nervous transmission topic 9

Nervous Transmission • Nerve cells are polarised in their resting state – difference in voltage across the neurone membrane with a value of -70mV o Resting potential • Resting potential is generated and maintained by a sodium-potassium pump which moves sodium ions out of the neurone and potassium pumps into the neurone o Creates electrochemical gradient as the concentration of sodium ions is higher outside the cell because the membrane is not permeable to sodium ions o Potassium ions diffuse back out because of potassium ion channels

Outside of the cell is positively charged due to the imbalance of positively charged ions

Depolarisation

1. The excitation of a neurone cell triggered by a stimulus causes the sodium channels to open, making it more permeable to sodium ions which subsequently diffuse into the neurone down the electrochemical gradient – makes inside less negative
2. Upon reaching threshold of -55mV even more sodium channels open, eventually giving a potential difference of +30mV which is the end of the depolarisation and start of repolarisation. Achieved due to sodium ion channels closing and potassium ion channels opening
3. Potassium ions diffuse out of the neurone down the concentration gradient and eventually restore the resting potential. Hyperpolarisation caused by delay in potassium ion channels closing – potential difference is greater than the resting potential
4. Resting potential achieved by sodium-potassium pump – returns potential difference to -70mV

• Action potential travels along the neurone as a wave of depolarisation where the sodium ions move to the adjacent resting region where they rigger a change in potential difference, thus stimulating another action potential
• Absolute refractory period = sodium ion channels are blocked and it is impossible for another action potential to be generated
• Relative refractory period = sodium channels are not blocked, but potassium ion channels are still open and effectively the threshold is raised

• Myelin sheath increases the speed at which the electrical potential is carried o Insulator of axons and dendrons, produced by schwann cells

Saltatory conduction::mechanism by which the speed is increased

Action potential jumps between gaps in the cells of the myelin sheath – nodes of ranvier because myelin sheath is impermeable

• Synapses are junctions between two neurones

• Presynaptic membrane depolarises upon the arrival of an action potential - Causes calcium channels to open – allow calcium ions to enter the neurone - -Presence of calcium ions in neurone causes the fusion of synaptic vesicles filled with a particular neurotransmitter e.g. acetylcholine, with the presynaptic membrane – causes release of neurotransmitter into the synaptic cleft o

Neurotransmitter binds to the receptors located on the postsynaptic membrane and:

• Excitatory post-synaptic potential::cation channels opened, enables sodium ions to enter the neurone – membrane depolarises and triggers another action potential
• Inhibitory post-synaptic potential::stimulated opening of anion channels, enables chloride ions to enter the neurone – hyperpolarisation of the post-synaptic membrane, making triggering a new action potential more difficult

Effects of Drugs on the Nervous System
• Nicotine mimics the effects of acetylcholine and triggers the release of dopamine and at height doses binds to and blocks acetylcholine receptors
• Lidocaine blocks voltage-gated sodium ion channels
• Cobra venom bings to blocks acetylcholine receptors