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(a) Introduction to the nervous system

The nervous system and hormones enable us to respond to external changes - external stimuli.

The nervous system and hormones also help us to control conditions inside our bodies.

Nerve cells (neurones) are those that communicate with each other in complex systems that include information messaging to all the structures of the body like organs, glands ad muscles etc.

Infancy and childhood are critical periods when most of these vital nerve cell connections are made and they are crucial to our physical and mental development and general wellbeing.

A 'simple' single celled organism can only respond to its immediate environment, but the cells of multicellular organisms must be able to communicate with each other before responding to any internal or external changes.

Therefore complex multicellular systems have evolved nervous and hormonal 'messaging' systems.

The nervous system enables humans to detect changes in their situation, react to their surroundings and coordinate their behaviour responses - which may be deliberate from the conscious mind or autonomic, meaning an automatic response of the nervous system like the reflex arc (described in detail later).

Organisms need to be able to respond to stimuli from changes in their environment, primarily to survive!

Any change in your surroundings eg temperature, visual, sound etc. is potentially a detectable stimulus to one of you sensory organs eg skin, eyes, ears etc. The stimulus might be chemical, light, pain, position, pressure, sound, temperature, touch etc.

You have five different sense organs ears, eyes, nose, skin and tongue which contain receptors (groups of cells) that are sensitive to particular stimuli.

In the receptor cells the stimulus input is converted into an electrical nerve signal - an electrical impulse which is sent to the central nervous system (CNS)

Although complex, the nervous system has two main groups of nerve cell connections:

(i) The central nervous system (CNS) for vertebrates (animals with backbones) consists of the brain and spinal cord.

The CNS is effectively the control centre.

(ii) The peripheral nervous system consists of all the rest of the nerve cells (neurones) that connect to all the rest of the parts of the body.

All of these peripheral nerve connections lead to and from the brain and spinal cord, so all the messaging from anywhere in the body must occur by way of the CNS.

The reflex actions that can happen by virtue of our central nervous system help prevent injury from various sources in potentially dangerous situations - details later.

With our varied receptor cells, we as humans can react to our surroundings and coordinate our behaviour to our best advantage - throughout millions of years all animals exhibit survival adaptations.

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(b) Reminder - comparing two control systems (hormones and nerve cells)

The nervous system and endocrine hormone system are two quite different mechanisms of control in the body, BUT, in principle they function in similar ways AND interact with each other too.

The endocrine system uses chemical molecule messengers (hormones) to communicate information.

See Homeostasis - introduction to how it functions (negative feedback systems explained) 

and Hormone system - Introduction to the endocrine system - role of thyroxine  gcse biology

The nervous system uses electrical impulse messages to communicate information (This page).

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(c) Function and types of receptor cells

Cells called receptors can detect stimuli (changes in the environment outside the organism).

Receptor cells and the stimuli they detect include:

Light receptor cells in the eyes that are sensitive to light, the light energy creates electrical signals that are sent to the brain for 'processing'.

Light receptor cells, like most animal cells, have a nucleus, cytoplasm and cell membrane.

Sound receptors in the ears that are sensitive to sound vibrations in the air

There are also balance receptors in the ears that are sensitive to changes in position and enable us to keep our balance.

The receptors on the tongue are sensitive to chemicals and enable us to taste, and therefore detect, a wide variety of different foods (bitter, salty, sour, sweet chemical stimuli etc.) or anything else in contact with the tongue - good or bad!

The receptors in the nose are also sensitive to chemicals and enable us to smell all sorts of different things which may be a pleasant or unpleasant experience.

The receptors in the skin that are sensitive to touch, pressure, pain and to temperature changes.

Receptors are sometimes grouped and function as one unit i.e. a sense organ.

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(d) The structure and function of the central nervous system (CNS)

All vertebrates, those animals with backbones, have a CNS consisting of the brain and spinal cord.

Information in the form of an electrical signal, from receptors, passes along cells in nerves (neurones) to the brain through the central nervous system (spinal cord ==> brain)  and ...

