4.7 THE NERVOUS SYSTEM
1. Responses to all stimuli involve the reception of information and its transfer from the receptor to an effector via the nervous system.
Neurones
-detect stimulants in the environment
-transmit signals
- integrate and process information
-activate appropriate responses in other body parts.
2. Effectors are either muscles or glands.
The effectors can either be muscles or glands
3. Candidates should be able to draw a labelled diagram of a section through the spinal cord; central canal, grey matter, white matter, dorsal root, ventral root, sensory neurone, dorsal root ganglion, connector neurone, motor neurone, effector, meninges.
White Matter – Myelin Sheaths. Grey Matter – Cell bodies. Dorsal Root – Receptors. Dorsal Root Ganglion –
Sensory cell body, Central Canal – Cerebrospinal fluid (CSF)
4. The reflex arc is the basis for protective involuntary actions.
A reflex arc is a rapid involuntary response resulting from nervous impulses initiated by a stimulus. The action is involuntary as the brain is not involved. Reflex arcs are mainly protective in function. However Blinking,
Coughing and Swallowing are reflexes.
5. The reflex arc is related to nerve pathways involved in voluntary actions via ascending and descending tracts in the spinal cord.
E.g.
Stimulus – Hot Surface
Receptor – Temperature and pain receptors in the skin
Sensory Nerve – sends impulse to spinal column
Relay Neurone – connects sensory to motor
Motor Neurone – Sends impulse to the effector
Effector – Muscle
Response – Move arm away from the surface as muscles contract.
Sometimes the impulses are sent to the brain via ascending nerve fibres that originate at synapses in the grey matter of the spinal chord. The brain may store this information, or it may relate it to other sensory information, which may modify the response to a certain action.
6. In simple organisms, such as Hydra, the sense receptors respond to a limited number of stimuli and so the number of effectors is small. Their nerve net system consists of simple nerve cells with short extensions joined to each other and branching in a number of different directions.
Simple organisms such as the hydra don’t have a nervous system but have a nerve net. This nerve net is simple compared to the mammalian structure, and they have no brain or true muscles. This network or nerves spreading across the body have a limited number of stimuli. The nerve nets connect sensory photoreceptors and touch-sensitive nerve cells located win the body wall and tentacles. The simple nerve cells with short extensions are all joined to each other and branching in a number of different directions. The transmission of nerve impulses is slow.
7. In mammals there are three types of neurones: sensory, motor and relay
(connector).

8. Candidates should be able to draw a labelled diagram of a mammalian motor neurone; dendrites, cell body, nucleus, axon, myelin sheath, Schwann cell, nodes of Ranvier, nerve/axon endings.
9. Candidates should be able to describe the functions of; dendrites (carry information towards the cell body), cell body, nucleus, axon, myelin sheath, Schwann cell, nodes of Ranvier, nerve/axon endings.
Dendrites –receive impulses and carry towards cell body
Myelin Sheath – Electrical insulator (speeds up impulse)
Node of Ranvier – Important in impulse transmission
Axon – Transmits impulse from the cell body
Axon Terminals – Boutons, Where Synapses arise
Schwann Cells – produces a large amount of plasma membrane which is wrapped around the axon in tight spiralled layers.
10. Microelectrodes and cathode ray oscilloscopes are used to measure the potential difference across the membrane of 'giant' axons.
The typical electrical charge associated with a typical nerve impulse is around 50mV. These impulses though small can be detected by using Cathode Ray Oscilloscopes and a pair of Microelectrodes. They measure the speed and magnitude of impulses and can analyse their patterns.
11. The resting potential is the potential difference between the inside and the outside of a membrane when a nerve impulse is not being conducted.
NO impulse being conducted – The Potential difference between the inside and outside of the membrane.
12. Resting potentials are typically minus values, the minus indicating the inside is negative with respect to the outside. The membrane is said to be polarised.
13. The sodium-potassium exchange pumps maintain the concentration and an uneven distribution of sodium ions and potassium ions across the membrane.
