Lower Motor Neuron Circuits

Review: Lower Motor Neuron Circuits

• Four motor systems contribute to motor control descending from upper motor neurons.
• Motor Cortex plans, initiates, and directs voluntary movements.
• Brainstem centers control rhythmic, stereotyped movements and postural control.
• Basal Ganglia initiates intended movement and suppresses unwanted movement.
• Cerebellum coordinates ongoing movement.
• Motor neuron pools consist of lower motor neurons.
• Sensorimotor integration and central pattern generation occurs in local circuit neurons.
• Sensory inputs feed information into local circuit neurons.
• Spinal cord and brainstem circuits influence skeletal muscles.
• Lower motor neurons are the final common pathway.
• Lower motor neurons innervate muscle fibers within a single muscle, forming a motor neuron pool.
• Alpha motor neurons innervate extrafusal muscle fibers to generate force.
• Gamma motor neurons innervate intrafusal muscle spindles to regulate the gain of the stretch reflex and set muscle tone.
• Motor neurons in the ventral horn are somatotopically organized.
• Local circuit neurons in the medial intermediate zone synapse with medial lower motor neurons.
• Lateral intermediate zone neurons synapse with lateral ventral horn neurons.

Review: Negative Feedback Systems

• Muscle spindles monitor and maintain muscle length
• Golgi tendon organs monitor and maintain muscle force.

Reflexes and Voluntary Movement

• Voluntary movements are purposeful, unlike reflexes which have a one-to-one stimulus-response mapping.
• In voluntary movements, multiple neurons in various brain regions coordinate muscles to achieve a goal.
• Effectiveness in voluntary movements increases with practice.
• Voluntary movements are internally generated, not necessarily responses to environmental stimuli.

Overview of Motor Control

• Frontal lobes' motor and premotor areas drive limb movements which includes planning and precise control of simple and complex actions.
• Brainstem motor control centers are important for postural control.
• Vestibular nucleus projects to the vestibulospinal tract.
• Reticular formation projects to the reticulospinal tract.
• Superior colliculus projects to the colliculospinal tract.
• Red nucleus projects to the rubrospinal tract.
• Upper motor neurons target medial or lateral ventral horns.
• The ultimate targets of upper motor neurons are the local circuit neurons which synapse with the lower motor neurons.
• Two collections of upper motor neurons exist, coming from the cortex or brainstem.

Somatotopic Organization

• Somatotopy of local circuit neurons exists in the intermediate zone (IZ) of the ventral horn.
• Medial IZ contains local circuit neurons synapsing with medial lower motor neurons.
• Lateral IZ neurons synapse with neurons in the lateral ventral horn.
• Lateral white matter contains axons from the motor cortex, controlling skilled voluntary movements.
• Medial white matter axons come from the brainstem which influences posture, balance, locomotion, and orienting.
• Brainstem pathways go to the medial ventral horn for posture and balance.
• Cortical pathways go to the lateral ventral horn for voluntary, skilled movements.
• Ipsilateral pathways refers to the same side of the body.
• Contralateral pathways refer to the opposite side of the body.

Primary Motor Cortex

• Upper motoneurons in the primary motor cortex mediate the planning and initiation of movements.
• Basal ganglia and cerebellum input into the thalamus which then projects to the motor cortex.
• Primary motor cortex is located in the precentral gyrus.
• Low current elicits movement and muscle contractions.
• Low threshold for eliciting movements indicates a large and direct pathway to lower motoneurons.
• Upper motor neurons are pyramidal cells and include Betz cells, which include largest cell soma in the CNS.
• Betz cells make up 5% of the projections to the spinal cord.
• Betz cells are most important for distal muscle control.
• Betz cells are only found in primary motor cortex.
• Non-Betz pyramidal cells are found in all divisions of both the motor and premotor cortex.

Corticospinal and Corticobulbar Tracts

• Motor cortical neuronal axons travel through the internal capsule.
• The axons then go from ventral surface of the midbrain to the cerebral peduncle on through to the pons to the ventral surface of the medulla and then onto the spinal cord.
• In the brainstem, corticobulbar axons give rise to bilateral collaterals that innervate multiple brainstem nuclei.
• The corticobulbar projections terminate on local circuit neurons.
• Corticobulbar projection controls muscles of the head, face, and neck, mediating facial expressions, chewing and tongue movements.
• The corticospinal tract is divided into two parts.
• Ninety percent of fibers decussate in the caudal medulla, forming the lateral corticospinal tract.
• The lateral corticospinal tract terminates in lateral ventral horn and intermediate zones synapsing on local circuit neurons.
• Some of these axons synapse directly onto alpha motor neurons to control the distal limbs, forearm, and hand.
• These axons dictate fine motor skills such as writing, playing instruments, etc.
• The other 10% of fibers do not cross, instead forming the ventral corticospinal tract, continuing and terminating bilaterally.
• Ventral corticospinal tract originates from the dorsal and medial regions of the motor cortex and serves axial and proximal muscles.