... the brain then coordinates the response,

... reflex actions are automatic and rapid,

... and often involve sensory, relay and motor neurones.

The 'components' of the nervous system

As mentioned already, the CNS of vertebrates consists of the brain and spinal cord only.

The CNS is then connected to the body by sensory neurones and motor neurones.

Brain reminder!

The spinal cord is a long column of neurones (nerve cells) that runs from the base of the brain down through the spine - physically protected by the bones of the vertebral column.

It is the spinal cord that relays information between the brain and the rest of the body.

The electrical nerve impulses ('information') are relayed via sensory neurones, relay neurones and motor neurones AND pretty fast too in a fraction of a second!

The structure and function of different parts of the nervous system are described below.

Nerve cells, also called neurons/neurones, are, elongated cells that carry electrical signals or impulses all around the body.

The diagram on the right shows the basic structure of a nerve cell or neurone.

Neurones can be very long! The cell body, containing the nucleus, is about 0.1 mm across, but the axon can be a meter long (1000 mm) - this single long nerve cell acts faster in relaying electrical signals than a series of individual smaller cells connected together.

The axon, which carries the electrical signal, is covered in a protective electrically insulating myelin sheath (not shown here, but see other cell diagrams below).

The cell body connects to lots of other neurones.

Some general points about nerve cell (neurone) structure:

(i) All neurones, like most other cells, have a cell body containing the nucleus in a membrane surrounded cytoplasm and other subcellular structures.

The cell body of all neurones is found in the central nervous system (CNS)

The cell body has fine tip extensions called dendrites/dendrons that connect to other neurones and carry the electrical impulses of the nerve signals.

As well as the dendrites for nerve cell communication, neurones have an extended shape so they can carry electrical nerve impulses from one part of the body to another.

(ii) Dendrites (dendrons) are branched protoplasmic extensions of a nerve cell that propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project.

(iii) An axon (nerve fibre), is a long, slender projection of a nerve cell (neuron), in vertebrates, that typically conducts electrical impulses known as action potentials away from the nerve cell body.

(iv) Neurones are relatively long cells which helps the fast electrical impulse transfer between one neurone and another - one long nerve cell transfer is faster than through lots of smaller-shorter cells.

(v) The myelin sheath is a fatty electrically insulating tissue layer around the axon connections between neurones. The myelin sheath also helps speed up the electrical impulse transfer and the axon in the neurone cells carries the electrical signal - if there was no myelin insulation, the signal will be lost.

Axon endings (axon terminals) are button-like endings of axons through which axons make synaptic contacts with other nerve cells or with effector cells.

Receptors - groups of cells that respond to a particular stimulus - e.g. they detects stimuli such as heat, light, pain, sound, taste, smell, pressure (see previous section for more details).

Receptors often form part of a larger complex organs e.g. the taste buds on your tongue or the retina cells of the eye which respond to light.

Receptors start what is known as the 'reflex arc' described in the next section.

Sensory neurones - the nerve cells that transmit the electrical impulse signal from the receptors in the sense organs to the spinal cord and brain of the central nervous system.

Sensory neurone structure

A long dendron carries nerve signals from receptor cells to the cell body which is at the centre of the neurone.

A shorter axon then transfers the electrical impulse from the cell body to the axon terminals that connect to the CNS.

The CNS processes the information and coordinates how the body should respond to the information received from nerve impulses.

Relay neurones - the nerve cells that transmit the electrical signals through the CNS (brain + spinal cord) from sensory neurones to the motor neurones.

Relay neurones have lots of dendrites spreading out that carry the nerve signals from the sensory neurones to the cell body and an axon carries the nerve impulses to the motor neurones.

A synapse (diagram below) is a connection between two neurones eg the thin gap of the junction between a sensory neurone and a relay neurone, it enables the electrical impulse signals from receptors  to reach the spinal cord and brain (i.e. the central nervous system) and on to the effectors.

Between the end of one neurone, and the start of another, chemicals are released in the gap that rapidly diffuse across the gap in the synapse, triggering the transfer the electrical signal.

