Nerves(a) Outline the roles of sensory receptors in mammals in converting different forms of energy into nerve impulses; Sensory receptors detect changes in our environment. They are energy transducers. Each sensory receptor is adapted to detect a change in a particular form of energy. When a stimulus is detected, the receptors convert the external or internal stimulus into an electrical signal a nerve impulse.

(b) Describe, with the aid of diagrams, the structure and functions of sensory and motor neurones; When the stimulus is detected and the energy converted into a depolarisation of the receptor cell membrane, the impulse must be transmitted to other parts of the body. The impulse is transmitted along the neurones as an action potential. Sensory neurones carry the action potential to the central nervous system. Motor neurones carry an action potential from the CNS to an effector e.g. muscle or gland. These two are connected by relay neurones. Neurones have a similar structure the structure allows them to carry the action potential from part of the body to another. They are long, so the AP can be transmitted over a long distance. The cell surface membranes have gated ion channels they control the movement of Na/K/Ca ions. They have Na/K pumps which use ATP to actively transport sodium ions out of the cell and potassium ions into the cell. They allow a potential difference to be maintained across the cell surface membrane. They are surrounded by a myelin sheath this acts an electrical insulator which insulates the neurone from electrical activity from other cells. The myelin sheath is a series of Schwann cells, where there are gaps where they meet, these are called nodes of Ranvier. The cell body contains a nucleus, ribosomes and many mitochondria for ATP. Motor neurones have a long axon which carries the AP to the effector, the cell body is in the CNS. Sensory neurones have a long Dendron which carry the AP from a sensory receptor to the cell body which is outside the cell body. A short axon then carries the AP into the CNS. Both neurones have dendrites which connect to other neurones.(c) Describe and explain how the resting potential is established and maintained; Resting potential the p.d. across the neurone cell membrane while the neurone is at rest. It is -60mV inside the cell, compared with the outside. Sodium/potassium ion pumps use ATP to pump 3 sodium ions out for every two potassium ions which are pumped in. The plasma membrane is more permeable to potassium ions and many diffuse out again. The cell cytoplasm has many large organic anions So therefore the interior of the cell is maintained at a negative potential compared to the outside. The cell membrane is polarised. The p.d. across the cell membrane = -60mv, this is the resting potential.

(d) Describe and explain how an action potential is generated; Action potential is the depolarisation of the cell membrane, so the inside is more positive than the outside, with a p.d. across the membrane of +40mV this can be transmitted along the axon. Threshold potential p.d. across the membrane of -50mV if the depolarisation of the membrane does not reach this then no AP is created. At rest the gated sodium ion channels are kept closed. As some potassium ions diffuse back out as some potassium channels are open, if some sodium ion channels open then sodium ions diffuse down their conc. grad. Into the cells from the tissue fluid. This causes a depolarisation of the membrane. In the generator region of receptor cells the gated channels are opened by energy changes in the environment. The gates further along the neurone (voltage-gated channels) are opened by changes in the p.d. across the membrane. These channels respond to depolarisations of the membrane.

Generator potentials in the sensory receptors are depolarisations of the cell membrane. If the depolarisation is large enough to reach the T.P. it will open voltage gated channels. This allows a large influx of sodium ions and the polarisation reaches +40mV. At this value the neurone transmits an A.P. as many voltage-gated sodium ion channels open. The A.P. is self perpetuating it will continue until one end of the neurone.

(e) Describe and explain how an action potential is transmitted in a myelinated neurone, with reference to the roles of voltage-gated sodium ion and potassium ion channels; Sodium ion channels opening at one point of the neurone, upset the balance of sodium and potassium ions (resting potential) created by the Na/K ion pumps. This creates local currents in the cytoplasm of the neurone. Local currents cause sodium ion channels further along the membrane to open. When an AP occurs the Na channels open at a particular point along the neurone This allows Na ions to diffuse across the membrane from the region of higher conc. outside the neurone into the neurone. The movement of Na ions into the neurone upsets the balance of ionic concentrations created by the pumps. The conc. of Na ions inside the neurone rises at the point where Na ion channels are open. This causes the Na ions to diffuse sideways, away from this region of increased conc. The movement of the charged particles is the local current.The sodium gate which was initially closed now opens due to the movement of sodium ions, their movement of Na ions along the neurone alters the p.d. across the membrane. When the p.d. across the membrane is reduced, the gates open. This allows Na ions to enter the neurone at a point further along the membrane. The AP has moved along the neurone. Na and K ions cannot diffuse through the myelin sheath. So the ionic movements which create the AP cannot occur. The movements only occur at the nodes of Ranvier, in myelinated neurones local currents elongate and Na ions diffuse along the neurone from one node of Ranvier to another. This is saltatory conduction as the AP appears to jump from one node to the next.(f) Interpret graphs of the voltage changes taking place during the generation and transmission of an action potential;

