NERVOUS SYSTEM
We can study behaviour and thought without necessarily knowing anything at all about the nervous system – or how the behaviour is generated. Cognitive psychologists studying slips of the tongue, for example, may not care whether the brain is made up of rubber bands or of neurons. In fact, if you simply count the cells in the brain, neurons are very much a minority group; most of them are non-neuronal, glial cells. [glial cells non-neuronal cells in the brain that provide ‘support’ for the neurons] But rubber bands are rarer still in there. Nevertheless, our interactions with the world around us depend crucially on the activity of the nervous system. Without it, we not only have no senses and no ability to move; we also have no thoughts, no memories and no emotions. These are the very essence of our selves, yet they can be disastrously changed, and even completely erased, by disorders of the nervous system. We can very effectively treat some psychological disorders simply by using words to change the ways in which patients think. But the only generally available palliatives for other conditions, like Parkinson’s disease or schizophrenia, are drug treatments. And some conditions, such as Alzheimer’s disease, are currently untreatable. The more we learn about how the nervous system operates, the better we can understand how it can go wrong. That, in turn, will increase our chances of finding out how to prevent, or even reverse, psychological disorders. If we do not understand the way that the nervous system works, then we, like our forebears throughout humankind’s history, are confined to a passive role as observers and documenters of the effects of nervous dysfunction.
The nervous system has both central and peripheral components. The central part includes the brain and the spinal cord; the peripheral part includes the nerves through which the central nervous system interacts with the rest of the body. ‘Nerve’ is a familiar word and is used in various ways in ordinary conversation. But in psychology we use it specifically to mean a cord of neuronal axons bundled together passing through the human body. We have probably all had the experience of hitting our ‘funny bone’ – the discomfort is due to the compression of the ulnar nerve. Nerves are typically sensory (afferent) – carrying information to the central nervous system from sensory neurons whose cell bodies are located in the periphery of the body – or motor (efferent) – extending out from the central nervous system to the organs and regulating muscular movement or glandular secretion.
The basic unit of the whole of the nervous system is the neuron. Neurons operate alongside various other types of cells, whose activity can be essential to normal neuronal function. Even in the brain, only about 10 per cent of the cells are neurons. Most are glial cells, which fall into several different classes, each with its own function. There are astrocytes, oligodendrocytes (in the central nervous system), microglia and ependymal cells. (The word ending -cyte means ‘cell’.) Glial cells were once thought of as the structural glue (that is what glia means in Greek) that holds the neurons in place, but their roles are proving to be far more complex. For example, astroctyes, which are the most common class, not only provide physical support to the neurons, but also help to regulate the chemical content of the fluid that surrounds the neurons. Astrocytes wrap closely round some types of synapses (the junctions between neurons) and help to remove glutamate (a neurotransmitter substance) from the synaptic cleft (the gap between neurons meeting at the synapse) via an active pumping system. If the pump fails, the system can become reversed, so that excess glutamate is released back into the synapse, which can be fatal to nearby neurons.
Neurons come in many shapes – or morphologies –which give them their different functions. For example, projection neurons have fibres that connect them to other parts of the nervous system. Even within this category, there are many different morphologies, but all projection neurons share some basic similarities. You can think of the neuron as having three essential components. The heart of the neuron is the cell body, where the cell’s metabolic activities take place. Input from other neurons typically comes via the dendrites. These can be a relatively simple tuft of fine, fibre-like extensions from the cell body, or highly complex branches like the twigs and leaves of a tree. The output of the neuron is transmitted via its axon to the dendrites of other neurons, or other targets such as muscles. Axons can be very long, reaching right down the spinal cord, or so short that it is difficult to tell them apart from the dendrites. Nerve cells with such short axons are called interneurons rather than projection neurons, because all their connections are local. Some neurons have just a single axon, although it may still make contact with a number of different target cells by branching out towards its end. Other cells have axons that are split into quite separate axon collaterals, each of which may go to an entirely different target structure.
