NRS 250 Final - Motor Systems

The Spinal Cord: Structures 

• The spinal cord is a long, thin structure of nervous tissue running from the brainstem to the lower back. 

• It is housed within the vertebral column and divided into four regions: cervical, thoracic, lumbar, sacral. 

• Each segment gives rise to afferent (sensory) and efferent (motor) roots. 

• Dorsal roots carry sensory input; ventral roots carry motor output. 

Autonomic Nervous System (ANS) 

• Composed of a two-neuron chain: a preganglionic neuron and a postganglionic neuron. 

• Preganglionic neurons release acetylcholine (ACh) onto nicotinic receptors in ganglia. 

• Postganglionic neurons release ACh or norepinephrine, depending on receptor type. 

Sympathetic vs Parasympathetic Divisions 

• Sympathetic (fight-or-flight): short preganglionic, long postganglionic neurons; release norepinephrine. 

• Parasympathetic (rest-and-digest): long preganglionic, short postganglionic neurons; release ACh. 

• Dual innervation of organs allows coordinated responses. 

The Motor Unit 

• A motor unit = one alpha motor neuron + all the muscle fibers it innervates. 

• Motor pools = all motor neurons innervating one muscle. 

Muscle Spindle & Myotatic Reflex 

• Muscle spindle detects stretch; contains intrafusal fibers. 

• Stretch activates Ia afferents, causing reflexive muscle contraction (e.g., knee-jerk). 

• Circuit: Ia sensory → alpha motor neuron → same muscle; inhibitory interneuron → antagonistic muscle. 

Withdrawal & Crossed Extension Reflex 

• Withdrawal: quickly pulls a limb from a painful stimulus. 

• Crossed extension: extends the opposite limb to support body weight during withdrawal. 

γ Motor Neurons 

• Innervate intrafusal fibers in muscle spindles. 

• γ co-activation ensures spindle sensitivity during movement. 

• γ loop: stretch → Ia afferent → α motor neuron → contraction → intrafusal adjustment via γ activation. 

Spinal Reflexes & Central Pattern Generators (CPGs) 

• CPGs = spinal networks generating rhythmic patterns like walking, independent of brain input. 

• In quadrupeds, CPGs coordinate alternating gait across limbs. 

Planning Movement 

• Premotor cortex: involved in preparing movement. 

• Prefrontal cortex: involved in intention and decision-making. 

• Monkey experiment: light panels showed premotor activation before movement. 

• fMRI study: imagined movement activated motor planning areas. 

• Mirror neurons: fire when observing or performing actions. 

Executing Movement 

• Primary motor cortex (M1): located in precentral gyrus; initiates voluntary movement. 

• Uses population coding: groups of neurons encode direction/intensity of movement. 

• Corticospinal tract: major descending motor pathway controlling distal muscles. 

Basal Ganglia Components

Includes: Striatum (caudate + putamen), Globus pallidus, Substantia nigra, Subthalamic nucleus. 

Basal Ganglia Pathways

• Direct pathway: facilitates movement via disinhibition of thalamus. 

• Indirect pathway: inhibits movement by enhancing thalamic inhibition. 

• Purpose: focused selection of motor programs, filtering inappropriate ones. 

Parkinson’s Symptoms

Bradykinesia, tremor, rigidity, postural instability.

Parkinson’s Connection to Basal Ganglia 

• Caused by loss of dopamine neurons in substantia nigra → imbalance of direct/indirect pathways. 

• Results in excessive output filtering, suppressing movement. 

Parkinson’s Treatment 

L-DOPA: precursor to dopamine that can cross blood-brain barrier and boost dopamine synthesis.