How nerves work

Question 1

Describe the subdivisions of the nervous system.

Answer 1

The central nervous system:
Brain and spinal cord
The peripheral nervous system:
Autonomic - sympathetic, parasympathetic and enteric (involuntary)
Somatic - voluntary

Question 2

Describe the anatomy of the brain.

Answer 2

The brain is composed of two parts:
Cerebellum - responsible for balance and coordination
Cerebrum - surface is called cerebral cortex, bumps are called gyrus, the grooves between gyri are called sulcus.

Question 3

Name 4 lobes of the brain and state their purpose.

Answer 3

Frontal lobe = speech and movement (primary motor centre)
Parietal lobe = sensation (somatic sensory cortex)
Temporal lobe = hearing
Occipital lobe = vision

Question 4

Describe the purpose of the meninges and the 3 different types.

Answer 4

The meninges are why the brain appears to look shiny, the meninges are 3 layers of connective tissue:
- Pia matter, is the deepest and thinnest layer
- Arachnoid is the middle layer.
- Dura matter, most superficial layer, thick tough layer.

Question 5

Describe the anatomical organisation of the brain and brain stem.

Answer 5

Diencephalon contains the thalamus and hypothalamus (regulation of temperature, thirst and hunger)
Inferior to the diencephalon is the brain stem (regulation of blood pressure, respiratory rate and vomit centre)
Then follows, (inferiorly):
Mid brain
Pons
Medulla oblongata
Then continuous with spinal cord.

Question 6

Describe the anatomical organisation of the spinal cord.

Answer 6

31 pairs of spinal nerves.
8 cervical
12 Thoracic
5 Lumbar
5 Sacral
1 Coccygeal

Question 7

Describe the microanatomy of the spinal cord.

Answer 7

The dorsal root contains sensory fibres that carry information from the periphery towards the CNS.
The dorsal root ganglia is where the cell bodies of sensory fibres are contained.
The ventral root contains motor fibres that carry signals away from CNS towards the periphery to give instructions.

Question 8

Describe white and grey matter in the vertebral column.

Answer 8

White matter is on the outside of the spinal cord contains mostly axons, white colour because some axons are myelinated.
Gray matter is on the inside of the spinal cord and contains all cell bodies that reside in the ventral and dorsal horns.

Question 9

Describe the general structure of a neuron.

Answer 9

Neurons have a soma (cell body) this contains the nucleus.
Neurons have dendrites that extend from the neuron and receive information.
The “initial segment” or the axon hillock triggers action potentials.
The axon is like a long wire connecting the neuron to all its different parts, it sends the action potential.
The release of the action potential comes at the end of the axon at synapses or ‘presynaptic terminals’.

Question 10

Describe the morphology of neurons.

Answer 10

Afferent (sensory) neurons can be bipolar or pseudounipolar.
Interneurons can be multipolar or anaxonic (no clear axon).
Motor neurons can be multipolar.

Question 11

Name the different kinds of Glia in the central nervous system

Answer 11

Astrocytes
Oligodendrites
Microglia
Ependymal cells

Question 12

Describe astrocytes

Answer 12

Astrocytes are one of the supporting cells in the central nervous system.
They maintain the external environment for neurons, by regulating the water and potassium concentrations in the extracellular fluid.
They also surround blood vessels and form the blood brain barrier.

Question 13

Describe oligodendrites.

Answer 13

These form myelin sheaths in the central nervous system.
These sheaths are wrapped around axons and can speed up the transmission of a signal.

Question 14

Describe microglia.

Answer 14

Microglia are the macrophages of the CNS, that hoover up infection (e.g. by phagocytosis).

Question 15

Describe ependymal cells.

Answer 15

These cells produce the cerebrospinal fluid that surrounds the brain and protects it.

Question 16

Describe the different types of glia in the peripheral nervous system.

Answer 16

Schwann cells -
These are the (PNS) equivalent of oligodendrites, they form myelin sheaths over axons, however they can only insulate 1 axon.
Satelite cells -
Support cell bodies.

Question 17

Describe what is meant by a resting membrane potential.

Answer 17

The resting membrane potential of a cell is a state where there is no net flow of ions across the cell membrane. This is important to keep the cell in a state where is it ready to respond.

Question 18

Describe the ionic basis of the resting membrane potential.

Answer 18

The resting membrane potential is generated by the selective permeability of the resting membrane to K+.
The resting membrane potential is therefore close to the K+ equilibrium potential.

Question 19

Explain functions of graded potentials.

