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Tissues > Tissues 6- Nerves > Flashcards

Tissues 6- Nerves Flashcards

1
Q

What does the CNS consist of

A

Two cerebral hemispheres, the brainstem (cerebellum) and spinal cord

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2
Q

What are the most prominent features of the brain

A

The two cerebral hemispheres

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3
Q

What is present below the hemispheres

A

The brainstem and spinal cord

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4
Q

Describe the Peripheral nervous system

A

Nerve fibres originating from the CNS

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5
Q

Describe the appearance of the cerebral hemispheres

A

The cerebral hemisphere (telencephalon) have a distinctive convoluted appearance where the edges are called gyri (singular gyrus). The valleys are called sulci (singular sulcus). The hemisphere is conventionally separated into four functionally distinct lobes.

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6
Q

What are the 4 lobes of the brain, describe their functions

A

Frontal: Responsible for executive functions, such as personality.
Parietal: Contains somatic sensory cortex responsible for processing tactile information
Temporal: Contains important structures such as the hippocampus ( short term memory). Amyglada (behaviour). Wernicke’s area (auditory perception and speech).
Occipital; Processing of visual information.

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7
Q

What does the brainstem consist of

A

The midbrain, pons and medulla in descending order. These structures have a multitude of important functions, such as control of respiration and heart rate. Target or source of all cranial nerves.

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8
Q

What is the cerebellum

A

Hindbrain structure attached to brainstem
Important role in motor coordination, balance & posture
Dorsal region of the CNS

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9
Q

What is the spinal cord

A

Spinal cord
Extends down from medulla:
Conduit for neural transmission
Co-ordinates some reflex actions

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10
Q

What are neurones

A

The communication cells of the CNS. A mature neurone is a non-dividing excitable cell whose main function is to receive and transmit information in the form of electrical signals.

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11
Q

Why can’t neurones be classified on the basis of shape, location or function

A

Due to their polymorphous nature

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12
Q

What features do all neurones share

A

Soma (cell body, perikaryon)- consists of nucleus and lots of ribosomes and neurofilaments for sructure and support accounts for 10% of total SA. Cell body is small compared to the size of the neurone. All neurones have axons and dendrites too.

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13
Q

What are axons

A

Long thin processes that arise from the axon hillock region of the soma. They can branch into collaterals and they are usually covered in myelin

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14
Q

What is the function of an axon

A

They are responsible for transmitting the electrical signals from neurones.

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15
Q

What is myelin and why is it important

A

A high resistance, low capacitance substance, which electrically insulates the axon, allowing for faster transmission of the electrical signal.

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16
Q

How many axons do all neurones have

A

1

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17
Q

Describe the polymorphous nature of neurones

A

Unipolar: 1 axonal projection (rare)
Psuedo-unipolar: Single axonal projection that divides into two
Bipolar: 2 projections from cell body- one axon, one dendrite
Multipolar: Numerous projections from cell body
Pyramidal cells: ‘pyramid’ shaped cell body
Purkinje cells: GABA neurones found in the cerebellum
Golgi cells: GABA neurones found in the cerebellum

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18
Q

What is current research showing about the characteristics of neurones in the hippocampus

A

That they can divide

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19
Q

What are dendrites

A

Thin, but highly branched outgrowths from the cell body, they receive input from other neurones, hence regulating the excitability of the cell body.
They are not covered in myelin.

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20
Q

What are astrocytes

A

Most abundant cell type in mammalian brain. They function as structural cells and they play an important role in cell repair, synapse formation, neuronal transmission and plasticity. They are also able to proliferate. They are also involved in immune reactions in the brains and hence are considered facultative macrophages, they also play a role in neurotransmitter release and reuptake- hence playing a role in homeostasis.

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21
Q

What is the role of foot processes on astrocytes

A

They play a role in maintaining the blood brain barrier

22
Q

What are the characteristics of oligodendrocytes

A

They have variable morphology and function

23
Q

What is the role of oligodendrocytes

A

Numerous projections that form internodes of myelin
One oligodenrodcyte  myelinates many axons
Responsible for myelinating the axons of the CNS.

24
Q

What is the role of Schwann cells

A

They myelinate the axons of the peripheral nervous system.

They can only myelinate a single axonal segment.

25
Q

What is meant by the neuroglia

A

Every other type of cell in the nervous system that is not a neurone.

26
Q

What are microglial cells and their function

A

Specialised cells - similar to macrophages

Perform immune functions in CNS

27
Q

What are ependymal cells

A

Epithelial cells - line fluid filled ventricles

Regulate production & movement of cerebrospinal fluid (CSF)

28
Q

Describe the ionic imbalance in neurones

A

There is an ionic imbalance between the ECF and ICF of a neurone, with an unequal distribution of the major physiological ions (Calcium, potassium, chloride and sodium).High extracellular - Na+ & Cl-
Low extracellular - K+
High concentration gradient for Ca2.

29
Q

Is the plasma membrane permeable to these ions

A

no
Channels cannot couple the ion flow to an energy source, hence they cannot participate in active transport, making the membrane transiently impermeable to these ions.

30
Q

What is the ionic imbalance caused by

A

The activities of various membrane-bound channels and transporters.

