Human Biology Test Revision

The nervous system is one of the body’s two communications systems. Along with the endocrine system, it coordinates all our voluntary and involuntary actions.

The nervous system receives and processes information from sense organs and brings about responses to the information received.

Nerve cells, or neurons, are the basic structural and functional units of the whole nervous system. They are highly specialised cells perfectly designed for rapid communication of messages in the body.

Structure of neurons

Neurons vary in size and shape, but they all consist of a cell body and two different types of extension from the cell – the dendrites and the axon.

Cell body

The cell body contains the nucleus and is responsible for controlling the functioning of the cell. Around the nucleus is cytoplasm containing the organelles that are found in most cells: mitochondria, endoplasmic reticulum, ribosomes and Golgi apparatus.

Dendrites

Dendrites are usually fairly short extensions of the cytoplasm of the cell body. They are often highly branched and they carry messages, or nerve impulses, into the cell body.

Axon

The axon is often a single, long extension of the cytoplasm. It usually carries nerve impulses away from the cell body. Although usually longer than the dendrites, the length of axons varies enormously. Those in the brain may be only a few millimetres long, while the axons that run from the spinal cord to the foot may be a metre or so in length.

At its end, the axon divides into many small branches. Each of these branches terminates at the axon terminal.

Myelin sheath

Most axons are covered with a layer of fatty material called the myelin sheath. The term nerve fibre is used for any long extension of a nerve cell, but usually refers to an axon. Those that have a myelin sheath are called myelinated fibres and those that do not are said to be unmyelinated.

Outside the brain and spinal cord, the myelin sheath is formed by Schwann cells, which wrap around the axon. At intervals along the axon are gaps in the myelin sheath, called nodes of Ranvier.

The myelin sheath has three important functions:

• It acts as an insulator.

• It protects the axon from damage.

• It speeds up the movement of nerve impulses along the axon.

In the brain and spinal cord, the myelin sheath is produced by oligodendrocytes.

Synapses

Nerve impulses have to be passed from neuron to neuron. This usually occurs where the axon terminal of one neuron joins with a dendrite or the cell body of another. This junction is called a synapse.

The neurons do not actually physically touch at the synapse; instead, there is a small gap between them. Messages have to be carried across this gap, which occurs by the movement of chemicals called neurotransmitters.

A similar synapse exists where an axon meets a skeletal muscle cell. This tiny gap is called the neuromuscular junction.

Types of neurons

Neurons can be classified based on their function or structure.

Functional types of neurons

•Sensory (also known as afferent or receptor) neurons carry messages from receptors in the sense organs, or in the skin, to the central nervous system (brain and spinal cord).

•Motor (also known as efferent or effector) neurons carry messages from the central nervous system to the effectors, the muscles and glands.

•Interneurons are located in the central nervous system and are the link between the sensory and motor neurons. Interneurons may also be called association neurons, connector neurons or relay neurons.

Structural types of neurons

This classification is based on the number of extensions from the cell body.

•Multipolar neurons have one axon and multiple dendrites extending from the cell body. This type of neuron is the most common and includes most of the interneurons in the brain and spinal cord as well as the motor neurons that carry messages to the skeletal muscles.

•Bipolar neurons have one axon and one dendrite. Both the axon and dendrite may have many branches at their ends. Bipolar neurons occur in the eye, ear and nose, where they take impulses from the receptor cells to other neurons.

•Unipolar neurons have just one extension, an axon. These types of neurons are not found in humans or other vertebrates, but in insects.

•Pseudounipolar neurons have properties of both unipolar neurons and bipolar neurons. There is a single axon from the cell body, which then separates into two extensions. One extension connects to dendrites, while the other ends in axon terminals. The arrangement of the cell body and axon means that the cell body lies to one side of the main axon. Most sensory neurons that carry messages to the spinal cord are of this type.

Nerve fibres

The axons and dendrites of nerve cells are known as nerve fibres. Outside the brain and spinal cord, nerve fibres are grouped together to form a nerve. Nerve fibres are arranged into bundles held together by connective tissue, with multiple bundles joining together to form a nerve.

