Week 12 - Excitable Tissues Flashcards

1
Q

What are leak channels

A

Pores in the cell membrane which is open all of the time

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2
Q

What are gated ion channels

A

Pores/transmembrane proteins in the cell membrane which is only open when the cell is stimulated. When they are not activated they are closed and ions cannot travel through

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3
Q

What is intracellular fluid

A

the fluid inside the cell, which is mainly a salt solution (ion solution)

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4
Q

What is extracellular fluid

A

the fluid which surrounds the cell which is also mainly a salt solution (ion solution)

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5
Q

What are the main ions which form action potential

A

sodium (Na+) and potassium (K+)

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6
Q

What is the proportion of Na+ and K+ ions inside and outside of the cell

A

there is a higher concentration of Na+ in the extracellular fluid and a higher concentration of K+ in the intracellular fluid

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7
Q

What is resting membrane potential

A

Resting Membrane potential is the electrical potential difference across the cell membrane of excitable cells when they are not actively transmitting signals. The potential difference exists because the inside of the cell is negatively charged relative to the outside.

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8
Q

How is resting membrane potential formed in excitable cells

A
  • K+ ions have a high permeability through leak channels. Since there is a high concentration of K+ inside the cell in comparison to outside, K+ diffuses out of the cell along its concentration gradient. This makes inside the cell more negatively charged and outside
  • The cell’s “battery” is now charged
  • When the 2 forces of positive and negative charges are balanced and there is no more net movement of K+ it is referred to as the equilibrium potential
  • However whilst potassium diffuses until its concentration it starts to get “pulled back” by the negative charges on the inside of the cell membrane as K+ is positively charged
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9
Q

In the formation of resting membrane potential what is the movement of K+ influenced by

A
  1. The concentration gradient is still driving K+ to leave the cell
  2. Electrical gradient that is pulling K+ back into the cell
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10
Q

What is action potential

A

A regenerating depolarization of membrane potential that propagates along an excitable membrane

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11
Q

What are the factors which allow for the generation of action potential (2)

A
  • A resting membrane potential (polarized capacitor)
  • The cell membrane can briefly become permeable to Na+ ions when the nerve is stimulated
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12
Q

How is action potential created

A
  1. Resting Stage
    - This is the resting membrane potential before the action potential begins
  2. Depolarization Stage
    - When the cell becomes stimulated Na+ ion channels open, and Na+ ions flow down its concentration gradient into the cell (also due to the electrical gradient)
    - This causes the cell to become increasingly more positively charged, and to the extent that it becomes more positively charged than outside the cell
    - This is the action potential
  3. Repolarization Stage
    - Shortly after the formation of the action potential the Na+ ion channels close and the cell membrane is impermeable to Na+
    - The K+ ion channels open and K+ ions pass out of the cell which re-establishes the normal negative resting membrane potential
    - As a result of the action potential the cell has gained some Na+ ions and lost some K+ ions. In oder to get things back to normal a sodium potassium pump (N+/K+ ATPase) uses ATP (active transporter) pumps Na+ ions out of the cell and K+ ions into the cell in order to restore normal concentrations
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13
Q

What are the 3 connective tissue sheath of skeletal muscle

A
  • Epimysium
  • Perimysium
  • Endomysium
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14
Q

What is epimysium

A

(most superficial) An overcoat of dense irregular connective tissue that surrounds the entire muscle. This tissue protects the muscle and reduces friction during contraction.

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15
Q

What is perimysium

A

Fibrous connective tissue that surrounds groups of muscle fibres called fascicles

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16
Q

What is endomysium

A

(deepest) Fine sheath of connective tissue composed of areolar and reticular fibres surrounding each muscle fiber

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17
Q

What do the 3 connective tissue sheaths of muscle form

A

Tendons. Tendons transmits forces across joints or bones.

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18
Q

What is sarcolemma

A

The plasma membrane of the muscle cell which is the outer covering of a muscle, enclosing the sarcoplasm (cell’s cytoplasm). Multiple nuclei lie just under the sarcolemma.

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19
Q

What are t-tubules

A

Specialized structures which penetrate deep into the interior of the muscle cell. It transmits action potential from the cell surface to the interior of the muscle fiber, allowing for muscle contraction.

