Week 2 - D - Pharmacology 4 - Motor units and muscle spindles, golgi tendons and joints

Question 1

The neural element of the motor system comprises upper motor neurones (UMNs) within the brain and lower motor neurones (LMNs) with soma within the brain stem and ventral horn of the spinal cord What supplies the input to the lower motor neuron to modulate its activity?

Answer 1

The upper motor neuron supplies the lower motor neuron with input

Question 2

Lower motor neurons are comprised of two separate neurons, the alpha and gamma neurons What do each of these supply?

Answer 2

The alpha motor neurons innervate the bulk of the muscle fibre that generate force The gamma motor neuron innervates a sensory organ within the muscle known as the muscle spindle with efferents

Question 3

Biceps brachii and brachialis (as flexors) oppose triceps brachii and anconeus (as extensors), these groups are antagonists to each other What are the movements that control movement of the trunk maintaining posture?

Answer 3

This would be the axial muscles

Question 4

What are the intrinsic and extrinisc muscles of the back?

Answer 4

Extrinsic back muscles - Trapezius, rhomboid, lattisimus dorsi, levator scapulae Intrinsic back muscles - erector spinae (iliocostalis, longissimus and spinalis) and the transversospinalis

Question 5

What muscles are the flexors and extensors of the arm? When the muscles in the compartment are working together, what is this known as?

Answer 5

The biceps brachii and brachialis flex the arm The triceps brachii and aconeus extend the arm Known as synergistic muscles when they work together

Question 6

Axons of lower motor neurons exit the spinal cord via which roots?

Answer 6

They exit the spinal cord via the ventral roots

Question 7

Each ventral (anterior) root joins with a dorsal (posterior) root to form a mixed spinal nerve containing sensory and motor fibres Which areas of the spinal cord have very large ventral horns and why?

Answer 7

The cervical and lumbar regions have large ventral horns as there are more cell bodies here as they supply the upper and lower limbs respectively

Question 8

Motor neurones (aka motoneurones) as said are not distributed evenly as there are a greater number in the cervical and lumbar enlargements What do vertebra these enlargements span to?

Answer 8

Cervical enlargement - C3-T1 Lumbar enlargement - L1-S3

Question 9

In the dorsal column, the fascicuclus cuneatus recieves the sensory info from the upper limbs and upper half, the fasciculus gracilis recieves info from the lower half and liimbs What vertebral level is the switch? Is the gracilis or cuneatus more medial in the dorsal column of the spinal cord? THIS IS A RANDOM FLASCHARD

Answer 9

T6 - T6 and above - fasciculus cuneatus T6 and below fasicuclus gracilis Fasiculus gracilis is medial to fasiciculs cuneatus in the spinal cord

Question 10

What is a motor unit? It is the smallest functional component of the motor system

Answer 10

A motor unit is an alpha motor neuron and all the skeletal fibres it collectively innervates

Question 11

Muscle contraction results from the individual and combined actions of motor units which must be co-ordinated What is the collection of a-MNs innervating a single muscle known as?

Answer 11

This is referred to as the motor neurone pool

Question 12

Force of muscle contraction is graded by α-MNs by two principle mechanisms: What are these two principles?

Answer 12

The freuqency of action potential discharge of each a-MN The recruitment of additional synergistic, motor neurons

Question 13

Each action potential released by each alpha motor neuron causes a muscle twitch What is a twitch?

Answer 13

A twitch is rapid muscle contraction followed by relaxation hence why more frequent release of action potentials is required to keep the muscle contracting for longer

Question 14

The cell bodies of LMNs show a distinct (somatotopic) distribution in the ventral (anterior) horn Where do the LMNs supplying the axial muscles, vs distal muscles and the flexors vs extensors lie in relation to one another?

Answer 14

LMNs supplying the axial muscles lie medially to those innervating distal muscles LMNs supplying flexors lie dorsal to the ones supplying extensors which lie ventrally

Question 15

What are the three input source to an alpha motor neuron that regulate its activity?

Answer 15

Input from the central terminals of dorsal root ganglion who innervate the muscle spindles The UMNs in the motor cortex and brainstem SPinal interneurons

Question 16

WHat neuron innervates the muscle spindles?

