LECTURE 6 Flashcards

1
Q

what is the order of skeletal muscle fibres?

A

muscle-> muscle fibers-> myofibrils-> sarcomere.

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2
Q

What is the sarcomere?

A

functional unit of muscle, has proteins M line, Z line and titin.

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3
Q

What is the sliding filament model?

A

Proposes that a muscle shortens or lengthens by the actin and myosin filaments sliding past each other without changing length
Myosin cross-bridges cyclically attach, rotate, and detach from actin filaments utilizing energy from ATP hydrolysis
Causes a relative change in sarcomere size with force produced at the Z bands (sarcomere border)

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4
Q

Describe the sequence of events in muscle contraction

A

Step 1:
Action potential travelling through axon of motor neuron reaches presynaptic terminal (voltage gated Ca2+ channels open)
Acetylcholine (ACh) is released into synaptic cleft and binds with ACh receptors on muscle membrane (sarcolemma)

Step 2:
Sarcolemma depolarizes
Sarcolemma has projections which extend into the muscle called T (transverse) tubules which allow depolarization within muscle
Step 3:
Depolarization of the T tubule system causes Ca2+ release from the adjacent sarcoplasmic reticulum (storage sacs for Ca2+)

Step 4:
Ca2+ binds to troponin–tropomyosin complex on the actin filaments, causing a structural change and exposing myosin binding site
Step 5:
Myosin head activated through splitting of attached ATP to ADP+P and binds to actin
Myosin head swivels causing release of ADP and sliding of actin to create tension

Step 6:
New ATP binds to the myosin cross-bridge; allowing dissociation of myosin from actin

Step 7:
Cross-bridge activation-dissociation cycle continues as Ca2+ concentration remains high enough to inhibit the troponin–tropomyosin system

Step 8:
Muscle stimulation ceases without motor neuron excitation
Intracellular Ca2+ concentration rapidly moved back into sarcoplasmic reticulum through active transport (requires ATP hydrolysis)

Step 9:
Ca2+ removal restores the inhibitory action of troponin–tropomyosin

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5
Q

How is voluntary movement controlled?

A

controlled from the top down, brain-spinal cord-muscle

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6
Q

What is the role of lower motor neurons?

A

Innervate skeletal muscle with excitation causing contraction
Subjected to excitatory and inhibitory input from upper motor neurons
Cell bodies are located in brainstem and ventral horn of spinal cord

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7
Q

What is the role of upper motor neurons?

A

Provide input to LMNs
Essential for initiation of voluntary movement in humans
Located within the cerebral cortex (motor areas) and areas of the brainstem
Most UMNs synapse with interneurons before synapsing with LMNs

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8
Q

What is the motor unit?

A

consists of the lower motor neuron and the specific muscle fibres it innervates

Provides nerve supply to muscle
It is the individual and combined actions of motor units which produce muscle action
Each muscle fibre generally receives input from only one neuron, yet a single motor neuron may innervate many muscle fibers

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9
Q

what information exits the ventral horn of spinal cord?

A

efferent

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10
Q

what are the three functional characteristics of the motor unit?

A

Twitch (speed) characteristics
Tension (force) characteristics
Fatigability (endurance)

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11
Q

What are the 4 common motor unit categories?

A

Type IIa: Fast twitch, moderate force, high fatigue resistance
Type IIx: Fast twitch, high force, moderate fatigue resistance
Type IIb: Fast twitch, very high force, low fatigue resistance
Type I: Slow twitch, low force, and very high fatigue resistance

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12
Q

What is the all-or none principle?

A

A stimulus strong enough to trigger an action potential in the motor neuron will activate ALL of the accompanying muscle fibres in the motor unit to contract synchronously

A single motor unit does not exert a force gradation; either the impulse elicits an action or it does not

Changes in overall muscle tension are generated through motor unit recruitment strategies

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13
Q

What determines the force of overall muscle action?

A
Increased number (recruitment) of motor units 
A muscle generates considerable force when activated by all of its motor units

Increased frequency of motor unit discharge
Repetitive stimuli reach a muscle before it relaxes to increase the total tension (temporal summation)

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14
Q

What is Hennemans size principles?

A

: is the orderly recruitment of specific motor units to produce a smooth muscle actions

Allows the CNS to fine tune skeletal activity to meet demands of the task

Within a single motor pool, the motor neurons are recruited in order of ascending size (small to large)

As muscle force requirements increase, motor neurons are recruited with progressively larger axons

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15
Q

What are the two purposes of hennemans principle?

A

It minimizes the development of fatigue by using the most fatigue resistant fibers most often (for smaller forces)
It permits equally fine control of force at all levels of force output

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16
Q

What are the three types of temporal summation?

A

wave, unfused tetanus, fused tetanus.

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17
Q

What is wave summation?

A

Second stimulus occurs before muscle fibre has completely relaxed
Increasing strength of second contraction

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18
Q

What is unfused tetanus?

A

Frequency of 20-30 stimuli per second

Sustained but wavering contraction

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19
Q

what is fused tetanus?

