An introduction to the nervous system

the reflex arc and response time experiments

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

 This page will answer many questions e.g.

 What do we mean by a sensory organ?

 What are your five sense organs?

 How are signals from sensory organs sent to the brain?

 What is a synapse? What is a sensory neuron?

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



Introduction

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.

The nervous system enables humans to react to their surroundings and coordinate their behaviour.

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)

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

The nerve cells (neurones) and connections are distributed to all parts of the body.

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

Note:

A single celled organism can only respond to stimuli from its immediate environment.

However, the cells of a multicellular organism must be able to communicate with each other.

To do this, multicellular organisms have evolved nerve and hormone communication systems.

With our varied receptor cells, we as humans can react to our surroundings and coordinate our behaviour to our best advantage.


Receptor cells

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

Receptors 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.


The central nervous system (CNS)

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.

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

Nerve cells are called neurons/neurones, 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.

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

Some general points about nerve cell structure:

(i) All neurons, like most other cells, have a cell body with a nucleus in a membrane surrounded cytoplasm and other subcellular structures. The cell body has extensions called dendrites/dendrons that connect to other neurones and carry the electrical impulses of the nerve signals.

(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 chord and brain of the central nervous system.

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.

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

Lots of dendrites spreading out from the relay neuron carry the nerve signals from the sensory neurones to the motor neurones.

Synapse - 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 (ie 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.

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 through the central nervous system from the brain via the spinal cord to the effector cells of the muscles or glands from one neurone to another (see diagram above).

Many short dendrites 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.

Receptors have already been discussed.

Effectors - the muscles or glands that respond in a variety of ways to the electrical signal from the brain - they respond to the nervous impulses.

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.

Effectors complete what is known as the 'reflex arc' described in the next section.


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.

Reflexes 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

The transfer of information in a reflex action, from a receptor to an effector, is called a reflex arc.

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.

In these 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 cells => sensory neurones + synapses => relay neurones + synapses in CNS (spinal chord or unconscious brain) => motor neurones + synapses  => effector cells/organ => response

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.

(i) Stimulus

The pain receptor cells are stimulated by the insect bite!

(ii) Sensory neurones

The sensory neurones send electrical nerve signals to the relay neurones in the CNS.

(iii) Transmission - synapses

When the impulses reach a synapse between a sensory neurone and a relay neurone in the CNS they trigger the release of a chemical (neurotransmitter) causing the impulse to be sent along relay neurons.

Note that a relay neurones connect sensory neurones to a motor neurones.

(iv) Pain experienced - decision automatically made!

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.

(v) Motor neurones

The motor neurons convey the response signal to the effectors, in this case the muscle cells of your arm.

(vi) Effector response

The effectors 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.

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!)

See also the iris reflex action The eye - structure, function and vision defects GCSE biology revosion notes


Comparing nerve and hormone functions

Hormones effectively act as 'chemical messages' to trigger particular biochemical reactions and their effects are ..

more general around the body, but tend to affect particular cells in particular organs,

and relatively long-lasting compared to eg the fast but short-term nervous impulses and responses of a reflex arc.

Compared to the hormone system of response and control in the body, nerve signals are electrical (not chemical), the nerves act very fast - a short burst of a nervous impulse for a short time, acting from one precise area to another in the body.


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,


 Simple physical response tests

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.

(a) 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!

 

(b) 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, 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.


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


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