Answer 19

Graded potentials depolarise the membrane to the threshold value that then allows the firing of an action potential.
Graded potentials can signal stimulus intensity in their amplification.

Question 20

Describe some properties of graded potentials.

Answer 20

Decremental, graded potentials become smaller as they travel along the membrane, so they are only useful in short distances.
Can be depolarising or hyperpolarising, neurotransmitters can open channels that either depolarise or hyperpolarise the cell, this means they can excite or inhibit a cell.
Can summate, the addition of two synapses that are happening at the same time.

Question 21

Describe generator potentials.

Answer 21

Generator potentials happen in sensory receptors.
There is a stimulus and ion channels open. This causes depolarisation of the membrane towards the threshold value. This then allows the firing of an action potential.

Question 21

Describe end-plate potentials.

Answer 21

End-plate potentials happen at neuromuscular junctions.
The action potential is fired out the ventral root, it then travels down the axon towards the synapse at the neuromuscular junction.
After diffusing across junction the action potential reaches the skeletal muscle membrane.

Question 22

Describe post-synaptic potentials.

Answer 22

Post-synaptic potentials happen at synapses.
The neurotransmitter is release at the presynaptic terminal, where it then diffuses across the synaptic cleft towards the membrane of the post-synaptic cell where it binds to the receptors. This then opens channels, causing depolarisation and eventual firing of an action potential.

Question 23

Describe pacemaker potentials

Answer 23

Pacemaker potentials occur in the pacemaker tissues of the heart,.

Question 24

Explain the role of synaptic integration in neuronal function.

Answer 24

The axon hillock is the overall decider whether the action potential is fired or not. The summation of the synaptic inputs decide if the axon hillock will reach threshold.

Question 25

Describe the three types of summation of synapses.

Answer 25

Axo-axonic synapse - One input synapses onto another. INHIBITORY
Axosomatic synapse - input synapses directly onto the cell body (soma) INHIBITORY
Axodendritic synapse - input synapses onto a dendrite. EXCITATORY

Question 25

Describe the ion channels involved in action potentials.

Answer 25

There are 3 types of channels that determine the ionic basis of the action potential.
Leaky K+ channels - open at all times.
Voltage-gated Na+ and K+ channels - open at threshold value (-55mV).
Sodium enters.
Potassium exits.

Question 26

Describe the Voltage-gated Na+ channels in action potentials.

Answer 26

At rest, voltage-gated sodium channels are closed.
The action gate opens in response to meeting the threshold value, and this allows sodium ions to flow into the cell.
The inactivation gate closes during the absolute refractory period, this prevents more ions flooding into cell. It begins opening again in the relative refractory period.

The voltage-gated sodium channels mediate depolarisation.

Question 27

Describe the process of depolarisation.

Answer 27

Depolarisation is mediated by the opening of voltage-gated sodium channels (due to threshold being reached).
Depolarisation of a patch of membrane causes the neighbouring regions of membrane to depolarise and so on and so forth. The depolarisation occurs slowly and does not lose its strength along the membrane.

Question 28

Describe the refractory period in depolarisation.

Answer 28

The refractory period is the period after a sodium channel has been opened and depolarised the next patch of membrane (carried out its function), the sodium channels are closed by inactivation gates.
This ultimately prevents the spread of the action potential in the wrong direction.

Question 29

Describe some characteristics of an action potential.

Answer 29

Needs a threshold value.
“all or none”
Self-propagating
Have a refractory period
Travel slowly

The stimulus intensity has an effect of firing frequency of action potential but not on amplitude.

Question 30

Describe subthreshold and suprathreshold stimuli.

Answer 30

Subthreshold stimuli are stimuli that are not sufficient enough to reach threshold to fire an action potential. They have small graded potentials.
Suprathreshold stimuli are sufficient enough to get cell to threshold, as the strength of the stimulus increases, the frequency of action potentials is increased.
They have larger graded potentials.

Question 31

Describe A alpha nerve fibres.

Answer 31

Largest nerve fibre
Myelinated
Velocity = 70-120 m/sec
Function - proprioception, motor neurons.

Question 32

Describe A beta nerve fibres.

Answer 32

Large
Myelinated
Velocity = 30-70 m/sec
Function - touch, pressure.

Question 33

Describe A gamma nerve fibres.

Answer 33

Small
Myelinated
Velocity = 15-30m/sec
Function - motor neurons of muscle spindles.

Question 34

Describe A delta nerve fibres.

