somatotopic organization of the ventral horn in the cervical enlargement. Locations of descending projections from the motor cortex in the lateral white matter and from the brainstem in the anterior-medial white matter are shown.
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## Arrangement of motor neurons and local circuit interneurons within the spinal cord
* Medial intermediate zone local circuit neurons project to medial ventral horn motor neurons.
* Medial local circuit neurons have axons that may project to targets along the entire length of the cord, and also cross the midline to innervate contralateral side.
* Lateral regions of the intermediate zone contain local neurons that synapse with motor neurons in the lateral ventral horn.
* Lateral circuit neurons project over a smaller area and do not cross the midline.
* Allows distal regions to act independently of each other.
med ventral horn has lower motor neurosn for posteure balance and orienting movements of head and neck during shits of visual gaze. receipve descending input from the pathways orginating mainly in the brainstem, course through the anterior medial white matter of the spional cord and terminate bilaterally.
lateral ventral horn contains lower motor neurons that mediate skilled voluntary movements of the distal extremities. Receive descending projection from the contralateral motor cortex via lateral division of the corticospinal tract.
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## Medial brainstem pathways modulate theaction of motor neurons in the ventromedial area
* Vestibular nuclei:
* Receive information from inner ear
* Project to medial regions of spinal gray matter.
## Medial brainstem pathways modulate theaction of motor neurons in the ventromedial area
* Superior colliculus
* Projects to medial cell groups in the cervical cord
* Influences neck muscles (colliculospinal tract)
* But major output of superior colliculus to spinal cord mediated by reticular formation. Axial musculature control of neck and performing orienting movements of head and eye movements.
* Able to predict changes in posture, and generate an appropriate stabilizing response.
* Some muscles fire in anticipation of a need for postural adjustment.
* Reticulospinal tract important for this process. If it is severed in a cat, no change in compensatory muscles occur during the process.
* Stimulate motor cortex in the right place can induce paw lifting, and several limb muscles to fire. Inhibition of the reticulospinal tract will allow the paw to move but will prevent the movement of other limbs.
Note:
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## Anticipatory maintenance of body posture
Severed reticulospinal tract will allow biceps to fire but
will not allow the gastrocnemius to fire for posture.
* Receives inputs from S1, posterior parietal structures (incorporates multiple sensory modalities, used for planning).
* Controls contralateral side of the body
* Topographic organization- body represented across the medial-lateral axis. More space given to areas of fine motor control. Multiple neurons can get the same muscle to fire- not located in exact same place in cortex.
* Primary motor cortex located in the precentral gyrus
* Gets input from sensory cortex, basal ganglion and cerebellum
* Has 6 layers, layer V is the output layer (pyramidal cells, including the large Betz cells consisting of about 5% of projection to spinal cord and concerned with fine distal movements)
* Primary pathway- the corticospinal tract. Axons cross in the caudal medulla, and innervate in lateral ventral horns
90% of corticaospinal axons at caudal end of medulla cross (decussate, lateral corticospinal tract). 10% remain ipsilaterally (ventral corticospinal tract).
most corticobulbar inputs (except lower face and tongue) terminate bilaterally.
left illustrates spike triggered averaging method for correlating muscle activity with the discharges of single upper motor neurons.
right shows the response of a thumb muscle by a fixed latency to the single spike discharge of a pyramidal tract neuron. This can be used to determine all muscles influenced by a given motor neuron.
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## Purposeful movements resulting from prolonged microstimulation of the primary motor cortex
Coordinated movements of hand and mouth after stimulation near the middle of the precentral gyrus towards head (like for eating).
Coordinated movements of hand towards belly as if inspecting an object. Notice clustering of centralized trajectories after many trials instead of just random movements.
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## Directional tuning of an upper motor neuron in the primary motor cortex
Monkey trained to move joystick in response to light
Summing response from a bunch of neurons shows that the direction is better encoded from an ensemble or population of neurons— so that different movement directions/sequences are represented by overlapping and distributed populations of neurons giving rise a series of neuronal population vectors rep all the different directions.
