Skeletal muscle is innervated by the somatic/autonomic nervous system
Somatic
Smooth and cardiac muscle are innervated by the somatic/autonomic nervous system
Autonomic
What is a motor unit?
One alpha motor neurone and all the muscle fibres it supplies
What is the correlation between precision control and motor unit size?
the more precise control the movement requires, the less muscle fibres are involved in the motor unit
Where has the largest ratio of neurones to muscle fibres in a motor unit in the human body?
The extraoccular muscles
Neurones:fibres = 1:3
Contrast with gastrocnemius
neurones:fibres = 1:1000-2000
What is the correlation between strength of muscle and motor unit size?
the more power (with less precision) required, the more muscle fibres are supplied by a single neurone.
Each muscle fibre is formed from many cells - true/false
False - each muscle fibre is a single cell
Each muscle fibre is formed from myofibrils - true/false
True
Myofibrils are the smallest contractile unit of a muscle - true/false
False
Sarcomeres are the smallest functional contractile units of skeletal muscle - true/false
True
Skeletal muscle usually attaches to the skeleton via _____ to give a lever system
Tendons
A myofibril contains alternating bands of thick ______ filaments and thin ______ filaments
Myosin
Actin
The A-band contains many/few myosin filaments with many/few actin filament
Many
Few
The H-zone is a region within the _____ ______ that appears slightly lighter
A-band
The H-zone contains only myosin/actin filaments
Myosin
The I-band consists of thin actin filaments and/without thick myosin filaments
Without thick filaments
The H-zone shortens in contraction, the A-zone remains the same/shortens and the I-zone stays the same - true/false
False H-zone shortens A-zone remains the same I-zone shortens Entire sarcomere shortens
ATP is required for both contraction and relaxation of smooth muscle. True/False
True
Ca2+ ions are required to allow ATP to bind to myosin heads. True/false
False - Ca2+ is required to allow cross bridge formation
How does Ca2+ affect muscle contraction
Ca2+ affects muscle contraction as it displaces the troponin-tropomyosin complex and exposes the myosin cross-bridge binding sites on the actin filaments
What are the two factors affecting the production of tension in a muscle?
Number of muscle fibres contracting
tension developed by each contracting fibre
In skeletal muscle the duration of the AP is much shorter/longer than the twitch.
Shorter
A shorter AP than a twitch means that your muscles can repeatedly contract to give the same amount of force. True/false
False - muscle repeatedly contracting gives an increased force - summation of twitches
What is tetanus?
Tetanus is when the muscle fibres are repeatedly stimulated to contract with no relaxation between stimuli and a maximum sustained contraction is generated
Cardiac muscle cannot be tetanized. True/false
true
Why can cardiac muscle not be tetanized?
Its relatively long refractory period prevents overstimulation
Tension within a contracting muscle increases/decreases with increasing frequency of the muscle stimulation
Tension within a contracting muscle increases with increasing frequency of the muscle stimulation
Give the relationship between length of fibre and contraction tension
Maximum contraction is achieved when the muscle fibre is at its optimum length before onset of contraction
Why does optimal length give maximum contraction?
the optimal length is the point at which the most actin binding sites and myosin cross bridges overlap.
How does a stretched fibre affect the strength of the contraction?
There is less/insufficient overlap of actin binding sites and myosin cross bridges and so contraction is less/doesnt occur
How does a shortened fibre affect the strength of the contraction?
there is plenty of overlap but the muscle cannot shorten much further and so contraction is decreased
In the body a resting muscle is approximately its optimal length - true/false
True
Skeletal muscle tension, resulting from cross bridge cycling, is transferred to bone via dilation and relaxation of tendons and muscle connective tissues - true/false
False - transferred via the stretching and tightening of connective tissues and tendons
There are two types of muscle contraction - name them
Isotonic and isometric
What is isotonic contraction?
Contraction where muscle tension remains constant but muscle length changes
What is isometric contraction?
Muscle tension develops across the muscle but fibre length remains constant.
Give TWO examples of when an isotonic contraction is used
(1) For body movements
(2) for moving objects
Give TWO examples of when an isometric contraction is used
(1) supporting objects in a fixed position
(2) maintaining body posture e.g. while walking
What is the relationship between load and the velocity at which muscles shorten?
the bigger the load, the slower the muscle shortens
What is a congenital myopathy?
A condition, present at birth, which shows microscopic histological changes leading to reduced contractility of muscles
Apart from congenital myopathy, name three (of four) other causes of intrinsic muscle diseases
Acquired myopathy e.g. polymyositis
Endocrine myopathy e.g. Cushing’s Syndrome, thyroid disease
Toxic myopathy e.g. from alcohol or statins
Chronic degeneration of contractile elements e.g. Muscular dystrophy
What is a reflex?
A stereotyped response to a specific stimulus
What is the simplest form of co-ordinated movement?
A reflex
Why are the neural pathways of reflexes important?
They allow for localising lesions in the motor system.
Where are pathways responsible for reflexes located?
At various levels along the motor system
What is the simplest example of a monosynaptic spinal reflex?
The stretch reflex.
Stretch reflexes are a positive feedback mechanism by which the muscle resists the passive change in muscle length to maintain optimum length - true/false
False - stretch reflex is a NEGATIVE feedback mechanism by which the muscle resists the passive change in muscle length to maintain optimal length.
The stretch reflex helps to maintain posture while standing/walking (choose one)
Walking
The sensory receptor in the stretch reflex is ________ activated by ________
The sensory receptor in the stretch reflex is muscle spindle activated by muscle stretch
Stretching the muscle increases/decreases firing in the afferent/efferent neurones.
Increases and afferent
Afferent neurones in the stretch reflex synapse in the ____ ____ with alpha/beta/gamma ______ neurones.
Afferent neurones in the stretch reflex synapse in the Spinal cord with alpha/beta/gamma motor neurones.
Activation of the stretch reflex results in contraction/relaxation of the stretched muscle
Contraction
In the stretch reflex, contraction/relaxation of antagonist muscles happens simultaneously with contraction/relaxation of the stretched muscle
relaxation of the antagonist
Contraction of the stretched muscle
Fibres in muscle spindles are known as extrafusal/intrafusal fibres. Fibres not part of muscle spindles are known as extrafusal/intrafusal fibres
Intrafusal fibres - muscle spindle
Extrafusal fibres - not muscle spindles
Muscle spindles are found within the ________ of the muscle and run parallel/anti-parallel/perpendicular to the muscle fibres
Within the belly of the muscle and run parallel to the muscle fibres
Muscle spindles have sensory nerve endings known as ________
Annulospiral fibres
Discharge from annulospiral fibres decreases when muscle is stretched - true/false
False - discharge from annulospiral fibres increases when muscle is stretched.
the afferent/efferent neurone supplying the muscle spindle is the alpha/beta/gamma motor neurone
efferent - gamma neurone
What is the function of the gamma motor neurone?
It adjusts the tension in the spindle to maintain sensitivity to stretch when the muscle is contracting.