Neuronal Structure and Function

Question 1

Describe the anatomical divisions of the nervous system

Answer 1

Brain and spinal cord make up the CNS. Peripheral nerves, autonomic and enteric nervous systems make up the PNS. The autonomic nervous system innervates blood vessels and internal organs and includes the paravertebral ganglia.

Question 2

What are the infoldings of the brain called?

Answer 2

Sulci, and the smooth spaces between are gyri

Question 3

Grey vs white matter?

Answer 3

grey contains lots of cell bodies, white matter is composed of bundles of nerve fibres.

Question 4

Describe a spinal cord cross section

Answer 4

Grey matter in the centre, white around the edges. The dorsal spinal root and horn (grey matter) are AFFERENT, while ventral are EFFERENT.

Question 5

What is the dorsal root ganglion

Answer 5

An enlargement of the dorsal root containing the cell bodies of the neurons that make it up. Fibres of the ventral root have their cell bodies in the ventral horn of the grey matter.

Question 6

What are the subdivisions of efferent nerves?

Answer 6

Those supplying the skeletal muscles are somatic, while those that supply the viscera and blood vessels are sympathetic efferent nerves.

Question 7

What are the two CNS cell types?

Answer 7

The neurons and the neuroglia

Question 8

What is the neuropil

Answer 8

This is the space between the neuronal cell bodies in the grey matter, and it contains cytoplasmic extensions of both neurons and glia

Question 9

What’s another name for a nerve terminal?

Answer 9

A synaptic bouton

Question 10

What is the name for axon branches?

Answer 10

collaterals

Question 11

Where in the axon are APs generated?

Answer 11

At the axon hillock in a phenomenon known as integration

Question 12

What does a change in dendrite number indicate?

Answer 12

A change in neuron activity over time, its either receiving less or more information in a neural pathway that’s being used more or less over time.

Question 13

Describe glial cells

Answer 13

these support CNS cells and make up about 90% of the cells in the nervous system.

Question 14

What are astroglia?

Answer 14

cells with long processes that attach firmly to blood vessels, joining up to cover the vessels and separate the extracellular fluid of the brain and the blood - the blood brain barrier.

Question 15

What are oligodendrocytes?

Answer 15

about 75% of all glial cells in CNS white matter where they form myelin sheaths - in the PNS this is done by Schwann cells.

Question 16

What are microglia?

Answer 16

Phagocytes scattered throughout white and grey matter

Question 17

What are ependymal cells?

Answer 17

ciliated cells that line the cerebral ventricles, the central fluid filled spaces of the brain, and the central canal of the spinal cord. they form a cuboidal columnar epithelium called the ependyma.

Question 18

What are peripheral nerve trunks?

Answer 18

Axons run to their destinations in these outside the CNS alongside major blood vessels, protected by connective tissue.

Question 19

Describe peripheral nerve trunk structure

Answer 19

Outermost layer is the epineurium, loos aggregate of connective tissue anchoring the nerve trunk to surroundings. Inside, the tough perineurium surrounds fascicles of axons. Endoneurium covers individual nerve fibres.

Question 20

Duration of an action potential?

Answer 20

I mammalian axons, about 0.5 to 1 ms.

Question 21

Absolute vs relative refractory period

Answer 21

If a stimulus is given immediately after an AP is generated, another one can’t be stimulated until after the absolute refractory period. Limits APs per second. Following this, a stronger stimulus is required to stimulate an AP during ~5ms known as the relative refractory period

Question 22

Why is the membrane potential positive at the peak of an AP?

Answer 22

Because the membrane is more permeable to sodium than potassium

Question 23

Why do APs repolarise?

Answer 23

The open state of the Na+ channels is unstable and they show a time dependent inactivation - beginning to inactive after the peak of the action potential. Meanwhile in response to the depolarisation voltage gated K+ channels begin to open and K+ ions move out down electrochemical gradient.

Question 24

Describe the ion movements of an AP

Answer 24

I ain’t writing that just know it

Question 25

Why does a subthreshold stimulus fail to trigger an action potential on an axon, and what is the ionic explanation?

Answer 25

A subthreshold stimulus does not cause enough depolarization to open a sufficient number of voltage-gated sodium channels. Without enough channels opening, the inward sodium current is not large enough to initiate further depolarization. This prevents reaching the threshold membrane potential needed to trigger the positive feedback loop of sodium channel opening, and thus no action potential occurs.

Question 26

What happens at the threshold level in terms of sodium channel activity, and how does this lead to an action potential?

Answer 26

At the threshold, enough voltage-gated sodium channels open to significantly increase sodium permeability. This initial depolarization causes even more sodium channels to open through a positive feedback mechanism. The process rapidly amplifies, leading to full depolarization of the membrane and the generation of an action potential. All available sodium channels eventually open in this cascade.

Question 27

What is the absolute refractory period, and what is its ionic basis?

Answer 27

The absolute refractory period is the time immediately following an action potential during which no new action potential can be initiated, regardless of stimulus strength. This occurs because most sodium channels are in an inactivated state and cannot reopen until they return to their closed (resting) state. They must spend a brief time at the resting membrane potential before they can be reactivated, ensuring a one-way propagation of the action potential.

Question 28

What causes the relative refractory period and why is a stronger stimulus needed during this time?

Answer 28

During the relative refractory period, some sodium channels have returned to their closed (but not inactivated) state and are ready to open again, but not all have recovered. Therefore, a stronger-than-normal stimulus is required to open enough of these recovered sodium channels simultaneously to reach threshold and initiate a new action potential. This period reflects a partial recovery of excitability in the neuron.

Question 29

How does the sodium-potassium pump influence action potential conduction, and what happens if it is inhibited?

Answer 29

The sodium-potassium pump maintains the ionic gradients (high extracellular sodium and high intracellular potassium) necessary for action potential generation by continuously transporting sodium out of the cell and potassium into the cell. During an action potential, small amounts of sodium enter and potassium leaves, but not enough to alter the overall gradient. If the pump is inhibited, such as by metabolic poisoning, the gradients eventually dissipate, and the neuron loses its ability to conduct action potentials. Thus, the sodium pump indirectly powers nerve signaling by preserving the essential ion gradients.

Question 30

What is sensory transduction

Answer 30

The process by a which a stimulus, mechanical, chemical whatever is converted into an AP. Stronger stimulus = more frequent APs

Question 31

What factors determine conduction velocity along an axon?

Answer 31

The internal electrical resistance of the axon, and its membrane's resistance and capacitance.

Question 32

Describe the effect of myelination

Answer 32

increases membrane resistance and decreases its capacitance, so that depolarising influence of an AP spreads further along an axon. With greater current spread, AP can jump between nodes of ranvier in saltatory conduction.

Question 33

What is a compound action potential?

Answer 33

the summed AP of all the fibres in a nerve - graded with stimulus strength unlike in a single axon.

Question 34

Why are several peaks seen when compound action potential is recorded far from its stimulus?

Answer 34

This reflects different conduction velocities of different axons in the nerve. A fibres are fastest and involved in motor control. C fibres are slowest and unmyelinated.