Properties of the Motor Unit Flashcards
define motor unit
The minimal functional unit of the motor system. It consists of a somatic efferent (alpha-motoneuron) and all the extrafusal muscle fibres it supplies e.g.
extra-ocular muscles (1 neurone :10 fibres)
quadriceps (1 neurone:1000 fibres)
what are the 4 components of the motor system?
- Motoneurons (Neuronal tissue)
- Skeletal Muscles (Muscular tissue)
- The Neuromuscular Junction (where the neurone and muscle connect – specialised synapse).
- Connective tissue
what do the motor systems of the body work together to do?
Set muscle tone of the body – a small amount of sustained muscle contraction responsible for body posture
Bring about voluntary movement via antagonistic muscle pairs
define innervation ratio
The number of muscle fibres innervated by a single motor neurone.
Generally, the smaller the innervation ratio the more precise the movement.
where do the motor neurones originate from in the NS?
NS –> PNS –> efferent system –> somatic motor efferents (for upper motor neurones) –> alpha motor neurone (for lower motor neurone)
what are the 2 types of muscle?
striated
non- striated
what are the 2 muscles that are striated?
skeletal
cardiac
what is the muscle that is non- striated?
smooth
what are both striated and non-striated muscles made of?
Striated and non-striated muscles are both made of muscle tubulars.
In striated, however the tubulars are arranged in a PARALLEL formation.
In non-striated they run in EVERY direction.
define myogenic
what muscles are myogenic
generate their own electrical activity - ANS only controls rate & strength of contraction
Both cardiac and smooth muscle are myogenic
what are the 4 important functions of the skeletal muscle?
Set muscle tone of the body – a small amount of sustained muscle contraction responsible for body posture
Allow for voluntary movement via antagonistic muscle pairs
Stabilises joints
Generate heat – mechanical efficiency of skeletal muscle is 20% so 80% is lost by heat.
describe the general anatomy of skeletal muscles
The single cell of muscle is known as a myocyte - muscle fibre
Myofibrils are the most abundant organelle within them
Myocytes collect together form a fascicle
The neurovascular bundle (blood vessels) of any muscle runs in between and in parallel with muscle fascicles
Fascicles collect to form a muscle
what is the order in the organisation of the muscle starting from the most superficial layer first?
muscle –> fascicles –> muscle fibres –> myofibrils –> myofilaments
what are the 3 tissues that envelope the skeletal fibres?
Epimysium – CT enveloping all fascicles
Perimysium – CT enveloping a fascicle
Endomysium – CT enveloping each muscle fibre
what is the importance of tendons?
A tendon is a tough band of fibrous connective tissue
Attaches muscle to bone
It forms a point of confluence, bringing together the individual contractions of the myocytes to produce a combined action at a usually a joint.
Skeletal muscle can be classified according to the assemblies of their fascicles… what are these 5 classifications?
convergent/triangular
circular/sphincteric
parallel/strap
fusiform
Pennated Muscles
what are convergent/ triangular muscles?
muscle fibres converge to a small point on a tendon
allows for very strong contraction (i.e. pectoralis major)
what are circular/sphincteric muscles?
surround openings at orifices (i.e. orbicularis oris)
what are parallel/strap muscles?
fibres run parallel to each other
these are long muscles that cause large movements - not strong but good endurance (i.e. sternocleidomastoid)
what are fusiform muscles?
spindle-shaped
belly wider than points of origin & insertion (i.e. biceps brachii)
what are pennated muscles and what are the 3 types?
a muscle with fascicles that attach obliquely (in a slanting position) to its tendon.
unipennate – fibres diagonal to tendon - allows great strength (i.e. lumbricals)
bipennate – bilateral fibres run in opposing diagonals to central tendon - more power but less motion range (i.e. rectus femoris)
multipennate - multiple rows of diagonal fibres, with a central tendon which branches into two or more tendons - produce powerful contractions (i.e. deltoid muscle w/ anterior, posterior & middle sections)
what is structure of actin?
the thinner filament. They are connected to the Z line.
Actin filaments have binding sites for myosin heads.
what is the structure of myosin?
the thicker filament. Myosin filaments have globular heads that bind to the actin binding sites.
what is the binding site on actin called?
actin—myosin binding site
when the muscle is relaxed, what is the actin- myosin binding site blocked by?
actin—myosin sites are blocked by tropomyosin
what is sarcoplasm?
the shared cytoplasm within a muscle fibre
why do myofibrils have light and dark chains?
Myofibrils have alternating light and dark bands due to the varied overlapping of actin and myosin
how are myocytes structured?
Each muscle fibre (myocytes) contains many myofibrils.
These are long cylindrical organelles made up of proteins which specialised for contraction.
They are lined up in parallel to provide maximum force when they all contract together.
The myofibrils have lots of adjacent contractile units called sarcomeres.