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20
Q

What is sarcoplasm

A

The cytoplasm of muscle cells. It’s the cellular material that surrounds the myofibrils, which are responsible for muscle contraction.

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21
Q

What are myofibrils

A
  • Myofibrils are the contractile elements of the cell which are densely packed, rod-like, running parallel to the length of the fiber
  • Mitochondria and other organelles are squeezed in between them
  • They make up 80% of the cell
  • Myofibrils within a fibre are perfectly aligned and have a series of repeating dark (a) bands and light (I) bands
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22
Q

What are sarcomers

A
  • The repeating segments of the myofibrils. The Sarcomere is the region of a myofibril between 2 successive Z discs
  • Sarcomeres consist of a dark A band in the middle and half light I bands at each end
  • Z discs are sheets of proteins (connectins) which anchor the muscle filaments and connects myofibrils to one another
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23
Q

What are myofilaments

A

the protein filaments that make up the structural components of myofibrils, the contractile units of muscle cells (thick and thin filaments)

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24
Q

What are the main types of myofilaments (3)

A
  • Thick filaments
  • Thin filaments
  • Elastic filaments
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25
Q

What are thick filaments

A
  • Composed of the protein myosin
  • Located in the centre of the sarcomere
  • They run the entire length of the dark A band
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26
Q

What are thin filaments

A
  • Composed of the protein actin
  • Think filaments are anchored to the Z disk
  • They extend across the light I band and part way into the dark A band
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27
Q

What are elastic filaments

A
  • Composed of protein titin
  • they extend from the Z disc to the thick filaments, then run within the thick filaments
  • They help hold the thick filaments in place, but they also help the muscle spring back into shape after it has been stretched or has contracted
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28
Q

What is the structure of thick filaments

A
  • Comprised primarily of protein myosin
  • Each myosin molecule has a rod like tail and 2 globular heads
  • The tails are attached via a hinge to the heads and are able to attach and form cross bridges with the thin filaments (this requires ATP)
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29
Q

What is the structure of thin filaments

A
  • Comprised of protein actin
  • They are actin subunits which form a long helical chain composed of 2 strands
  • The globular actin subunits contain active sites which myosin heads attach to during contraction however there are regulatory proteins tropomyosin and troponin which covers/blocks the active sites on the actin
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30
Q

What is the H zone

A

only thick filament and no thin filament causing it to have a lighter image

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31
Q

What is A band

A

Where the thick filaments are found

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32
Q

What is I Band

A

where the thick filaments are not

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33
Q

What is M line

A

anchoring area for thick filaments

34
Q

What is Z disc

A

anchoring area for thin filaments - the thick filaments are also attached to the Z disc via titin (elastic filamentous structural protein)

35
Q

In order to contract a skeletal muscle must

A
  1. Be stimulated by a nerve ending
  2. Propagate an electrical current, or action potential, along its sarcolemma
  3. Have a rise in intracellular Ca2+ levels, the final trigger for contraction
36
Q

What is excitation contraction coupling (EC coupling)

A

the process by which an electrical signal triggers a contraction in muscle cells

37
Q

What is the sarcoplasmic reticulum

A
  • Comprised of terminal cisternae and longitudinal tubules
  • terminal cisternae form junctional feet adjacent to the t-tubule membrane
  • It’s an intracellular storage compartment for Ca2+
38
Q

What is the triad

A

The combination of the 3 structures t-tubules, sarcoplasmic reticulum (terminal cisternae and longitudinal tubules) is referred to as the “Triad”

39
Q

How does excitable-contraction coupling work

A
  1. Stimulation of skeletal muscle by acetylcholine (ACh) results in the generation of an action potential in the sarcolemma, shown by the net entry of Na+ ions.
  2. The Action potential (Na+ ions) travels down the T-tubules and triggers the opening of voltage gated calcium channels in the sarcoplasmic reticulum. This leads to an influx of Ca2+ ions.
  3. Ca2+ binds to troponin which causes the blocking action of tropomyosin to cease on think filaments, exposing actin active binding sites
  4. Contraction - myosin heads attach to actin, forcing cross bridges. ATP hydrolysis then powers the movement of the myosin heads, causing the actin filaments to slide relative tot he myosin filaments. This sliding filament mechanism results in muscle contraction
  5. Once action potential ceases Ca2+ ions are pumped back into the sarcoplasmic reticulum
  6. Tropomyosin blockage restored, blocking myosin binding sites on actin; contraction ends and muscle fiber relaxes
40
Q

