Muscular system

Question 1

What are the components of a basic motor unit?

Answer 1

Central motor control area
Primary neurone to synapse
Spinal chord
Motor neuron
Neuro-muscular junction
Muscle

Question 2

Brain control of movement

Answer 2

Lateral pathways = voluntary muscle movements from direct cortical control
Ventromedial pathways = posture and locomotion from brain stem control

Question 3

Why is white matter in the spinal chord so important?

Answer 3

White matter = information highway
Ascending tracts: sensory inputs
Descending tracts: motor inputs (autonomic and somatic)
Cell bodies of motor neurons lie within the ventral horn of the spinal chord (grey matter)

Question 4

Neurotransmission at the neuromuscular junction

Answer 4

A neuron is activated when the membrane potential reaches a threshold level, triggering an action potential.
This triggers an action potential.
1. Action Potential
2. Ca2+ entry
3. Vesicle fusion
4. ACh release
5. ACh receptor activation
6. Current Propogation
7. ACh removal

Question 5

Excitation-contraction coupling

Answer 5

Excitation of a muscle cell causes its contraction nearly simultaneously
Made possible by:
- close proximity between sarcolemma/T tubule, sarcoplasmic reticulum and myofibrils
Myofibrils surrounded by sarcoplasmic reticulum
T tubules continuous with extracellular fluid
Depolarisation of t-tubule membrane = big release of Ca+ into cystol from different sources

Question 6

Describe a myofibril

Answer 6

Sarcoplasm contains many long, protein-rich structures called myofribrils
Made up of:
- actin (thin strands)
-myosin (thick strands)
which are organised into sarcomeres = basic contractile unit of a muscle fibre

Question 7

Sliding filament hypothesis

Answer 7

AT REST: troponin (protein) covers actin filament

• when calcium is released from SR due to action potential, calcium binds to troponin exposing the actin
• myosin head binds to exposed sites and pivots, shortening myosin filaments = contraction
• ATP is needed to disengage the myosin head = muscle relaxation

Question 8

What are the two components needed for muscle contraction and their importance?

Answer 8

ATP
- obtained from creatinine phosphate, aerobic and anaerobic metabolism
- allows for detachment of myosin head from actin binding sites
- rigor mortis if no ATP
CALCIUM
- at rest is stored in sarcoplasmic reticulum
- action potential triggers release from SR, binding to actin to expose actin binding sites, myosin head attaches = contraction
- active re-uptake by SR
- myopathies, statins-induced myotoxicity

Question 9

The importance of acetylcholine at the neuromuscular junction

Answer 9

Is the neurotransmitter released at the synapse to propagate action potential
Broken down by ACETYLCHOLINESTERASE
into choline and acetic acid which is reabsorbed (choline transporter)

Question 10

What is neuropathy?

Answer 10

Disease or dysfunction of peripheral nerves

Question 11

What is myopathy?

Answer 11

Disease of muscle tissue

Question 12

Which part of the brain is responsible for Parkinson’s disease?

Answer 12

Basal ganglia - facilitates wanted movements, suppresses unwanted movements
Parkinson’s = degeneration of dopaminergic neurones

Question 13

Which part of the brain is responsible for MS?

Answer 13

Cerebellum - coordination action in posture, movement pathways may be affected

Question 14

How does botox work?

Answer 14

Disrupts SNARE complex formation - synaptic vesicles can’t release acetylcholine into the NMJ = muscles do not get signal to contract
Therapeutic application for cerebral palsy

Question 15

Myasthenia gravis

Answer 15

Autoimmune disease
Antibodies against acetylcholine receptors are produced
- inefficient detection of acetylcholine leads to muscle weakness and fatigue

Question 16

Motor neurone disease

Answer 16

Progressive muscle weakness and atrophy
Degeneration of large alpha motor neurons and neurons od the motor cortex that synapse onto these
Causes: excitotoxicity, build up of harmful free radicals

Question 17

Sciatica

Answer 17

Compression of spinal roots by herniated intervertebral discs

• weakness of leg muscles
• pain in back of thigh

Question 18

Describe cardiac muscle

Answer 18

• Small, branched cells interconnected to other
  cardio myocytes, centrally located nucleus, striated
• rely on cardiac self-contractile pacemaker cells for contractions
• functions are blood circulation and to maintain hydrostatic BP

Question 19

Describe smooth muscle

Answer 19

• Small, spindle-shaped cells with tapered ends, centrally located nucleus, non-striated
• located in walls of blood vessels, hollow organs, respiratory, digestive, CV and reproductive tracts
• can be self-contractile/ require innervation from autonomic nerves
• functions are movement of food, urine and reproductive tract secretions, regulation of diameter of airways/ blood vessels

