Module 13 - Lecture 9 - Movement - Reflexes and Gait Flashcards
What is the general outline or concepts of a reflex?
A reflex is the simplest level of movement where only a small number of synapses participate. It is controlled by the spinal cord instead of the brain. Most reflexes have a stereotyped sensory input that results in a distinct motor out (patellar tendon reflex).
What are the 5 general components of a reflex?
5 Components of a reflex:
- Receptor
- Afferent
- CNS (spinal cord) - technically optional but almost always used via interneurons
- Efferent LMN fibers
- Muscle
What are the parts of the stretch reflex? Compare and contrast them with reciprocal inhibition.
Monosynaptic Stretch Reflex
The excitatory pathway from a spindle to the α motor neurons innervating the same muscle is unusual in that it is a monosynaptic reflex; in most cases, sensory neurons from the periphery do not contact lower motor neurons directly but instead exert their effects through local circuit neurons.
This monosynaptic reflex arc is variously referred to as the “stretch,” “deep tendon,” or “myotatic” reflex, and it is the basis of the knee, ankle, jaw, biceps, or triceps response tested in a routine physical examination.
The tap of the reflex hammer on the tendon stretches the muscle, which evokes an afferent volley of activity in the Ia sensory axons that innervate the muscle spindles. The afferent volley is relayed to the α motor neurons in the brainstem or spinal cord, which then deliver an efferent volley to the same muscle.
How the monosynaptic stretch reflex violates the 5 components of a reflex?
The monosynaptic reflex synapses directly onto the lower alpha motor neuron (versus using interneurons)
Compare and contrast the stretch reflex to the reciprocal inhibition.
RECIPROCAL INHIBITION: The pattern of connectivity in local circuits of the spinal cord involving excitatory and inhibitory interneurons arranged to ensure that contraction of agonistic muscles produces forces that are opposite to those generated by contraction of antagonistic muscles; thus, reciprocal innervation mediates the simultaneous relaxation of antagonists during contraction of agonists.
Ex: Holding a cup and adding soda to it will cause a change in force which in turn results in the contraction of the biceps and relaxation of the triceps to adjust for the increased external load.
The monosynaptic reflex synapses directly onto the lower alpha motor neuron (versus using interneurons).
Why is there additional time between the monosynaptic reflex and reciprocal inhibition?
Stretch Reflex: Excitation of the agonist efferent LMN takes about 40-50ms and uses the axon type: Ia These axon diameters are slightly larger.
Reciprocal Inhibition: Inhibition of the antagonist alpha motor neuron takes about 55ms and uses the axon type: Ib.
but it is also due to the fact that the reciprocal inhibition has an extra step → because they are synapsing on interneurons that can take a little more time. -
What are the properties of the golgi tendon organ (GTO) reflex?
Components:
GTO
1b afferent
CNS (spinal cord using interneurons)
Alpha motor neurons
Muscle
The Golgi tendon circuit is thus a negative feedback system that regulates muscle tension; it decreases the activation of a muscle when exceptionally large forces are generated and, in this way, protects the muscle.
This reflex circuit also operates at lower levels of muscle force, counteracting small changes in muscle tension by increasing or decreasing the inhibition of α motor neurons. The same Ib afferents also make synaptic connections with excitatory interneurons that increase the excitability of α motor neurons that innervate the antagonistic muscle.
Be prepared to compare and contrast the GTO with the stretch reflex (including reciprocal inhibition).
What are the key components to the flexor withdrawal reflex?
What are the key components to the cross-extension reflex?
Flexor Withdrawal Reflex: SAME SIDE
Ipsilateral Pain receptor (nociceptor) Ex: Left foot
Group II afferent fibers send into to CNS
CNS: use of an IPSILATERAL interneuron network
Connect to an alpha motor fiber (efferent)
EXCITES FLEXORS muscles
INHIBITES EXTENSORS muscles
Cross-Extension Reflex: OPPOSITE SIDE
Ispilateral pain receptor (nociceptor) Ex: Left foot
Group III afferent fibers send into to CNS
CNS: use of an CONTRALATERAL interneuron network
Connect to an alpha motor fiber (efferent)
EXCITES EXTENSORS muscles
INHIBITES FLEXORS muscles
Results: You don’t fall on the floor
What are the implications of the cross extensor reflex and the flexor withdrawal reflex as evidence towards the complexity of local spinal cord circuits (Hint: listed as notable in lecture).
Spinal cord circuits: Clusters of interneurons that connect to a LMN
Local Spinal cord circuits are complex because:
- Connecting multiple muscles that achieve the same output (all the flexors activate)
- These circuits span many levels in the spinal cord (T12-L3 etc)
- These circuits cross sides of the spinal cord
What are the general ideas of the spinal cord interneuron (IN) circuits?
Long and short ..
(# segments, laterality (sides), MN pools, knowing the descending UMN links will be helpful for future lectures)
The properties of long distance networks (spinal cord circuits):
Span multiple spinal levels
Can function bilaterally
Primarily activates the medial aspect of the ventral horn therefore control the proximal (postural) muscles (MN pool)
Associated with:
Anterior Corticospinal tract
Tecto/vestibulo/reticulospinal tract
Extrapyramidal
The properties of short networks (spinal cord circuits)
Spans only a few spinal segments (2-3)
Almost only ipsilateral
Primarily actives lateral aspect of the ventral horn therefore control the distal (fine motor control) muscles (MN Pool)
Associated with:
Lateral Corticospinal tract
Pyramidals
- What are Central Pattern Generators?
- What are the 3 factors that can “activate” a central pattern generator (CPG)?
- CPG are the neural circuits that underlie built-in programs
- CPGs consist of neurons in the CNS that produce the appropriate pattern of muscle contraction to generate a functional movement
- 3 factors are :
- Spontaneously active and rhythmic (respiration)
- Triggered by a specific sensory stimulus (swallowing)
- Voluntary and triggered by higher motor centres (locomotion)
What are the 3 pieces of evidence to convey the CPG for locomotion is not a series of reflexes?
- The timing of the different muscles contractions is complex and fast
- It is not dependent on signals from the brain = supraspinal structures
- It is not dependent on sensory information (stepping can occurs if dorsal roots are cut)
What is the UMN control (what part of the BS) that can be involved in controlling/ modulating locomotion?
Mesencephalic Locomotor Region (MLR): located in the reticular formation in the caudal pons and medulla. This area can trigger locomotion and change the speed and pattern of the movement by changing the level of activity delivered to the spinal cord.
***The idea is that in these UMN, when they are excited, they have axons that travel to these interneuron networks. This is occurring through the reticulospinal tract.
High intensity of stimulus results in an increase in gait speed.
What are the 5 roles of supraspinal (UMN) input to the locomotor CPG?