What is the molecule which stimulate the skeletal muscle

A

acetylcholine (ACh) - type of neurotransmitter

41
Q

What are the molecules which block the binding to the active sites of thin filament

A

troponin and tropomyosin

42
Q

What is contraction

A

refers to the activation of myosin’s cross bridges - force-generating sites

43
Q

What is shortening

A

occurs when the tension generated by the cross bridge exceeds forces opposing shortening

44
Q

When do you know that contraction has ended

A

when cross bridges become inactive the tension generated decline and relaxation is induced

45
Q

What is a motor unit

A

the single motor nerve and all the muscle fibers it supplies

46
Q

what are muscle twitches

A

the response of a muscle to a single, brief threshold stimulus.

47
Q

What are the 3 phases to a muscle twitch

A
  1. Lag Period - Time between stimulation and excitation-contraction coupling
  2. Period of Contraction - Cross bridges form, tension increases and muscle may shorten
  3. Period of Relaxation - Ca2+ re-absorbed and muscle tension returns to zero
48
Q

What is the graded muscle response

A

the variations in the degree of muscle contraction, which is required for proper control of skeletal movement

49
Q

What impacted the degree of muscle contraction

A
  • frequency of stimulation
  • Strength of stimulus
50
Q

How does the frequency of stimulation impact degree of muscle contraction

A

If there are very frequent stimuli the muscle doesn’t have time to completely relax resulting in wave summation (the waves superimpose). If stimuli are given quickly enough it results in tetanus, which is continuous contractions.

51
Q

How does the strength of stimulus impact the degree of muscle contraction

A

The strength of the stimulus is proportionate to the force of the contraction. And force is proportional
If the strength of the stimulus increases so does the force of the contraction once over the threshold stimulus.

The force of contraction increased through the phenomenon called recruitment, which means that more muscle fibers are brought into play.

52
Q

What is the threshold stimulus

A

stimulus strength at which the first observable muscle contraction occurs

53
Q

Does Action potential have a constant amplitude

A

yes

54
Q

What are the properties of action potential (4)

A
  • all or nothing event (either over or under the threshold voltage
  • Have constant amplitude - AP do not summate unlike contractions
  • Have constant conduction velocity
  • Initiated by depolarization which in induced by extrinsic stimulation
55
Q

What initiates an action potential

A

depolarization of membrane potential which is induced by by extrinsic stimulation

56
Q

What is conduction velocity

A

the speed at which the action potential propagates down a neural pathway

57
Q

What influences the conduction velocity of an AP

A
  • Myelination
  • Fiber diameter (larger the diameter the faster it conducts)
  • Temperature
58
Q

What is the absolute refractory period

A

the period after the generation of an action period where it is not possible to generate a subsequent action potential. It occurs due to voltage inactivation of Na+ channels.

59
Q

What is the function of absolute refractory period (3)

A
  • Limits maximum frequency of action potential
  • Ensures that each action potential is separate
  • Enforces one way transmission of nerve impulses
60
Q

What is the relative refractor period

A

where an action potential can be generated if there is a large enough stimulus. An action period can be generated but its more difficult

61
Q

How does action potential propagate down a bare plasma membrane

A

voltage of the action potential will decay quickly and will not be able to propagate down the dendrite as the current leaks across the membrane

62
Q

How does action potential propagate down a un-methylated axon

A

axon which contains voltage gated Na+ and K+ channels but is unmyelinated, it can regenerate the action potential at each point there is an ion channel, so that the voltage doesn’t decay. However the conduction of the action potential is slow because of the movements of ions and of the gates.

63
Q

How does action potential propagate down methylated axons

A

maintains the current of the action potential. Action potential is only generated at the ‘Node of Ranvier’, which are the gaps in between myelin sheath which contain voltage gated ion channels. This enables the AP to prorogate quickly.

