Week 3 - Lab on reflexes Flashcards
What are the alpha motor neurones that innervate a muscle referred to as?
A muscle’s motorneurone pool
Where do the cell bodies of motor neurones of the limbs and axial skeleton lie?
In the ventral horn of the spinal cord
What is a motor unit made up of?
The motorneurone and the group of muscle fibres it is connected to
From which pathways for motor neurons receive synaptic input?
- Descending pathways from higher centres involved in voluntary movement and postural control
- Sensory neurones whose receptive endings lie in muscles, tendons, skin and joints
Why do all muscles of a motor unit discharge together?
The excitatory synapse between a motor neurone and each muscle fibre is an extremely secure and efficient one, which means each presynaptic action potential that arrives at the motor endplate ultimately gives rise to a postsynaptic myscle action potential
How are the size of the motor unit action potential, force generated and size of motor units connected?
The bigger the motor unit, the larger the motor unit action potential and the force generated
How does the body ensure smaller motor units are used for smaller forces?
As strength of excitation of the motorneurone pool is increased, small motor units are fired off first. For stronger excitation, more and larger motor units are recruited into the active population
Order of recruitment is fixed
WHat is a tendon reflex/myotatic?
Tapping on the knee reflex
- tap imparts a stretch on the muscle. That change in length is sensed by the muscle spindle sensory neurone, causing a reflex excitation of the motorneurones, opposing the stretch by active contraction of the muscle
How does the tendon reflex help to automatically maintain posture?
Automatically regulates muscle length in large postural muscles
How is the inverse myotatic reflex different to the myotatic reflex?
Occurs in response to large rises in muscle tension, not length per se
AND
Sensory neurone is located in muscle tendon, not muscle itself
AND
Causes reflex inhibition of motorneurones –> sudden relaxation of muscles
Describe the well known crossed extensor reflex
Mediates limb withdrawal from a painful stimulus, while through reciprocal excitatory and inhibitory connections to the motorneurones pool on the opposite side of the spinal cord, it results in automatic extension of the other limb
What can cause hypoflexia?
Damage to dorsal or ventral roots, or degradation of afferent or motorneuones
This will also cause a reduction in muscle tone and resistance to stretch
WHat is muscle tone?
The continuous and passive partial contraction of the muscles, or the muscle’s resistance to passive stretch during resting state
What could lesions to descending neural pathways cause?
- Descending neural pathways exert effects either on the alpha motorneurones, or on the small gamma motorneurones that act to regulate sensitivity of the muscle spindles
- Therefore, lesions in descending neural pathways can lead to either hyperreflexia or hyporeflexia, and also to changes in muscle tone and resistance to stretch
What are the electrical events as a subject voluntarily contacts their muscle?
Voluntary recruitment means muscle force is modulated by the CNS - combines recruitment with the frequency of MU activation and synchronization