Each sarcomere unit is made up of two types of protein filament:
actin and myosin
name each part of a sarcomere
z- line: mark the boundary of each sarcomere
l- band (light band): contains thin actin filaments and no myosin filaments
a- band (dark band): contain thick myosin filaments and actin at the outer edges
h- zone: only myosin filaments
explain the steps in muscle contraction (21 hefty steps)
- Muscle contraction is triggered when an action potential arrives at a neuromuscular junction — the point where a motor neurone and a skeletal muscle fibre meet.
- When an action potential reaches the neuromuscular junction, it stimulates the release of Acetylcholine.
- Acetylcholine binds to receptors on the postsynaptic membrane (the sarcolemma), opening sodium ion channels, and resulting in depolarisation.
- Acetylcholine is then broken down by enzymes preventing the muscle being overstimulated.
- When the action potential reaches the sarcoplasmic reticulum in the sarcoplasm it stimulates calcium ion channels to open.
- The calcium ions diffuse down their concentration gradient flooding the sarcoplasm with calcium ions.
- The calcium ions bind to troponin causing it to change shape.
- This pulls on the tropomyosin moving it away from the actin—myosin binding sites on the actin filament.
- Now that the binding sites have been exposed the myosin head binds to the actin filament forming an actin—myosin cross-bridge.
- Once attached to the actin filament the myosin head flexes, pulling the actin filament along.
- The molecule of ADP bound to the myosin head is released.
- An ATP molecule can now bind to the myosin head.
- This causes the head to detach from the actin filament.
- The calcium ions present in the sarcoplasm also activate the ATPase activity of the myosin.
- This hydrolyses the ATP to ADP and phosphate, releasing energy which the myosin head uses to return to its original position.
- The myosin head can now attach itself to another actin—myosin binding site further along the actin filament and the cycle is repeated.
- This is repeated up to 100 times per second.
- The cycle continues as long as the muscle remains stimulated.
- During the period of stimulation many actin—myosin bridges form and break rapidly, pulling the actin filament along.
- This shortens all the sarcomere in the myofibril and causes the muscle to contract.
what are the 2 divisions of a motor neurone?
Upper motoneurone
Lower motoneurone
what is a lower motor neurones function?
Lower motor neurones are the same thing as alpha motor neurones
It’s cell body is in either:
- Lamina IX of the grey matter of the spinal cord
- Cranial nerve motor nucleus - grey matter of the brain stem
Its axon supplies skeletal muscles of the body directly
what is an upper motor neurones function?
Upper motor neurones command lower motor neurones to bring about contraction. They do not have access to skeletal muscle.
what does random distribution of muscle fibres of a single motor unit allow for?
Muscle contraction to be distributed uniformly throughout the muscle which allows for smoother movement (all parts are contracting not just a small section concentrated with muscle fibres if the structure was not random)
The nervous system to regulate the rate of contraction.
what is the same between all muscle fibres in a motor unit?
Each muscle fibre is innervated by only 1 motoneurone
All the muscle fibres of a single motor unit have the same physiological profile
same contraction speeds
same susceptibility to fatigue
They have the same histochemical profile
Same myosin fibre types
Same enzyme expression profile - same Metabolic profile
(All black myocytes are innervated by same motor neurone etc.)
what is a twitch?
A twitch is a brief contraction of a muscle in due to electrical stimulation
muscle fibres can be classified according to…?
How quickly they develop force
How long they take to relax
(Y axis is Force measured with strain gauge)
what are the 3 broad classes of twitch fibres (myocytes)?
Fast twitch (Type FF): Muscle develops force at a high speed but fatigues very quickly (shown by width of curve) (e.g. in eye)
Intermediate Twitch (Type FR): Muscle develops force at a medium speed and fatigues at a moderate rate (shown by width of curve) (Gastrocnemius)
Slow Twitch (Type S): Muscle develops force at a slow speed but fatigues very slowly (shown by width of curve) (soleus)
All twitch fibres belonging to a single motor unit at the same type!!
how does summation of twitches occur?
When you increase the frequency of twitches, summation occurs and you get a tetanus.
The Tetanus contraction is sustained and smooth.
Our body stimulates muscles at a very high frequency meaning summation occurs much more quickly removing the ups and downs resulting in a smoother contraction.
describe the experiment that shows that the motor neurone determines the characteristics of the muscle it innervates
The motor neuron determines the characteristics of the muscle it innervates
This was worked out in an experiment where you took two motor units. One which had a type S motor neurone and a type S muscle and another with a Type FF motor neurone and a type FF muscle.
The motor neurones were switched (i.e. S motor neurone connected to FF muscle).
The Muscles changed their twitch properties and biochemistry and conformed to the same muscle type of the neurone connected.
lower motor neuron = involuntary
upper motor neuron = voluntary
