All living things – from the simplest unicellular protozoan to insects, birds and mammals – respond to stimuli such as light and chemicals. Animals have a nervous system, which is an efficient way of detecting changes and transmitting information around the body. Neurobiology is the study of the structure and functioning of the nervous system.

The human nervous system is made of two parts:

  • Central nervous system (CNS) – brain and spinal cord : role of coordination.
  • Peripheral nervous system (PNS)- nerves: connect all parts of the body to the CNS.

Together they coordinate and regulate body functions.

Human sensory receptors

Receptors are the parts of a nervous system that detect a stimulus and initiate a nerve impulse. There are many types of receptor. On the surface of our bodies, we have thermoreceptors in our skin that respond to temperature, photoreceptors in the retina of each eye that respond to light, and chemoreceptors in our noses and on our tongues that respond to chemical substances. We also have internal chemoreceptors. Those in our blood vessels detect the pH or carbon dioxide concentration of our blood to help to regulate our breathing.

Mechanoreceptors are another group of receptors, stimulated by pressure of forces. Some respond to changes in blood pressure, others to the movement of fluid in the inner ear. Whenever we move any part of our body (e.g. a leg to kick a ball, or an arm and fingers to pick up a pen) we need to know exactly where the part of the body is. We receive this information from mechanoreceptors known as stretch receptors, found in muscles. Stretch receptors respond to stretching of the muscles and allow the brain to work out the positions of all parts of the body.

Components of the nervous system

Nerve impulses are carried by neurons to effectors, which are either muscles or glands. The effectors carry out the response. This pathway usually involves the central nervous system (CNS) – either the brain, the spinal cord, or both.

Nerves carry electrical impulses from the central nervous system to all parts of the body, making muscle contract or glands produce enzymes or hormones. Electrical impulses are electrical signals that pass along nerve cells.

Glands and muscles are called effectors because they go into action when they receive nerve impulses or hormones. The biceps muscle is an effector that flexes the arm; the salivary gland is an effector that produces saliva when it receives a nerve impulse from the brain.

The nerves also carry impulses back to the central nervous system from receptors in the sense organs of the body. These impulses from the eyes, ears, skin etc make us aware of changes in our surrounding or in ourselves.

The neuron that carries the impulse from the receptor to the CNS is called the sensory neuron, and the one that carries the impulse from the CNS to the effector is called the motor neuron. These sensory and motor neurons throughout the body make up the peripheral nervous system (PNS). Within the CNS, relay neurons connect the sensory and motor neurons via synapses.

The Central Nervous System (CNS) coordinates the response

The CNS is a coordination centre, it receives information from the receptors and then coordinates a response. The response is carried out by the effectors. For example a small bird is eating some seed when out of the corner of its eye, it spots a cat skulking towards it (this is stimulus). The receptors in the bird’s eye are stimulated. Sensory neurones carry the information from the receptors to the CNS. The CNS decides what to do about it. The CNS sends information to the muscles in the bird’s wings (the effectors) along motor neurones. The muscles contract and the bird flies away to safety.

Nerve cells (neurones)

The central nervous system and the peripheral nerves are made up of nerve cells called neurones. Motor neurones carry impulses from the central nervous system to the muscles and glands. Sensory neurones carry impulses from the sense organs to the central nervous system. Relay neurones also called multi-polar or connector neurones are neither sensory not motor but make connections to other neurones inside the central nervous system.

Each neurone has a cell body consisting of a nucleus surrounded by a little cytoplasm.

Structure of a motor neurone
Structure of a sensory neurone

Branching fibres called dendrites, from the cell body make contact with the neurones. A long filament of cytoplasm, surrounded by an insulating sheath runs from the cell body of the neurone. The cell bodies of the neurones are mostly located in the brain or in the spinal cords and it is the nerve fibres that run in the nerves. A nerve is easlily visible, white, tough and stringy and consists of hundreds of microscopic nerve fibres bundled together. Most nerves will contain a mixture of sensory and motor fibres. So a nerve can carry many different impulses. These impulses will travel in one direction in sensory fibres and in the opposite direction in motor fibres. Some of the nerve fibres are very long. The nerve fibres to the foot have their cell bodies in the spinal cord and the fibres run inside the nerves, without a break, to the skin of the toes or the muscle of the foot. A single nerve cell may have a fibre 1 m long.

