Lecture 14 - Reflexes Flashcards
Organization of neural structures involved in the control of movement. Four systems— local spinal cord and brainstem circuits, descending control centers in the cerebral cortex and brainstem, the cerebellum, and the basal ganglia—make essen- tial and distinct contributions to motor control.
Distribution of lower motor neurons in the ventral horn of the spinal cord. Motor neu- rons were identified by injecting a retrograde tracer into either the medial gastrocnemius or soleus muscle of the cat, thus labeling neuronal cell bodies and re- vealing their spatial distribution. A transverse section through the lumbar level of the spinal cord (A) shows lower motor neurons forming distinct, rod-shaped clus- ters (motor neuron pools) in the ipsilateral ventral horn. Spinal cord cross sections (B) and a reconstruction seen from the dorsal surface (C) illustrate the distribu- tion of motor neurons innervating individual skeletal muscles in both axes of the cord. The rodlike shape and distinct distribution of different motor neuron pools are especially evident in the dorsal view of the reconstructed cord. The dashed lines in (C) represent the locations of individual lumbar and sacral spinal cord sections shown in (B). (After Burke et al., 1977.)
Since there are far more muscle fibres than motor neurons, individual motor axons branch within muscles to synapse on multiple extrafusal fibres; and the fibres that a motor neuron synapses onto are typically distributed over a wide area within the muscle => ensuring smoother movement because the contractile force is spread evenly.
Most extrafusal skeletal muscle fibres are innervated by how many alpha motor neurons?
Only one.
Benefit of arrangement of motor unit
Reduces the chance that damage to one or a few alpha motor neurons will significantly alter a muscle’s action.
What is a motor unit?
An alpha motor neuron and the muscle fibres it innervates; The smallest unit of force that can be activated by the muscle, because an AP generated by a motor neuron usually reaches the contraction threshold of all the muscle fibres in the motor unit.
Where do we see an orderly relationship between the locations of motor neuron pools and the muscles they innervate
Along the length of the SC and across the medial-to-lateral dimension of the cord. Provides a spatial map of the body’s musculature.
How do we see this topography of motor neuron pools along the length of the SC?
Each lower motor neuron innervates muscle fibres within a single muscle, and all the motor neurons of the motor neuron pool are grouped together into a rod-shaped cluster that runs parallel to the long axis of the SC for one or more spinal cord segments.
Eg. the motor neuron pools that innervate the arm are located in the cervical enlargement of the cord and those that innervate the leg are located int eh lumbar enlargement.
How do we see this topography of motor neuron pools in the medial to lateral dimension of the SC?
Motor neurons that innervate the axial musculature (like the postural muscles of the trunk) are located most medially in the ventral horn of the SC, whereas neurons that innervate the muscles of the shoulders are lateral to the axial neurons. Those that innervate the proximal muscles of the arm are the next most lateral, while those that innervate the distal parts of the extremities lie farthest from the midline.
What is the implication of the spatial organisation of motor neuron pools in the ventral horn?
Provides a framework for understanding how descending projections of upper motor neurones and intersegmental SC circuits control posture and modulate movement.
Somatotopic plan reflected in upper motor neuron pathways
Medial lower motor neuron pools governing postural control etc receive input from upper motor neurons that comprise long pathways running in the medial and ventral white matter of the SC. More lateral lower motor neuron pools that innervate the distal extremities (concerned with the execution of skilled behaviour) are governed by projections that run through the lateral white matter of the SC.
Somatotopic plan reflected in the location of local SC circuits that interconnect the lower motor neuron pools distributed along the longitudinal axis of the SC
The medial local circuit neurons, supplying lower motor neurons in the medial ventral horn, have axons that project to many SC segments (some between the cervical and lumbar enlargements => coordination of rhythmic movements of upper/lower limbs) while other axons terminate along the entire length of the cord and help mediate posture. Many also have axonal branches that cross the midline in the ventral commissure of the SC to innervate lower motor neurons in the medial part of the contralateral hemicord. This ensures that groups of axial muscles on both sides of the body act in concert to maintain and adjust motor activity that requires synchronous bilateral coordination of muscles (posture and breathing).