You can think of the released chemical as 'messenger molecule', and technically it is called a neurotransmitter because it triggers the electrical signal from one nerve cell (neurone) to another.

The transfer of the nerve impulses is quite fast, but the diffusion of the neurotransmitter molecules across the synapse gap does take a short time, so things are slowed down a bit.

Synapse structure

Neurotransmitter - chemicals produced that transmit the electrical signal across a synapse gap between one neurone cell and another (see text and diagram under synapse).

Motor neurones - the nerve cells that transmit the electrical signals from the central nervous system of the brain and spinal cord from one neurone to another to the effector cells of the muscles or glands (see diagram above).

The signals trigger the appropriate response by the muscles (e.g. contract) or gland (e.g. hormone secretion)

Motor neurone structure

A motor neurone has many short dendrites that carry nerve impulses from the CNS to the cell body, then one long axon carries the signal from the cell body to the effector cells.

The branches (dendrites) connect with other nerve cells.

Receptor cells have already been discussed.

Effector cells in the muscles or glands that respond in a variety of ways to the electrical signal from the brain or spinal cord (CNS) - the cells respond to the nervous impulses and cause things to happen.

Nervous impulses cause muscles to respond and contract e.g. from receptors detecting heat or pain.

Nerve impulses cause glands to secrete hormones - chemical messengers to effect a response.

Reflex arc

Effectors complete what is known as the 'reflex arc' described in the next section which fully describes the sequence:

stimulus  ===>  receptor  ===>  CNS coordinator  ===>  effector  ==> response

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(e) The central nervous system (CNS) and reflex actions - the reflex arc

You should know and understand the role of receptors, sensory neurones, motor neurones, relay neurones, synapses and effectors in simple reflex actions.

The CNS coordinates the response when it receives information from the receptors and causes the effectors to respond to the stimulus detected e.g.

(i) suppose you start to cross the road, but your eye detects a car coming along - the visual stimulus.

(ii) Your receptor cells (retina of eye) send nerve impulses to the brain which builds up an image of the environment - including the approaching car.

(ii) The sensory neurones convey the information from the receptor cells of the eye to the CNS.

(iv) The CNS then decides what to do e.g. how you will your brain respond to the stimulus detected.

(v) The CNS then sends impulses via the motor neurones which transmit the 'instructions' from your brain, through the spinal cord, to your muscles.

(vi) Your effectors, that is your muscles, contract and you step back from being hit by the car, job done!

In this example both your brain and spinal cord of your CNS are involved, and you have made a conscious decision to avoid being hit by the car.

BUT, sometimes your body reacts without any apparent conscious thought, but the CNS is still involved either through the spinal cord or an unconscious part of the brain (see next section on the reflex arc).

Reflex actions are automatic responses to stimuli detected by the receptors in the organs of the body.

Reflex actions are rapid automatic responses to particular stimuli, that do NOT involve the conscious part of the brain - they are an important defence mechanism of our body to prevent injury eg

You don't have to think about reflex actions, given a stimulus, they just happen!

Reflex action occur in simpler organisms than humans and in evolutionary terms, they can be considered an aid to survival,

e.g. if in danger, especially if you get a shock - experience a traumatic situation, your body automatically releases the hormone adrenaline to heighten your mental and physical response to the new situation.

If the intensity of light impacting on your eye is too great, your pupil automatically gets smaller to allow less light. In a dimly lit room, the opposite response occurs and your pupil widens to let more light in.

If something hot touches your skin, on feeling pain you immediately try to recoil from the heat source eg on burning your hand, the muscles rapidly contract to take your hand away.

A baby grips a finger placed near its hand - a grasping reflex.

In these reflex action situations, not involving the conscious brain functions, the transfer of information from the receptor to the effector is called a reflex arc.

Know and understand that in a simple reflex arc action from a receptor to an effector - by way the spinal cord or an unconscious part of the brain):

A stimulus detected by receptors (receptor cells) causes impulses from a receptor to pass along a sensory neurone (nerve cell) to the central nervous system.

