Motor Units, Muscle Spindles, Golgi Tendon Organs and Joint Receptors Flashcards
Describe what makes up the somatic motor system
- Consists of the skeletal muscles and the elements of the nervous system that control them - these elements being UMN’s and LMN’s
- UMNs supply input to LMNs to modulate their activity
- LMNs command muscle contraction they are comprised of comprise alpha (α) motor neurones (α-MNs) that innervate the bulk of fibres within a muscle that generate force. Gamma (γ) motor neurones (γ-MNs) innervate a sensory organ within the muscle known as a muscle spindle
This is essentially mainly in reference to the corticospinal tract
What do axons of LMN’s exit the spinal cord via ?
Ventral rootlets/roots (ventral = anterior)
What is an alpha motor neuron and all the skeletal muscle fibres it innervtes defined as ?
A motor unit - this is the smallest functional unit of the motor system
What does muscle contraction result from ?
The individual and combined contractions of motor units which must be co-ordinated
What is the collection of alpha motor neurons that innervate a single muscle termed as ?
Motor neuron pool
What 2 main factors determine the force of muscle contraction ?
- Frequency of AP discharge of the α-MN
- The recruitment of additional, synergistic, motor units
Where are the cell bodies of LMN’s located ?
The ventral (anterior) horn of the spinal cord
What are the 3 sources of input which regulate the activity of an alpha motor neuron ?
- Central terminals of dorsal root ganglion cells whose axons innervate the muscle spindles
- UMNs in the motor cortex and brain stem
- spinal interneurones
What does a single AP produced in an alpha motor neuron result in and what does summation of this result in ?
- Single AP produced causes a muscle fibre to twitch
- Summation of twitches causes a sustained contraction as the number of incoming AP’s increases
What are the main types of skeletal muscle ?
Fast and slow fibre types
Slow-oxidative (Type I) Fibres:
- ATP derived from oxidative phosphorylation.
- Slow contraction and relaxation.
- Fatigue resistant.
Type IIa (fast fibres):
- ATP derived from oxidative phosphorylation
- Fast contraction and relaxation
- Fatigue resistant
Type IIb (fast fibres):
- ATP derived from glycolysis
- Fast contraction
- Not fatigue resistant
Define what the stretch reflex (myotatic reflex) is
A muscle contraction in response to stretching within the muscle. It is a monosynaptic reflex which provides automatic regulation of skeletal muscle length.
What is the muscle spindle made up of ?
- A fibrous capsule
- Intrafusal muscle fibres
- sensory afferents (Ia class) that innervate/detect sensations in the intrafusal muscle fibres
- gamma motor neurone (γ-MN) efferents that innervate the intrafusal fibres
Describe the stages of the stretch reflex
What 2 things are intrafusal fibres of muscle spindles innervated by and what is the purpose of this ?
Innervated by both:
sensory afferents (Ia class) fibres and gamma motor neurone (γ-MN) efferents
- The sensory afferents detect stretch of the muscle spindle and are involved in stretch reflex,
- During voluntary movement, α- and γ-MNs are normally co-activated (by higher centres), so that the intrafusal muscle fibres contract in parallel with the extrafusal fibres (essentially normal muscle fibres). Serves the function of maintaining the sensitivity of the spindle
Where are golgi tendon organs located and what is their function ?
located at the junction of muscle and tendon
Function:
They monitor changes in muscle tension and act to regulate muscle tension to:
- Protect muscle from overload (in extreme circumstances – e.g. weight lifting)
- More generally regulate muscle tension to an optimal range
Their sensory afferents enter the spinal cord and synapse upon inhibitory interneurones which, in turn, synapse upon the alpha motor neurones of the homonymous muscle resulting in an inhibitory response
