Lecture 11: Motor Systems

Motor Control System

Network of CNS and PNS structures that generate and regulate movement.

Somatic Motor System

The voluntary motor system controlling skeletal muscle contraction.

Basal Ganglia

Subcortical nuclei that regulate initiation and suppression of voluntary movement.

Cerebellum

Coordinates movement accuracy, balance, and motor learning.

Descending Systems

Pathways from the motor cortex and brainstem that control spinal motor circuits.

Local Circuit Neurons

Interneurons in the spinal cord that integrate sensory input and descending commands.

Skeletal Muscle

Striated muscle under voluntary control.

Motor Neuron

A neuron that transmits signals from the CNS to a muscle fiber, causing contraction.

Upper Motor Neuron (UMN)

Originates in the motor cortex or brainstem; sends descending signals to spinal circuits; does not exit the CNS.

Lower Motor Neuron (LMN)

Found in spinal cord ventral horn or cranial nerve nuclei; directly innervates muscles; leaves CNS.

Crossing the Midline (Decussation)

The point where upper motor neuron axons cross to the contralateral side of the body.

α-Motor Neuron (Alpha)

Innervates extrafusal fibers responsible for muscle contraction and force generation.

β-Motor Neuron (Beta)

Innervates both extrafusal and intrafusal fibers; regulates both contraction and spindle sensitivity.

γ-Motor Neuron (Gamma)

Innervates intrafusal fibers; adjusts spindle sensitivity during muscle contraction.

Extrafusal Muscle Fiber

Contractile skeletal muscle fiber producing movement.

Intrafusal Muscle Fiber

Specialized fibers within muscle spindles that detect stretch.

Proprioceptor

Sensory receptor detecting body position and movement.

Afferent Fiber

Sensory neuron carrying signals to the CNS.

Efferent Fiber

Motor neuron carrying signals from the CNS.

Motor Unit

A single α-motor neuron and all muscle fibers it innervates.

Motor Pool

All motor neurons innervating a single muscle.

Fast-Fatigable (FF) Motor Unit

Generates high force quickly but fatigues rapidly; used for brief, intense actions.

Fast Fatigue-Resistant (FR) Motor Unit

Intermediate; moderate force, moderate endurance.

Slow (S) Motor Unit

Generates low force but resists fatigue; for posture and sustained activity.

Recruitment

Progressive activation of more motor units to increase muscle force.

Rate Coding

Increased firing rate of a motor neuron increases force output.

Size Principle

Motor units are recruited from smallest (S) to largest (FF) based on force demand.

Neuromuscular Junction (NMJ)

The synapse between a motor neuron’s axon terminal and a muscle fiber.

Synaptic Bouton

Presynaptic terminal of the motor neuron containing acetylcholine (ACh).

Synaptic Vesicle

Membrane-bound structure storing ACh for release into the synaptic cleft.

End Plate

Specialized postsynaptic region of the muscle fiber membrane with junctional folds.

Junctional Folds

Invaginations of the sarcolemma that increase the surface area for ACh receptors.

Acetylcholine (ACh)

Neurotransmitter released by motor neurons that triggers muscle depolarization.

Acetylcholine Receptor (AChR)

Ligand-gated ion channel on muscle membrane that opens upon ACh binding.

End Plate Potential (EPP)

Depolarization at the end plate due to AChR activation.

Voltage-Gated Na⁺ Channel

Opens after EPP to generate a muscle action potential.

Action Potential (AP)

Electrical signal that travels along the muscle fiber membrane.

Sarcolemma

Plasma membrane of a muscle cell.

All-or-Nothing Response

Once threshold is reached, a full contraction occurs; if not, none does.

Propagation

The spread of an action potential across the muscle fiber to trigger uniform contraction.

Reflex

Involuntary, automatic motor response to a sensory stimulus.

Spinal Reflex

A reflex mediated at the spinal cord level without cortical input.

Reflex Arc

Pathway consisting of a receptor, sensory neuron, interneuron, motor neuron, and effector.

Postsynaptic Reflex

Reflex involving one or more interneurons.

Proprioception

Sensory awareness of body and limb position.

Muscle Spindle

Proprioceptor sensitive to muscle stretch and rate of length change.

Golgi Tendon Organ (GTO)

Proprioceptor sensitive to muscle tension.

Nuclear Bag Fiber

Intrafusal fiber detecting dynamic stretch (velocity).

Nuclear Chain Fiber

Intrafusal fiber detecting static stretch (length).

γ-Motor Neuron Activation

Adjusts spindle tension to maintain sensitivity during contraction.

Stretch Reflex (Myotatic Reflex)

Reflex contraction of a muscle in response to stretching (e.g., knee-jerk).