At a nerve junction (synapse) between a sensory neurone and a relay neurone in the central nervous system, a chemical is released that causes an impulse to be transmitted by a relay neurone,

A chemical is then released at the synapse between a relay neurone and motor neurone in the central nervous system, causing impulses to be sent along by a motor neurone to the organ (the effector) that brings about the response (of the effector cells).

The effector is either a muscle or a gland, a muscle responds by contracting or a gland responds by releasing (secreting) chemical substances called hormones.

 The central nervous systems decides what is to be done depending on what stimulus is received

Examples of reflex arc responses:

Muscles in your arm may contract to withdraw your hand from a heat source, sharp point or wasp/bee sting!

Glands may secrete a particular hormone in response to a particular stimulus eg adrenalin in a 'flight response' from a dangerous situation.

The pupils in  your eyes respond by decreasing/increasing in size if the light level is too high/low.

Summary of the reflex arc sequence via the central nervous system:

stimulus  ===>  receptor  ===>  coordinator  ===>  effector  ==> response

and in a little more detail ...

stimulus ==> receptor cells ==> sensory neurone ==> synapse ==> relay neurone and synapse in CNS (spinal cord or unconscious brain) ==> motor neurones ==> effector cells/organ => response

Note the three neurones in the reflex arc do NOT link physically, there is a gap, the synapse, between each pair enabling lots of neurones to be connected together.

The reflex arc action is automatic and fast, no thinking involved - doesn't involve the conscious brain, just a rapid automatic response on the part of your body!

Another good example is when facing and experience a threat situation! When an insect bites your hand, the reflex arc goes into action and your body muscles (e.g. in your arm) rapidly withdraw your hand from the threat - descriptive details to go with the diagram above, are set out below. CNS = central nervous system.

(i) Stimulus A stimulus is detected by receptor cells. The pain receptor cells in your skin are stimulated by the insect bite! Its the same 'painful' sequence as if your hand touches a hot surface. (ii) Sensory neurones The receptor cell response triggers a response by sensory neurones. The stimulated receptor cells cause the sensory neurones to send electrical nerve signals to the relay neurones in the CNS - impulse transmission to the spinal cord. (iii) Nerve impulse transmission by relay neurones and synapses When the nerve impulses reach a synapse (gap) between a sensory neurone and a relay neurone in the CNS (or between a relay neurone and a motor neurone), they trigger the release of a chemical (neurotransmitter molecules) causing the impulse to be sent across the gap (diagram below). In other words, when the electrical impulse reaches the end of the 1st neurone, it triggers the release of chemical transmitter molecules into the gap and they diffuse across the gap and bind to receptor sites on the next neurone. This triggers the 2nd neurone to re-transmit the nerve impulse signal. So, the synapses allow the relay neurones in the spinal cord to transmit the nerve impulse from the sensory neurones to the motor neurones. Although neurones themselves transmit the impulses quickly because of their electrical charge nature, synapses do slow down the transmission of impulses because the diffusion of the neurotransmitter molecules takes a little time. However, the overall process is fast, the average time for 'reflex arc' reaction to take place is 0.25 seconds, 0.17 s for audio stimulus and 0.15 s for a touch stimulus. (iv) Motor neurones - pain experienced - decision automatically made! The motor neurons convey the response signal to the effector cells In this case the biceps muscle cells of your arm, but it could be to a gland to secrete hormones. The spinal cord of the CNS processes the nerve signals and starts the response 'procedure'. When the impulses reach a synapse between a relay neuron in the CNS and a motor neurone they trigger the release of a chemical (neurotransmitter) causing the impulse to be sent along motor neurones. Note that other neurones in the spinal cord via synapses also send a nerve impulse message to your brain after your hand withdraws - which is when you actually experience the pain of a bite or hot surface, but everything is so fast that your hand effectively withdraws at the same time as you feel the pain. (v) Effector response The motor neurone signals triggers the response of effector cells. On receiving the nerve signal from the motor neurones, the effector cells act i.e. your muscles contract to produce the automatic response - the rapid recoil of your arm and hand from the vicinity of the insect or hot surface. If you experience danger, the body's adrenal gland responds by secreting the hormone adrenaline - makes you more alert and increases metabolic rate, particularly in the muscles. and this is how a reflex arc works and its faster than normal conscious decision making processes BECAUSE you don't have to think about it !!!! (procrastination is NOT part of a reflex arc action!)

