MOTOR UNIT Flashcards
What does motor unit consist of?
Motor neurone
Skeletal muscle
NMJ
Connective tissue
Purpose of motor unit
Set muscle tone of the body
Bring about voluntary movement via antagonistic muscle pairs
Definition of motor unit
Minimal functional unit of motor system
Innervation ratio?
The number of muscle fibres innervated by a single motor neurone
Smaller the ratio the more precise the movement
Examples of innervation ratios. Extra-ocular and quadriceps ?
Extra ocular 1 neurone: 10 fibres
Quadriceps 1:1000
Where motor unit originates from?
From somatic motor efferents
Myogenic?
Generate own electrical activity
Importance of skeletal muscle
Movement
Stability of joints
Heat generation (only 20% efficient)
Posture
Single cell of a muscle known as?
Myocyte
What is a tendon?
Organised tough band of fibrous connective tissue
Attaches muscle to bone
Importance of tendon
Forms a point of confluence bringing together the individual contractions of the myocytes to produce a combined action usually at a joint
How can skeletal muscles be classified?
According to the assemblies of their fascicles
Types of skeletal muscles
Convergent
Circular
Strap
Fusiform
Pennated
Three types of pennated muscles
Unipennate
Bipennate
Multipennate
What are myocytes?
Muscle cells
Skeletal myocytes are tubular
Made from myofibrils
Role of lower motor neurone
Transmit signals from upper MN to effector
Where are lower motor neurones located
Cell body in lamina IX of the grey matter of the spinal cord then to effector
Characteristics of muscle fibres of a motor unit
Same contraction speed
Same susceptibility to fatigue
Same myosin fibre typing
Importance of the motor unit
Allows uniform development of force throughout muscle
Nervous system regulate rate and speed of contraction
Muscle contraction to be distributed throughout the muscle
How muscle fibres categorised
How quickly they develop force
How long they take to relax
Three broad classes of muscle fibres?
Fast twitch
Intermediate twitch
Slow twitch
Fast twitch?
Muscle develops force at a high speed but fatigues very quickly
E.g. eye
Intermediate twitch
Muscle develops force at medium speed and fatigues at moderate rate
E.g.
Slow twitch
Muscle develops force at slow speed and fatigues very slowly
Tetanus?
When increase frequency of twitches
Summation of twitches occurs
Tetanus contraction is sustained and smooth
Body stimulates muscle at high frequency so summation occurs quicker and contraction smoother
Stretch reflex?
Contraction of muscle in response to its passive stretching
Increase length of muscle automatically by increasing contractility
Muscle tone?
Amount of tension or resistance to movement in the muscle
Change in muscle tone allows us to move
Neurone pathway of skeletal muscles
Somatic
Neurone supply of glands and smooth muscle
Autonomic
Causes for degeneration of nerves
- Injury
- Genetics of the body
- Aging
Injured peripheral nerves have a capacity to regenerate and re-
innervate effectors. True or False
True
Do CNS or PNS regenerate more easily
PNS
Structural features of neurone likely to be affected by insult:
- Epineurium - (interfascicular bands attach adjacent nerve fascicle)
- Perineurium
- Endoneurium
- Myelin sheaths
- Axon (deepest)
Neuropraxia ?
Mildest form of traumatic peripheral nerve injury
Temporarily loses its ability
Injury would be at level of myelin sheath
Axonotomesis
Axon and myelin sheath is damaged but surrounding CT is intact
Result of severe crush injury
Can be recovered
Neurotmesis
Entire nerve fibre is completely transected or severed
Most sever class of nerve damage
Recovery of function doesn’t occur
Seddons classification of nerve injuries
Neuropraxia
Axonotomesis
Neurotmesis
When nerve damaged, divided into 2 segments:
Proximal segment - receives support of cell body
Distal segment - cut off from cell body
Distal segment of neurone?
• often cut off from cell body
• Loses potential for repairs
• Loses potential for nutritional support
•Becomes vulnerable to phagocytosis by glia
What 3 glial cells are important when nerve is damaged
- Myelin forming cells
Oligodendrocytes + Schwann cells - Astrocytes - create environment for neurones
- Microglia - immune cells of NS
Reaction to injury (minutes after)
- neurone stop conducting AP
- cut axon start leaking Intracellular fluid
- cut ends pull apart and start swelling
Reaction to injury an hour after injury sustained
- Synaptic terminal degenerates. Accumulation of neurofilaments (significant role in determining the shape of cells, caliber of axonal projections, and maintenance of axonal transport)
- Astroglia surround terminal
- Microglia infiltration
Fate of distal segment of a severed nerve?
- Not viable
- Dies due to nutritional loss
- axonal segment undergoes Wallerian degeneration
- axon digested by phagocytes
Tissues that might be preserved after damage:
Myelin sheaths
Epineurium
Perineurium
Endoneurium
Importance of tissues that may be preserved after damage
These form hollow tubes to guide any new regrowth of the end of the proximal end
Reactions to injury after days/ weeks?
The distal segment undergoes wallerian degeneration
Digested by phagocytes
Fate of proximal segment of a severed nerve?
- Undergoes process chromatolysis
- Cell body very active and prod. Proteins for repair
- Soma becomes bloated
-Nucleus displace to periphery - Injured nerve seals wounded stump: neuroma
- sometimes nerve stump regenerates to innverate peripheral structures
What is neuroma?
Injured nerve seals the wounded stump
Regeneration of axonal stump in proximal segment
Forms many sprouts
Some which find Schwann cell tubes
What is innervation?
Term used to describe the normal state of nerve supply to a muscle or other target
Alpha motorneurones innervates skeletal muscle
Re innervation?
Re growth of the nerve supply to the muscle
Is re innervation successful?
Reinnveration to the right organ is not always successful
Most nerves reinnervate a different target
This leads to unexpected results (or syndromes)
Acute phase of denervated muscle that is not reinnervated?
- Muscle is immediately paralysed (flaccid paralysis)
- muscle becomes areflexic (absence of deep tendon reflexes)
- Muscle start to fasciculate (twitching)
If muscle not reinnervated the fasciculations will subside
Chronic phase of denervated muscle that is not reinnervated?
Fasciculations subside and muscles:
- loose bulk due to lack of use and dennervation
Muscle will die
Muscle replaced with CT and fat
State of fibrosis
Processes that distal and proximal segment undergo when nerve damaged
Proximal - chromatolysis (redistribution of nissl substance in the cell body to prod proteins for regeneration)
Distal - wallerian degeneration