Conduction of a nerve impulse

A nerve impulse is an electrochemical change that travels along a nerve fibre. It is described as electrochemical because it involves:

• a change in electrical voltage

• that is brought about by changes in chemicals (specifically, the concentration of ions inside and outside the cell membrane of the neuron).

Electrical charge and potential difference

Electrical charges can be positive or negative. Two positive or two negative charges repel each other. A positive and a negative charge attract each other.

When positive and negative charges come together, energy is released. If a group of positive and negative charges are separated, they have the potential to come together and release energy. The potential, or potential difference, between two places can be measured. It is called the voltage and is measured in volts (V) or millivolts (mV). 

Potential difference across a cell membrane

When some chemical substances are dissolved in water, they break up into electrically charged particles called ions. This happens to some of the substances dissolved in the fluid around and inside cells.

• The fluid outside the cell, the extracellular fluid, contains a high concentration of sodium chloride, and so most of its charged particles are positive sodium ions (Na+) and negative chloride ions (Cl–).

• The fluid inside the cell, the intracellular fluid, has a low concentration of sodium ions and chloride ions. Its main positive ions are potassium (K+), and the negative ions come from a variety of organic substances made by the cell.

Differences in the concentration of ions mean that there is a potential between the inside and the outside of the cell membrane. This potential difference is called the membrane potential. It occurs in all body cells, but is particularly large in nerve and muscle cells. The membrane potential of unstimulated nerve cells, known as the resting membrane potential, can be measured and is about –70 mV.

Ions are unable to diffuse through the phospholipid bilayers of the cell membrane directly. Instead, they move through protein channels. Some channels, called leakage channels, are open all the time; others, called voltage-gated channels, only open when the nerve is stimulated.

Sodium and potassium ions also move across the cell membrane through a carrier protein known as the sodium–potassium pump. The pump moves two potassium ions into the cell for every three sodium ions that are removed. Therefore, there is a net reduction of positive ions inside the cell. This movement is against the concentration gradient and, therefore, is active transport and uses adenosine triphosphate (ATP).

The combination of the location of the ions, the permeability of the cell membrane and the sodium– potassium pump means that there is a net flow of positive ions out of the cell because more potassium ions are diffusing out of the cell than there are sodium ions diffusing into the cell. This, in addition to the negative organic ions inside the cell, results in the inside of the cell being more negative than the outside. This produces a negative resting membrane potential, and the membrane is said to be polarised.

Action potential

If the stimulus to a neuron is sufficient, the signal will be passed along the neuron. This happens due to the opening and closing of voltage-gated channels, which causes the rapid depolarisation and repolarisation of the membrane. This lasts approximately 1 millisecond and is called an action potential.

Depolarisation is the sudden increase in membrane potential. This occurs if the level of stimulation exceeds about 15 mV, or the threshold.

After a short period, repolarisation occurs. The sodium channels close, which stops the influx of sodium ions. At the same time, voltage-gated potassium channels open, increasing the flow of potassium ions out of the cell. This makes the inside of the membrane more negative than the outside and decreases the membrane potential. The membrane is repolarised.

The potassium channels remain open longer than what is needed. This results in the membrane potential dropping lower than the resting membrane potential, and the membrane is hyperpolarised. This process is called hyperpolarisation.

Once the sodium channels have opened, they quickly become inactivated. This means that they are unresponsive to stimulus. Therefore, for a brief period after being stimulated, the membrane will not undergo another action potential. This period, called the refractory period, lasts from when the membrane reaches the threshold of –55mV until it returns to the resting membrane potential of –70mV.

Transmission of the nerve impulse

A single action potential occurs in one section of a membrane. However, it triggers an action potential in the adjacent membrane. This process continues along the length of the neuron and is called a nerve impulse.

Conduction along unmyelinated fibres

In an unmyelinated nerve fibre, depolarisation of one area of the membrane causes a movement of sodium ions into the adjacent areas. This movement stimulates the opening of the voltage-gated sodium channels in the next part of the membrane, which initiates an action potential in that area of the membrane.

The process repeats itself along the whole length of the membrane so that the action potential moves along the membrane away from the point of stimulation.