A

Frequency of 80-100 stimuli per second

Sustained contraction without changes in force

20
Q

what is rate coding?

A

refers to the motor unit firing rate

Active motor units discharge at higher frequencies to generate greater muscle tension

21
Q

What are the two types of lower motor neurons?

A

alpha and gamma

22
Q

what are alpha motor neurons?

A

associated with large, myelinated axons
Axons project to extrafusal skeletal muscle
Alpha Motor neuron creates motor unit

23
Q

What are gamma motor neurons?

A

consist of medium-sized myelinated axons

Axons of gamma motor neurons project to intrafusal fibers in the muscle spindle

24
Q

What are myotomes?

A

are groups of muscles innervated by a single spinal nerve

25
Q

What is LMN pool?

A

are groups of cell bodies whose axons project to a single muscle (may utilize multiple spinal nerves)

26
Q

What are the 4 anatomical positions in LMN pool and what do they stimulate?

A

Medial: Axial and proximal muscles
Lateral: Distal muscles
Anterior: Extensors
Posterior: Flexors

27
Q

What is reciprocal inhibition?

A

inhibition of antagonist muscles during agonist contractions
Important with voluntary movement to prevent stretch reflex of antagonist muscles

28
Q

What is proprioceptive body schema?

A
spinal cord creates a complete proprioceptive model of the body in real time (schema)
Proprioceptive input (afferent) to  spinal cord relayed via interneurons to lower motor neurons (muscle synergies) and to the cerebellum (anterior spinocerebellar and rostrospinocerebellar  tracts)
29
Q

What is muscle synergies?

A

term indicating coordinated muscular action or groups of muscles that are activated together
Proprioceptive input is interpreted at the level of the spinal cord to alter muscle patterns to perform task

30
Q

what is phasic stretch reflex?

A

muscle contraction in response to quick stretch (also called - Myotatic reflex, deep tendon reflex, tendon tap reflex)

Reflex arc is monosynaptic indicating the reflex utilizes one synapse between the primary afferent neuron and the efferent motor neuron

During the stretch reflex a quick muscle stretch is detected by muscle spindles

31
Q

What is withdrawal reflex and what is the result?

A

Involves painful stimulus detected by nociceptor
Results in:

Activation of flexor and inhibition of extensor motor neurons (ipsilateral – same side of body)
Moves body away from painful stimulus

32
Q

What is crossed extension reflex and what does it result in?

A

Activated simultaneously when a withdrawal (flexion) reflex is initiated
Is a crossed reflex, meaning the effect is contralateral to the stimulus
Results in:

Activation of extensor and inhibition of flexor motor neurons (contralateral – opposite side of body)
Weight support for contralateral limb allowing the other limb to engage in a flexion reflex

33
Q

What are the three tracts for UMNs?

A

medial, lateral and anterior nonspecific

34
Q

what is the role of medial UMNs?

A

innervate postural and girdle muscles

35
Q

what is the role of lateral UMNs?

A

innervate muscles used for fine control (fractionated movement) of muscles of body and face

36
Q

What is the role of anterior nonspecific UMNs?

A

contribute to background levels of excitation in the spinal cord and reflex arcs

37
Q

What tracts are involved with Medial UMNs?

A

recticulospinal tracts, medial vestibulospinal tracts, lateral vestibulospinal tract and medial corticospinal tract

38
Q

What tracts are involved in lateral UMNs

A

Lateral Corticospinal Tract

Rubrospinal tract

39
Q

What is fractionation?

A

is the ability to activate individual muscles independently of other muscles
Is essential for normal fine movement and control of the hands
Without fractionation, the fingers and thumb would act as a single unit, as they do when picking up a water bottle

40
Q

What is the lateral corticospinal tract?

A

Most important pathway controlling voluntary movements

Tract arises from primary motor cortex , and motor planning areas

Axons of UMNs descend in tracts to the medulla where they decussate and travel down the spinal cord in the white matter

Synapse with LMNs to activate muscle for contraction and movement

Provides fractionation by stimulating LMNs to specific muscles and activating inhibitory neurons to prevent unwanted muscles from contracting

41
Q

What is the rubrospinal tract?

A

has minor contribution to motor control in humans

Primarily facilitates extensor muscles of upper limbs

42
Q

What is the primary motor cortex?

A

Located in the precentral gyrus (anterior to the central sulcus)
Neurons organized somatotopically in the motor homunculus

43
Q

What is the premotor area?

A

Controls large muscle groups that span multiple joints (trunk and girdle)
Utilized in anticipatory postural adjustments
Associated with reticulospinal tract

44
Q

What is the supplementary motor area?

A

Area is activated before performing many movements that require coordination of both hands
Example: buttoning your shirt
Organizes order of sequential movements
Example: socks before shoes

45
Q

What is the Broca’s area?

A

Plans the motor movements of speech

46
Q

What tracts are involved with nonspecific UMNs and where do they originate?

A

Raphespinal Tract: arises from Raphe nuclei in the brainstem
Primary source of serotonin in the spinal cord

Ceruleospinal Tract: arises from coeruleus nuclei in the brainstem
Releases norepinephrine in the spinal cord