Reciprocal Inhibition

Activation of agonist muscle accompanied by inhibition of antagonist muscle.

Golgi Tendon Reflex (Inverse Myotatic Reflex)

Protective reflex causing muscle relaxation when excessive tension is detected.

Flexor Withdrawal Reflex

Rapid withdrawal of limb from a painful stimulus.

Crossed Extensor Reflex

Contralateral limb extension to support body weight when the opposite limb withdraws.

Proprioceptive Feedback

Continuous sensory feedback about muscle state sent to CNS.

Medial-Lateral Map

Organization of motor neurons in the ventral horn: medial neurons control proximal muscles, lateral neurons control distal muscles.

Longitudinal Map

Organization of motor neurons along the spinal cord; groups of neurons innervate specific muscles across segments.

Cervical Enlargement

Spinal region with increased motor neurons for arm and hand muscles.

Lumbar Enlargement

Region with increased motor neurons for leg and foot muscles.

Corticospinal Tract (Pyramidal Tract)

Descending motor pathway controlling voluntary movement.

Pyramidal Decussation

Point in the medulla where ~90% of corticospinal fibers cross to the contralateral side.

Lateral Corticospinal Tract

Controls fine distal movements (hands, fingers).

Anterior (Ventral) Corticospinal Tract

Controls axial and proximal limb muscles bilaterally.

Brainstem Descending Tracts

Include vestibulospinal, reticulospinal, and colliculospinal tracts.

Vestibulospinal Tract

Controls posture and balance through reflex adjustments.

Reticulospinal Tract

Coordinates posture and locomotion; anticipatory postural control.

Colliculospinal Tract (Tectospinal)

Coordinates head and eye movements toward stimuli.

Primary Motor Cortex (M1)

Frontal lobe region generating neural impulses for movement execution.

Precentral Gyrus

Gyrus housing M1.

Premotor Cortex

Plans and organizes movements before execution.

Supplementary Motor Area (SMA)

Involved in planning complex, bilateral, and internally generated movements.

Betz Cells

Large pyramidal neurons in layer V of M1 with long axons descending to the spinal cord.

Motor Map (Homunculus)

Somatotopic representation of body parts proportional to control precision.

Penfield Map

Experimental mapping of motor cortex using electrical stimulation.

Motor Plasticity

The ability of motor maps to reorganize following injury or training.

Purposeful Movement Mapping

Motor cortex organization corresponds to coordinated, behaviorally relevant movements.

Autonomic Nervous System (ANS)

Controls involuntary functions; divided into sympathetic and parasympathetic divisions.

Visceral Motor System

Another term for the ANS; regulates smooth and cardiac muscle, and glands.

Homeostasis

Maintenance of internal equilibrium through autonomic regulation.

Preganglionic Neuron

Neuron in the CNS projecting to autonomic ganglia.

Postganglionic Neuron

Neuron in ganglia projecting to target tissues.

Autonomic Ganglion

Cluster of postganglionic neuron cell bodies outside the CNS.

Lateral Horn (Spinal Cord)

Location of sympathetic preganglionic neuron cell bodies.

Sympathetic Division

'Fight or flight'; increases energy expenditure.

Thoracolumbar Outflow

Sympathetic preganglionic fibers arise from T1–L2 spinal segments.

Sympathetic Chain Ganglia

Series of ganglia parallel to spinal cord housing postganglionic sympathetic neurons.

Prevertebral Ganglia

Sympathetic ganglia near major abdominal arteries.

White Rami Communicantes

Myelinated axons carrying preganglionic fibers to sympathetic ganglia.

Gray Rami Communicantes

Unmyelinated axons carrying postganglionic fibers back to spinal nerves.

Epinephrine/Norepinephrine

Neurotransmitters that increase heart rate, blood glucose, and blood flow to muscles.

Adrenergic Receptors

Receptors activated by norepinephrine or epinephrine on target organs.

Parasympathetic Division

'Rest and digest'; conserves energy and restores homeostasis.

Craniosacral Outflow

Parasympathetic preganglionic fibers originate in brainstem and sacral spinal cord.

Cranial Nerves III, VII, IX, X

Carry parasympathetic fibers to head and thoracic/abdominal organs.

Parasympathetic Ganglia

Located near or within target organs.

Muscarinic Receptor

Metabotropic ACh receptor on target organs; mediates slow, modulatory effects.

Nicotinic Receptor

Ionotropic ACh receptor on postganglionic neurons; mediates fast synaptic transmission.

Varicosity

Swelling along autonomic axon terminals releasing neurotransmitter diffusely.

Diffuse Junction

Broad neurotransmitter release zone in smooth muscle; no defined NMJ.