Reflex and Voluntary Control of Movement

Editor's Notes

• #3: Gutyon is best
• #4: Vestibulospinal Reflexes Senses falling/tipping contracts limb muscles for postural support Vestibulocollic Reflexes acts on the neck musculature to stabilize the head if body moves Vestibulo-ocular Reflexes stabilizes visual image during head movement causes eyes to move simultaneously in the opposite direction and in equal magnitude to head movement Locomotor regions: midbrain locomotor regions: when stimulated, leads to sustained locomotion. Involved in initation of movement.
• #5: When one motor neuron is active, the other is inhibited
• #6: Modification by afferent input ensures can adapt to certain situations (terrain for example) can see CPG activity in transected cat The simplest movements are reflexes (knee jerk, pupil dilation), which are involuntary, stereotyped and graded responses to sensory input, and have no threshold except that the stimulus must be great enough to activate the relevant sensory input pathway. Fixed action patterns (sneezing, orgasm) are involuntary and stereotyped, but typically have a stimulus threshold that must be reached before they are triggered, and are less graded and more complex than reflexes. Rhythmic motor patterns (walking, scratching, breathing) are stereotyped and complex, but are subject to continuous voluntary control. Directed movements (reaching) are voluntary and complex, but are generally neither stereotyped nor repetitive. Rhythmic motor patterns comprise a large part of behaviour. They are also complex (unlike reflexes) yet stereotyped (unlike directed movements) and, by definition, repetitive (unlike fixed action patterns). As a consequence of this combination of behavioural importance and experimental advantage, rhythmic motor pattern generation has been studied extensively. Key to understanding rhythm generation in this (and many other network-based CPGs) is the concept of a half-centre oscillator. A half-centre oscillator consists of two neurons that individually have no rhythmogenic ability, but which produce rhythmic outputs when reciprocally coupled. Several types of interacting processes can support this rhythm generation (see
• #7: showed that when a portion of the brain stem of a cat was cut across the middle—thus severing any connections between the brain and the spinal cord—the cat was still capable of standing. Furthermore, if a specific region of the brain stem was stimulated, the cats could be induced to walk on a treadmill, and alternating bursts of muscle activity could be recorded in extensors and flexors in conjunction with walking (Shik et al., 1966). These series of experiments led to the conclusion that each limb is controlled by a central pattern generator (CPG) in the spinal cord, which controls rhythmic motor activity, including walking. Shik and colleagues experimented with a cat whose brain stem was severed but that was still able to walk on a treadmill when a specific region of the brain stem was stimulated. The top of the figure shows the brain and the spinal cord. The muscle activity recorded from the flexors and extensors demonstrates that they are contracting and relaxing at opposite times from each other, consistent with normal function.
• #8: Refer to overhead slide # 1 regarding these notes: interneurons located in medial part of spinal cord make bilateral synaptic connections with motoneurons on axial muscles interneurons located more laterally make ipsilateral synaptic connections with motoneurons of girdle muscles (shoulder and pelvis) interneurons located in lateral part of spinal cord make synaptic connections with distal limb muscles The Propriospinal system is a series of neurons whose axons run up and down (rostral-caudal direction) the spinal cord. They connect different segmental levels of the spinal cord together by synapsing with interneurons and motoneurons. Medial propriospinal neurons have long branching axons – some extend the length of the spinal cord – to coordinate movements of the neck and pelvis allowing the axial muscles to be coordinated. They are located in the ventral and medial columns. Lateral propriospinal neurons interconnect a smaller number of segments and have more focused connections. This explains the greater interdependence of action of more distal muscles at the hand and wrist. The shoulder and elbow have more stereotyped movements and thus are more interconnected than the hand and wrist but less than the axial muscles. They are located in the dorsal and lateral columns.
• #9: Direct: input to lower motor neurons via axons that extend directly from the cerebral cortex. Indirect pathway: input to lower motor neurons from motor centers in the brainstem. These brain stem centers in turn receive input from the basal gangli, thalamus, brainstem nuclei, reticular formation, cerebellum and cerebral cortex.
• #10: Some axons of corticobulbar tract cross over, some do not
• #11: Rubrospinal red nucleus located in midbrain Lateral Vestibulospinal Tract primarily excites proximal extensor motoneurons & inhibits flexor motoneurons of both upper and lower limbs through interneurons & propriospinal neurons Medial Vestibulospinal Tract makes synaptic connections with medial motoneuron groups (neck and back muscles) and with nearby interneurons and propriospinal neurons some monosynaptic excitatory connections to ipsilateral neck motoneurons and monosynaptic inhibitory connections to contralateral neck motoneurons Both are important in the reflex control of balance and posture in response to head movements. Medial Reticulospinal Tract axons from pontine reticular formation descend in ventral columns on ipsilateral side of spinal cord make excitatory synaptic connections with axial muscles and proximal limb extensor muscles Functions to support posture Neurons in the reticulospinal tract rise from a cluster of nuclei in the reticular formation in the pons and medulla. Lateral Reticulospinal Tract axons from medullary reticular formation descend bilaterally in ventral part of lateral columns make inhibitory synaptic connections with neck and back motor neurons has widespread polysynaptic inhibitory connections with extensor motor neurons & excitatory connections with flexor motor neurons Tectospinal Tract originates in deep layers of superior colliculus axons cross to contralateral side of body just below periaqueductal gray matter project to medial interneurons in upper cervical segments regulates contralateral movements of head in response to visual, auditory and somatic stimuli Receives inputs from the cortex via a cortico-tectospinal pathway
• #12: The motor cortex itself is subdivided into three areas, each with its own topographical representation of muscle groups and specific motor functions; M1 is primary motor cortex, PMA is premotor area, and SMA is supplementary motor area, initially subdivided based on electrophsyiological experiments: which area can evoke movement with the lowest stimulation
• #13: Motor cortex is plastic: meaning the representation of body parts can change with practice or disuse. Important for rehabilitation after stroke. motor regions of the cortex contain somatotopic motor map of body (mapping from cortical activation site to muscles on contralateral side of body) - found in primary motor cortex, premotor cortex, and supplementary motor areas. orderly arrangement of control areas for face, digits, hand, arm, trunk, leg and foot fingers, hands and face (used in tasks requiring greatest precision and finest control) have disproportionately large representation
• #14: Voluntary eye movement controls tracking or moving eyes voluntarily to different objects. Damage results in locking involuntarily on certain objects
• #15: commands from the brain got to spinal motor neurons or interneurons carried by descending pathways (e.g. corticospinal tract as discussed earlier) posterior parietal cortex = association area motor plans include: choice of muscles, strength of contraction and sequence of contraction
• #16: This is also true for correction of errors (more later with basal ganglia and cerebellum).