Neuro Quiz 4

Prefrontal + parietal systems

goal, context, action selection

Premotor cortex + SMA

planning and sequencing

M1 (primary motor cortex)

generates major descending motor commands

Basal ganglia

selection, scaling, suppression

Cerebellum

timing, coordination, error correction

Descending pathways + LMNs

carry commands to muscle

Location: Primary Motor Cortex (M1)

Precentral gyrus.

Role: Primary Motor Cortex (M1)

Major cortical source of descending motor commands, important for voluntary movement initiation and execution.

Organized somatotopically: Cortical representation reflects motor-control demands, not physical body size.

Controls voluntary movement primarily on the contralateral side.

Role: Premotor Cortex (PMA)

Contributes to motor planning and helps select/prepare movements.

Integrates sensory/contextual information (generally external) into a motor plan.

Important for movements guided by external sensory cues

Location: Premotor Cortex (PMA)

Anterior to M1.

Location: Supplementary Motor Area (SMA)

Medial/superior frontal surface

Role: Supplementary Motor Area (SMA)

Contributes to planning and sequencing.

Helps organize internally generated movements.

Important for internally generated and learned movement sequences, and for coordinating multi-step and bilateral movements.

Upper motor neuron (UMN)

Neuron with cell body in cortical motor regions; descends via corticospinal/corticobulbar pathways to influence lower motor neurons.

Lower motor neuron (LMN)

Neuron with cell body in the spinal ventral horn or a cranial motor nucleus; axon exits the CNS and directly innervates skeletal muscle.

Primary motor cortex (M1)

Precentral gyrus; major cortical source of descending motor commands; especially important for voluntary movement execution; organized somatotopically

Premotor cortex (PMA)

Cortex anterior to M1; contributes to motor planning, especially for movements guided by external sensory cues.

Supplementary motor area (SMA)

Medial/superior frontal motor region; organizes internally generated and learned movement sequences.

Corticospinal tract

Descending pathway from motor cortex to the spinal cord; controls trunk/limb musculature; major pathway for skilled voluntary movement.

Corticobulbar tract

Descending pathway from motor cortex to brainstem cranial motor nuclei; controls head, face, jaw, tongue, pharyngeal, and laryngeal musculature.

Pyramidal decussation

The crossing point of most corticospinal fibers, located in the lower medulla.

Lateral corticospinal tract

~90% of corticospinal fibers; crosses at the pyramidal decussation; especially important for skilled, precise distal limb movement

Anterior corticospinal tract

Smaller set of corticospinal fibers; descends initially ipsilaterally; more involved in axial/trunk and postural control.

Neuromuscular junction (NMJ)

Synapse where a lower motor neuron releases acetylcholine onto a muscle fiber, triggering depolarization and contraction

Motor unit

One lower motor neuron plus all the muscle fibers it innervates

Spasticity

Increased resistance to passive movement that increases with faster stretch; a UMN sign.

Flaccidity

Decreased resistance to passive movement due to reduced muscle tone; an LMN sign

Fasciculation

Visible muscle twitching, seen with LMN lesions.

Myasthenia gravis

Autoimmune disorder of the neuromuscular junction producing fatigable weakness with intact sensation

Basal ganglia

Subcortical structures that select, facilitate, suppress, and scale movement via loops through the thalamus back to cortex

Substantia nigra pars compacta

Source of dopamine that modulates striatal activity to help facilitate and scale intended movement

Bradykinesia / Akinesia

Slowed, reduced-amplitude voluntary movement / absence or marked reduction of voluntary movement — hypokinetic signs of basal ganglia dysfunction.

Rigidity

Increased resistance to passive movement, not velocity-dependent; a basal ganglia (Parkinsonian) sign.

Chorea / Dystonia

Hyperkinetic patterns: brief irregular unpredictable movements / sustained or intermittent involuntary contractions producing abnormal postures

Cerebellum

Structure that compares intended vs. actual movement to support timing, coordination, error correction, and motor learning.

Dysmetria

Inability to accurately scale the range or distance of a movement (overshoot/undershoot); a cerebellar sign.

Dysdiadochokinesia

Impaired ability to perform rapid alternating movements smoothly and rhythmically; a cerebellar sign.

Ataxia

Impaired coordination of voluntary movement, producing unsteady, poorly coordinated movement or gait

Intention tremor

Tremor that increases during voluntary movement as the target is approached; a cerebellar sign (distinct from Parkinsonian resting tremor)

Ataxic dysarthria

Motor speech disorder from impaired coordination of speech movements, producing irregular timing, force, and precision — a cerebellar sign.

