by Sandi Mann
The other part of the CNS is the spinal cord. This is a cylindrical bundle of nerves that runs down the middle of the protective bony spinal column from the medulla oblongata in the brainstem to the PNS. The spinal column is made up of bones called vertebrae and although the spinal column is fairly flexible, some of the vertebrae in the lower parts of the spinal column become fused. The spinal cord is surrounded by a clear fluid called cerebral spinal fluid (CSF), that acts as a cushion to protect the delicate nerves.
Spinal cord nerves relay information via millions of nerve fibres from inside and outside the body to the brain and back again; the nerves connecting the spinal cord to the rest of the body are referred to as the peripheral nervous system (PNS).
Spotlight: The spinal cord
The spinal cord is about 45 cm (18 inches) long in men and around 43 cm (17 inches) long in women. It is about the diameter of a human finger.
The peripheral nervous system
The peripheral nervous system (PNS) refers to all the neurons of the body outside the brain and spinal cord (the central nervous system). It is the brain that makes the decisions about how to respond to the information that it receives via the PNS – and the brain uses the PNS to send out responses for action to muscles, glands and so on.
All cells of the nervous system are comprised of neurons (or nerve cells), which are the basic units of the nervous system. The nervous system contains around 10–12 billion neurons and around 80 per cent of them are found in the brain. Neurons contain nerve processes, which extend from the nerve cell body outwards. The nerve processes consist of axons and dendrites, which are able to conduct and transmit signals; axons transmit signals away from the cell body while dendrites, which are shorter than axons, carry signals towards the cell body. There are three kinds of neuron:
• Motor (or efferent) neurons transmit signals from the CNS to organs, glands and muscles.
• Sensory (or afferent) neurons send information to the CNS from internal organs or from external stimuli.
• Interneurons (relay) transmit information between motor and sensory neurons.
‘In proportion to our body mass, our brain is three times as large as that of our nearest relatives. This huge organ is dangerous and painful to give birth to, expensive to build and, in a resting human, uses about 20 per cent of the body’s energy even though it is just 2 per cent of the body’s weight. There must be some reason for all this evolutionary expense.’
S. Blakemore, ‘Meme, myself, I’, New Scientist, 13 March 1999
The other kind of cell within the NS other than neurons are glial cells, which provide structural support to the neurons, insulate them, nourish them and remove waste products.
All neurons share the same structure, described in the table below.
Neuron component Function
Cell body or soma Contains the nucleus
Dendrites Branch out from the cell body allowing connection with other neurons
Axon Transports signals received. Nerves are bundles of axons coming from many neurons.
Myelin sheath Insulates the axon and speeds up rate of conduction of signals down the axon. The myelin is produced by the glial cells (see earlier).
Node of Ranvier These are the gaps formed between myelin sheath cells along the axons. Since fat serves as a good insulator, the myelin sheaths speed the rate of transmission of an electrical impulse along the axon. The electrical impulse jumps from one node to the next and thus speeds the rate of transmission.
Terminal buttons At the end of each axon, they release neurotransmitters (see below). The junction between an axon of one neuron and the cell body or dendrite of a neighbouring neuron is called a synapse.
The somatic and autonomic nervous systems
The PNS consists of two systems: the somatic and the autonomic nervous systems. The somatic nervous system (SNS) transmits sensory information as well as that needed for voluntary movement via the sensory and motor neurons referred to earlier.
The autonomic nervous system (ANS) regulates the internal body functions that are not under voluntary control, such as blood flow, heartbeat, digestion and breathing, and is further subdivided into the sympathetic and parasympathetic nervous systems. The sympathetic nervous system controls activities that prepare the body for action, such as increasing the heart rate and increasing the release of sugar from the liver into the blood – activities generally considered to be part of the fight-or-flight reaction (used to cope with emergency situations – see Chapter 14). The parasympathetic nervous system activates less active functions needed when the body is at rest, such as the production of saliva and digestive enzymes. The ANS produces its effects in two ways: either by direct stimulation of body organs or by stimulating the release of hormones from the endocrine glands (see below).
Neurotransmitters
Nerve cells or neurons communicate and send messages by transmitting nerve impulses. It was once believed that neurons communicated with one another by sending electrical impulses across the gaps (the synapses) between them. However, it is now known that, while the electric impulses travel down the nerves, they do not jump across the gaps between the nerves; instead, neurons mostly communicate across synapses via electrochemical means by releasing chemicals called neurotransmitters. They are produced by glands such as the pituitary and the adrenal glands.
The synapse has three parts to it:
• the presynaptic membrane of the neuron sending the message
• the postsynaptic membrane of the receiving neuron
• the synaptic cleft.
When a nerve signal reaches the presynaptic membrane of the presynaptic neuron, neurotransmitter molecules are released into the synaptic cleft. The neurotransmitter molecules then diffuse and float across the synaptic cleft, so that they can bind to the receptors in the postsynaptic membrane. These receptors are shaped to receive only one type of neurotransmitter, which fits it like a key in a lock. This then sends the signal across to the new neuron.