The sodium-Potassium pump moves 3 Na+ to the outside of the cell and 2 K+ into the cell. This requires ATP
14. Potassium (K
+
) and organic anions (COO
-
) are higher inside the neurone while the concentration of sodium (Na
+
) is higher outside. The membrane is more permeable to K
+
than any of the others.
Living Cells maintain an uneven distribution of inorganic ions across the membrane. Inside, the axon has a high concentration of K+ and low concentration of Na+. Whereas the outside is the opposite.
15. Since the concentration of K
+ is higher than outside they diffuse out. This outward movement of positive ions means that the inside becomes slightly negative.
However, concentration gradients encourage diffusion in the opposite direction that that stated above. The
K+ ions diffuse out more readily than the Na+ in. This leads to the inside being negative relative to the outside.
16. Nerve impulses are due to changes in the permeability of nerve cell membrane to K
+ and Na
+ which leads to changes in the potential difference across the membrane and the formation of an action potential.
The changes can be –Mechanical
-Electrical
-Chemical
17. Suitable stimulation of an axon results in change of potential across the membrane from a negative inside value of about –70mV to a positive inside value of +40mV. This change is called an action potential. The membrane is said to be depolarised.
18. The action potential is the result of a sudden increase in the permeability of the membrane to Na
+
. This allows a sudden influx of
Na
+ which depolarises the membrane.
At the site of the stimulus there is a brief change in permeability to
Na+ ions. Na+ channels open. The membrane is now more

permeable to Na+ than K+ and so Na+ diffuses INTO the cell down the concentration gradient. This disturbs the resting potential and the inside becomes positive relative to the outside.
19. A fraction of a second after this, depolarisation the K
+ diffuse out and repolarises the membrane. There is an overshoot of K
+ leaving as the K
+
/Na
+ pump restores the ionic balance. When a certain amount of Na+ move into the cell and the inside reaches a particular voltage (-50mV) the Action Potential arises. When the Action Potential reaches its peak, the Voltage Gated Sodium Channels close, so no more ions enter. Repolarisation is when
Voltage Gated Potassium Channels open and Potassium leaves the cell down its concentration gradient.
The Ions are then in the wrong places for another action potential to occur, so the Na+/K+ pump restores the original positioning by moving Na+ out and K+ in by active transport.
This is called the refractory period during which another action potential cannot be generated so ensuring a unidirectional impulse and limiting frequency. These points should be applied to the oscilloscope trace.
The membrane repolarises in two phases: - Absolute and Relative. Absolute occurs whilst the action potential is occurring. During this period, no more impulses can be generated. Relative occurs during the restoration of the ionic balance across the membrane, brought about by the Na+/K+ Pumps. During this stage an Action
Potential can be generated if the stimulus is strong enough.
20. The Na
+
/K
+ pumps and the sodium and potassium channels are transmembrane proteins.
Transmembrane Proteins
21. Changes in the intensity of the stimulus do not affect the size of the action potential and below a threshold intensity no action potential takes place. This is the ' all or nothing law'.
The size of the impulse is independent of the size of the stimulus. If the stimulus is below a certain threshold no action potential is initiated. If it exceeds the threshold, and action potential IS initiated.
22. Propagation of the nerve impulse involves the stimulation of the next part of the membrane by local electric currents which cause depolarisation.
Propagation is when the impulse is passed to the next part of the cell. During depolarisation the inside is positive relative to the outside. Whilst one portion is depolarised, depolarisation of the next portion is initiated. There is a series of electrical currents propagated along the axon. This means that the impulse can only travel in one direction. As the impulse is progresses, the action of the Na+/K+ pump causes the neurone to be repolarised behind the impulse.
23. Myelination speeds up the rate of transmission of impulses by increasing the distance over which the local currents can bring about depolarisation. The speed of conduction is also affected by the diameter of the axon.
Impulses are generated at a faster rate in myelinated axons because less of the axon is used. The impulse cannot pass through the myelin, and so has to jump from one node of ranvier to the next. This is called
Saltatory Conduction.
The diameter of the axon also contributes to the rate of transmission. If the diameter is doubled the velocity of the impulse increases.
Temperature can also contribute to rate. At lower temperatures the rate is slowed, and as the temperature increases the rate increases. However Pump can be denatured at higher temperatures.