Route: Corticospinal Pathway

Motor cortex → spinal cord

Corticospinal Pathway Controls Primarily

Trunk/limb muscles

Clinical Relevance: Corticospinal Pathway

Major pathway for skilled voluntary movement

Route: Corticobulbar Pathway

Motor cortex → brainstem cranial motor nuclei

Corticobulbar Pathway Controls Primarily

Head, face, jaw, tongue, pharyngeal, and laryngeal muscles

Clinical Relevance: Corticobulbar Pathway

Directly relevant to speech and swallowing motor functions

Corticospinal tract

UMN → LMN → muscle

Lower Motor Neuron (LMN)

Cell body in the ventral horn of the spinal cord.

Lower Motor Neuron (LMN)

Axon exits through a spinal nerve.

Lower Motor Neuron (LMN)

Directly innervates skeletal muscle.

Upper Motor Neuron (UMN)

Cell body in cortical motor regions.

Upper Motor Neuron (UMN)

Axon descends through: corona radiata, internal capsule, brainstem, and medullary pyramids.

Upper Motor Neuron (UMN)

Most fibers cross at the pyramidal decussation (medulla).

Upper Motor Neuron (UMN)

Descends in the spinal cord

Lateral Corticospinal Tract

~90% of corticospinal fibers

Lateral Corticospinal Tract

Crosses at the pyramidal decussation in the medulla then descends in the contralateral lateral spinal cord

Lateral Corticospinal Tract

Especially important for skilled, precise distal limb movement

Anterior Corticospinal Tract

Smaller proportion of fibers

Anterior Corticospinal Tract

Descends initially ipsilaterally; many fibers cross near their spinal level of termination

Anterior Corticospinal Tract

More involved in axial/trunk and postural control

Lesion above the pyramidal decussation (motor cortex, internal capsule, brainstem above the crossing)

weakness primarily on the contralateral side of the body

Lesion below the pyramidal decussation (spinal cord)

weakness primarily on the ipsilateral side below the lesion

Neuromuscular junction

the site of the synapse between the LMN and the muscle: motor neuron releases the neurotransmitter acetylcholine; signal crosses the synapse to the muscle fiber; triggers muscle fiber depolarization and contraction

Motor unit

one LMN plus all the muscle fibers it innervates.

Upper Motor Neuron Lesion

increased tone/spasticity (increased resistance to passive movement that increases with faster stretch)

Upper Motor Neuron Lesion

weakness

Upper Motor Neuron Lesion

Hyperreflexia (exaggerated reflexes)

Upper Motor Neuron Lesion

Mild disuse atrophy

Lower Motor Neuron Lesion

weakness

Lower Motor Neuron Lesion

decreased tone/flaccidity (decreased resistance to passive movement due to reduced muscle tone)

Lower Motor Neuron Lesion

Hyporeflexia/areflexia (decreased/absent reflexes)

Lower Motor Neuron Lesion

More prominent neurogenic atrophy

Lower Motor Neuron Lesion

Fasciculations may occur (twitching)

Myasthenia Gravis symptoms

ptosis/diplopia

dysarthria (motor speech disorder from weakness, incoordination, or impaired

control of speech muscles, in MG, weakness and fatigability)

dysphonia (impaired voice production affecting loudness, pitch, or vocal quality)

dysphagia (difficulty swallowing safely and efficiently)

weakness that worsens with sustained or repeated activity

Myasthenia Gravis

Autoimmune disorder affecting the neuromuscular junction where antibodies disrupt normal acetylcholine receptor function.

Myasthenia Gravis

Neural signal reaches the NMJ, but muscle activation becomes less reliable. It is characterized by fatigable weakness. Sensation remains intact

The basal ganglia help determine which movements are expressed and how strongly they are expressed, by:

• Selecting desired motor programs

• Facilitating intended movement

• Suppressing competing or unwanted movement

• Scaling movement amplitude and vigor

Basal ganglia motor loop:

Cortex → Basal Ganglia → Thalamus → Cortex

Hypokinetic Patterns

Difficulty with movement initiation, scaling, or sustaining movement reduced rate and amplitude of movement

Hypokinetic Patterns Examples

Akinesia, Bradykinesia, Rigidity

Hyperkinetic Patterns

Excessive or involuntary movement characterized by involuntary movements

Hyperkinetic Patterns Examples

Tremor, Chorea, Dystonia

Akinesia

absence or marked reduction of voluntary movement

Bradykinesia

slowness and reduced amplitude of voluntary movement

Rigidity

increased resistance to passive movement

Tremor

rhythmic, oscillating involuntary movement

Chorea

brief, irregular, unpredictable involuntary movements

Dystonia

sustained or intermittent involuntary contractions producing abnormal movements or postures