Muscular system Flashcards
What are the components of a basic motor unit?
Central motor control area Primary neurone to synapse Spinal chord Motor neuron Neuro-muscular junction Muscle
Brain control of movement
Lateral pathways = voluntary muscle movements from direct cortical control
Ventromedial pathways = posture and locomotion from brain stem control
Why is white matter in the spinal chord so important?
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)
Neurotransmission at the neuromuscular junction
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
Excitation-contraction coupling
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
Describe a myofibril
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
Sliding filament hypothesis
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
What are the two components needed for muscle contraction and their importance?
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
The importance of acetylcholine at the neuromuscular junction
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)
What is neuropathy?
Disease or dysfunction of peripheral nerves
What is myopathy?
Disease of muscle tissue
Which part of the brain is responsible for Parkinson’s disease?
Basal ganglia - facilitates wanted movements, suppresses unwanted movements
Parkinson’s = degeneration of dopaminergic neurones
Which part of the brain is responsible for MS?
Cerebellum - coordination action in posture, movement pathways may be affected
How does botox work?
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
Myasthenia gravis
Autoimmune disease
Antibodies against acetylcholine receptors are produced
- inefficient detection of acetylcholine leads to muscle weakness and fatigue
Motor neurone disease
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
Sciatica
Compression of spinal roots by herniated intervertebral discs
- weakness of leg muscles
- pain in back of thigh
Describe cardiac muscle
- 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
Describe smooth muscle
- 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
Describe skeletal muscle
- 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
Definition of synergist muscles
Muscles that work together/in the same direction
Definition of an antagonist muscles
Works in the opposite direction to a given muscle eg. triceps, quadriceps, extensor digitorum
Definition of agonist muscles/prime mover
Muscle whose contraction is responsible for producing a particular movement eg. biceps, hamstrings, flexor digitorum superficialis and flexor digitorum profundus
Name the 3 layers surrounding a muscle
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
Motor unit
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
Define a muscle unit
Muscle fibres innervated by a single motor neuron
Define a motor unit
Muscle unit plus its motor neuron
Define a motor neurone pool
Collection of neurons innervating a single muscle
What are the 3 types of skeletal muscle fibres?
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
What is muscular dystrophy?
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
What are muscle spindles?
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
Simple tendon jerk reflex
Also known as myotatic/stretch reflex
Reflex can be normal, absent, hyper or hypo-reactive
Describe a tendon reflex arc
- Activation of sensory receptors by muscle stretch with reflex hammer
- Sensory message transported to spinal chord via myelinated afferent axons (in peripheral nerves)
- Afferent neurons synapse onto lower motor neurons in the spinal chord = activation
- Motor message is transported to muscle fibres via myelinated axons
- Contraction of muscle fibres = reflex limb movement
Describe the normal withdrawal reflex
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
Name 4 clinical neurological exams
Tendon reflexes - ankle jerk
Muscle tone
Muscle strength
Pyramidal drift
Observations in clinical neurological exams
Observations - resting posture, symmetry, hypertrophy, wasting, involuntary movements
Coordination, balance, proprioception - heel toe walking
Muscle strength- resistance to applied force
Lower motor neuron lesions
Innervate skeletal muscle directly, located in ventral horn of spinal chord or cranial nerve nuclei
Lesion = weakness, atrophy, decrease muscle tone and tendon reflexes
Upper motor neuron lesions
Originate in higher motor centres ( eg. cerebral cortex), synapse with LMN, relay descending commands
Lesion: weakness, increase tendon reflexes and tone, Babinski