24. Candidates should be able to draw a labelled diagram of a synapse; synaptic knob, synaptic vesicle, mitochondria, pre-synaptic membrane, synaptic cleft, post-synaptic membrane.

25. Transmission of the impulse across the synapse is chemical rather than electrical. The chemical, or neurotransmitter, is enclosed in synaptic vesicles at the end of the axon before the synapse. The arrival of a nerve impulse depolarises the presynaptic membrane of this axon and calcium ions rush in. This influx of calcium causes the vesicles to fuse with the presynaptic membrane and releases their contents into the synaptic cleft. The neurotransmitter diffuses across the cleft and depolarises the postsynaptic membrane, propagating the impulse along the axon of the adjacent nerve cell.
When the impulse reaches the end of the Axon it depolarises the presynaptic membrane, causing Voltage gated Calcium channels to open. Calcium Ions diffuse into the presynaptic terminal and cause storage Synaptic Vesicles containing Neurotransmitter to fuse with the plasma membrane (exocytosis). The Neurotransmitter is then released into the Synaptic Gap and diffuses across the gap to the postsynaptic membrane. Here, it binds with protein receptor sites in the membrane and causes the Sodium Channels to open – restarting the depolarisation process.
The Refractory Period and the Synapse stop impulses going the wrong way (only the post synaptic knob has the correct binding sites for the neurotransmitter)
26. The most common transmitter substances are acetyl choline and noradrenaline.
Acetylcholine
Noradrenaline
27. Candidates should be able to describe the role of cholinesterase in synapses.
Following the release of neurotransmitter, the substance is quickly degraded by enzymes such as
Cholinesterase. The products of this breakdown are Recycled by the neurone into new vesicles ready for when the next impulse arrives. The recycling process needs energy and this is where there are many Mitochondria
28. Candidates should be able to describe the effects of chemicals on synaptic transmission as illustrated by organophosphorus insecticides acting as cholinesterase inhibitors. Psychoactive drugs such as cocaine and cannabis affect synapses in the brain.
Stimulants – Stimulate the sympathetic nervous system
Depressants – Inhibit passage of pain etc.
Organophosphates block cholinesterase, meaning the neurotransmitters aren’t broken down and so causes repeated firing of the nerve impulse. The Nervous system becomes overactive.
Psychoactive drugs such as Cocaine and Cannabis work in a similar way, but they block the break down of neurotransmitter Dopamine.
Drug dependency – increasing tolerance to drug, more drug has to be taken for the same effect.
Physical dependency – changes in the structure and the way the neurones of the brain work – Withdrawal symptoms.
Psychological dependency – constantly crave the drug in question.
E.g. Alcohol
29. Plants are responsive to the environment. However, their responses are slow because coordination is achieved by hormones.
30. Photoperiodism describes the influence of relative periods of light and darkness on flowering.
Plants are Long Day, Short Day or Day neutral.
31. Phytochrome is the photoreceptor responsible for absorbing light. The photoperiodic stimulus is detected by the leaves of a plant.
A blue green pigment called Phytochrome in leaves is responsible for the control of flowering in some plants via the hormone Florigen.
Phytochrome has 2 forms
-PR – absorbs Red Light
-PFR – Absorbs Far Red Light

In the Light PR is converted to PFR
In the Dark PFR is converted to PR
32. Flowering plants are divided into day neutral, long-day and short-day plants according to their photoperiodic requirements prior to the production of flowers.
Plants measure day length by the amount of phytochrome existing in each of the two forms.
Long Night Plants (Short Day)– More than 14 Hours of Darkness
More PR (RedLight) - Causes Florigen to be released.
However in these Long Night (Short Day) plants, if the Dark period is interrupted by a flash of red light then flowering is inhibited, (but this can be reversed by a flash of far red light) LOW LEVEL OF PFR
Short Night Plants (Long Day) – less than 10 hours of Night
more PFR – Causes Florigen to be released
Short Night plants will also flower if they are in Long Nights if the Night is interrupted with a flash of Far Red light. Breaking the night up into 2 short nights. HIGH LEVELS OF PFR.
Day Neutral plants flower without reference to the day length.