MOTOR & REFLEX SYSTEM

Anatomy and Functional Overview

The motor system is responsible for voluntary movement, posture, and muscle tone. Lesions of the motor system produce weakness, abnormal tone, altered reflexes, and involuntary movements.

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Components of the Motor Pathway

1. Upper motor neuron (UMN)

   • Originates in motor cortex

   • Descends via corticospinal tract

2. Lower motor neuron (LMN)

   • Anterior horn cell

   • Peripheral nerve

   • Neuromuscular junction

   • Muscle

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Damage at different levels produces distinct clinical patterns, which is the foundation of neurological localization.

Motor System Examination

Motor examination should be systematic and includes assessment of:

1. Muscle bulk

2. Muscle tone

3. Muscle power

4. Reflexes

5. Involuntary movements

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1. Muscle Bulk

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Normal Bulk

Muscles appear symmetrical with well-defined contours.

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Wasting (Atrophy)

Occurs due to:

• LMN lesions

• Disuse

• Muscle disease

• Chronic neuropathy

Fasciculations indicate anterior horn cell disease or LMN pathology.

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Hypertrophy

• True hypertrophy → exercise, myotonia

• Pseudohypertrophy → Duchenne muscular dystrophy

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2. Muscle Tone

Tone is the resistance felt during passive movement of a relaxed limb.

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Spasticity

• Seen in UMN lesions

• Velocity-dependent

• “Clasp-knife” phenomenon

• Predominantly affects:

  • Flexors in upper limb

  • Extensors in lower limb

Common causes:

• Stroke

• Spinal cord lesions

• Multiple sclerosis

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Rigidity

• Seen in extrapyramidal disorders

• Velocity-independent

• Types:

  • Lead-pipe rigidity

  • Cogwheel rigidity (Parkinson disease)

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Hypotonia

Seen in:

• Acute UMN lesions (spinal shock)

• LMN lesions

• Cerebellar disease

Muscle Power

Muscle strength is graded using the Medical Research Council (MRC) scale:

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Grade           | Description                      

0                      | No movement                     
1                      | Flicker                          
2                      | Movement with gravity eliminated 
3                      | Movement against gravity       
4                      | Movement against resistance     
5                      | Normal power                    

Patterns of Weakness

Pattern                                   | Likely lesion         
Hemiparesis                        | Cerebral hemisphere    
Paraparesis                          | Spinal cord               
Quadriparesis                    | Cervical cord / brainstem 
Distal weakness                | Peripheral neuropathy     
Proximal weakness          | Myopathy                  
 

Upper Motor Neuron Syndrome

UMN lesions occur above the anterior horn cell.

Clinical Features

• Weakness

• Spasticity

• Hyperreflexia

• Clonus

• Extensor plantar response

• Minimal wasting (late)

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Pathophysiology

Loss of inhibitory cortical control over spinal reflex arcs leads to:

• Increased tone

• Exaggerated reflexes

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Common Causes

• Stroke

• Spinal cord compression

• Tumors

• Multiple sclerosis

​Lower Motor Neuron Syndrome

LMN lesions involve:

• Anterior horn cell

• Nerve root

• Peripheral nerve

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Clinical Features

• Flaccid weakness

• Hypotonia

• Hyporeflexia or areflexia

• Fasciculations

• Early muscle wasting

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Common Causes

• Motor neuron disease

• Poliomyelitis

• Peripheral neuropathy

• Radiculopathy

Reflexes

A reflex is an involuntary, stereotyped response to a stimulus.

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Classification

1. Deep tendon reflexes

2. Superficial reflexes

3. Pathological reflexes

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Deep Tendon Reflexes (DTRs)

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Reflex              | Root 
Biceps             | C5–C6
Triceps            | C7–C8 
Knee                | L3–L4 
Ankle               | S1    

Grading of Reflexes

• 0 – absent

• 1+ – diminished

• 2+ – normal

• 3+ – exaggerated

• 4+ – clonus

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Clonus

• Sustained clonus indicates UMN lesion

• Ankle clonus is most commonly tested

Superficial Reflexes

Abdominal Reflex

• T8–T12

• Lost early in UMN lesions

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Cremasteric Reflex

• L1–L2

• Absent in UMN lesions above L1

Plantar Reflex

Normal response: flexion of toes

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Abnormal response (Babinski sign):

• Extension of great toe

• Fanning of other toes

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Seen in:

• UMN lesions

• Infants (physiological)

Involuntary Movements

Presence of involuntary movements during motor examination suggests basal ganglia or cerebellar involvement.

Examples:

• Tremor

• Chorea

• Myoclonus

• Dystonia