VCE Biology Unit 2 - 2010
Functioning Neurons
Three basic steps involved in the function of nerve cells
- Generation of a nerve impulse (action potential) by sensory neurons.
- Conduction (propagation) of an impulse along axons.
- Chemical transmission of a signal to another cell across a synapse.
Sensory nerves respond to stimuli by depolarising (i.e. decreasing the negative charge inside the cell. If the depolarisation is great enough, an action potential is generated.
The action potential is conducted along the length of the axon to the axon terminal which forms a synapse with an effector cell. When an action potential enters the axon terminal, neurotransmitter molecules are released. These diffuse across the synapse to specific receptors in the postsynaptic cell’s plasma membrane.
The action potential
A nerve impulse or action potential is a wave of electrical change that passes rapidly along an axon membrane. The initial stimulus is either large enough to generate an action potential or it is not; it is an all or nothing situation.
The intensity of the stimulus (e.g. heat or stretching) is indicated by the increased rate of action potentials generated and the number of sensory nerves that respond.
Once an action potential has been generated, another action potential cannot be produced for a brief period even if it is depolarised.
If depolarisation is sufficient to reach the threshold potential Na+ (sodium ions) channels in the membrane open and Na+flood into the cell along a concentration gradient causing an action potential. Because Na+are positive the inside of the cell becomes briefly positive which causes the K+ channels to open and the K+ diffuses out of the cell along its concentration gradient (i.e. it is passive). The inside of the cell becomes negative again. The electrical change to the inside of the cell from being briefly positive to being negative again is the action potential. After the action potential the Na+ and K+ channels are closed and the intra-cellular ion concentrations are returned to their initial values by the Na+ - K+ pumps (requires energy). The refractory period is the time after the action potential that the axon is unable to generate another action potential.
Conduction
During an action potential NA+ diffuses sideways inside the axon depolarising the adjacent regions of the axon membrane.
Increasing the speed of signal can be achieved by increasing the diameter of the neuron and increasing the insulation. Both these factors decrease the resistance of the neuron resulting in an increased rate of conduction.
Insulation of axons evolved in a number of invertebrates and vertebrates. Myelin sheaths are the most affective. These are plasma membranes composed predominantly of phospholipids are wrapped around the axon. Myelin sheaths are made from Schwann cells which wrap around the axon. Every millimetre there is a bare region of axon called the ‘node of Ranvier’. An action potential moves more quickly along a myelinated axon because the ‘depolarisation’ jumps more quickly from one node to the next (salutatory conduction).
Synaptic transmission
Synaptic transmission occurs between the presynaptic nerve terminal and a specialised region of the postsynaptic cell.
When an action potential passes into a nerve terminal, vesicles containing neurotransmitter molecules move to the nerve cell membrane and release their content into the synapse (about 20 nm). The neurotransmitters diffuse across the gap to specialised receptors in the membrane of the dendrites of the postsynaptic nerve cell. The neurotransmitters may act as an excitatory or an inhibitory agent. That is, it may provoke or inhibit activity in the postsynaptic cell. The released neurotransmitters are quickly broken down by enzymes or taken back up into the nerve terminal. The nerve terminals is packed with vesicles and mitochondria. Remember a synapse can only transmit a signal in one direction, therefore nerves conduct in only one direction.
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