Question 20

Describe skeletal muscle

Answer 20

• Very long, striated cells, multinucleated
• located throughout the body, associated with bones and connective tissue
• innervated by somatic and voluntary motor nerves
• functions are movement of skeleton, guards entrances to respiratory/digestive/urinary tracts, protects internal organs, nutrient store

Question 21

Definition of synergist muscles

Answer 21

Muscles that work together/in the same direction

Question 22

Definition of an antagonist muscles

Answer 22

Works in the opposite direction to a given muscle eg. triceps, quadriceps, extensor digitorum

Question 23

Definition of agonist muscles/prime mover

Answer 23

Muscle whose contraction is responsible for producing a particular movement eg. biceps, hamstrings, flexor digitorum superficialis and flexor digitorum profundus

Question 24

Name the 3 layers surrounding a muscle

Answer 24

Epimysium: (outer layer) dense collagen layer surrounding the muscle, separating it from the nearby tissue
Perimysium: (middle layer) divides muscles into fascicles, contains collagen, elastic fibres, nerves and blood vessels
Endomysium: (inner layer) flexible layer inside fascicles, contains fine capillaries and nerve supply

Question 25

Motor unit

Answer 25

Basic unit of contraction

• typical muscle controlled by 100 motor neurons
• each muscle fibre is normally innervated by only one motor neuron in only one place

Question 26

Define a muscle unit

Answer 26

Muscle fibres innervated by a single motor neuron

Question 27

Define a motor unit

Answer 27

Muscle unit plus its motor neuron

Question 28

Define a motor neurone pool

Answer 28

Collection of neurons innervating a single muscle

Question 29

What are the 3 types of skeletal muscle fibres?

Answer 29

Slow - slow contraction, low force generated but high endurance (fatigue resistance), red muscle, oxidative enzymes aerobic metabolism eg. marathon
Fast (a) - fast contraction, medium force developed and endurance, (fatigue resistance), oxidative and glycolytic enzymes, mixed metabolism
Fast (b) - fast contraction, high force generated but low endurance(fast fatiguing), glycolytic enzymes, anaerobic metabolism eg. sprint

Question 30

What is muscular dystrophy?

Answer 30

Total lack of mRNA for the dystrophin protein
X linked inheritance
Long term damage caused from friction of fibres without proper anchoring causes muscle degeneration

Question 31

What are muscle spindles?

Answer 31

Sensory organs within our muscles
Mechanosensitive channels = cause sensory neurons to fire more when muscle spindle is stretched
Causes muscle to contract in response so muscles aren’t damaged by excessive stretch

Question 32

Simple tendon jerk reflex

Answer 32

Also known as myotatic/stretch reflex

Reflex can be normal, absent, hyper or hypo-reactive

Question 33

Describe a tendon reflex arc

Answer 33

1. Activation of sensory receptors by muscle stretch with reflex hammer
2. Sensory message transported to spinal chord via myelinated afferent axons (in peripheral nerves)
3. Afferent neurons synapse onto lower motor neurons in the spinal chord = activation
4. Motor message is transported to muscle fibres via myelinated axons
5. Contraction of muscle fibres = reflex limb movement

Question 34

Describe the normal withdrawal reflex

Answer 34

Crossed-extensor reflex: reciprocal inhibition inhibits flexors on opposite side of the body.
Extensor muscle activation helps stabilise extra load on the opposite limb.
Complex reflex arc involving excitatory and inhibitory interneurons.
Pain causes all flexors to be stimulated, moving limb away
Clinical testing:
- flexor plantar reflex/ Babinski sign
- short instrument stroked on sole of foot:
> big toe flexion - normal
> big toe extension - abnormal

Question 35

Name 4 clinical neurological exams

Answer 35

Tendon reflexes - ankle jerk
Muscle tone
Muscle strength
Pyramidal drift

Question 36

Observations in clinical neurological exams

Answer 36

Observations - resting posture, symmetry, hypertrophy, wasting, involuntary movements
Coordination, balance, proprioception - heel toe walking
Muscle strength- resistance to applied force

Question 37

Lower motor neuron lesions

Answer 37

Innervate skeletal muscle directly, located in ventral horn of spinal chord or cranial nerve nuclei
Lesion = weakness, atrophy, decrease muscle tone and tendon reflexes

Question 38

Upper motor neuron lesions

Answer 38

Originate in higher motor centres ( eg. cerebral cortex), synapse with LMN, relay descending commands
Lesion: weakness, increase tendon reflexes and tone, Babinski