Transmission along myelinated fibres

In a myelinated fibre, the myelin sheath insulates the nerve fibre from the extracellular fluid. This does not occur at the nodes of Ranvier because the myelin sheath is absent from the nodes. Therefore, where the nerve fibre is surrounded by myelin, ions cannot flow between the inside and outside of the membrane and an action potential cannot form. Instead, the action potential jumps from one node of Ranvier to the next. This ‘jumping conduction’, known as saltatory conduction, allows the nerve impulse to travel much faster along myelinated fibres than along unmyelinated ones.

Size of the nerve impulse 

A nerve impulse that travels along a fibre is always the same size, regardless of the size of the stimulus. A weak stimulus, provided it exceeds the threshold, produces the same action potential as a strong one. This is called an all-or-none response – a stimulus is either strong enough to trigger an impulse, or it is not. The magnitude of the impulse is always the same.

Two things enable us to determine the strength of a stimulus: a strong stimulus causes depolarisation of more nerve fibres than a weak stimulus; and a strong stimulus produces more nerve impulses in a given time than a weak stimulus.

Transmission across a synapse 

The synapse is the very small gap between adjacent neurons. Nerve impulses travel from one neuron to the next by neurotransmitters diffusing across the synapse.

Effect of chemicals on the transmission of nerve impulses

There are many chemicals that influence the transmission of nerve impulses, mostly at the synapse or at the neuromuscular junction. Stimulants such as caffeine and benzedrine stimulate transmission at the synapse. Other drugs, such as anaesthetics or hypnotics, depress the transmission. Venom from certain species of snakes and spiders also affects the neuromuscular junction.

Nerve agents (also called nerve gases) contain organophosphates, which cause the build-up of acetylcholine at the neuromuscular junction. All muscles in the body then try to contract and the loss of muscle control prevents breathing. Organophosphates are also used in some insecticides.

A receptor is a structure that is able to detect a change in the body’s internal or external environment. When a receptor is stimulated, the body is able to respond to the change.

Types of receptors

There are different types of receptors to be able to detect the different types of stimuli.

Thermoreceptors are located in the skin and hypothalamus and respond to heat and cold and allow us to regulate body temperature 

Osmoreceptors are located in the hypothalamus and respond to osmotic pressure, allowing the body’s water content to be maintained  

Chemoreceptors are present in the nose (sensitivity to odours), mouth (sensitivity to taste), and in certain blood vessels, which are involved in the regulation of the heartbeat and breathing  Touch receptors are found mainly in the skin. Some are close to the surface of the skin and are sensitive to very light touch (e.g. lips, fingertips). Other touch receptors are located deeper in the skin and are sensitive to pressure and vibrations.

Pain receptors are stimulated by damage to the tissues, such as from a cut, by poor blood flow to a tissue, or by excessive stimulation from stimuli such as heat or chemicals. The receptors for pain are especially concentrated in the skin and the mucous membranes. They occur in most organs, but not in the brain.

Reflexes 

A reflex is a rapid, automatic response to a change in the external or internal environment. All reflexes have four important properties.

A stimulus is required to trigger a reflex – the reflex is not spontaneous.

A reflex is involuntary – it occurs without any conscious thought.

A reflex response is rapid – only a small number of neurons are involved.

A reflex response is stereotyped – it occurs in the same way each time it happens.

Protective reflexes are present from birth. 

Learnt reflexes 

More complex motor patterns appear during a baby’s development, including reflexes such as suckling, chewing or following movements with the eyes. These innate reflexes are determined genetically.

Some complex motor patterns are learnt and are called acquired reflexes, e.g. Muscular adjustments required to maintain balance while riding a bike.

Both the endocrine system and the nervous system are involved in communication within the body. However, they do not duplicate each other’s roles; rather, they complement and reinforce each other.

The central nervous system (CNS) consists of the brain and spinal cord and is where incoming messages are processed and outgoing messages are initiated.

Protection of the CNS

Three structures protect the CNS:

• bone

• membranes called meninges

• cerebrospinal fluid.

Cranium and vertebrae

The outermost protective layer is bone. The brain is protected by the cranium, the part of the skull that houses the brain, while the spinal cord runs through the vertebral canal, an opening in the vertebrae.

These bones provide a strong, rigid structure to protect the structures underneath.

Meninges 

Inside the bones, and covering the entire surface of the CNS, are three layers of connective tissue forming membranes called the meninges.