31
Q

What is the importance of this ionic imbalance

A

The relative concentrations of these ions is one of the factors that gives the cell membrane an electromotive force or potential difference between the inside and outside of the cell. The outside of the cell is referred to as the zero reference point (0mv). The inside of the cell, in particular the area adjacent to the cell membrane has a negative membrane potential of -50mV- -90mV. RMP of around -70mV. This makes neurones excitable

32
Q

Why is a neurone with an RMP of -90mV less likely to generate an action potential

A

Needs a larger depolarisation.

33
Q

Which two ions play an important role in generating an action potential

A

Na+ and K+ ions

34
Q

Describe RMP in terms of the voltage gated ion channels

A

Voltage-gated Na+ channels (VGSCs) & voltage-gated K+ channels (VGKCs) are closed

35
Q

What happens during depolarisation

A

Membrane depolarisation - opening of VGSC  Na+ influx  further depolarisation

36
Q

What happens during repolarisation

A

VGKCs opens at a slower rate and causes  efflux of K+ from cell  membrane repolarisation

37
Q

Are VGSCs and VGKCs stimulated at the same time to open

A

Yes

38
Q

What is a consequence of the action potential in terms of the ion gradients

A

It leaves an imbalance in the ion gradients, with more Na+ on the inside and more K+ on the outside, need to reverse this to allow another stimulus to generate a new action potential. These ion gradients are restored by the action of the Na+/K+ ATPase pump.

39
Q

Describe the action of the Na+/K+ ATPase pump.

A

Na+ ions enter the vestibule, Na+/K+ ATPase pump is phosphorylated, changing the pump into its active configuration- transporting Na+ to the ECF. K+ then enters the vestibule, the pump returns to its resting configuration- K+ returned into the neurone.

Ion balanced is restored.

40
Q

Describe the nature of the Na+/K+ ATPase pump

A

Reasonably permanent fixture in the membrane- it does not appear to restore ionic concentrations- but it does have a protein turnover time, which is longer than the ion rebalance stage.

41
Q

Describe saltatory conduction

A

AP spreads along the axon by ‘cable transmission’
Myelin prevents AP spreading because it has - high resistance & low capacitance
Nodes of Ranvier - Small gaps of myelin intermittently along axon: These areas have a high density of VGSCS and VGKCs.
AP ‘jumps’ between nodes -saltatory conduction
AP is unable to ‘jump’ across the gap at the axon terminal

42
Q

Can action potentials jump from one neurone to the other

A

No-synapses are required

43
Q

What is the synapse

A

A junction consisting of the pre-synaptic terminal (axon terminal)- separated from the post-synaptic cell (dendrite of another neurone) by an extracellular space known as the synaptic cleft.

44
Q

Describe the transmission of an action potential at the synapse

A

Action potential leads to the opening of voltage gated calcium ion channels at the presynaptic neurone
Influx of Ca2+ ions into presynaptic terminal
This elevation causes synaptic vesicles containing neurotransmitters to fuse with the plasma membrane of the presynaptic neurone.
Fusion of the synaptic vesicles with the terminal membrane- neurotransmitter released by exocytosis.
Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the membrane of the post-synaptic neurone.
Binding of the neurotransmitter to postsynaptic receptors causes VGSCs in the post-synaptic neurone to open/close thus regulating the activity of the post-synaptic neurone.

45
Q

Describe neurotransmitter reuptake

A

NT dissociates from receptor and can be:
Metabolised by enzymes in synaptic cleft
Recycled by transporter proteins

46
Q

Describe the role of astrocytes in nervous transmission across a synapse

A

They are able to release neurotransmitters in response to Ca2+ influx, and are involved in the reuptake of the neurotransmitter.

47
Q

Describe the importance of the Na+/K+ ATPase pump

A

The concentration of ions is far from equilibrium across the membranes, hence when an ion channel opens, ions flow rapidly down their concentration gradients, changing the membrane potential.

48
Q

What are K+ leak channels, and why does K+ not diffuse out of the cell

A

These channels randomly flicker between an open and closed state. When open, they allow K+ to diffuse freely down their electrochemical gradient.
However the transfer of k+ across the plasma membrane leaves unbalanced negative charges on the inside- creating a membrane potential.
This charge imbalance opposes any further movement of K+ out of the cell, hence an equilibrium is established where the membrane potential keeping K+ inside the cell is just strong enough to counteract the tendency of K+ to move down its electrochemical gradient.

49
Q

Describe what is meant by RMP and why it is not zero

A

The flow of negative and positive ions is precisely balanced- so no further difference accumulates across the membrane- Nerst equation can express this quantitatively. Negative in animal cells due to K+ leak channels

50
Q

What is the significance of the change in membrane potential

A

It changes the probability of how many voltage gated ion channels will be open- not how wide they are open.

51
Q

What is the importance of the timer on Na+ ions

A

If the channels continued to respond to altered membrane potential, the cell would get stuck with most of its Na+ channels open. The cell is saved from this state due to the automatic inactivating mechanism that Na+ channels display, a timer that allows them to rapidly adopt a special inactivated conformation even though the membrane is still depolarised. They remain in this inactivated state until the membrane is repolarised. This period of time is known as the refractory period.

Tissues (12 decks)

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