Whereas, local circuit neurons in the lateral region of the intermediate zone have shorter axons that typically extend fewer than 5 segments and are predominantly ipsilateral. More restricted pattern of connectivity = finer and more differentiated control that is exerted over muscles of distal extremities on one side.
Two types of lower motor neurons in the motor neuron pools of the ventral horn
Alpha motor neurons and gamma motor neurons
Alpha motor neurons
Large, innervate striated muscle fibres, which actually generate the forces needed for posture and movement.
Gamma motor neurons
Interspersed among the alpha motor neurons; smaller; innervate specialised muscle fibres; actually sensory receptors arranged in parallel with the force-generating striated muscle fibres.
Function is to regulate sensory input to the muscle spindles by setting intramural muscle fibres to an appropriate length.
Muscle spindles
Specialised muscle fibres; actually sensory receptors arranged in parallel with the force-generating striated muscle fibres. Embedded within connective tissue capsules int he muscle. AKA intrafusal muscle fibres.
Innervated by sensory axons that send info to the SC and brainstem about the length of the muscle.
Variation in size of motor units and small alpha neurons
Small alpha neurons innervate relatively few muscle fibres to form motor units that generate small forces, whereas large motor neurons innervate larger, more powerful motor units.
Slow motor units/fibres
Comprise small “red” muscle fibres that contract slowly and generate relatively small forces; rich myoglobin content, a lot of mitochondria, and rich capillary beds, so resistant to fatigue.
Have lower thresholds for activation.
Fast fatigue-resistant motor units
Intermediate size; not quite as fast as fast-fatiguable; generate about 2X force of slow motor unit; resistant to fatigue
Fast fatigable motor units
innervated by larger alpha motor neurons; pale muscle fibres that generate more force; sparse mitochondria, so easily fatigued. Required for brief exertions that require large forces like running or jumping.
Higher thresholds for activation; reached only during rapid movements requiring great force.
Motor unit plasticity
The myofibril and neuronal properties of motor units are subject to use-dependent plasticity. Can change proportion of different muscle fibres. Muscle biopsies show that sprinters have a large proportion of powerful but rapidly fatiguing pale fibres in their leg muscles than do marathon runners - underlying mechanism of neuromuscular adaptions to physical exercise and training.
Size principle
Henneman. As the synaptic activity driving a motor neuron pool increased (by stimulating sensory nerves or upper motor pathways that project to a lower motor neuron pool), low-threshold S motor units are recruited first, then FR motor units, and finally the FF motor units (at the highest level of activity).
==> Ordered recruitment of motor units.
Offers a simple solution to the problem of grading muscle force. The combination of motor units activated by such orderly recruitment optimally matches the physiological properties of different motor unit types with the range of forces required to perform different motor tasks.
Structure of the muscle spindle
Group Ia afferent sensory axons are coiled around the middle region of each class of intrafusal fibre = annulospiral primary endings.
Group II afferents form secondary endings on mostly nuclear chain fibres = flower spray endings.
Reciprocal innervation + example
Leads to rapid and efficient adjustments to changes in the length of the muscle.
Centrally projecting branch of the sensory neuron forms monosynaptic excitatory connections with those alpha motor neurons in the ventral horn of the SC that innervate the same muscle and also forms inhibitory connections (via local circuit neurons) with the alpha motor neurons that innervate antagonistic muscles.
Stretching a muscle spindle leads to increased activity in group Ia afferents and an increase in the activity of alpha motor neurons that innervate the same muscle. Group Ia afferents also excite the motor neurons that innervate synergistic muscles, and they indirectly inhibit the motor neurons that innervate antagonists via intervening reciprocal Ia-inhibitory interneurons.
What kind of loop is the stretch reflex?