Answer 16

The gamma motor neuron (a type of LMN)

Question 17

Neuromuscular activation depends on The firing rates of the LMNs involved The number of a LMNs that innervate a muscle (i.e. number of motor units) What are the total number of motor units innervating a single muscle known as?

Answer 17

The motor neuron pool

Question 18

Force production by innervated muscle fibres depends on what two things?

Answer 18

The fibre size (hypertrophy) The fibre phenotype - fast or slow contracting muscle

Question 19

As the degree of tension in the muscle increases, small motor units is recruited first and larger motor units are recruited later What do we mean by small and large motor units?

Answer 19

A small motor unit means there is a small number of fibres supplied by a single alpha motor neuron A large motor unit means there is a large number of fibres supplied by a single alpha motor neuron

Question 20

Give an example of which bodily parts would have small motor units or large motor units?

Answer 20

Small motor units for when a specific function is required -eg extraocular muscles Large in large muscles eg the quadriceps femoris

Question 21

Small motor units are innervated by small α-MNs (soma diameter) whereas the converse is true for large motor units Force of contraction that each motor unit produces is dependent upon its size as well as dependent on its phenotype What are the three phenotypes of fibre?

Answer 21

Type 1 - Slow oxidative Type IIa - Fast oxidative Type IIb - Fast glycolytic

Question 22

Two major types of muscle fibre: Fast and Slow Differ in how fast myosin ATPase splits ATP (different myosin ATPase isoforms and activities) to provide energy for cross bridge cycling. Reflected in time to develop peak tension Which skeletal muscle fibre is not fatigue resistant? Which recieve their ATP via oxidative phosporylation? Which is not very well vascularised and is thereofre white and not red?

Answer 22

Slow oxidative - well vascularised, therefore red, receives ATP via oxidative phosphorylation and is fatigue resistant Fast oxidative - reasonably well vascularised, therefore still red, receives ATP via oxiative phosphorylation and is fatigue resistant Fast glyoclytic - poorly vascularised and therefore white, recessives ATP via glyoclysis and is not fatigue resistant

Question 23

What stores the oxygen in the muscle?

Answer 23

Myoglobin

Question 24

Slow fatiguing Intermediate size a-MNs Type IIa fibres What fibre is this?

Answer 24

This is Fast oxidative

Question 25

Typically this fibre is used in sprinting and is not fatigue resistant Which fibre?

Answer 25

Type IIb - Fast glycolytic

Question 26

Low tension Fatigue resistant Small α-MN, low threshold Type I fibres Which one?

Answer 26

Type I - Slow oxidative

Question 27

Separate point from lecture Just revision How long does ATP provide energy for? What provides energy for 15 seconds? What provides energy for 4 minutes? What provides energy for 77 miinutes? How long does fat stores provide energy for?

Answer 27

• ATP - energy for 4 seconds
• Phosphocreatinine - energy for 15 seconds
• Free-circulating glucose - energy for 4 minutes
• Glycogen stores - energy for 77 minutes
• Fat stores - energy for 4+ days

Question 28

Motor units when generating force in a muscle are recruited in a specific order As tension increases which motor units are recruited? and therefore which fibres?

Answer 28

Smaller motor units are recruited as small tension and therefore as tension increases larger and larger motor units are increased Hence slow oxidative fibres are recruited initially before fast oxidative and finally fast glycolytic at high tensions

Question 29

When a skeletal muscle is pulled, it pulls back What is this reflex known as?

Answer 29

This is the myotatic reflex (stretch reflex)

Question 30

Change in length (and rate of change) is registered by a sensory organ within the muscle, the muscle spindle. Contributes to non-conscious proprioception What do the spindles consist of that contribute to this moyotatic reflex?

Answer 30

The spindle consists of A fibrous capsule INtrafusal muscle fibres (extrafusal generate force of contraction) Sensory afferents Gamma motor neurons efferents innervating the intrafusal fibres

Question 31

Which type of sensory receptors are the ones for proprioception?

