Structure of a nerve

Nerves and Nerve impulses “Nerve impulse: a self-propagating wave of electrical disturbance which travels along the surface of a neuron membrane””

Neurons Motor neuron – connects central nervous system with effectors

Sensory Neuron – connects sensory receptors to the central nervous system

Myelin Axons may be myelinated. Myelinated neurons are surrounded by a layer of fatty myelin. Myelin is secreted by Schwann Cells which completely surround the axon. Myelin is an electrical insulator which prevents ions leaking out of the axon. It also insulates an axon from electrical activity in adjoining cells.

Neuron membranes Neuron membranes have various types of protein carrier molecules embedded within them. The ions the carriers transport help generate nerve impulses 1)Sodium-potassium pumps (active transport) 2)Carriers which allow facilitated diffusion of ions. 3)Voltage gated channels.

Resting and Action Potentials Resting Potential The resting potential is the state of the membrane when no impulse is passing along it. The inside of the axon is negatively charged with respect to the outside. There is a potential difference (voltage) of -70 mV. Membrane is said to be polarized.

Action Potential During an impulse, or action potential, the charge is reversed and the membrane becomes depolarized. Resting and Action potentials animation

How is the resting potential established? There are two processes which maintain the resting potential: Active transport of Na + and K + ions by specialized protein pumps in the axon membrane. Facilitated diffusion of Na + and K + ions through channel protein molecules.

Active Transport Sodium-potassium pumps move pump Na + out of the axon and K + into the axon. 3 Na + are pumped out and 2 K + are pumped into the cell. Movements are against a concentration gradient so ATP is required.

Facilitated diffusion The ions tend to diffuse back through channel proteins down their concentration gradients. However….. the membrane is more permeable to potassium than sodium, so more K + move back out of the axon than Na + move in….. so more positive ions accumulate on the outside, which becomes more positive than the inside = RESTING POTENTIAL.

How is an action potential generated? The axon membrane also contains sodium and potassium voltage-gated channels. At resting potential, the voltage-gated channels are closed. Voltage-gated channels open only when the potential difference across the membrane reaches a certain specific value – the threshold value (approx -50 mV). If threshold is reached, voltage-gated Na + channels open and Na + ions flood into the axon. (attracted by build up of negative charge) Resting potential is reversed (-70mV to +40 mV) and the membrane becomes depolarised. An action potential is an “all or nothing” response – if threshold is reached an action potential will occur. Action potential

Repolarization The Na + voltage-gated channels close at approx +30 mV. The K + voltage-gated channels open, and K + flow out of the axon, causing the potential difference to become more negative again. The Na/K pump begins again and resting potential is restored. The K + channels stay open for a few milliseconds after repolarisation, so the membrane becomes more negative than resting potential – hyperpolarised.

Refractory period For a brief period following an action potential, the axon is not excitable. The refractory period is necessary as it allows the proteins of voltage sensitive ion channels to restore to their original polarity.

Advantages of a refractory period: Ensures nerve impulses flow in one direction only. Ensures action potentials are separated, with no overlap. This is important because the brain uses the frequency of impulses to determine the strength of a stimulus. More frequent impulses = strong stimulus.

Factors affecting nerve impulse speed Temperature - The higher the temperature, the faster the speed. Ions have more kinetic energy so diffusion rates are faster. However, channel proteins and pumps become denatured at excessive temperatures. Membrane fluidity and structure is also disrupted at high temperatures. Axon diameter - The larger the diameter, the faster the speed. So marine invertebrates, who live at temperatures close to 0°C, have developed thick axons to speed up their responses. This explains why squid have their giant axons. Myelination – myelinated axons show saltatory conduction, which speeds up transmission.

Transmission of nerve impulses In unmeyelinated neurons, depolarisation of one area of membrane sets up local currents which stimulates the next patch of membrane to open Na + channels. Speed = 1-3 ms -1. In myelinated neurons, depolarisation occurs only at Nodes of Ranvier. Action potentials jumps from one node to the next. This is known as Saltatory Conduction. Speed = up to 120 ms -1 Saltatory Conduction is metabolically more efficient as fewer ions move across the membrane, so less energy is expended restoring the resting potential. Conduction of impulses

How are action potentials started? A nerve impulse is created by altering the permeability of the membrane to Na +. When the gated sodium channels open, Na + flood into the cell. A small influx of Na + causes a generator potential. The larger the stimulus, the more channels will open. If enough Na + enter, threshold potential will be reached and an action potential is generated.

How do receptors generate nerve impulses? Pacinian corpuscles are normally round. In this state the Na + channels are closed. When corpuscles are under pressure, the membranes become stretched and this opens the Na + channels.

Photoreceptors The retina at the back of the eye contains specialised receptor cells called rods and cones. Rods and cones contain molecules which change shape when light hits them. The change in shape opens Na + channels in the cell membranes.