A negative feedback loop used to maintain muscle length at a desired value The appropriate length is specified by the activity of descending upper motor neuron pathways that influence the lower motor neuron pool. Deviations from the desired length are detected by the muscle spindles because increases or decreases in the stretch of the intrafusal fibres alter the level of activity in the sensory axons that innervate the spindles. These changes lead to adjustments in the activity of alpha motor neurons which returns the muscle to the desired length by contracting the stretched muscle and relaxing the opposite muscle group and by restoring the level of spindle activity and sensitivity.
Spinal cord circuitry for the flexion-crossed extension reflex
Involves slowly conducting afferent axons and several synaptic links.
Simulation of nociceptive sensory fibres leads to withdrawal of the limb from the source of pain by excitation of ipsilateral flexor muscles and reciprocal inhibition of ipsilateral extensor muscles. Contralateral extensor muscles are also excited while flex muscles are inhibited, providing postural support during withdrawal of the affected limb.
Step cycle - locomotion
Diagram and electromyographic recordings of the step cycle, showing leg flexion (F) and extension (E) and their relation to the swing and stance phases of locomotion.
Changes in the sequence of limb movements accompanying changes in speed of locomotion
At the highest speeds, the movements of the two front legs are synchronised, as are the movements of the two hindlimbs.
CPGs and locomotion
Transection of the spinal cord at the thoracic level isolates the hindlimb segments of the cord. After recovering from surgery, the hindlimbs are still able to walk on a treadmill, and reciprocal bursts of electrical activity can be recorded from flexors during the swing phase and from extensors during the stance phase of walking.
Combined with experiment where dorsal roots are sections too, in which locomotion can still be induced by the activation of local circuits either by the act of transecting the SC or by the stimulating the release of NT from the now-transected upper motor neuron pathway. Rhythmic patterns are not just dependent on sensory inputs (you can sever the dorsal roots and still have locomotion) or input from descending projections from higher centres (you can transect the SC and still have locomotion).
So, Seems like local circuitry provides for each limb a CPG responsible for the alternating flexion and extension of the limb during locomotion.
CPGs are comprised of
CPG comprise local circuit neurons that include excitatory glutamateric neurons coupled to one another and a variety of inhibitory GABAergic and glycinergic neurons.
Control theory
Proportional-integral-derivative PID controller.
Minorsky.
Open loop - no feedback.
Closed loop - feedback.
Has a set point and feedback signal. New inputs generate a change in the system that compensates for the input and maintains the system state. Can think of the knee-jerk reflex in this way.
Control theory
Proportional-integral-derivative PID controller.
Minorsky.
Open loop - no feedback.
Closed loop - feedback.
Has a set point and feedback signal. New inputs generate a change in the system that compensates for the input and maintains the system state. Can think of the knee-jerk reflex in this way.
How do the brainstem and motor cortex control locomotion?
The spinal locomotor system is activated by signals from the mesencephalic locomotor region MLR, relayed by neurons in the medial reticular formation MRF. The cerebellum receives signals from both peripheral receptors and spinal CGPs and adjusts the locomotor pattern through connections in brain stem nuclei. Visual information conveyed to the motor cortex can also modify stepping movements.
How does the mesencephalic locomotor region motify stepping patterns?
Stimulation of the MLR excites interneurons in the MRF whose axons descend in the ventrolateral funicular VLF to the spinal locomotor system. When the strength of stimulation of the MLR in a decerebrate cat walking on a treadmill is gradually increased, the gait and rate of stepping change from slow walking to trotting and finally to galloping. As the cat progresses from trotting to galloping, the hind limbs shift from alternating to simultaneous flexion and extension.
How the neural structures involved in the control of locomotion are organised
Once activity is initiated, cortical systems play a relatively minor role in sustaining CPG. Cortical control is most relevant for conveying motor intention (like spatial navigation, walking or running) and visual guidance of locomotion thru complex environments.
