Lower motor neuron pools & muscle stretch reflex Flashcards
Where do motor neurons come from?
The ventral horn of the spinal cord
What types of motor neuron do we have?
Alpha and gamma
What does the dorso-ventral arrangement of motor neurons reflect?
The nature of the muscle
- flexors are located more dorsally and extensors more ventrally
What does the medal-lateral arrangement reflect?
More proximal muscles medially
More distal muscles laterally (as these will travel furthest in descent and will be the last to leave the spinal cord)
Target of alpha motor neurones
extrafusal fibres which mediate contraction
Target of gamma motor neurones
intrafusal fibres whcih are part of the muscle spindle and have a role in maintaining spindle sensitivity
What is the neurotransmitter of spinal motor neurones in the ventral horn?
ACh
What are the different types of skeletal muscle fibre? What are their different innervations?
- slow contracting (type 1) → innervated by the smallest alpha neurons, recruited first and used in all forms of locomotion
- fast contracting fatigue resistant (type 2a) → innervated by medium sized alpha motor neurons
- fast contracting easy fatigued (type 2b) → innervated by the largest alpha motor neurons and used in short, powerful movements (e.g. sprinting)
What is a motor unit?
The one alpha motor neuron and the muscle fibres it innervates
What is the collection of alpha motor neurones which innervated a single muscle called?
Alpha motor neuron pool
What is the size principle?
Motor units are generally recruited in order of smallest to largest (smallest motor neurons to largest motor neurons) as contraction increases
Describe reciprocal inhibition
It is the process of muscles on one side of a joint relaxing to accomodate contraction on the other side of that joint
it is accomplished by the actions of an inhibitory interneuron in the spinal cord
How does reciprocal inhibition work?
The afferent of the muscle spindle bifurcates in the spinal cord. One branch innervates the alpha motor neuron that causes the homonymous muscle to contract, producing the reflex. The other branch innervates the inhibitory interneuron, which in turn innervates the alpha motor neuron that synapses onto the opposing muscle. Because the interneuron is inhibitory, it prevents the opposing alpha motor neuron from firing, thereby reducing the contraction of the opposing muscle. Without this reciprocal inhibition, both groups of muscles might contract simultaneously and work against each other.
What are central pattern generators?
Biological neural circuits that produce rhythmic outputs in the absence of rhythmic input
- e.g. for walking, swimming, etc
What are the simplest central rhythm generators?
Individual neurons which allow pacemaker properties due to their membrane physiology
What is the main characteristic symptom of lower motor neuron lesion?
Flaccid paralysis
What are all the characteristics of lower motor neuron lesion?
Muscle weakness or paralysis Hypotonia Hyporeflexia Initial 'fasciculation' (a brief spontaneous contraction affecting a small number of muscle fibres, often causing a flicker of movement under the skin) Longer term muscle wasting
What receptors in muscles are used for proprioception?
muscle spindles
Golgi tendon organs
Describe muscle spindles
Situated in the fleshy part of muscles
in parallel with the extrafusal fibres and attached to muscle connective tissue
Describe Golgi tendon organs
situated in the tendons at the ends of the muscle
they are in series with the extrafusal fibres
What do muscle spindles signal?
The length and the rate of change of length of the muscle in which they are located
What do Golgi tendon organs signal?
The tension in the tendon
What does the muscle stretch reflex do?
Contracts the muscle after it has been stretched to prevent injury
What type of fibres are muscle spindles?
Intrafusal fibres (these are fibres that serve as proprioceptors, where extrafusal fibres are contractile elements)
Which way do the muscle spindles lie in relation to the muscle fibres?
In parallel so that the force acting on the muscle acts similarly on the spindle
What is the structure of a muscle spindle?
Fluid-filled capsule of connective tissue
Contains 7 modified intrafusal fibres with contractile ends and a non-contractile centre, detecting length and rate changes
- the non-contractile centre is innervated by terminating sensory fibres
- the contractile ends are innervated by motor axons
What happens to the muscle spindle when the muscle elongates?
the intrafusal fibres also elongate
sensory ending which wrap around the centre are deformed and increase their firing rate
What are the two types of sensory afferent that innervate muscle spindles?
Ia - primary afferent
II - secondary afferent
What are the two types of muscle spindle fibre?
nuclear bag fibres (b)
nuclear chain fibres (c)
Discuss nuclear bag fibres (b)
- swollen in centre
- may be dynamic (Ia primary afferent) or static (II secondary afferent, as well as Ia)
Discuss nuclear chain fibres (c)
- smaller than b
- both Ia and II afferents
- stiff in a similar way to static b fibres meaning they signal muscle length also
What type of fibres does a muscle spindle typically containing?
one dynamic b1, one static b2 and 5c fibres
How can the muscle spindle reflex be strengthened if required?
a single sensory afferent may form synapses with multiple motor neurons in order to strengthen the response
How many neurons constitute the muscle spindle relfex?
2 - it is monosynaptic
Describe the muscle spindle relfex
- stretch imposed on muscle deforms the intrafusal muscle fibres of the muscle spindles, which initiate APs by activating mechanically-gated ion channels in afferent Ia. axons
- Ia for synapse with alpha motor neurons that innervate same (homonymous) muslce and, via local circuits, form inhibitory connections with the alpha motor neurons of antagonistic (heternymous) muslces - this is called reciprocal innervation
Give an example of reciprocal inhibition
Knee jerk reflex
quads contract and swing leg forwards
Hamstring relaxes, allowing extension of leg
What do gamma neurons do in the muscle spindle reflex?
adjust sensitivity of the muscle spindles
their activation causes contraction of the polar regions of the intrafusal fibres
this stretches the central region from both ends, leading to an increase in firing rate of the sensory endings or greater likelihood that the sensory endings will fire in response to stretch of muscle
Which motor neurons are therefore involved in the muscle spindle reflex?
Coactivation of alpha and gamma motor neurones - allows spindles to function at all muscle lengths during movements and postural adjusments
What do Renshaw inhibitory interneurons do?
associate with alpha motor neuron and provide neg feedback to inhibit firing in order to prevent fatigue
Discuss the latency components of the muscle spindle reflex
- M1 - short latency, involved in proximal muscle control
- M2 - long latency component, slow and involved in fine voluntary distal limb movements
Level of jaw jerk
trigeminal
level of biceps reflex
C5-C6
level of brachioradialis reflex
C5-C6
Level of triceps reflex
C6-C7
Level of finger jerk reflex
C8-T1
Level of knee jerk reflex
L3-L4
Level of ankle jerk reflex
S1
Discuss flexor withdrawal reflex
Painful stimulus percieved and limb withdrawn by flexor action
Discuss crossed-extensor reflex
A reflex in which the contralateral limb compensates for loss of support when the ipsilateral limb withdraws from painful stimulus in a withdrawal reflex
- opposite of the flexor withdrawal reflex
What does the Golgi tendon reflex do?
Protective mechanism to control the tension of an active muscle by causing relaxation before the tension becomes high enough to cause damage
How does the Golgi tendon reflex work?
Ib sensory fibres are sent through the dorsal root into the spinal cord to synapse on Ib inhibitory interneurones that in turn terminate directly on the motor neurons that innervate the same muscle
The fibres also make direct excitatory synapses onto motoneurons that innervate the antagonist muscle
Structure of GTO
strands of collagen that are encapsulated
in series with a group of muscle fibres at one end and merge with the tendon proper at the other
