Lecture 6

Question 1

What are motor neurons responsible for?

Answer 1

Activating skeletal muscle cells

Motor neurons are specialized nerve cells that control muscle fibers.

Question 2

How many motor neurons innervate each skeletal muscle cell?

Answer 2

One motor neuron

Each skeletal muscle fiber is innervated by a single motor neuron.

Question 3

Where does the process of muscle contraction begin?

Answer 3

Higher centers like the brain or spinal cord

Question 4

What initiates the transmission of an action potential down a motor neuron?

Answer 4

A conscious decision to contract a muscle is made by CNS

Message is sent via A alpha fibers. Causing Ca+ to enter motor neuron causing release of Ach by vesicles

Question 5

What is released at the presynaptic terminal when an action potential reaches it?

Answer 5

Neurotransmitters

Question 6

What is the primary neurotransmitter at the neuromuscular junction?

Answer 6

Acetylcholine (ACh)

Question 7

What type of receptors are found on the skeletal muscle cell membrane?

Answer 7

Nicotinic Acetylcholine Receptors (nAChR)

Question 8

What shape do nAChRs have?

Answer 8

Cylinder-shaped ion channels each with 2 Ach receptor sites

Question 9

How many binding sites for acetylcholine does each nAChR have?

Answer 9

Two binding sites

Question 10

What ion primarily moves through the nAChR in skeletal muscle?

Answer 10

Sodium (Na⁺)

Question 11

What happens when ACh binds to nAChR?

Answer 11

The channel opens, allowing sodium ions to flow in

Question 12

What is the result of sodium ions flowing into the skeletal muscle cell?

Answer 12

Depolarization of the skeletal muscle cell membrane

Question 13

What is triggered if depolarization reaches a critical threshold?

Answer 13

Action potential in the muscle cell

Question 14

What are voltage-gated sodium channels crucial for?

Answer 14

Generating and propagating action potentials

Question 15

What are the two main gates of voltage-gated sodium channels?

Answer 15

Activation gate (M gate) and Inactivation gate (H gate)

Question 16

What is the resting state of voltage-gated sodium channels?

Answer 16

Activation gate closed, Inactivation gate open

Question 17

What occurs during the open/activated state of voltage-gated sodium channels?

Answer 17

Both gates open, allowing sodium influx

Question 18

What is the inactivated state of voltage-gated sodium channels?

Answer 18

Activation gate open, Inactivation gate closed

Question 19

What must occur for a voltage-gated sodium channel to reset?

Answer 19

The cell must repolarize to the resting potential

Question 20

Why are these channels called ‘fast sodium channels’?

Answer 20

They open and close very quickly

Question 21

What do local anesthetics typically block?

Answer 21

Fast sodium channels

Question 22

What is the mechanism of action potential propagation?

Answer 22

Local depolarization triggers adjacent voltage-gated sodium channels to open

Once one na channel opens it opens the ones next to it

Question 23

What factors influence the speed of action potential propagation?

Answer 23

• Nerve Diameter
• Nerve Length
• Myelination

Question 24

What is saltatory conduction?

Answer 24

Action potentials jump from one Node of Ranvier to the next

Question 25

What is the primary benefit of myelination?

Answer 25

Increases conduction speed and improves efficiency preventing loss of ions

Question 26

What are the Nodes of Ranvier?

Answer 26

Small, unmyelinated gaps along a myelinated axon

Question 27

What cells produce myelin in the CNS?

Answer 27

Oligodendrocytes

Question 28

What cells produce myelin in the PNS?

Answer 28

Schwann cells

Question 29

What is a consequence of demyelinating diseases?

Answer 29

Failure of action potential propagation Multiple sclerosis , Guillian barre, Neuromyelitis optica

Question 30

What is the function of gap junctions?

Answer 30

Direct electrical connections between neighboring cells Move ions especially na and ca 6 connexins make 1 connexon

Question 31

What are connexons?

Answer 31

Cylinders formed by 6 connexin proteins

Question 32

What ion predominantly moves through gap junctions?

Answer 32

Sodium (Na+)

Question 33

Why are gap junctions important in the heart?

Answer 33

They allow rapid depolarization spread between cardiac cells

Question 34

What determines the electrical resistance through gap junctions?

Answer 34

The number of gap junctions between two cells

Question 35

What is a key feature of chemical synapses?