If not enough neurotransmitters are produced for some reason (e.g. stress, drugs or poor nutrition might affect neurotransmitter production), or if they are blocked from reaching their proper receptors, this can result in certain difficulties and conditions for the individual.
Spotlight: Acetylcholine
The first neurotransmitter was discovered in 1921 by the German pharmacologist Otto Loewi (1873–1961) and was named acetylcholine. It is responsible for stimulating muscle activity (among other functions). Loewi won the Nobel Prize in Physiology or Medicine in 1936 for his work on neurotransmitters (shared with another scientist, Sir Henry Dale).
There are three types of neurotransmitter:
• amino acids
• peptides
• monoamines.
Within each category are different neurotransmitters that are each responsible for specific functions. Neurotransmitters can also be classified as excitatory or inhibitory. Excitatory neurotransmitters (e.g. epinephrine and norepinephrine) are those that excite the neurons and stimulate the brain, while inhibitory neurotransmitters (such as GABA and serotonin) have a calming effect on the brain. Inhibitory neurotransmitters help control the spread of excitation through the nervous system.
There are four main criteria for identifying chemicals as neurotransmitters:
1 The chemical must be synthesized in the neuron or otherwise be present in it.
2 When the neuron is active, the chemical must be released and produce a response in some target.
3 The same response must be obtained when the chemical is experimentally placed on the target.
4 A mechanism must exist for removing the chemical from its site of activation after its work is done.
Spotlight: Epilepsy
Epilepsy is a disorder of the central nervous system characterized by recurrent seizures. Excitation and inhibition of electrical activity in the brain are normally carefully balanced. Neurons will usually fire singly or in small groups in order to achieve
a desired aim and then stop firing. In epilepsy, a seizure happens if too many neurons fire at once so that neuron excitation and inhibition become unbalanced; either there is too much excitation, or too little inhibition. This causes a wave of abnormal activity between neurons or groups of neurons and can lead to other neurons nearby or throughout the brain also firing. If the electrical activity is confined to one part of the brain, this results in a partial seizure. If it spreads through the entire brain, this results in a generalized seizure.
Many of the drugs that treat epilepsy act by either increasing activity in the inhibitory systems or decreasing activity in the excitatory systems. The drugs may affect the neurotransmitters responsible for sending messages, or attach themselves to the surface of neurons and alter the activity of the cell by changing how ions flow into and out of the neurons.
The endocrine system
The endocrine system is made up of glands that produce and secrete hormones – chemical messengers that regulate the activity of cells or organs. The endocrine system is the collection of glands that produce hormones that regulate, among other things, metabolism, growth and development, tissue function, sexual function, reproduction, sleep and mood. The endocrine and nervous systems are intimately linked; the endocrine glands are controlled directly by stimulation from the nervous system. By regulating the functions of organs in the body, these glands help to maintain the body’s homeostasis. The nervous system provides a very fast system to activate specific glands and muscles throughout the body. The endocrine system, however, is much slower-acting, but has very widespread, long-lasting effects. Hormones are distributed by glands through the bloodstream to the entire body, affecting any cell with a receptor for a particular hormone.
The main glands that make up the endocrine system are outlined below:
• Hypothalamus: this is located in the forebrain (see above) and is responsible for the direct control of the endocrine system through the pituitary gland (see below) by secreting important hormones.
• Pituitary gland: this is a small area connected to the hypothalamus of the brain. The pituitary gland is actually made of two completely separate structures: the posterior and anterior pituitary glands.
• Pineal gland: The pineal gland is found just posterior to the thalamus of the brain and produces the hormone melatonin that helps to regulate the human sleep–wake cycle known as the circadian rhythm (for more on the sleep–wake cycle, see Chapter 18).
• Thyroid gland: this is located at the base of the neck and produces three major hormones.
• Adrenal glands: the two adrenal glands are located just above the kidneys and are each made of two distinct layers, each with their own functions: the outer adrenal cortex and the inner adrenal medulla.
• Pancreas: this is a large gland that produces the hormones glucagon and insulin, which are responsible for maintaining blood glucose levels. When the pancreas fails to produce insulin, diabetes results.
• Gonads: the gonads – ovaries in females and testes in males – produce the sex hormones of the body, testosterone in males and oestrogen in females.
Dig deeper
Epilepsy and medications:
http://www.epilepsy.com/learn/treating-seizures-and-epilepsy/seizure-and-epilepsy-medicines
Henry Molaison – the amnesiac we’ll never forget:
http://www.theguardian.com/science/2013/may/05/henry-molaison-amnesiac-corkin-book-feature
Mo Costandi, ‘Phineas Gage and the Effect of an Iron Bar through the Head on Personality’, The Guardian, 8 November 2010:
http://www.theguardian.com/science/blog/2010/nov/05/phineas-gage-head-personality
