nervous system The nervous system is a highly specialised body system that enables the body to respond to changes in the environment. It enables communication and coordination throughout the body, thus maintaining a constant internal environment. It is made of two parts, the central nervous system and the peripheral nervous system. On successful completion of this topic you will: understand the transmission of nerve impulses, diseases that affect transmission and their modification by drugs (LO3). To achieve a Pass in this unit you need to show that you can: explain key stages in the transmission of nerve impulses (3.1). 1

2 1 Central nervous system The central nervous system (CNS) is made up of the spinal cord and the brain. The spinal cord is approximately 5 mm in diameter. It starts at the base of the brain and it ends at the lumbar vertebrae. It is not only protected by the vertebrae there are membranes called spinal meninges and cerebrospinal fluid that also provide protection. It extends to the peripheral nervous system (PNS) through the gaps on either side of the vertebrae. The brain is protected by the cranium and the cranial meninges. It receives many impulses and controls involuntary movements such as heart rate and breathing rate. The hypothalamus is the control centre of the brain and it controls the autonomic nervous system, helping to regulate factors such as temperature and blood glucose levels. 2 Peripheral nervous system Key terms Central nervous system (CNS): Consists of the brain and spinal cord. Peripheral nervous system (PNS): The extensive network of nerves that extend from the central nervous system. Hypothalamus: The control centre of the brain. Afferent: Carry nerve impulses from receptors to the central nervous system. Efferent: Carry nerve impulses from central nervous system to effectors. Receptor: The part of the body that detects a change in the environment. Effector: The muscle or organ which produces an effect in response to a stimulus. Neurones: Cells that emit electrical and chemical impulses. The peripheral nervous system (PNS) is the extensive network of nerves that extends from the central nervous system. Afferent nerves carry information from receptors to the central nervous system and efferent nerves carry information to effector organs from the central nervous system. Somatic nervous system The somatic nervous system is part of the peripheral nervous system made of both afferent and efferent nerves. The somatic nervous system is involved with voluntary control of body movements via skeletal muscles. It processes information from receptors in skin, tendons, eyes, nose, ears, tongue and more that gives us sensations of sight, smell, taste, sound and touch. Autonomic nervous system This is made up of only efferent nerves. It consists of the sympathetic and parasympathetic systems opposing each other. For example, it controls heart rate the sympathetic system increases heart rate and the parasympathetic decreases it. The sympathetic system can be seen to prepare the body for flight or action, for instance, in stressful or frightening situations. It is the parasympathetic system that will return the body back to normal. 3 Neurones Neurones are specialised cells that make up the nervous system, carrying electrical impulses from one part of the body to another. Nerves are bundles of neurones. Sensory nerves take information into the central nervous system and motor nerves take information from the brain to effectors such as muscles. A motor neurone consists of a cell body with extensions called dendrites and a long axon insulated by a myelin sheath (see Figure 8.4.1). 2

3 Figure 8.4.1: The structure of a neurone showing the cell body, dendrites and the axon. Cell body Dendrites Dendrites Axon Axon Synapses A sensory neurone is structurally similar it contains a smaller cell body, dendrites, a dendron (carries impulse from receptor to cell body) and a short axon. Resting potential The resting potential is the state that each neurone is in when it is ready to conduct an electrical impulse. During this time the fluid inside the axon is negatively charged due to an unequal distribution of ions, so the axon is polarised. Active transport and facilitated diffusion of sodium and potassium results in an unequal exchange of positive ions leaving the cell, hence leaving the inside negatively charged. Key terms Resting potential: A nerve cell resting and ready for action but not active. Action potential: The change in electrical potential of an impulse along a nerve cell. Repolarisation: Change in membrane potential that returns the membrane potential to negative after the depolarisation. The action potential A receptor detects a change in the environment and generates an action potential. A wave of depolarisation occurs along the axon: the permeability of the axon is reversed and sodium ions suddenly flow into the axon, making it positive in relation to the outside. The sodium and potassium channels are gated and can limit or allow more molecules through when needed. Some of the gated channels detect the change in the voltage and open to allow sodium ions to diffuse in. If the threshold value is reached then all the gates open for approximately 0.5 milliseconds. This allows more sodium ions to diffuse inside quickly, producing a positive charge inside the axon. Repolarisation takes place when the membrane potential is reduced to zero the potassium channels open for 0.5 milliseconds so potassium ions diffuse out, leaving the charge inside the axon negative again, allowing the neurone to return to resting potential. 3