Functional Organization of Motor Cortex

• In the 1870s, Fritsch and Hitzig demonstrated that electrical stimulation of motor cortex caused muscle contractions on the opposite side of the body.
• Hughlings Jackson proposed that motor cortex had a representation of musculature.
• Epilepsy can induce partial seizures ("marches") that move systematically from one body part to another.
• Wilder Penfield developed the "Montreal Procedure" which allowed for reactions from patients while the surgeon stimulated different areas of the brain.
• He found that the human motor cortex had a map of musculature.
• Penfield correlated location of muscle contractions to the site of electrical motor cortex stimulation.
• Penfield mapped the site of the precentral gyrus in over 400 neurosurgical patients.
• Intracortical electrical stimulation in the 1960's showed upper motor neurons in layer V of the motor cortex projecting to the lower motor neuron.
• Axons can be focally stimulated.
• When microstimulation was combined with muscle electrical activity recordings, small currents elicited excitation of several muscles.
• Movements, not just muscles, might be organized.
• Movements were elicited even when stimulating sites were far from the original stimulation site.
• Local circuits are involved in controlling movements.
• In the 1960s, Evarts developed a system for recording from motor cortex neurons.
• Firing rate of motor neurons increased in frequency as force increased.
• Firing rates increased in frequency prior to the development of force.
• Evarts proposed that motor cortex contributes to the early phase of movement generation and planning.
• Spike triggered averaging is used to correlate activity of individual neurons to muscle activity.
• If animals perform a simple movement such as wrist flexion or extension, then a peripheral muscle group can be shown to be activated by one motoneuron.
• Observations confirm that single upper motoneurons contact several lower motoneuron pools.
• Cortical neurons encode sets of movements instead of individual muscles.
• Michael Graziano used longer microstimulations in awake behaving monkeys to show fine-mapping of behaviorally relevant movements.
• Small currents that elicited muscle electrical activity responses showed that motor maps are organized sets of complex movement rather than individual muscles.
• Central space/manipulation movements are directed toward the reaching area nearby the body.
• Contralateral hand movements can be blue, the elicited movements are illustrated with the curved black lines, and the final positions are red dots.
• Prolonged stimulation elicits coordinated movements of the hand to the mouth.
• Activity can be seen in the neuron increased before movements between 90 and 225 degrees.
• Activity in that same neuron decreased where movements of the hand were indicated between 45, and 315 degrees.
• Neurons discharge activity during movement allowing identification of their directional preference in 1986.
• The neuron's discharge rate was greatest before movements in a particular direction.
• Movement is represented by the integrated activity of a population of broadly tuned upper motor neurons.

Directional Tuning of Neurons

• The starting positions of the contralateral hand are shown in blue.
• Elicited movements are illustrated as curved black lines.
• The final positions are shown via red dots for hand movements.

Premotor Cortex

• The premotor cortex exerts both direct and indirect influences on motor behavior.
• Indirect effects are mediated via reciprocal projections to the primary motor cortex.
• Direct effects are mediated by axons projecting to corticobulbar and corticospinal tracts.
• Lateral divisions serve different functional specializations.
• Lateral premotor cortex is important in 'closed-loop' motor tasks.
• A monkey is trained to reach in different directions depending on visual cue observed.
• Lateral premotor neurons fire at the appearance of a cue and increase firing rates between cue and signal to move, encoding intention to move.
• Ventrolateral premotor cortex also contains neurons that respond to observed movements performed by another individual.
• Mirror neurons are neurons that fire when both the animal acts, and the animal observes the same action performed by another.
• Findings suggest that the premotor cortex plays a role in encoding the observed actions of others.

Effects of Cortical Damage

• The lateral premotor cortex requires external cues for movement selection.
• Lesions in monkeys impair "closed loop" tasks and the ability to perform visual cue-conditioned tasks.
• This occurs even if they can see the visual cue and perform the motor response.
• Frontal lobe patients have trouble with initiating movement upon visual cue.

Summary: Cortical Upper Motor Neurons

• Corticobulbar and corticospinal tracts activate muscles of the head, face, and body
• The premotor cortex is critical for action and direction selectivity.
• Some premotor neurons respond to observed movement in others.
• Pre-motor Cortex also is specialized for lateral closed and medial open loop control.

Brainstem Pathways

• Brainstem motor pathways enable posture, balance and locomotion
• These tracts are located in both Ipsilateral and Contralateral parts of the body.

Vestibular Nuclei

• Brainstem motor centers normally work in concert with motor cortex.
• The reticular formation receives vestibular, balance, posture and control input, providing feedback from sensory systems.
• Medial VS tract, mediates, feedback, or responding to a disturbance of body posture and stability signaled by the semi-circular canals.
• Extending your arms and the dorsiflexion of your neck when you trip gives rise to postural control

Reticular Formation

• Reticular formation neurons project to the medial ventral horn and modulate reflexes for stereotyped movements.
• These neurons receive input from the cortex, hypothalamus, or brainstem.
• These connections can initiate feedforward adjustments to stabilize posture during ongoing movements
• In feedforward postural control the cortical upper motor neurons initiate the primary and compensatory movement.
• The compensatory movements usually precedes the primary movement.

Colliculus

• Superior colliculus pathways direct the head to specific movement locations.
• Direct pathways via the spinal cord
• Indirect pathways through the reticular formation input to help function in head and neck orientation.

Types of Postural Control

• Feedforward postural responses are “preprogrammed” and typically precede the onset of limb movement.
• Feedback responses are initiated by sensory inputs that detect postural instability.

Red Nucleus

• Projects into the cervical level cord.
• These tracts terminate into lateral ventral horn and intermediate zones.
• Controls arm and hand moments prior to onset through rubrospinal tract.