The relationship between sensory neurones, the CNS and motor neurones ( A neural pathway)
StructureSensory neuroneMotor neurone
Cell bodyNear end of neurone, in a ganglion (swelling) just outside the spinal cordAt start of neurone, inside the grey matter of the spinal cord
DendritesPresent at the end of neuroneAttached to the cell body
AxonVery shortVery long
DendronVery longNone

The nerve impulse

The nerve fibres do not carry sensations like pain or cold. These sensations are felt only when a nerve impulse reaches the brain. The impulse itself is a series of electrical pulses that travel down the fibre. Each pulse lasts about 0.001 s and travels at speeds of up to 100 ms-1. All nerve impulses are similar; there is no difference between nerve impulses from the eyes, ears or hands.

How a synapse transmits an electrical impulse

Neurones transmit information very quickly to and from the brain, and the brain quickly decides how to respond to the stimulus. At a synapse, a branch at the end of one fibre is in close contact with the cell body or dendrite of another neurone.

Synapse between nerve neurones

When an impulse arrives at the synapse, vesicles in the cytoplasm release a tiny amount of the neurotransmitter substance. It rapidly diffuses across the gap (also known as synaptic cleft) and binds with neurotransmitter receptor molecules in the membrane of the neurone on the other side of the synapse. This then sets off an impulse in the neurone. Sometimes several impulses have to arrive at the synapse before enough transmitter substance is released to cause an impulse to be fired off in the next neurone.

Synapses control the direction of impulses because neurotransmitter substances are only synthesised on one side of the synapse, while receptor molecules are only present on the other side. They slow down the speed of nerve impulses slightly because of the time taken for the chemical to diffuse across the synaptic gap.

Many drugs produce their effects by interacting with receptor molecules at synapses. Heroin, for example stimulates receptor molecules in synapses in the brain, triggering the release of dopamime (a neurotransmitter), which gives a short-lived ‘high’.

Reflexes help prevent injury

Reflexes are rapid, automatic responses to certain stimuli that don’t involve the conscious part of the brain – they can reduce the chances of being injured. For example if someone shines a bright light in your eyes, your pupils automatically get smaller so that less light gets into the eye – this stops it getting damaged. Or if you get a shock , your body releases the hormone adrenaline automatically – it doesn’t wait for you to decide that you’re shocked. The passage of information is a reflex (from receptor to effector) is called a reflex arc.

The reflex arc

One of the simplest situation where impulses cross synapses to produce action is in the reflex arc. A reflex action is an automatic response to a stimulus. A stimulus is a change in the external or internal environment of an organism. It provides a means of rapidly integrating and co-ordinating a stimulus with the response of an effector ( a muscle or a gland) without the need of a thought or a decision. When a particle of dust touches the cornea of the eye, you will blink, you cannot prevent yourself from blinking. A particle of food touching the lining of the windpipe will set off a coughing reflex that cannot be suppressed. When a bright light shines in the eye, the pupil contracts. You cannot stop this reflex and you are not even aware that it is happening. The nervous pathway for such reflexes is called a reflex arc.

One leg is crossed over the other and the muscles are totally relaxed. If the tendon just below the kneecap of the upper leg is tapped sharply, a reflex arc makes the thigh muscle contract and the lower part of the leg swings forward.

The pathway of this reflex arc is traced in Figure across. Hitting the tendon stretches the muscle and stimulates a stretch receptor. The receptor sends off impulses in a sensory fibre. These sensory impulses travel in the nerve to the spinal cord.

The reflex tree jerk

In the central region of the spinal cord, the sensory fibre passes the impulse across a synapse to a motor neurone, which conducts the impulse down the fibre, back to the thigh muscle (the effector). The arrival of the impulses at the muscle makes it contract and jerk the lower part of the limb forward. You are aware that this is happening which means that the sensory impulses must be reaching the brain, but there is nothing you can do to stop it.

The sequence of events in a simple reflex arc

Voluntary and involuntary action

Voluntary actions

A voluntary action starts in the brain. It may be the result of external events, such as seeing a book on the floor, but any resulting action, such as picking up the book, is entirely voluntary. Unlike a reflex action it does not happen automatically, you can decide whether or not you carry out the action.

The brain sends motor impulses down the spinal cord in the nerve fibres. These make synapses with motor fibres, which enter spinal nerves and make connections to the sets of the muscles needed to produce effective action. Many sets of muscles in the arms, legs and trunk would be brought into play in order to stoop and pick up the book, and impulses passing between the eyes, brain and arm would direct the hand to the right place and ‘tell’ the fingers when to close the book.

One of the main functions of the brain is to co-ordinate these actions so that they happen in the right sequence and at the right time and place.

Involuntary actions

The reflex closure of the iris protects the retina from bright light; the withdrawal reflex removes the hand from a dangerously hot object; the coughing reflex dislodges a foreign particle from the windpipe. Thus, these reflexes have a protective function and all are involuntary actions.