For a detailed description of the eye's iris reflex action see

 The eye - structure, function, reflex action, vision defects and correction

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(f) Simple physical response tests - how fast is your reaction time!

Copied and re-edited from Reaction times and vehicle stopping distances  gcse physics revision notes

Simple reaction time experiments

Your reaction time to a situation may be typically 0.2 to 0.8 seconds when fully alert. However your reaction time can be affected by  tiredness, feeling unwell, drugs, alcohol, in other words anything that affects the speed of your brain function.

You can conduct quite simple experiments to test your reaction time to a particular situation. However, since the reaction time is too short.

(i) Computer screen reaction test - responding as quickly as possible to something appearing on the screen.

In this situation, the computer software generates something up on the screen and your click the mouse or tap the keypad in response to the visual (or sound?) stimulus.

The computer automatically times your response by monitoring your contact with the keyboard or by clicking the mouse - its more accurate, especially as it can measure reaction times in milliseconds.

Computer generated stimuli give more accurate response reaction times than e.g. the dropped ruler experiment described in section (b) which potentially involves human error - computer experiments avoid the possibility of the person anticipating when the event is to happen e.g. reading the body language of someone dropping the ruler in experiment (b) described below.

I've quickly written an extremely simple computer programme to test your response to a X appearing on the screen.

Response time test: It probably only works on Microsoft platforms, and maybe not all of them?

Your anti-virus protection might query it, because it is a .exe file, but its written with compiled BBC BASIC and should not pose any threat. Unfortunately I never learned to write in a multi-platform professional computer programming language, but I'm not exactly short of website projects!

 

(ii) Catching a falling object test

Fraught with human error, but a bit of classroom fun!

You get someone to hold a ruler vertically, with thumb and first finger, above someone else's hand, who is ready to catch it with their thumb and first finger.

First image on the right. The ruler should be held at the top of the scale and steady hands from both people.

The catching person should have the middle of their thumb and finger adjacent to zero on the cm scale - squat down to make sure you are reading the scale horizontally.

Then, without warning, the person holding the ruler, lets go of it. The second person has to react as fast as possible and catch the dropped ruler between their thumb and first finger.

Second image on the right.

The longer the distance d, the slower your reaction time!

When caught, you then read how far the ruler as fallen by taking the reading, to the nearest centimetre, from where the middle of their thumb and finger are.

You repeat the experiment a number of times to get an average, but its not a particularly accurate experiment.

You need to have steady hands and not let the ruler wobble about or fall at an angle other than vertical.

Controlling variables - fair test criteria:

You should drop the ruler from the same height each time the experiment is performed.

You should also use the same ruler and the same hand to catch the ruler.

Use the same person/people dropping the ruler and catching it though, obviously, you can compare one person's results with another.

The slower your response time, the further the ruler falls before being caught.

You might repeat the experiment by having e.g.

having some background distractions - a group of people talking nearby, or somebody trying to engage you in conversation or music playing,

or taking a caffeinated drink like coffee or cola to act as a stimulant. - a drug that speeds up neural activity in the central nervous system.

 

Extension of experimental results

You can pool class results and produce a histogram of number of pupils versus equally spaced reaction times.

You can do the same thing with a computer screen test too.

You can investigate the effect of stimulant like caffeine in coffee.

i.e. do the test 10 times, have a rest, drink a cup of coffee and later repeat the test.

You should find that you a bit faster, a smaller response time because caffeine is a central nervous stimulant. It makes you more alert.