1. The membrane starts in its resting state polarised with the inside being -60mV compared to the outside.2. Na ion channels open and some Na ions diffuse into the cell. 3. The membrane depolarises it becomes less negative than the outside, the threshold value of -50mV is reached.4. Voltage-gated sodium ion channels open, Na ions flood in. As more Na ions enter, the cell becomes +vely charged inside compared with the outside. 5. The p.d. across the membrane reaches +40mV. The inside is +ve and the outside off the cell is ve.6. The Na channels close and the K channels open.7. K diffuse out of the cell bringing the p.d. back to ve inside compared with outside this is repolarisation.8. The p.d. overshoots the cell is hyperpolarised.9. The original p.d. is restored so that the cell returns to its resting state. After an AP the Na/K ions are in the wrong places. The concentrations of these ions inside + outside of the cell is restored by the pumps. After each AP the cell membrane cannot be stimulated to reach an AP. This is the refractory period and allows cells to recover after an AP. It ensures APs are transmitted in 1 direction only.(g) Outline the significance of the frequency of impulse transmission; When a stimulus is at higher intensity the sensory receptors will produce more generator potentials. This causes more frequent APs in the sensory neurone. When these arrive at a synapse they will cause more vesicles to be released. This creates a higher frequency of APs in the postsynaptic neurone. Our brain can determine the intensity of the stimulus from the frequency of signals arriving. A higher frequency of signals means a more intense stimulus.

(h) Compare and contrast the structure and function of myelinated and non-myelinated neurones; Myelinated neurones Make up one-third of neurones. They are insulated by myelin sheaths, the sheaths are created by separate Schwann cells. These wrap around the neurone, so the sheath consists of several layers of membrane and thin cytoplasm from the Schwann cells. At intervals of 1-3mm along the neurone there are gaps in the sheath these are the nodes of Ranvier. Each node is very short, 2-3m long.Non-myelinated neurone Several neurones may be enshrouded in one loosely wrapped Schwann cell. So the AP moves along the neurone in a wave rather than jumping from node to node as seen in myelinated neurones. Advantage of myelination Myelinated neurones transmit an AP more quickly than non-myelinated neurones. For Mneurones the speed is 100-120 ms1, an NMneurone has a speed of 2-20 ms1. Myelinated neurones carry signals from sensory receptors to the CNS and from the CNS to effectors. They carry signals over long distances. The increased speed of transmission means that the signal reaches the end of the neurone more quickly this allows a more rapid response. Non myelinated neurones are shorter and carry signals over a short distance. They are used in coordinating body functions e.g. breathing so a quick speed of transmission is not so important.

(i) Describe, with the aid of diagrams, the structure of a cholinergic synapse; Cholinergic synapse use acetylcholine as their neurotransmitter substance. The synaptic knobThe presynaptic neurone ends in a swelling called the synaptic knob. This knob has specialised features: Many mitochondria indicates and active process needing ATP is involved. Large number of SER. Vesicles of acetylcholine, the transmitter substance that will diffuse across the synaptic cleft Voltage-gated Ca ion channels in the membrane.

The postsynaptic membrane Contains Na ion channels which respond to acetylcholine. The channels have 5 polypeptide molecules, two have a receptor site which is specific to acetylcholine. When ACH binds to the two receptors the sodium ion channel opens.

(j) Outline the role of neurotransmitters in the transmission of action potentials; Transmission across the synapse: An AP arrives at the synaptic knob The voltage-gated Ca ion channels open Ca ions diffuse into the synaptic knob The Ca ions cause the synaptic vesicles to move to + fuse with the presynaptic membrane ACH is released by exocytosis ACH molecules diffuse across the cleft ACH molecules bind to the receptor sites on the Na ion channels in the postsynaptic membrane Na ion channels open Na diffuse across the postsynaptic membrane into the postsynaptic NEURONE A generator potential is created If sufficient generator potentials combine then the potential across the postsynaptic membrane reaches the threshold potential A new AP is created in the postsynaptic neurone

(k) Outline the roles of synapses in the nervous systemThe main role of synapses is to connect two neurones together so a signal can be passed from one to another. Although there are other functions: Several presynaptic neurones converge to one postsynaptic neurone. This would allow signals from different parts of the nervous system to create the same response useful where different stimuli are warning us of danger.

One presynaptic neurone may diverge to several postsynaptic neurones allows 1 signal to be transmitted to several parts of the nervous system useful for reflex arc. One postsynaptic neurone elicits the response while another informs the brain Synapses ensure signals are transmitted in the correct direction. They can filter out unwanted low-level signals if a low level stimulus creates an AP in the presynaptic neurone it wont pass to the next neurone because several vesicles of ACH must be released to create an AP in the postsynaptic neurone

Low-level signals can be amplified by Summantion the way several small potential changes can combine to produce one larger change in p.d across the membrane If a low level stimulus is present it generates successive APs in the presynaptic neurone the release of many vesicles of ACH over a short period of time will enable the postsynaptic GPs to combine to make AP Acclimatisation after repeated stimulation a synapse may run out of vesicles containing ACH. The nervous system no longer responds to stimuli helps to avoid overstimulation of an effector which could damage it.

The pathways created enable the nervous system to convey lots of messages, the brain knows where the signals are coming from as the neurones from specific receptors connect to specific regions of the brain.