Physiology Flashcards
Physiological functions of skeletal muscles
Maintain posture Purposeful movement in relation to the external environment Respiratory movement Heat production Contribution to whole body metabolism
Describe the three muscle types with regards to action and striations
Skeletal - striated, voluntary
Cardiac - striated, involuntary
Smooth - non-striated, involuntary
What causes a striated appearance under a microscope?
Alternating myosin thick (dark) and actin thin (light) filaments within the muscle
What type of innervation applies to each muscle type?
Skeletal - somatic
Cardiac and smooth - autonomic
What are the main differences between skeletal and cardiac muscle?
Initiation - neurogenic vs. myogenic Motor units vs no motor units NMJ vs gap junctions Ca++ from SR vs ECF + SR Motor unit recruitment and summation vs Frank-Starling
What is the neurotransmitter at the NMJ?
ACh
What is the definition of a motor unit?
A single alpha motor neuron and all of the skeletal muscle fibres it innervates
What differences are there in motor units with regards to function?
No. of fibres within unit - fine actions have fewer fibres e.g. external eye, facial expression, hand muscles.
Precision vs Power
What is the functional unit of skeletal muscle?
Sarcomere
NB: sarcomeres –> myofibrils –> fibres
Function of Tendons
attachment of muscle to bone - lever systems
What is a myofibril?
Specialised contractile intracellular structures, containing alternating thick (myosin) and thin (actin) filaments arranged into sarcomeres
What makes up the boundaries of a sarcomere?
Z lines - connect the thin filaments of two adjoining sarcomeres
What are the 4 zones of a sarcomere?
A band - entire span of thick filaments, including where thin overlap at ends
H zone - lighter area in the middle of the A band where the thin filaments do not reach
M line - middle of A band and H zone
I band - remaining portion of thin filaments not overlapping any thick
Is ATP required for contraction or relaxation (sliding filaments)?
Both
Contraction - to power cross bridges
Relaxation - releases cross bridges and pumps Ca++ back into SR
Define Excitation Contraction Coupling
the process whereby the surface action potential results in the activation of contractile structures of the muscle fibre
What triggers the release of Ca++ from the lateral sacs of the SR?
The spread of the surface action potential down the transverse T-tubules
NB - t tubules are extensions of the surface membrane which stretch into the fibre
How does calcium switch on cross bridge formation?
Ca++ binds with troponin, pulling the troponin-tropomyosin complex aside to expose the cross bridge binding site
Two primary factors influencing the gradation of skeletal muscle tension -
No. of fibres in the muscle - motor unit recruitment allows for stronger contraction
The tension developed by each contracting fibre - depends on the frequency pf stimulation, the summation of contraction, the length of muscle fibres and the thickness of muscle fibres.
Which physiological mechanism helps prevent muscle fatigue?
During submaximal contractions, asynchronous motor unit recruitment prevents muscle fatigue
In skeletal muscle, summation of contraction can occur through repetitive fast stimulation (AP) -T/F?
True
Define summation
If a muscle fibre is restimulated before it has fully relaxed, the second “twitch” is added on top of the first = summation
Define tetanus
Maximum sustained contraction due to rapid fire stimulation with no opportunity to relax
MB - remember that cardiac muscle cannot be tetanised due to refractory period
What is the optimal length and how does this effect contraction?
Optimum length is the point of optimal overlap of thick filament and thin filament cross bridging sites. This allows for maximal tetanic contraction to be achieved.
In the body, resting length is roughly optimal.
How does the contractile component result in the movement of the limb?
Skeletal muscle tension id transmitted to the bone via the stretching and tightening of the elastic component (connective tissue/tendon)
What are the two types of skeletal muscle contraction?
Isotonic - body movements etc, constant tension as length changes
Isometric - supporting fixed objects and posture, constant length
What are the main differences between types of skeletal muscle fibre?
ATP synthesis pathways
Resistance to fatigue
Activity of myosin ATPase (determining speed of contraction via energy available for cross bridge cycling)
What metabolic pathways supply ATP in the muscle fibre?
Immediate - transfer of phosphate from creatine phosphate to ADP
Aerobic - Ox Phos
Anaerobic - glycolysis
3 types of skeletal muscle fibre
Slow Oxidative type I - slow twitch fibres
Fast oxidative type IIa - intermediate twitch
Fast glycolytic type IIx - fast twitch
Main uses of the three muscle fibre types
Slow oxidative - prolonged low work aerobic activity e.g. walking, maintain posture
Fast oxidative - prolonged activity with moderate work e.g. jogging
Fast glycolytic - anaerobic, short term high intensity e.g. jump
Reflex action (def)
a stereotyped response to a specific stimulus
Give an example of a monosynaptic spinal reflex
The stretch reflex
How does the stretch reflex occur?
Negative feedback resisting passive change in length, resulting in maintenance of optimal length sensory receptor (muscle spindle) is activated by muscle stretch, increasing firing in the afferent neurons.
These in turn synapse with alpha motor neurons (efferent limb) in spinal cord to innervate stretched muscle.
What spinal segment and peripheral nerves are involved in each reflex initiated by rubber hammer?
Knee - L3,4 - femoral Ankle - S1,2 - tibial Biceps - C5,6 - musculocutaneous Brachioradialis - C5,6 - radial Triceps - C6,7 - radial
What are muscle spindles?
Sensory receptors for stretch reflex - a collection of specialised fibres, called intrafusal fibres (ordinary fibres = extrafusal), with sensory nerve endings called annulospiral fibres.
What makes up the efferent motor supply of muscle spindles?
gamma motor neurons - adjust tension to maintain sensitivity during shortening
What might impair skeletal muscle function?
Intrinsic muscle disease
Disease of NMJ
Disease of lower motor neurons
Disruption of input to motor nerves
List types of myopathies which may cause intrinsic muscle disease
Congenital e.g. muscular dystrophy, myotonia Inflammatory e.g. polymyositis Non-inflammatory e.g. fibromyalgia Endocrine e.g. Cushings, thyroid disease Toxic e.g. alcohol, statins
Symptoms of muscle disease
Muscle weakness
Delayed relaxation after voluntary contraction (myotonia)
Muscle pain (myalgia)
Muscle stiffness
Ix for neuromuscular disease
EMG Nerve conduction studies (determine functional integrity) Muscles enzymes (CK) Inflammatory markers (CRP, PV) Muscle biopsy
What is EMG?
Electromyography - records the frequency and amplitude of muscle fibres APs to help differentiate between primary muscle disease and neurological disease - non-diagnostic