Answer 31

This would be Type 1a sensory receptors

Question 32

Stretch of muscle spindle Activation of Ia afferent Excitatory synaptic transmission in spinal cord (monosynaptic) mediated by which neurotransmitter? This leads to activation of the a-MN efferent Contraction of homonymous muscle

Answer 32

The excitatory synaptic transmission is mediated by glutamate

Question 33

The Myotatic Reflex Can be Used to Assess the Functional Integrity of the Spinal Cord at Specific Levels What muscle/joint is assessed at these levels C5/6 C7 L3/4 S1

Answer 33

C5/6 biceps (elbow) C5/6 brachiordialis (wrist) C7/(8) triceps (elbow) L3/4 quadriceps (knee) S1/(2) Gastrocnemius (ankle)

Question 34

The Intrafusal Fibres of Muscle Spindles are Innervated by Gamma Motor Neurones (γ-MNs) The intrafusal fibres also consits of a non contactile equatorial region innervated by what?

Answer 34

This region is innervated by Type 1a sensory afferents

Question 35

The gamma motor neurones are stimulated to fire by higher centres in the CNS and they are subject to descending control rather than simple reflex control In normal motor movmeent, which neuron is active causing contraction of the intra or extrafusal fibres? DO intrafusal and extrafusal fibres contract in series or in parallel?

Answer 35

In normal motor movement both alpha and gamma motor neurons are active causing contraction of the intrafusal and extrafusal fibres together Intrafusal and extrafusal fibres contract in parallel

Question 36

Stimulation of alpha and motor cause the contraction to be maintainted What defecit would stimulation of the alpha motor neuron alone cause?

Answer 36

This would cause the loss of the 1a sensory afferent and the loss of the proprioception in the muscle/joint

Question 37

Golgi tendon organs: are located at the junction of muscle and tendon monitor changes in muscle tension, they are in series with, rather than parallel to, extrafusal fibres What group of snesory afferents are the golgi tendon organs innervated by?

Answer 37

The golgi tendon organs are innervated by Type Ab (group II) sensory afferents

Question 38

What do golgi tendon organs act to prevent the muscles from doing? (they activate a reflex)

Answer 38

The golgi tendon organs regulate tension in the muscle to prevent the muscle being overstretched and will trigger a reflex should the muscles overstretch (eg in weight lifting)

Question 39

What is the reflex triggered by the golgi tendon known as?

Answer 39

This is the inverse myotatic reflex

Question 40

When the Ab (group II) afferent neurones carrying the sensory information from the golgi tendons enter the spinal cord, what happens to activate the inverse myotatic reflex?

Answer 40

The afferent neurones synapse on inhibitory interneurons which in turn synapse on the alpha motoneurons of the homonymous muscle forming the basis of the inverse myotatic reflex Due to the inhibitory interneurones synapsing on the same contracting muscle, this will hypepolarizze the muscle stopping contraction

Question 41

Golgi tendon organs gauge the force generated by a muscle by measuring the tension in its tendon What neruotransmitter does the inhibitory interneurone release to hyperpolarize the alpha motor neuron and cause relaxation of the muscle?

Answer 41

The neurotransmitter released by the interneuron is glycine Extreme examples of this are weight lifters – when a massive force is put on the muscle (eg trying to life the weight, to protect the tendon form rupture the inverse myotatic reflex will reduce contraction of the muscle causing it to drop the weight

Question 42

Proprioceptive axons are present in connective tissue of joints (e.g. joint capsules and ligaments) This also prevents excessive flexion and extension of the joint Thereofore give the three places proprioceptive information arises from?

Answer 42

Proprioceptive information arises from The muscle spindle (no gamma motor neurones means no proprioception here) The golgi tendons - provide the reflex myotatic reflex The connective tissue of joints

Question 43

It is important that to ensure overstretching of the muscle does not occur there is the reverse (inverse) myotatic reflex - internuerones play an important role in this During the myotatic reflex (jerk reflex), the antaonist of the contracting muscle must relax. What is this process known as?

Answer 43

This is recpirocal inhitbiton - basically need the excitatory interneurones to send signals to the muscles that are contracting whilst send inhibitory interneurones to the muscles that need to relax in order for the reflex to occur

WILL SPEAK MORE ABOUT THIS IN WEEK 4