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Nervous system: 5. The central nervous system (CNS) and reflex actions - the reflex arc

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There are various sections to work through, after 1 they can be read and studied in any order.

Sub-index of biology notes on the nervous system

(5) 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 muscle cells, 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.

The same reflex action applies to when your hand accidently touches a hot surface.

1. Your hand touches a hot surface.

2. The pain receptor cells are stimulated by a relatively high temperature well over the body temperature of 37oC.

3. The pain signal is sent along a sensory neurone.

4. These impulses are passed along via synapses to a relay neurone in the spinal CNS.

5. The CNS responds by sending a signal along a motor neurone via synapses to the muscles.

6. When the signal reaches the effector cells, the muscles respond and you quickly withdraw your hand from the source of heat to complete the reflex arc autonomic response.

Note that in a reflex arc the synapses ensure the nerve signal impulses only travel in one direction.

Other examples of the CNS coordinating the response to a situation.

Many animals react instinctively to movement of an object nearby e.g.

The receptors in the animals eye are stimulated.

Sensory neurons transmit the information from the receptors to the CNS-brain.

The CNS-brain decides how to respond.

The CNS sends signals through the motor neurons to the effector cells in the animals muscles (e.g. of arms, legs or wings - depending on the animal).

The muscles contract and the animal responds by moving towards a prey or fleeing from danger to safety.

SUMMARY of some important points with more detail on nervous impulse transmission

(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

The electrical signal connections

Summary of the overall process and some technical terms:

In a neuron, synaptic vesicles (or neurotransmitter vesicles) store various neurotransmitter molecules that are released at the synapse.

The space between two neurons across which the impulse is transmitted by a neurotransmitter molecules is known as a synaptic cleft (see diagram below).

Neurotransmitter receptor molecules are released in the synaptic gap.

An impulse triggers the release of neurotransmitter molecules from vesicles into the synaptic gap and the neurotransmitter diffuses across to bind with receptor molecules, in the membrane of the neurone after the synaptic gap, causing the impulse to continue.

More detailed description of the process:

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 and in one direction only.

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.

(Note: Opioid drug molecules like heroin bind to receptors and control how the body perceives pain via neurotransmitters.)

(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.

Note that in a reflex arc the synapses ensure that impulses travel in one direction only.

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

Keywords, phrases and learning objectives for this part on how the central nervous system and reflex arc

Be able to describe what a reflex arc action is and how it works.

Be able to describe a simple reflex arc in terms of receptor, sensory neurone, relay neurone, motor neurones and effector.

Know that in a reflex arc the synapses ensure that impulses travel in one direction only.

Know what the following are and their function (what they do) sensory neurones, receptor cells, effector cells, motor neurones, the hormone adrenaline, coordinator of central nervous system CNS.



INDEX of biology notes on the nervous system


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