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## Section of pyramidal tracts in monkeys produces loss of independent digit control
Divisions of the motor cortex in the macaque monkey brain.
lateral premotor and supplementary motor areas are involved in selecting and organizing purposeful movements of the limbs and face.
the frontal eye fields organize voluntary gaze shifts. The cingulate motor areas are involved in expression of emotional somatic behavior.
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## The premotor cortex
* Lies adjacent (rostral) to the primary motor cortex
* Makes extensive reciprocal connections with the primary motor cortex
* Projects directly to spinal cord (30% of axons in the corticospinal tract).
* Lateral premotor cortex- has neurons that are tuned to a particular direction of movement (like primary motor cortex) but differs in that they fire earlier than neurons in the primary motor cortex. This is especially important in conditional motor tasks, that pair a movement with a visual cue.
* During the pairing of a visual cue with a motor task, the neurons will fire before any initiation of the task. This is used for intentions.
* Lesions in monkey prevent vision conditioned tasks, although vision is fine and the task can be done in other ways.
Note:
thes neurons encode intention to perform a movement rather than just the movement itself.
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## Mirror motor neuron activity in a ventral-anterior sector of the lateral premotor cortex
Indeed a nice way to understand this is by examining portions of the lateral premotor cortex that contain so called mirror neurons that have been focus of a bit of attention over recent years.
peristimulus response histograms
passive observation of human hand interacting with (placing food on) tray and also during motor monkey’s own movement to retrieve food
based on Giacomo Rizzolatti et al, 1996
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## Mirror motor neuron activity in a ventral-anterior sector of the lateral premotor cortex
Also fires when the behavior is executed behind a visual barrier.
Suggests that parts of the premotor cortex play a role in encoding the actions of others.
Studies of this mirror neuron system is an active area of neurosci research and some hypotheses anticipate that this connections in the mirror neuron system could be disrupted in neurodevelopmental disorders such as autism or schizophrenia— but it is still important to note that these are active investigations and hypotheses still be tested.
* By investigating patients with various types of brain damage we can see how the various components of motor performance may be affected. Examples:
* Lesions to primary motor cortex (e.g. from a stroke) result in loss of voluntary movements on the contralateral (opposite) side of the body.
* Apraxia is the specific loss of the ability to plan and correctly perform co-ordinated motor skills, mainly as a result of damage to the supplementary motor area. Speech disorders result from damage to motor cortex.
* Patients can move muscles, and walk on command but can no longer link gestures to a coherent act, or to recognize the appropriate use of an object even though they can recognize what an object is.
Note:
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## Damage to cortex: alien limb syndrome
* A disorder in which person feels unable to control movements of a body part, believes that the limb is alien, or believes that the body part has its own personality
* It is typically associated with lesions in the supplementary motor area or those affecting blood flow to the anterior regions of the corpus callosum and the anterior cingulate
* Man who simultaneously tried to strangle and save his wife from himself.
normal response on left. Following damage to descending corticospinal pathways stroking sole give abnormal fanning of toes.
Also common in infants during maturation of the descending corticospinal pathways
spinal shock and decr activity deprived of input from motor cortex and brainstem
after several days recovery begins (not fully understood) and includes
-babinski sign
-spasticity (decerebrate rigidity). Cause by removal of suprresive infl by cortex on postural centre of vesitbulaer nuclei and reticular formation.. Rep abnormal incr in the gain of th spinal corste strech reflesxes.
* 1. Ventromedial system for balance, posture and controlling head & eye movements. Important for muscles of legs & trunk needed for walking.
* 2. Dorsolateral system for controlling movements of upper limbs & extremities such as fingers and toes as well as movement of facial muscles.
*
* Sensory
* Input to primary somatosensory area via dorsal root
* Two main pathways:
* 1. Dorsal spinothalamic tract for proprioception (body awareness and position in space) and haptic feedback (sensation of fine touch and pressure)– crosses in medulla
* 2. Ventral spinothalamic tract for nocioceptive information– crosses over in spinal cord