 

How to calculate the response time from your results

There is a simple formula you can use to calculate the actual reaction time (t in s) from the distance the ruler falls (d in m), under the acceleration due to gravity (a = 9.81 m/s²).

t = √(2d / a) =  √(2d / 9.81)

e.g. if the ruler falls 10 cm (0.10 m), reaction time = √(2 x 0.1 / 9.81) = √ = 0.14 s

Where does the reaction time formula come from?

Don't worry, you don't have to know this for either your GCSE/IGCSE biology or physics exams!, but here is the derivation for my own satisfaction and perhaps some keen students too?

KEY: a = acceleration (= g = 9.8 m/s²), u = initial velocity (m/s), v = final velocity (m/s), t = time (s), d = distance (m)

If a body is moving with an initial velocity of u and accelerates in a uniform manner (constant acceleration a),

the increase in velocity in a time is given by:  t = at.

Therefore the final velocity v after time t is given by

(equation 1) v = u + at

Now, if a body is moving with uniform acceleration its average velocity is equal to half of the sum of the initial velocity u and final velocity v.

average velocity = (u + v) / 2

but from equation (1), v = u + at, and substituting in the above equation gives

(equation 2) average velocity = (u + u + at) / 2 = u + ½at

Now the distance d, moved (displacement) = average velocity x time  (remember v =  ∆d / ∆t)

so, d = (u + ½at) x t

and, multiplying out gives (3): d = ut + ½at²

now in the experiment, u = 0, so the equation simplifies to (4): d = ½at²

this can now be rearranged to give equation (5): t = √(2d / a)

which is the equation you can use to calculate your actual reaction time from how far the ruler fell before you stopped it.

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See also The brain - what the different parts do and the dangers if damaged gcse biology revision notes

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Any practical work and investigations you did should also be revised

(which should also be revised, helps in understanding 'how science works' and context examination questions):

reaction times – measuring reaction times using metre rules, stop clocks or ICT,

using forehead thermometers before and after exercise,

demonstrating the speed of transmission along nerves by candidates standing in a semi-circle and holding hands and squeezing with eyes closed,

designing an investigation to measure the sensitivity of the skin,

demonstrating the knee jerk reaction,

investigation to measure the amount of sweat produced during exercise,

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General HUMAN BIOLOGY revision notes

See also cell biology index above

Introduction to the organisation of cells => tissues => organs => organ systems (e.g. in humans)

Examples of surfaces for the exchange of substances in animal organisms   gcse biology revision notes

See also Enzymes - section on digestion and synthesis  gcse biology revision notes

The human circulatory system - heart, lungs, blood, blood vessels, causes/treatment of cardiovascular disease

Homeostasis - introduction to how it functions (negative feedback systems explained)  gcse biology revision notes

Homeostasis - control of blood sugar level - insulin and diabetes  gcse biology revision notes

Homeostasis - osmoregulation, ADH, water control, urea and ion concentrations and kidney function, dialysis

Homeostasis - thermoregulation, control of temperature  gcse biology revision notes

The brain - what the different parts do and the dangers if damaged gcse biology revision notes

An introduction to the nervous system including the reflex arc  gcse biology revision notes

Hormone systems - Introduction to the endocrine system - adrenaline & thyroxine hormones  gcse biology revision

Hormone systems - menstrual cycle, contraception, fertility treatments  gcse biology revision notes

Respiration - aerobic and anaerobic in plants and animals.  gcse biology revision notes

Keeping healthy - communicable diseases - pathogen infections   gcse biology revision notes

Keeping healthy - non-communicable diseases - risk factors for e.g. cancers   gcse biology revision notes

Keeping healthy - diet and exercise  gcse biology revision notes

Keeping healthy - defence against pathogens, infectious diseases, vaccination, drugs, monoclonal antibodies

See also Culturing microorganisms like bacteria - testing antibiotics/antiseptics  gcse biology revision

Food tests for reducing sugars, starch, proteins and lipids  gcse biology revision notes

The eye - structure and function - correction of vision defects  gcse biology revision notes

Optics - lens types (convex, concave, uses), experiments, ray diagrams, correction of eye defects (gcse physics)