Module 5 Neuronal Communication

Question 1

Neurones

Answer 1

• Neurones make up the peripheral nervous system and carry electrical impulses around the body. A bundle of neurones makes up a nerve.
• All neurones contain a long fibre called an axon, a cell body containing the nucleus, and axon terminals that connect to other neurones and receive impulses.
• Myelinated neurones have axons that are insulated by a myelin sheath with small unmyelinated sections called nodes of Ranvier. Schwann cells wrap around the axon and form the myelin sheath. Impulses jump from one node to the next, which increases the speed of the impulse.
• Non-myelinated neurones are uninsulated, and the impulse is slower as it must travel across the entire length of the neurone.
• Sensory neurones transmit impulses from receptors to the CNS. They have one single dendron leading towards the cell body and one axon carrying impulses away from the cell body, which is in the middle of the cell.
• Relay neurones connect sensory neurones and motor neurones in the CNS. They have many branched axons and dendrites and are short.
• Motor neurones transmit impulses from the CNS to effectors, like muscles or glands. They have a large cell body in the spinal cord/brain at one end of the neurone, with many dendrites branching from it. They then have an axon leading away from the cell body.

Question 2

Sensory Receptors

Answer 2

• A receptor is a cell that responds to a stimulus. Transducers convert energy in one form into an electrical impulse. Every receptor responds to a specific stimulus.
• Photoreceptors – light. Chemoreceptors – chemicals. Mechanoreceptors – mechanical strain. Baroreceptors – blood pressure. Osmoreceptors – body fluids.
• Pacinian corpuscle is a mechanoreceptor located deep in the skin, found in fingertips, soles of feet, joints, tendons and ligaments.
• They respond to changes in pressure and establish a generator potential located at the end of sensory neurone axons.
• They are comprised of layers of membrane separated by a gel that contains sodium ions. Pressure causes the layers to stretch and sodium ion channels to open. Sodium ions flow into the neurone and create an electrical potential difference across the axon, which is a generator potential.

Question 3

Resting/ Action potentials

Answer 3

• At resting potential, the inside of an axon has a negative electrical potential compared to outside of the axon at approximately -70mV.
• The resting potential is maintained by the active transport of sodium and potassium ions by sodium/potassium pumps. 3 sodium ions are pumped out of the axon, for every 2 potassium ions that are pumped in. Therefore, there is a larger concentration of positive ions outside the axon, establishing an electrochemical gradient.
• Protein channels in the axon membrane are more permeable to potassium ions than sodium ions. More potassium ions move down their concentration gradient out of the axon at a faster rate, than sodium ions move in.
• An action potential is generated by a change in distribution of electrical charge caused by rapid movement of sodium and potassium ions.
• Voltage gated channel proteins that allow sodium and potassium ions through are closed at resting potential.
• A stimulus triggers sodium channels to open and sodium moves down its electrochemical gradient into the neurone, until a threshold of -55mV is reached.
• Depolarisation causes voltage gated sodium ion channels to open, and sodium ions move into the neurone. The inside of the axon becomes less negative, and this triggers further depolarisation, as an example of positive feedback.
• Once +30mV is reached, repolarisation occurs and all sodium voltage gated channels close. Voltage gated potassium channels open, and potassium ions move out of the axon, until at -70mV repolarisation is reached, as an example of negative feedback.
• Hyperpolarisation occurs as potassium channels are slow to close and too many potassium ions leave the axon, leaving the inside more negative than at resting potential. Once the potassium channels close, the sodium/potassium pump returns the resting potential to normal.

Question 4

Transmission of nerve impulses

Answer 4

• Transmission of nerve impulses occurs as depolarisation moves in one direction along the length of an axon. Repolarising sections of axons are unresponsive to depolarisation, which creates a wave of depolarisation in one direction.
• The all or nothing principle means that if a stimulus doesn’t reach the threshold for depolarisation, an impulse will not be transmitted. As the strength of the stimulus increases, the frequency of impulse increases, instead of the amplitude.
• A refractory period occurs after an action potential. Both sodium and potassium channels are closed. This allows action potentials to be discrete and impulses to travel in one direction.
• Different factors affect the speed of conduction and how fast an impulse is transmitted. Myelination means that action potentials only occur at nodes of Ranvier and jump from node to node. Saltatory conduction increases the speed of conduction. An axon with a thicker diameter will have a faster speed of conduction, as it has a larger surface area, so an increased rate of diffusion of ions. In lower temperatures decreased kinetic energy means a slower rate of diffusion of ions.

Question 5

Synapses

Answer 5

• Synapses consist of the presynaptic knob, the synaptic cleft, and the postsynaptic membrane. A synapse is the junction where two neurones meet. Cholinergic synapses use ACh as the neurotransmitter.
• An impulse arrives at the presynaptic membrane by an action potential, causing depolarisation of the membrane. Voltage gated calcium ion channels open, and calcium ions move down their electrochemical gradient into the presynaptic knob from tissue fluid.
• ACh containing vesicles then fuse with the presynaptic membrane, and ACh is released into the synaptic cleft by exocytosis.
• ACh diffuses across the synaptic cleft and binds to cholinergic receptors on the postsynaptic membrane. Sodium ion channels open and sodium ions diffuse into the neurone, causing depolarisation and an impulse.
• ACh is broken down and recycled so sodium ion channels don’t remain permanently open and prevents permanent depolarisation. Acetylcholinesterase catalyses the hydrolysis of ACh into choline and acetate. Choline is reabsorbed into the presynaptic membrane and reacts with acetyl coenzyme A to reform ACh.
• Synapses are unidirectional as neurotransmitters are only released from one synaptic membrane.
• Summation occurs when multiple impulses are added together, as a single impulse is insufficient to generate an action potential in the post synaptic neurone.
• Benefits of summation include that it can magnify the effects of a stimulus, respond to a combination of stimuli, and prevents the nervous system from becoming overwhelmed.
• Temporal summation occurs when multiple impulses arrive in quick succession. Spatial summation occurs when multiple impulses arrive simultaneously at different synaptic knobs but effect the same cell body.
• Excitatory neurotransmitters stimulate the generation of an action potential in the post synaptic neurone.
• Inhibitory neurotransmitters inhibit the generation of an action potential by opening potassium ion channels, causing hyperpolarisation.
• If both inhibitory and excitatory neurotransmitters act no action potential is generated as the entry of sodium ions is cancelled out by potassium ions that are leaving.