The outer layer, the dura mater, is tough and fibrous, and therefore provides a layer of protection for the brain. 

The middle layer, the arachnoid mater, is a loose mesh of fibres.

The inner layer, the pia mater, is far more delicate. It contains many blood vessels and sticks closely to the surface of the brain & spinal cord.

Cerebrospinal fluid (CSF) 

CSF occupies a space between the middle and inner layers of meninges. It also circulates through cavities in the brain and through a canal in the centre of the spinal cord. The CSF is a clear, watery fluid containing a few cells and some glucose, protein, urea and salts.

The CSF has three functions:

Protection: the CSF acts as a shock absorber, cushioning any blows or shocks the CNS may sustain.

Support: the brain is suspended inside the cranium and floats in the fluid that surrounds it.

Transport: the CSF is formed from the blood, and circulates around and through the CNS before eventually re-entering the blood capillaries. During its circulation it takes nutrients to the cells of the brain and spinal cord and carries away their wastes.

Cerebrum 

The cerebrum is the biggest part of the brain and consists of an outer surface about 2–4 mm thick of grey matter known as the cerebral cortex. The grey matter consists of neuron cell bodies, dendrites and unmyelinated axons. Below the cortex is white matter, which is made up of myelinated axons. Deep inside the cerebrum is additional grey matter called the basal ganglia.

The cerebral cortex is folded in patterns that greatly increase its surface area. In this way the cortex contains 70% of all the neurons in the central nervous system. The folding produces rounded ridges called convolutions. The convolutions are separated by either shallow downfolds called sulci or deep downfolds called fissures.

The deepest fissure, the longitudinal fissure, almost separates the cerebrum into two halves – the left and right cerebral hemispheres. Joining the two hemispheres, at the base of the longitudinal fissure, is the corpus callosum.

Each cerebral hemisphere is divided into four lobes – the frontal, temporal, occipital and parietal lobes. Another part of the cerebrum, the insula, is deep inside the brain and is regarded as a fifth lobe.

The cerebral cortex is involved in mental activities such as thinking, reasoning, learning, memory and intelligence; and perception of the senses and the initiation and control of voluntary muscle contraction.

The cortex can be roughly divided into three functional areas.

sensory areas, which interpret impulses from receptors

motor areas, which control muscular movements

association areas, which are concerned with intellectual and emotional processes.

One of the important functions of the cerebrum is memory. The association areas of the cerebral cortex are involved in memory.

Although the two sides of the cerebrum appear to be very similar, close inspection shows that they are not identical. Many specialised functions occur in only one hemisphere.

Corpus callosum

The corpus callosum is a wide band of nerve fibres that lies underneath the cerebrum at the base of the longitudinal fissure. Nerve fibres in the corpus callosum cross from one cerebral hemisphere to the other and allow the two sides of the cerebrum to communicate with each other.

Cerebellum

The cerebellum lies under the rear part of the cerebrum. It is the second largest part of the brain and its surface is folded into a series of parallel ridges. The outer folded part of the cerebellum is grey matter. Inside is white matter that branches to all parts of the cerebellum.

It is responsible for posture, balance and the fine coordination of voluntary muscle movement. All the functions of the cerebellum take place below the conscious level.

Hypothalamus 

The hypothalamus lies in the middle of the brain and cannot be seen from the outside. Although small, the hypothalamus controls many bodily activities, but it is mostly concerned with maintaining homeostasis.

Its functions include the regulation of:

•the autonomic nervous system, including heart rate and blood pressure

•body temperature

•food and water intake

•patterns of waking and sleeping

•contraction of the urinary bladder

•emotional responses, such as fear, anger, pleasure

•the secretion of hormones and coordination of parts of the endocrine system; acting through the pituitary gland, the hypothalamus regulates metabolism, growth, reproduction and responses to stress.

Medulla oblongata 

The medulla oblongata is a continuation of the spinal cord. It is about 3 cm long and extends from just above the point where the spinal cord enters the skull. Many nerve fibres simply pass through the medulla going to or from the other parts of the brain, but it does have an important role in automatically adjusting body functions.

The medulla oblongata contains:

•the cardiac centre, which regulates the rate and force of the heartbeat

•respiratory centres, which control rate and depth of breathing

•the vasomotor centre, which regulates the diameter of blood vessels.

