Nerves

Question 1

Central nervous system (CNS)

Answer 1

It consists of the brain and spinal cord. The brain is enclosed in a cling film like substance called the meninges. The spinal cord consists of 31 pairs of spinal nerves and 12 cranial nerves.

Question 2

Peripheral nervous system (PNS)

Answer 2

Consists of the nerves and ganglia outside of the brain and spinal cord. The PNS is divided into the somatic nervous system (voluntary) and the atonomic nervous system (involuntary). The autonomic exists in 2 states; the sympathetic (fight/flight) nd parasympathetic (rest/digest).

Question 3

Neurone types

Answer 3

1. Afferent (sensory) neurones in the PNS
2. Interneurones CNS
3. Efferent (motor) neurones in the PNS

Question 4

Glia

Answer 4

Aprroximately 90% of cells in the CNS.

• Astrocytes maintain the external environment for neurones, surround blood vessels and produce the blood brain barrier.
• Oligodendrocytes form the myelin sheath in the CNS.
• Microglia are phagocytic hoovers mopping up infection.

Question 5

Resting membrane potential

Answer 5

RMP keeps the cell ready to respond like a charged up battery. It is determined by concentration gradients of ions across the membrane and by membrane permeabilitt of each ion. These ions are Na+, K+ and Cl-. Ions move down their gradient through laky channels leading to a seperation of charge. The membrane is much more permeable to K+ so the RMP.is close to the equilibrium of K+. The movememt of K+ out of the cell creates a charge imbalance which opposes the flow of K+ down the concentration gradient. The Na+/K+ pump also makes a small contribution as it doesn’t pump one for one so each turn makes the inside more negative. Depolarise makes positive. Hyperpolarise makes negative.

Question 6

Graded potential

Answer 6

Graded potentials determine when to fire an action potential. These include generator potentials, postsynaptic potentials, endplate potentials and pacemaker potentials. They are decremental/non-propagated. They are graded so stimulus intensity is indicated by amplitude. They may excitatory or inhibitory. They can summate. There may be synaptic integration.
Graded potentials give rise to action potentials when they reach threshold.

Question 7

Action potential

Answer 7

These are all or none and self-propagating. The AP is passed along by Na+ channels depolarising their neighbouring channel. As channels need to recover there is a refractory period so the AP can’t go backwards. They are mediated by voltage gated channels.

1. At RMP K+ is leaking out of the cell.
2. If a graded potential reaches threshold (~55mV) the voltage gated Na+ channels open amd Na+ flows down its concentration gradient causing depolarisation.
3. Extra voltage gated K+ channels open at threshold but more slowly than Na+ channels. K+ flows out of the cell and re/hyperpolarises it.
4. Absolute refractory period. Na+ channels need time to recover so the cell wom’t respond.
5. Relative refractory period. Na+ channels are still recovering however they can fire another AP in response to a strong depolarisation.

Question 8

Nerve fibre types

Answer 8

A nerve contains bundles of nerve fibres. The compound AP is the result of all the nerve fibres. The fastest conducting nerve fibres arrive first (Aalpha). Nerves with large, myelinated axons conduct fastest as there is less friction. Nerves with small, unmyelinated axons conduct slowest. (C (most abundant)

Question 9

Describe the consequences of demyelinating disease.

Answer 9

Myelination increases the rate of conduction. Schwann cells (PNS) and oligodendrocytes (CNS) wrap around the axon creating insulation which stops current leaking out. Myelination speeds up conduction by spreading out the Na+ channels which exist at the nodes of Ranvier.
Demyelination creates gaps in the myelin sheath so the action potentials can’t be conducted effectively. Resulting in conditions like MS and Guillain-Barre syndrome.

Question 10

The neuromuscular junction (NMJ)

Answer 10

The NMJ is a synapse between a motor neuron and skeletal muscle. As the axon of a motor neuron enters the structure of skeletal muscle it forms many branches called axon terminals. There is a bulbous swelling called a synaptic end bulb at the extreme end of each axon terminal. Each bulb contains many synaptic vesicles containing acetylcholine. The gap between the terminal and sarcolemma is known as the synaptic cleft. The reciveing muscle is aka the motor emd plate.

Question 11

Describe the process of neuromuscular transmission

Answer 11

1. AP in a motor neuron
2. Opens V.gated Ca2+ channels in the presynaptic terminal.
3. Triggers the fusion of vesicles by Ca2+ dependant exocytosis.
4. Acetylcholine is released which diffuses across the synaptic cleft.
5. Binds to ACh nicotinic receptors.
6. Opens L.gated Na+/K+ channels.
7. Evokes a graded end plate potential.
8. Always depolarises adjacent membrane to threshold.
9. Opens v.gated Na+ channels, evoking a new AP.
10. Muscle contraction
11. ACh is removed by acetylcholineterase to stop continuous firing.

Question 12

CNS synaptic transmission

Answer 12

1. AP reaches axon terminal
2. V.gated Ca2+ channels open.
3. Ca2+ causes vesicles to release the neurotransmitter.
4. Neurotransmitter crosses the synapse
5. Neurotransmitter binds to neuroreceptors.
6. Triggers signal in post-synaptic neuron.
7. Neurotransmitter removed from synaptic cleft.

Question 13

Axo-synaptic

Answer 13

Synapse onto soma

Question 14

Axo-dendritic

Answer 14

Synapse onto dendrites

Question 15

Axo-axonal

Answer 15

Inhibitory synapses onto presynaptic terminal of axon.

Question 16

Postsynaptic potentials

Answer 16

Fast EPSP (ionotropic)
Slow EPSP (metabotropic)
Fast/slow IPSP
These are generally small so never reach threshold but enable complex synaptic integration.

Question 17

Synaptic integergration

Answer 17

The computational process by which an individual neurone processes its synaptic inputs and converts them into an output signal.

Question 18

Excitatory neurotransmitters

Answer 18

Acetylcholine, norepinephrine, dopamine, NO, glutamate, ATP, adenosine, peptides, histamine.

Question 19

Inhibitory neurotransmitters

Answer 19

GABA, glycine, dopamine, seratonin, peptides.

Question 20

Signal transduction in the CNS

Answer 20

There are multiple neurotransmitters. The postsynaptic potential is made up of EPSPs and IPSPs which are small. Anatomical arrangement is axo-somatic, axo-dendritic or axo-axonal. Connectivity varies and so does the clean up enzyme.

Question 21

Signal transduction in the NMJ

Answer 21

Acetylcholine is the neurotransmitter. The postsynaptic potential is an end plate potential. And acetylcholinesterase is the clean up enzyme.

Question 22

EPSP

Answer 22

Excitatory postsynaptic potential. Depolarise. Fast ones are created by ligand gated Cl- channels. Slow; G protein K+ channels.

Question 23

IPSP

Answer 23

Inhibitory postsynaptic potential. Hyperpolarises. Fast; ligand gated Na+/K+ channels. Slow; close (G protein) leaky K+ channels.