There are many other reflexes going on inside our bodies. We are usually unaware of these, but they maintain our blood pressure, breathing rate, heart beat etc. and so maintain the body processes.

Comparison of voluntary and involuntary actions

FeatureVoluntary actionInvoluntary action
Nature– Conscious thought (make decision about making action)
– Free will
– Consciously control skeletal muscles
– Does not involve thought
– Not under the control of the will
– Cannot control the activites
ExamplesIf we want to ask question we raise our handsInvolving
– skeletal muscle (e.g. knee jerk)
– smooth muscles (e.g. peristalsis)
– cardiac muscles (e.g. pumping of the heart)
RoleRespond with the benefit of experienceRespond quickly to avoid danger
Controlled by
Forebrain (Cerebrum):
– coordinates incoming information, initiates impulses sent to the effectors.
– may spontaneously initiate actions without any sensory stimulation
– Hind-brain (cranial reflex action)
– Spinal cord (spinal reflex action) e.g. blinking of the eyes
Speed of actionSlow response, as the cerebrum needs time to “think” before an action is carried outRapid response as the cerebrum is not involved
Response to the same stimulusThe same stimulus may produce various responses e.g. when you are hungry, you may decide to eat or not to eat, or just need to drink waterThe same stimulus always results in the same response (stereotyped response) e.g. the knee jerk reflex


Co-ordination is the way all the organs and systems of the body are made to work efficiently together. If for example the leg muscles are being used for running, they will need extra supplies of glucose and oxygen. To meet this demand, the lungs breathe faster and deeper to obtain the extra oxygen and the heart pumps more rapidly to get the oxygen and glucose to the muscles more quickly.

The brain detects changes in the oxygen and carbon dioxide content of the blood and sends nervous impulses to the diaphragm, intercostal muscles and heart. The co-ordination of the systems is brought about by the nervous system.

The extra supplies of glucose needed for running come from the liver. Glycogen in the liver is changed to glucose, which is released into the bloodstream. The conversion of glycogen to glucose is stimulated by among other things, a chemical called adrenaline. Coordination is by chemicals is brought about by the endocrine system.

The nervous system works by sending electrical impulses along nerves. The endocrine system depends on the release of chemicals, called hormones, from endocrine glands. Hormones are carried by the bloodstream. For example, insulin is carried from the pancreas to the liver by the circulatory system.

The spinal cord

Like all other parts of the nervous system, the spinal cord consists of thousands of nerve cells. All the cell bodies, apart from those in the dorsal root ganglia, are concentrated in the central region called the grey matter. The white matter consists of nerve fibres. Some of these will be passing from the grey matter to the spinal nerves and others will be running along the spinal cord connecting the spinal nerve fibres to the brain. The spinal cord is thus concerned with:

  • reflex actions involving body structures below the neck
  • conducting sensory impulses from the skin and muscles to the brain and
  • carrying motor impulses from the brain to the muscles of the trunk and limbs
The light area is the white matter, consisting largely of nerve fibres running to and from the brain. The darker central area is the grey matter, consisting largely of nerve cell bodies

In Figure below the spinal cord is drawn in transverse section. The spinal nerve divides into two roots at the point where it joins the spinal cord. All the sensory fibres enter through the dorsal root and the motor fibres all leave through the ventral root, but both kinds of fibre are contained in the same spinal nerve. This is like a group of insulated wires in the same electric cable.

Reflex arc (withdrawal reflex)

The cell bodies of all the sensory fibres are situated in the dorsal root and they make a bulge called a ganglion, see Figure below.

Cell bodies forming a ganglion

In even the simplest reflex action, many more nerve fibres, synapses and muscles are involved.

The structure and function of the brain

The brain is the most complex organ in the body. It consists of billions of neurons and hundreds of thousands of different connections, which are responsible for learning, memory and our individual personalities. Each part has a particular function, regulating some automatic processes, such as heart beat and balance, and controlling our speech and ability to reason.

The human brain
  • The cerebral hemispheres are the coordinating centre for learning, memory, language and reasoning. These regions receive information from the sense organs and coordinate and organise motor functions.
  • The hypothalamus controls the autonomic nervous system. It coordinates the endocrine and nervous system by regulating the secretions of the pituitary gland.
  • The cerebellum coordinates movement, posture and balance.
  • The medulla oblongata (brain stem) controls automatic and homeostatic activites such as breathing, swallowing, digestion and heart rate.
  • The pituitary gland has two parts – the posterior lobe stores and releases the hormones oxytocin and ADH from the hypothalamus, while the anterior lobe produces and secretes seven hormones, including FSH and growth hormone, which regulate many of the body’s functions.