PERPHERAL NERVOUS SYSTEM
Peripheral nerves are just bundles of axons. They appear as white matter, because most mammalian axons have a white myelin sheath around them, which helps to speed up nerve conduction. Although many neurons have cell bodies located in the central nervous system, there are clusters of cell bodies in the peripheral nervous system too. The simplest type of cluster is called a ganglion (plural, ganglia). The sensory division of the peripheral system deals with inputs from receptors sensitive to pressure on your skin, for example. The motor division deals with outputs, or signals, causing muscles to contract or relax. Together, these divisions make up the somatic nervous system, which enables you to interact with your external environment. The autonomic nervous system is the manager of your internal environment. It controls activity in structures like your heart and your gut and some endocrine glands (which secrete regulatory hormones), and it governs sweating and the distribution of blood flow. The autonomic nervous system is itself divided into the sympathetic and parasympathetic nervous systems, which have essentially opposite functions. The sympathetic system prepares you for emergency action. It redirects blood from your skin and your gut to your muscles, raises heart rate, dilates air passages to your lungs and increases sweating. These changes help you to run faster or fight more vigorously, and explain why people sometimes go white when they are really angry. The parasympathetic system calms you down: it slows heart rate, increases blood flow to the gut to facilitate digestion, and so on. Your bodily state in part reflects the balance between these two systems.
CENTRAL NERVOUS SYSTEM
The brain sits at the top of the spinal cord like a knotted end on a string or a walnut on a stick, with a smaller knot at the back (the cerebellum – Latin for ‘little brain’) which plays a key role in making movement smooth and efficient. The spinal cord, made up of both axons and ganglia, gives us some essential reflexes. You can withdraw your hand from a fire before the information from your fingers has reached your brain: the spinal circuitry is complex enough to go it alone. It is also complex enough to contribute to other motor sequences, like those involved in walking. Mammalian brains are made in two halves – or hemispheres –again like a walnut. The brain surface as viewed from the side or above is deeply wrinkled. This outer layer is the cortex (plural cortices), which comes from the Latin word meaning ‘bark of a tree’. What this view hides are the numerous subcortical structures. These process sensory input and relay it to appropriate areas of the cortex, or process motor output before relaying it to the spinal cord and from there to the peripheral nervous system.
But the brain should not be thought of as a sort of cognitive sandwich, with sensory information as the input, motor responses as the output, and cognition as the filling. Brain function is much more highly integrated than that. The motor and sensory systems are interactive, and each can directly modify activity in the other, without having to go through a cognitive intermediary. A cluster of cell bodies in the brain might form a blob, or nucleus (plural, nuclei), or be organized into an extended layer like the cortex. These nuclei are often connected by clusters of axons, called fibre bundles. If you cut into a nucleus, or into the cortex, the exposed surface does not appear white, but grey. The term grey matter, sometimes used colloquially, refers to areas that are composed
primarily of cell bodies rather than axons.
We can study behaviour and thought without necessarily knowing anything at all about the nervous system – or how the behaviour is generated. Cognitive psychologists studying slips of the tongue, for example, may not care whether the brain is made up of rubber bands or of neurons. In fact, if you simply count the cells in the brain, neurons are very much a minority group; most of them are non-neuronal, glial cells. [glial cells non-neuronal cells in the brain that provide ‘support’ for the neurons] But rubber bands are rarer still in there. Nevertheless, our interactions with the world around us depend crucially on the activity of the nervous system. Without it, we not only have no senses and no ability to move; we also have no thoughts, no memories and no emotions. These are the very essence of our selves, yet they can be disastrously changed, and even completely erased, by disorders of the nervous system. We can very effectively treat some psychological disorders simply by using words to change the ways in which patients think. But the only generally available palliatives for other conditions, like Parkinson’s disease or schizophrenia, are drug treatments. And some conditions, such as Alzheimer’s disease, are currently untreatable. The more we learn about how the nervous system operates, the better we can understand how it can go wrong. That, in turn, will increase our chances of finding out how to prevent, or even reverse, psychological disorders. If we do not understand the way that the nervous system works, then we, like our forebears throughout humankind’s history, are confined to a passive role as observers and documenters of the effects of nervous dysfunction.
The nervous system has both central and peripheral components. The central part includes the brain and the spinal cord; the peripheral part includes the nerves through which the central nervous system interacts with the rest of the body. ‘Nerve’ is a familiar word and is used in various ways in ordinary conversation. But in psychology we use it specifically to mean a cord of neuronal axons bundled together passing through the human body. We have probably all had the experience of hitting our ‘funny bone’ – the discomfort is due to the compression of the ulnar nerve. Nerves are typically sensory (afferent) – carrying information to the central nervous system from sensory neurons whose cell bodies are located in the periphery of the body – or motor (efferent) – extending out from the central nervous system to the organs and regulating muscular movement or glandular secretion.