64
Q

what is saltatory conduction

A

the mechanism where action potential is propagated along myelinated axons, where the the action potentials basically ‘jump’ from one node to the next

Conduction velocity is high because action potential are only generated at the ‘Nodes of Ranvier, which is faster is compared to continuous conduction along unmyelinated axons.

65
Q

what are the gaps in between myelin sheaths called

A

Nodes of Ranvier

66
Q

How does action potential propagate

A
  1. At a small area of the axon there is an Na+ influx causing the axonal membrane to depolarize, and the positive ions in the nerve axoplasm move towards the negative portion of the membrane
  2. Similarly, positive ions in the extracellular move towards the newly negatively charged area in the excellular fluid.
  3. A current is created that depolarizes the adjacent membrane in a forward direction. This impulse propagates away from its point of origin.
67
Q

What are synapses

A

the specialized junctions between neurons and where they communicate with each other or an effector cell (skeletal muscle cell)

68
Q

What is the process of synaptic signaling (formation of stimulus from nerves)

A
  1. Neurotransmitter synthesis
    - Synaptic vesicles are formed from budding Golgi and are transported to the terminal
    - Acetylcholine (ACh) is the neurotransmitter and is formed in the cytoplasm and is transported into the vesicles
  2. Action Potential Propagation
    - (1)Action potential begins in the ventral horn of the spinal cord and it depolarizes the axon membrane and opens voltage gated Ca2+ channels
  3. Neurotransmitter Release
    - (2)The influx of Ca2+ ions trigger the fusion of the synaptic vesicles with the pre-synaptic membrane (end of the axon) and ACh (neurotransmitters) are released into the synaptic cleft (space in between the end of the axon and the skeletal muscle fiber) via exocytosis
  4. Neurotransmitter Binding
    - (3)Released ACh diffuse across the synaptic cleft and binds to specific receptors which are typically ligand-gated ion channels or G protein couple receptors.
    - This can be a stimulus for the formation of action potential in skeletal muscle
69
Q

What molecule stimulates the release of neurotransmitters

A

calcium

70
Q

What are 2 methods to block the formation of action potential by synaptic signaling

A

This is done by the use of drugs which can either:

  • Are antagonist and blocks the post synaptic binding sites such as the voltage gated Na+ channel e.g. Curare
  • Decrease or prevent the release of neurotransmitters (ACh) from nerve vesicles e.g. Botox
71
Q

How does the release of neurotransmitters produce an action potential in muscle cells

A

the ACh activates the nicotinic channels which cause an initial depolarization in the cell membrane, increasing the voltage to -55mv. This acts as a stimulus which triggers the opening of the voltage gated Na+ channels which create the actual action potential

72
Q

How does botox work

A

In a normally functioning axon there are SNARE proteins in the nerve terminal which bind together to form a synaptic fusion complex. This complex enables the release of the neurotransmitters from the vesicles.

However when the nerve is exposed to botox, the botox inhibit the SNARE proteins from forming the complex, which prevents the release of neurotransmitters into the synaptic clef.

73
Q

Botox works by interrupting which proteins

A

SNAR protein

74
Q

What are nicotinic receptors

A

receptors which respond to the neurotransmitter AcetylCholine (ACh)

75
Q

what are dihydropyridine receptors

A

The voltage sensor which stimulate the ryanodine receptor to release Ca2+ intracellularly

76
Q

what is ryanodine channel

A

it the voltage gated Ca2+ channel in myocytes (muscle cells)

77
Q

What are stimuli which can operate gated channels

A
  • mechanical
  • temperature/heat
  • voltage (action potential, ions)
  • ligands
78
Q

What is the average resting membrane

A

-74mV or 70mv

79
Q

what is the average voltage of action potential

A

30 mV

80
Q

What is voltage which leads to opening the voltage gated sodium channels in a neuron (threshold)

A

-55mV

81
Q

How does the amplitude of the action potential impact the strength of muscle contractions

A

Does not impact it. The amplitude of action potential is constant.

82
Q

What is strength of contraction determined by

A

frequency of action potential