<figure><figcaption class="big">Somatotopic organization of lower motor neurons</figcaption><img src="figs/Neuroscience5e-Fig-16.03-0_e7b6c42.jpg" height="200px"><figcaption>Neuroscience 5e Fig. 16.3</figcaption></figure>
* 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
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<div><img src="figs/Neuroscience5e-Fig-16.04-0_a8cdffa.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 16.4</figcaption></div>
<figure><figcaption class="big">Somatotopic organization of descending upper motor neuron projections</figcaption><img src="figs/Neuroscience5e-Fig-17.01-1R_66b8f22.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 17.1</figcaption></figure>
**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.
<figure><figcaption class="big">Upper motor neurons, light red. Lower motor neurons, dark red</figcaption><img src="figs/Neuroscience5e-Fig-17.01-2R_fee0e45.jpg" height="500px"><figcaption>Neuroscience 5e Fig. 17.1</figcaption></figure>
Medial ventral horn has lower motor neurons for posture, balance, and orienting movements of head and neck during shits of visual gaze. Receive descending input from the pathways originating mainly in the brainstem, course through the anterior medial white matter of the spinal 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.
Info from semicircular canals in inner ear. Provides sensory information for self motion, head position, and body position relative to gravity. It is neural feedback for postural control
There are actually both crossed and uncrossed vestibular projections.
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## Medial brainstem pathways modulate the action of motor neurons in the ventromedial area
*Also rubrospinal tract in non-human primates and other mammals. Red nucleus in midbrain tegmentum. Axons terminate in lateral ventral horn region (distal extremities control). Few if any large neurons in red nucleus in human and don't project to spinal cord. Instead project to inferior olive in human (for learning signals/error control with cerebellum)*
* 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
<div><figcaption class="big">topographic representation in motor cortex</figcaption><img src="figs/Neuroscience5e-Fig-17.05-0_80cd89b.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 17.5</figcaption></div>
* 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)
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.
: test function of direct vs indirect pathways from motor cortex
: transect spinal cord at medulla, leaving indirect path to spinal cord via brainstem reticular formation
: stand walk run climb intact with proximal and axial muscles, but precise distal limb usage with hands impaired (e.g. can't pick up food objects). Independent use of fingers doesn't return
Corticobulbar is yellow, corticospinal in red. Note that most corticospinal axons cross the midline in the caudal medulla. Corticobulbar is for facial muscles.
internal capsule to cerebral peduncle at base of midbrain to scatter among pontine fibers and basal pontine gray matter then coalesce at ventral surface medulla to form medullary pyramids
*corticobulbar axons terminate primaryly on local circuit neurons rather than brainstem motor neurons*
*For animals with little silled movements of distal limbs/paws corticospinal projections mostly directed to the dorsal horn of spinal cord to modulate proprioceptive and mechanosensory inputs relevant to body movements. Projection to the ventral horn from corticospinal tract largest for animals with skilled fractionated movements of hands and forepaws.*
<figure><figcaption class="big">microstimulation of awake, behaving monkeys</figcaption><img src="figs/Neuroscience5e-Box-17B-0_6babded.jpg" height="400px"><figcaption>Neuroscience 5e Box 17B. Graziano et al., J. Neurophysiol 2005</figcaption></figure>
More complex maps (not just connected to id. motor pools) in cortex than appreciated by Penfield stimulation studies (Penfield and Boldrey *Brain* 1937). Woolsey 1958. Lemon TINS 1988
<div><img src="figs/Neuroscience5e-Fig-17.06-2R_22d459b.jpg" height="300px"><figcaption>Neuroscience 5e Fig. 17.6. Porter and Lemon, 1993</figcaption></div>
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.
<figure><figcaption class="big">Stimulation of limb movements. Blue crosses are start positions. Reds dots are final positions.</figcaption><img src="figs/Neuroscience5e-Fig-17.07-0_6c43e7d.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 17.7. Graziano et al, 2005</figcaption></figure>
*stimualtion that more roughly corresponds to volitional movements (hundreds of ms to sec), Graziano 2005.* With these stimus, movements are sequentiall distrubted across mutliple joints and purposeful.
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.
Blue crosses are start positions, curved black lines are final positions are reds dots.
<figure><figcaption class="big">Monkey trained to move joystick in response to light</figcaption><img src="figs/Neuroscience5e-Fig-17.08-1R_0f3b75c.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 17.8, adapted from Georgeopoulos et al, 1986</figcaption></figure>
is dependent on the direction of the future movement.
Red line indicates movement onset, blac
</figcaption><img src="figs/Neuroscience5e-Fig-17.08-2R_8f25ffd.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 17.8, adapted from Georgeopoulos et al, 1986<</figcaption></figure>
Population vector (red) for a population of simultaneously
recorded upper motor neurons (black lines indicate each id. neuron's spike rate)
</figcaption>
<img src="figs/Neuroscience5e-Fig-17.08-4R_57ae3e0.jpg" height="200px"><figcaption>Neuroscience 5e Fig. 17.8, Georgeopoulos et al., 1986</figcaption></div>
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.
* 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
<div><figcaption class="big">Monkey mirror neuron for hand reaching is active while observing a human hand reach</figcaption><img src="figs/Neuroscience5e-Fig-17.10-1R_57bd769.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 17.10. Rizzolatti et al., 1996</figcaption></div>
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.
Found in two cortical areas-- the posterior part of the inferior frontal cortex and the anterior part of the inferior parietal lobule [Rizzolatti:2004](http://www.ncbi.nlm.nih.gov/pubmed/15217330)
<figure><figcaption class="big">Mirror neuron for hand reaching not active while observing pliers reaching</figcaption><img src="figs/Neuroscience5e-Fig-17.10-2R_a2f4703.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 17.10. Rizzolatti et al., 1996</figcaption></figure>
<figure><figcaption class="big">Mirror neuron for hand reaching active even when not observing self reaching</figcaption><img src="figs/Neuroscience5e-Fig-17.10-3R_a0ef0dd.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 17.10. Rizzolatti et al., 1996</figcaption></figure>
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.
* 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
-spasticity (decerebrate rigidity). Cause by removal of supressive influence by cortex on postural centers of vestibular nuclei and reticular formation. Represents abnormal increase in the gain of the spinal cord stretch reflexes.
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
