Neurotransmission

Page 1

Nerve Conduction

• The conduction of an electrical impulse along a nerve is a complex process which is achieved through the movement of polarised ions in and out of sections in the neuron.

• A neuron has a cell membrane. The inside of the membrane is negatively charged because of the uneven distribution of positively charged ions (more outside than inside).

• Inside the neuron is an ion pump, which actively transports sodium ions outside of the cell membrane of the axon.

• The sodium is blocked from reentering the cell leaving more sodium ions outside. As such, the inside of the axon is slightly negatively charged which creates the potential for an electrical impulse to form.

• While the sodium ions are being pushed out of the cell, potassium ions are allowed into the cell. Even though potassium is positively charged, there are 3 sodium ions being pumped out for every 2 potassium ions entering the cell which creates a negative charge inside the cell.

• The result of this process means that the inside of the cell has an electrical potential of -70 millivolts. The condition of being negatively charged inside the cell is the neurons normal or resting state and is called resting potential.

• While a neuron is sitting at its resting potential, it is now ready for a stimulus to occur which will cause it to act - the generation and conduct of an impulse down the nerve is called action potential.

Page 2

Action Potential

• Now that the axon is charged with a -70mV charge with sodium outside and potassium inside, it is ready to conduct an impulse

• A nerve impulse can be thought of like a wave being passed down the neuron. A change in each section of the neuron causes the adjacent section to also change - leading to a cascade effect.

• An action potential occurs through the following multi-step process

1. At rest, the -70mV charge is maintained by the sodium-potassium pump which moves 3 sodium ions outside and 2 potassium ions inside during each pump. As a result of the difference in electrical charge, the resting state is polarised.

2. A stimulus is received which causes the sodium channels in the cell membrane to open resulting in a sudden and large amount of sodium rushing into the cell. As the sodium ions are positively charged, this causes the section of the axon to lose its polarity (charge) meaning it depolarises. Depolarisation from the sodium causes a slight overshoot of positive charge (+40mV) which causes a burst of electrical energy to be released and passed along the axon. The release of electrical energy is called action potential.

3. The reversal of charge (from -70mV to +40mV) is then passed along the next region of the neuron, causing it to also undergo an action potential. The process of the impulse being passed on is called propagation.

4. After the action potential has been reached, the sodium channels close and the potassium channels open. The potassium ions then flood out of the cell and restore the positive charge on the outside of the membrane. This section of the membrane is now undergoing the process of repolarization.

5. The movement of potassium ions causes a temporary overshoot of the resting electrical gradient (approx 75 mV) making the inside of the cell more negatively charged than usual. The time where it is more negatively charged than desired is called the relative refractory period and represents the time taken for the cell membrane to restore its resting potential (get back to -70mV). When the neuron is in the relative refractory period it cannot generate another another action potential (it can't pass information along).

6. After the potassium has flooded out, the sodium pump is restored (3 sodium fro 2 potassium ions) and the resting potential of more sodium outside the cell is restored.

• Takes 6 milliseconds, meaning a neuron can only fire every 6 milliseconds.

Page 3

Conduction Velocity

• How quickly an action potential moves along an axon is dependent on a few factors.

1. If a neuron has a large diameter the message will be relayed more quickly.

2. If the axon has myelin sheathes, the process will be sped up also. This is because it is only in the nodes of Ranvier that the action potentials are generated meaning sections of the axon are 'skipped'.

Page 4

Neurotransmission

• Once an action potential has reached the end of an axon, it stimulates structures within the axon terminals or synaptic knobs.

• Since the axon terminal and dendrite are not touching each other, the continuation of an action potential is not possible. Instead neurotransmitters are released from vesicles after being stimulated to do so by the action potential.

• The process goes on as follows:

1. Action potential stimulates synaptic vesicle.

2. Neurotransmitter within vesicle is released.

3. Neurotransmitter travels across synaptic cleft.

4. Neurotransmitter binds with receptor on ion channel.

5. Ion opens in response to neurotransmitter causing sodium ions to diffuse in

6. New action potential begins.

• Different neurotransmitters cause different responses in the dendrites of the next cell.

• Common neurotransmitters include adrenaline and acetyllcholine. Neurotransmitters usually excite or inhibit the response of a neighbouring neuron.

• They are not endless and when a lot of activity occurs, they can become depleted - this is known as synaptic fatigue.