4 Action potentials are repeatedly fired along the nerve fibres but there is a time delay between each action potential so the channels can close. The time between each action potential is called the refractory period. The speed of the action potential depends on the diameter of the axon the larger the axon, the faster the action potential. If the neurone is myelinated the impulse will travel faster because the Schwann cells that form around the axon provide insulation. If there are many synapses involved, this slows down the action potential. In the propagation of an action potential along an axon the channels are shut when the membrane potential is near the resting potential of the cell, but they rapidly begin to open if the membrane potential increases. When the channels open, they allow an inward flow of sodium ions, which changes the electrochemical gradient, which in turn produces a further rise in the membrane potential. This then causes more channels to open, producing a greater electric current, and so on. Key terms Refractory period: The period following the stimulation of a nerve cell. Synapse: The gap between two neurones. Synapses Axons are not attached to one another; where one axon ends and another starts there are small gaps. These gaps are called synapses (see Figure 8.4.2). Figure 8.4.2: Diagram showing a gap between two neurones where neurotransmitters diffuse across to transport the chemical message to the next neurone, where the electrical impulse will continue. Axon Synaptic vesicle Neurotransmitter Synapse Receptor Dendrites 4

5 The synapse will not allow the action potential to jump across instead neurotransmitters transport the message across the synapse and initiate an action potential in the next axon. Synaptic vesicles carry neurotransmitter chemicals that diffuse easily across the synaptic cleft. When the action potential reaches the synaptic bulb, calcium channels open in the presynaptic membrane, and because of the concentration difference calcium ions diffuse in. As the concentration increases, it causes the synaptic vesicles to fuse with the membrane releasing the neurotransmitter into the synapse. The neurotransmitter will reach the next synaptic bulb or effector within a millisecond, where the neurotransmitter binds to receptors on the surface. The neurotransmitter chemicals are released and initiate the action potential that excites the next neurone. There are four main groups of neurotransmitters, acetylcholine, amino acids, monoamines and neuropeptides. Acetylcholine acts in the brain to change the activity of other neurotransmitters, usually involved with memory and motivation. Acetylcholinesterase breaks down acetylcholine into acetic acid and choline, which can be reabsorbed through the presynaptic membrane where they join back together to produce acetylcholine this can then be used again. See the Case study to see how drugs can affect this process. Synapses allow information to be passed from one axon to another ensuring that the electrical impulse travels in the right direction. Portfolio activity (3.1) You can generate evidence for your portfolio by doing the following: draw a diagram of four axons include the synapses illustrate the initial action potential illustrate the activity at the synapse. Checklist In this topic you should now be familiar with the following ideas about the nervous system: the central nervous system consists of the brain and the spinal cord the peripheral nervous system is the network of nerves that extends out throughout the body connecting extremities to the CNS neurones are specialised cells that carry electrical impulses as information bundles of neurones are called nerves sensory neurones sense the change in stimuli from a receptor cell and send the information to the brain the brain processes the information and sends out a response via the motor nerve to an effector effectors are usually muscles or glands synapses are small gaps between neurones where neurotransmitters transmit the action potential to excite the next neurone action potentials are created by a change in the charge across the membranes of the axon the speed of action potentials depends on thickness of axon, number of synapses and whether there is a myelin sheath present. 5

6 Further reading Olesky, W. (2000) The Nervous System Rosen Publishing Group. Case study The first nerve gas produced for military use was in 1936 it was used in the war to directly affect the nervous system. Nerve agents are similar to insecticides and they can be deadly if people are exposed to them. This nerve gas contained organophosphates that blocked acetylcholinesterase. Therefore, there was no way to stop the effect of acetylcholine and it built up in the synapses because it was not reabsorbed. This could cause symptoms such as twitching, diarrhoea and sickness. Acknowledgements The publisher would like to thank the following for their kind permission to reproduce their photographs: Shutterstock.com: Photostock 10; Science Photo Library Ltd: JACOPIN 3, Henning Dalhoff 4 All other images Pearson Education Every effort has been made to trace the copyright holders and we apologise in advance for any unintentional omissions. We would be pleased to insert the appropriate acknowledgement in any subsequent edition of this publication. 6