Answer 34

Smallest
Unmyelinated
Velocity = 12-39m/sec
Function - touch, cold, “fast” pain.

Question 35

Describe the C nerve fibres.

Answer 35

Unmyelinated
Velocity = 0.5-2m/sec
Function - warmth, slow pain.

Question 36

Explain how conduction velocities are altered by diameter of axons.

Answer 36

Large diameter axons have a lower axial resistance so Na+ channels can be more spaced out along the membrane.
The conduction velocity is faster because of lowered axial resistance.

Question 37

Explain how conduction velocities are altered by myelination.

Answer 37

Myelination is carried out by Schwann cells in the PNS and Oligodendrocytes in the CNS.
Both wrap layers of myelin sheath around axons, insulating them.
This causes increased membrane resistance, so less current leaks out.
Also causes decreased capacitance, so less current is wasted charging up membrane.

Myelination allows action potentials to spread from node to node (Ranvier) and still reach threshold.
- this is known as SATATORY CONDUCTION.

Question 38

Describe the consequences of demyelinating diseases.

Answer 38

Demyelination in the CNS can cause multiple sclerosis.
In the PNS it can cause Guillain Barre syndrome.
Both diseases attack the myelin sheath and decrease the membrane resistance and increase membrane capacitance, so, conduction fails.

Question 39

Describe the structure of the neuromuscular junction.

Answer 39

Neuromuscular junction is the synapse between a motor neuron and skeletal muscle.
It therefore consists of the presynaptic terminal of the motor neuron, filled with (ACh) acetylcholine-containing vesicles.
There is a synaptic cleft between the motor neuron and muscle cell.
There is the post-synaptic end-plate of the skeletal muscle fibre. This post-synaptic end-plate has folds which increase ethe surface area and allow for more sodium channels.
The sarcomere is within the end-plate.

Question 40

Describe the process of neuromuscular transmission.

Answer 40

1 - The AP in the motor neuron is mediated by Na+ depolarisation.
2 - This depolarisation opens voltage-gated Calcium ion channels in the presynaptic terminal, so calcium enters the cell.
3 - The ACh filled vesicles fuse with the membrane (Calcium-dependent exocytosis)
4 - ACh diffuses across synaptic cleft
5 - ACh binds to ACh (nicotonic) receptors (these are ionotropic or ligand-gated ion channels)
6 - This opens ligand gated sodium and potassium channels.
7 - Evokes end-plate potential (graded potential).
8 - Depolarisation of membrane to threshold
9 - Opening of voltage-gated sodium channels
10 - Evokes action potential
11 - Muscle contracts
12 - Acetylcholine is cleared up by acetylcholinesterase.

Question 41

Describe the ultrastructure and functions of synapses between neurones.

Answer 41

Axo-dendritic (excitatory) - a presynaptic terminal synapses onto a dendrite.
Axo-somatic (inhibitory) - a presynaptic terminal synapses onto the soma.
Axo-axonal (excitatory and inhibitory) - a presynaptic terminal synapses onto another presynaptic terminal.

Question 42

Describe the process of synaptic transmission in the CNS.

Answer 42

(same as neuromuscular junction) + added complexities.
1 - AP reaches terminal
2 - Voltage - gated calcium channels open
3 - Calcium enters axon terminal
4 - Neurotransmitters release and diffuse
5 - Neurotransmitter binds to post-synaptic receptors
6 - Neurotransmitter removed from synaptic cleft.

Question 43

Describe the most common excitatory neurotransmitter.

Answer 43

Glutamate is the most common excitatory neurotransmitter.
It is responsible for fast inhibition in the spinal cord.

Question 44

Describe the most common inhibitory neurotransmitter.

Answer 44

GABA is the most common inhibitory neurotransmitter.
It is responsible for fast inhibition in the brain.

Question 45

Explain the role of synapses in integration of neuronal function.

Answer 45

Synaptic connectivity - divergence and convergence.
Convergence - multiple neurons combine to give just one signal.
Divergence - one neuron verges off into many different signals.
CNS is more complex to has both divergence and convergence ** one cell can be influenced by many others.
Neuromuscular junction only has divergence.

Question 46

Describe EPSPs and IPSPs.

Answer 46

EPSP = excitatory post-synaptic potential.
IPSP = inhibitory post-synaptic potentials
Can be fast (ionotropic)
or slow (metabotropic)

Question 47

Name some of the wide ranging neurotransmitters in the CNS.

Answer 47

Amines
Amino acids
Purines
Peptides
Gases - Nitric oxide.