Answer 35

They utilize neurotransmitters for communication

Question 36

What is the reason for the high electrical resistance in the AV node?

Answer 36

The AV node has a very small number of gap junctions compared to other tissues in the heart ## Footnote This results in a delay in signal transmission through the AV node.

Question 37

What type of receptors are found in skeletal muscles that respond to neurotransmitters?

Answer 37

nACh-R ## Footnote nACh-R stands for nicotinic acetylcholine receptors.

Question 38

What type of receptors are found in the heart that respond to acetylcholine?

Answer 38

mACh-R ## Footnote mACh-R stands for muscarinic acetylcholine receptors.

Question 39

What effect does acetylcholine have at the SA node or AV node?

Answer 39

It slows heart rate ## Footnote This occurs by making the membrane potential more negative, making it harder to excite the cells.

Question 40

What is the effect of opening a K+ channel in a cell?

Answer 40

It makes the cell more negatively charged, leading to suppression ## Footnote Closing K+ channels leads to depolarization of the cell.

Question 41

How do Na+ channels affect cell excitability?

Answer 41

Open Na+ channels make cells more positive (excited) ## Footnote Closed Na+ channels result in less positive, less excited cells.

Question 42

What is the Nernst potential for sodium (Na+)?

Answer 42

+61 mV

Question 43

What is the Nernst potential for potassium (K+)?

Answer 43

-90 mV

Question 44

What are the two gates of slow calcium channels in the heart?

Answer 44

* Inactivation gate (inside gate) * Activation gate (outside gate) ## Footnote At rest, the inactivation gate is open while the activation gate is closed.

Question 45

How does calcium affect muscle contraction?

Answer 45

Calcium is involved in depolarizing cells and muscle contraction ## Footnote Calcium influx is crucial for initiating muscle contraction.

Question 46

What happens when potassium concentration increases to 8?

Answer 46

The K+ Nernst potential becomes more positive than normal (normally -90 moves to around -77) ## Footnote This affects the driving force for potassium to exit the cell. Outside now not as negative, so gradient pull is less on K, leading to less leaking of k. Initially more excited can lead to muscle weakness and arrtyhmias and peaked t waves ,but if becomes too high, Na wont be able to depolarize due to no repolarization

Question 47

What is the treatment for high potassium levels?

Answer 47

* Diuretics * Insulin * Calcium supplements - stabilizes membrane - blocks na channels, making vrm more negative which increases threshold for depolarization . Less excited. Does not lower potassium levels ## Footnote These treatments help manage potassium levels in the body.

Question 48

How does calcium stabilize cells with Na+ leak channels?

Answer 48

Calcium blocks Na+ leak channels, making the membrane potential more negative ## Footnote This results in decreased excitability of the cells.

Question 49

What is the role of magnesium in relation to calcium?

Answer 49

Magnesium likely has a stabilizing effect similar to calcium ## Footnote The exact mechanism of magnesium's action is not fully understood.

Question 50

What is the function of skeletal muscles?

Answer 50

* Locomotion * Communication * Body temperature regulation * Storage of glycogen ## Footnote Skeletal muscles are essential for various bodily functions and responses.

Question 51

What is the structure that connects motor neurons to skeletal muscle cells?

Answer 51

Neuromuscular Junctions

Question 52

What is the sarcolemma?

Answer 52

The cell wall of skeletal muscle cells

Question 53

What is the sarcoplasmic reticulum?

Answer 53

The robust endoplasmic reticulum of skeletal muscle cells

Question 54

What is the difference between small and large motor units?

Answer 54

* Small motor units have small motor neurons and are activated first * Large motor units have large motor neurons and are activated if more force is needed ## Footnote Small motor units are easier to excite, while large motor units are harder.

Question 55

What can excessive positive charge in a cell lead to?

Answer 55

It can wipe out the ability for fast Na+ and slow K+ channels to open, leading to V-Fib ## Footnote V-Fib is a life-threatening condition with no heartbeat and no action potential.

Question 56

What is the average resting membrane potential (vrm) of a typical cell?

Answer 56

-80 mV

Question 57

What happens to the excitability of a cell when it becomes excessively depolarized?

Answer 57

It can lead to a lack of action potentials and muscle contractions ## Footnote This can result in dangerous arrhythmias.