The basic unit of the whole of the nervous system is the neuron. Neurons operate alongside various other types of cells, whose activity can be essential to normal neuronal function. Even in the brain, only about 10 per cent of the cells are neurons. Most are glial cells, which fall into several different classes, each with its own function. There are astrocytes, oligodendrocytes (in the central nervous system), microglia and ependymal cells. (The word ending -cyte means ‘cell’.) Glial cells were once thought of as the structural glue (that is what glia means in Greek) that holds the neurons in place, but their roles are proving to be far more complex. For example, astroctyes, which are the most common class, not only provide physical support to the neurons, but also help to regulate the chemical content of the fluid that surrounds the neurons. Astrocytes wrap closely round some types of synapses (the junctions between neurons) and help to remove glutamate (a neurotransmitter substance) from the synaptic cleft (the gap between neurons meeting at the synapse) via an active pumping system. If the pump fails, the system can become reversed, so that excess glutamate is released back into the synapse, which can be fatal to nearby neurons.
Neurons come in many shapes – or morphologies –which give them their different functions. For example, projection neurons have fibres that connect them to other parts of the nervous system. Even within this category, there are many different morphologies, but all projection neurons share some basic similarities. You can think of the neuron as having three essential components. The heart of the neuron is the cell body, where the cell’s metabolic activities take place. Input from other neurons typically comes via the dendrites. These can be a relatively simple tuft of fine, fibre-like extensions from the cell body, or highly complex branches like the twigs and leaves of a tree. The output of the neuron is transmitted via its axon to the dendrites of other neurons, or other targets such as muscles. Axons can be very long, reaching right down the spinal cord, or so short that it is difficult to tell them apart from the dendrites. Nerve cells with such short axons are called interneurons rather than projection neurons, because all their connections are local. Some neurons have just a single axon, although it may still make contact with a number of different target cells by branching out towards its end. Other cells have axons that are split into quite separate axon collaterals, each of which may go to an entirely different target structure.
PERPHERAL NERVOUS SYSTEM
Peripheral nerves are just bundles of axons. They appear as white matter, because most mammalian axons have a white myelin sheath around them, which helps to speed up nerve conduction. Although many neurons have cell bodies located in the central nervous system, there are clusters of cell bodies in the peripheral nervous system too. The simplest type of cluster is called a ganglion (plural, ganglia). The sensory division of the peripheral system deals with inputs from receptors sensitive to pressure on your skin, for example. The motor division deals with outputs, or signals, causing muscles to contract or relax. Together, these divisions make up the somatic nervous system, which enables you to interact with your external environment. The autonomic nervous system is the manager of your internal environment. It controls activity in structures like your heart and your gut and some endocrine glands (which secrete regulatory hormones), and it governs sweating and the distribution of blood flow. The autonomic nervous system is itself divided into the sympathetic and parasympathetic nervous systems, which have essentially opposite functions. The sympathetic system prepares you for emergency action. It redirects blood from your skin and your gut to your muscles, raises heart rate, dilates air passages to your lungs and increases sweating. These changes help you to run faster or fight more vigorously, and explain why people sometimes go white when they are really angry. The parasympathetic system calms you down: it slows heart rate, increases blood flow to the gut to facilitate digestion, and so on. Your bodily state in part reflects the balance between these two systems.
CENTRAL NERVOUS SYSTEM
The brain sits at the top of the spinal cord like a knotted end on a string or a walnut on a stick, with a smaller knot at the back (the cerebellum – Latin for ‘little brain’) which plays a key role in making movement smooth and efficient. The spinal cord, made up of both axons and ganglia, gives us some essential reflexes. You can withdraw your hand from a fire before the information from your fingers has reached your brain: the spinal circuitry is complex enough to go it alone. It is also complex enough to contribute to other motor sequences, like those involved in walking. Mammalian brains are made in two halves – or hemispheres –again like a walnut. The brain surface as viewed from the side or above is deeply wrinkled. This outer layer is the cortex (plural cortices), which comes from the Latin word meaning ‘bark of a tree’. What this view hides are the numerous subcortical structures. These process sensory input and relay it to appropriate areas of the cortex, or process motor output before relaying it to the spinal cord and from there to the peripheral nervous system.
But the brain should not be thought of as a sort of cognitive sandwich, with sensory information as the input, motor responses as the output, and cognition as the filling. Brain function is much more highly integrated than that. The motor and sensory systems are interactive, and each can directly modify activity in the other, without having to go through a cognitive intermediary. A cluster of cell bodies in the brain might form a blob, or nucleus (plural, nuclei), or be organized into an extended layer like the cortex. These nuclei are often connected by clusters of axons, called fibre bundles. If you cut into a nucleus, or into the cortex, the exposed surface does not appear white, but grey. The term grey matter, sometimes used colloquially, refers to areas that are composed
primarily of cell bodies rather than axons.
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