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Reflex and Voluntary Control of Movement
1. Keyboard cat, a fine example of voluntary control of movement in
the animal kingdom.
Reflex & Voluntary
Control of Movement
Csilla Egri, KIN 306, Spring 2012
2. Outline
Locomotion reflexes
Central pattern generators (CPGs)
Descending tracts
Pyramidal tracts
Extrapyramidal tracts
Cortical control of movement
Motor cortex
2
3. Motor Control
3
There are three levels in the hierarchy of motor control:
Spinal cord
Locomotion reflexes
CPGs
Brain stem
Talked about these during
the vestibular and visual
system lectures
Postural reflexes
Locomotor regions
Voluntary control of movement
Cortical motor areas
Voluntary control of movement
These different areas are highly interdependent and movements result
from the coordinated action of these three regions.
4. Spinal cord: locomotion reflex
example 4
Stepping reflex in newborns
Disappears around 6 weeks,
gradually replaced by voluntary walking
behaviour
Simplified half-center model for alternating rhythm generation
B&B Figure 14-8
5. Spinal cord: central pattern
generators (CPGs) 5
Neuronal network capable of generating a
rhythmic pattern or motor activity in the
absence of sensory input
Walking, swimming, respiration
Simplest CPGs contain spontaneously bursting or
reciprocally innervated neurons
Basic firing pattern modified by sensory or
descending inputs
6. Spinal cord: CPG example
6
Transect afferent input, and decerebrate cat:
With support, walking motion is reproducible with stimulation of brainstem
Each limb is controlled by its own CPG
7. Spinal Cord: organization of motor
tracts 7
Motor neurons of ventral horn organized topographically
Descending upper motor neurons control lower motor neuron
firing
Also influenced by the activity of interneurons & peripheral
sensory receptors
B&L Figure 9-12
8. Spinal Cord: organization
of motor tracts
8
Medial
reticulospin
al tract
Direct (pyramidal) pathway
lateral and anterior corticospinal tract
corticobulbar tract
Indirect (extrapyramidal) pathway
rubrospinal tract
tectospinal tract
vestibulospinal tract
reticulospinal tract (lateral & medial)
9. Direct (pyramidal)
pathways 9
Pyramidal tracts originate primarily in
motor, premotor and supplementary motor
areas of cortex
~ 90% of descending axons cross in the
medulla and descend in lateral columns
of spinal cord (lateral corticospinal tract)
Control distal muscles; precise,
agile, skilled movements
~ 10% cross over in spinal cord (anterior
corticospinal tract)
Control proximal trunk muscles;
coordinate movements of axial
skeleton
Corticolbulbar tracts also originate in
motor cortex, descend to brainstem
Axons terminate in motor nuclei of
cranial nerves
Control precise, voluntary movement of
head, neck and tongue
10. Indirect (extrapyramidal)
pathways 10
Complex, polysynaptic circuits involving
motor cortex, basal ganglia, thalamus,
cerebellum, and reticular formation
Rubrospinal: from red nucleus to
contralateral muscles controlling precise
movements of distal parts of upper limbs
Tectospinal: from superior colliculus to
contralateral muscles controlling reflexive
movements of head and neck to auditory
or visual stimuli
Medial
reticulospinal
tract
Vestibulospinal: from vestibular nucleus to ispsilateral skeletal muscles of
trunk and proximal parts limbs for maintaining posture and balance
Medial and lateral reticulospinal: from reticular formation to ipsilateral
skeletal muscles of trunk and proximal parts of limbs for maintaining posture
and regulating muscle tone in response to body movements.
11. Motor areas of cerebral cortex
11
Primary motor cortex
Main motor area involved in executing
voluntary movements
Movement elicited with least amount of
electrical stimulation
Pre motor cortex
Sensory guidance of movement (receive
input from posterior parietal cortex
Contributes to extrapyramidal pathways
Lesion impairs ability to develop strategy
for movement
Supplementary motor cortex
Involved in planning of complex and two
handed movements
Coordinates posture
13. Specialized cortical motor areas
13
Broca’s area (area 44, 45)
Coordinated movements of
tongue and vocal cords for
word formation
Lesion results in expressive
aphasia
Voluntary eye movement
field
Also controls voluntary blinking
Area for hand skill
Lesions result in motor apraxia:
uncoordinated, nonpurposeful
hand movements
Guyton Figure 55-3
14. Voluntary movement: simplified
linear sequence of events 14
Command must be organized by the brain
1. Identify target in space
a) objective is identified in posterior parietal cortex
which receives input from somatosensory, visual,
vestibular and auditory systems
b) Sense of body position in relation to target also required
2. info transmitted to supplementary & premotor areas
where the motor plan is developed
a) Choice of muscles, sequence of contractions, required
force and trajectory computed
3. Motor plan transmitted to primary motor cortex and
down descending pathways to interneurons & motor neurons
15. Motor Plan
15
Sensory feedback provided through ascending
afferent pathways
Transmitted to motor cortex either directly from
thalamus or indirectly thru connections between
somatosensory & visual cortex
Motor cortex has bidirectional connections with
thalamus, cerebellum & basal ganglia
Important in planning & execution of movement
(more in next lecture)
16. Objectives
After this lecture you should be able to:
Give an example of a locomotion reflex and an activity
governed by a CPG
Discuss the organization of the spinal cord and how it
relates to voluntary control of movement
Include both pyramidal and extrapyramidal descending tracts
Describe the organization of the motor cortices
Outline the sequence of events involved in initiation of
voluntary movement
16
17. 17
Test your knowledge
1. A lesion to Broca’s area results in
_____________________
2. Precise, voluntary movements of the head, neck and
tongue are controlled by descending inputs via the
__________________________ tract
3. Motor neurons descending in the pyramidal tracts
originate in ________________________ whereas motor
neurons descending in the extrapyramidal tracts
originate in
_______________________________________________.
Hinweis der Redaktion
Gutyon is best
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.
When one motor neuron is active, the other is inhibited
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
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.
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.
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.
Some axons of corticobulbar tract cross over, some do not
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
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
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
Voluntary eye movement controls tracking or moving eyes voluntarily to different objects. Damage results in locking involuntarily on certain objects
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
This is also true for correction of errors (more later with basal ganglia and cerebellum).