Fact-check
1 Which of the following is not part of the CNS?
a The hindbrain
b The spinal cord
c The sympathetic nervous system
d The forebrain
2 Which of the following is not part of the forebrain?
a The medulla oblongata
b The thalamus
c The hypothalamus
d The limbic system
3 Which of the following descriptions of the cerebrum is correct?
a The parietal lobe contains the visual cortex
b The occipital lobe contains the auditory cortex
c The temporal lobe contains the somatosensory cortex
d The frontal lobe contains the motor cortex
4 Which of the following statements about the spinal cord is correct?
a The spinal cord connects the midbrain to the hindbrain
b The spinal cord connects the medulla oblongata in the brainstem to the PNS
c The spinal cord is part of the sympathetic nervous system
d The spinal cord contains the visual cortex
5 Which of the following is not a kind of neuron?
a Dopamine
b Motor
c Sensory
d Inter
6 Which of the following is not a component of a neuron?
a Pons
b Dendrite
c Terminal button
d Axon
7 The purpose of the nodes of Ranvier are to:
a Slow impulses down
b Speed impulses up
c Provide insulation
d Release neurotransmitters
8 Which of the following statements about the PNS is correct?
a The sympathetic NS is part of the somatic NS
b The somatic NS controls involuntary processes
c The parasympathetic NS controls voluntary movements
d The sympathetic nervous system controls activities that prepare the body for action
9 Which of the following is part of a synapse?
a The post-synaptic membrane
b The neurotransmitter
c The axon
d The electric impulse
10 Which of the following is not a part of the endocrine system?
a The hypothalamus
b The thyroid gland
c The pancreas
d The kidney
18
Sleep
Sleep, essential for health and functioning, is a naturally recurring state in which we experience skeletal muscle relaxation and an altered state of consciousness. During sleep we are less reactive, experience altered hormone production and are unable to use our voluntary muscles or interact with the environment. Three additional criteria – reversibility (i.e. sleep is not permanent), recurrence (it occurs regularly) and spontaneity (it is not always intended) – distinguish sleep from that of other dormant states such as hibernation or coma. Human sleep occurs as repeating periods, in which the body alternates between two distinct modes known as non-REM (rapid eye movement) and REM sleep.
Why we sleep
Scientists do not know exactly why we sleep but there are three key theories:
• The Repair and Restoration Theory of Sleep: according to this theory, sleep allows our physiological and psychological functioning to be maintained – for example, it is known that during sleep the body increases its rate of cell division and protein synthesis. This could explain why newborns sleep so much – an average of about 16 hours in each 24-hour period; the first few months of a baby’s life are marked by rapid growth so this suggests that sleep allows these functions to occur.
• The Evolutionary Theory of Sleep: also known as the Adaptive Theory of Sleep, this approach suggests that periods of activity and inactivity evolved as a means of conserving energy and that we sleep at a time when being active would be most dangerous for us (if we cannot see predators as easily, or if it is colder so we would use up more resources if we were active). Support for this theory comes from the fact that animals with few natural predators, such as bears and lions, sleep for much longer periods (between 12 and 15 hours each day) than those with many natural predators (four or five hours each day). On the other hand, if sleep protects us from predators (by keeping us curled up and out of harm’s way), you might expect tho
se lower in the food chain to sleep more, so this theory is conflicting.
‘If sleep does not serve an absolutely vital function, then it is the biggest mistake the evolutionary process has ever made.’
A. Rechtschaffen, quoted by E. Mignot in ‘Why We Sleep: The Temporal Organization of Recovery’, PLOS Biology 6/4 (April 2008): 106
• The Information Consolidation Theory of Sleep: this theory suggests that sleep allows us learn and to process information from the day and to prepare for the day to come. Long-term memories are thought to be laid down at night and, indeed, studies of sleep deprivation suggest that a lack of sleep can have a detrimental effect on memory.
Spotlight: Different animals need different amounts of sleep
Adults typically sleep between six and nine hours per night, while bats sleep more than any other animal – a massive 20 hours per day. This could be due to the fact that their food source is only available for a short period of time (at dusk), so they are awake long enough to feed – then sleep to conserve energy. This supports the evolutionary theory that sleep allows us to conserve energy when food is scarce.
We may not know exactly why we sleep but we do have a good idea of how we sleep. The ventrolateral preoptic nucleus (VLPO or VLPN) in the hypothalamus is one area of the brain that is important in the sleep response. Neurons here induce sleep by inhibiting activity in areas of the brainstem that maintain wakefulness. Various neurotransmitters are also involved in the sleep–wake response, including histamine, dopamine, norepinephrine, serotonin, glutamate and acetylcholine. Histamine is sometimes thought to play a particularly vital role as it shows high activity during wakefulness, decreasing activity during non-REM sleep, and its lowest levels during REM sleep (which is why histamine-blocking antihistamine medications cause drowsiness and increase non-REM sleep). Another neurotransmitter, serotonin, is also released in the brain throughout the day; serotonin is used to produce melatonin, sometimes called the ‘sleep hormone’. Melatonin production is inhibited by light (and therefore stimulated by lack of light, or darkness) and jetlagged travellers who have difficulty sleeping may take melatonin to help adjust their sleep cycles to their new location.