In addition, other centres regulate the reflexes of swallowing, sneezing, coughing and vomiting.

All the centres in the medulla oblongata are influenced and controlled by higher centres in the brain, particularly the hypothalamus.

Spinal cord

The spinal cord is a roughly cylindrical structure about 44 cm long in an adult. It extends from the foramen magnum to the second lumbar vertebra.

The cord is enclosed in the vertebral canal, and inside the ring of bone are the three meningeal layers.

The spinal cord consists of areas of grey matter in the centre and is surrounded by areas of white matter.

The grey matter contains the central canal, which runs the length of the spinal cord and contains cerebrospinal fluid.

The myelinated nerve fibres of the white matter are arranged in bundles known as ascending and descending tracts. Ascending tracts are sensory axons that carry impulses upwards, towards the brain. Descending tracts contain motor axons that conduct impulses downwards, away from the brain.

One of the functions of the spinal cord is to carry sensory impulses up to the brain and motor impulses down from the brain. The second function of the spinal cord is to integrate certain fast, automatic responses called reflexes.

The peripheral nervous system (PNS) takes messages from receptors to the CNS and from the CNS to muscles and glands. It is composed of:

nerve fibres that carry information to and from the CNS

groups of nerve cell bodies, called ganglia, which lie outside the brain and spinal cord.

Types of nerves 

The nerve fibres are arranged into nerves, which arise from the brain and the spinal cord.

Cranial nerves

Twelve pairs of cranial nerves arise from the brain. Most cranial nerves are mixed nerves; they contain fibres that carry impulses into the brain, as well as fibres that carry impulses away from the brain. Fibres that carry impulses into the CNS are called sensory fibres; those that carry impulses away from the CNS are motor fibres. 

Spinal nerves

31 pairs of spinal nerves arise from the spinal cord. They are all mixed nerves containing both sensory and motor fibres. Each nerve is joined to the spinal cord by two roots. The ventral root contains the axons of motor neurons that have their cell bodies in the grey matter of the spinal cord.

The dorsal root contains the axons of sensory neurons that have their cell bodies in a small swelling on the dorsal root known as the dorsal root ganglion.

Divisions of the PNS

The nerves that make up the PNS contain fibres carrying nervous impulses to and from all parts of the body. The PNS is divided into parts, each with a particular function.

Afferent division 

The afferent (or sensory) division of the PNS has fibres that carry impulses into the CNS by sensory neurons from receptors in the skin and around the muscles and joints. These neurons can be further divided into:

• somatic sensory neurons, which bring impulses from the skin and muscles

• visceral sensory neurons, which bring impulses from the internal organs.

Efferent division 

The efferent (or motor) division has fibres that carry impulses away from the CNS. It is subdivided into:

• the somatic division (sometimes called the somatic nervous system), which takes impulses from the CNS to the skeletal muscles

• the autonomic division (autonomic nervous system), which carries impulses from the CNS to heart muscle, involuntary muscles and glands. The autonomic division is further subdivided into the:

– sympathetic division (sympathetic nervous system)

– parasympathetic division (parasympathetic nervous system).

Autonomic nervous system (ANS)

The ANS controls the body’s internal environment and is involved in many of the mechanisms that keep it constant. It usually operates without conscious control and is regulated by groups of nerve cells in the medulla oblongata, hypothalamus and cerebral cortex. Some of the body functions regulated by the autonomic division include:

• heart rate

• blood pressure

• body temperature

• digestion

• release of energy

• pupil diameter

• air flow to the lungs

• defecation

• urination.

The nerve fibres of the ANS make up part of the spinal nerves and part of some of the cranial nerves. They carry impulses to the heart muscle, other muscles of the internal organs and the glands.

The impulse travels along two neurons from the CNS to an organ controlled by the ANS. The first neuron is myelinated and has its cell body in the CNS. The second neuron is unmyelinated and has its cell body in a ganglion. 

The pathway from the CNS to heart muscle, involuntary muscle or glands is an important difference between the autonomic division and the somatic division. Where there are two motor neurons involved in the autonomic pathway, the somatic division has just one motor neuron carrying impulses from the CNS to the effector.