Spinal Cord:
- Central gray matter – neuronal cell bodies and synapses
- Peripheral white matter – ascending/descending fiber pathways (motor and sensory)
Muscle Unit:
- A single motorneuron and all of its muscle fibers
- Alpha motorneuron – largest cells of spinal cord and “final common pathway” from CNS to periphery. Each one usually innervates many muscle fibers – each muscle fiber innervated by only ONE alpha motorneuron
Spinal Reflex:
- Stereotyped motor responses to stimuli
- Afferents from periphery -> synapse(s) in spinal cord -> alpha motorneurons -> muscle fibers
General Info:
- Motor (corticospinal) pathway extends from motor area of cortex through brain stem and crosses between brainstem and spinal cord
- Fibers synapse in anterior horn (just prior to leaving cord)
- Decorticate Posturing – Cerebral injury (e.g., CVA due to carotid occlusion) – flexion of wrist and elbow and extension of ankle and knee
- Decerebrate Posturing – due to midbrain injury – posturing is similar, but elbow is extended
- Upper Motor Neurons (i.e. first order neurons) – are neurons above the synapse in the anterior horn
- Lower Motor Neurons (i.e., second order neurons) – peripheral motor neurons
- Polio – attacks anterior horn cells – LMN disease
- Gullian-Barre Syndrome – sensory and LMN loss due to peripheral nerve involvement
Abnormalities of Movement
- Fasciculation – visible, continuous, and rapid twitching of a muscle or part of muscle without movement of a limb
- Tremor – rhythmic involuntary back-and-forth movement that may be as rapid as trembling. It usually involves movement of a limb or body part. It may be represent only when the muscle is at rest, during a voluntary movement, or both.
- In Parkinson’s tremor is present at rest
- In Cerebellar disorder, tremor is intentional (i.e., occurs during intentional movements, such as drawing)
- Tics – repetitive twitching of a muscle group such as, facial muscle twitching resulting in grimaces. They may be emotional or neurological origin
- Chorea – obvious, rapid, sudden, involuntary, jerky movements that may involve the limbs, trunk, or face. They occur at irregular, unpredictable intervals and are not rhythmic or repetitive
- Athetosis – differs from chorea primarily in that it is slow, writhing, and twisting, rather than rapid and jerky (may occur with cerebral palsey)
- Myoclonus – sudden, rapid, unpredictable and involuntary jerking movements (e.g., a hiccup is a myoclonic movement of the diaphragm)
Upper Motor Neuron (UMN) Defects
- Spastic paralysis
- No significant muscle atrophy
- No fasciculations/fibrillations
- Hyperreflexia
- Babinski may be present
Lower Motor Neuron (LMN) Defects
- flaccid paralysis
- significant atrophy
- fasciculations/fibrillations
- hyporeflexia
- No Babinski
Ventromedial Tracts
Axial musculature involved in maintain posture (righting reflex and whole body orientation)
Dorsolateral Tracts
Distal musculature, initiation and control of voluntary movements.
Basal Ganglia
- Subcortical telencephalic nuclei involved in initiation and control of movement
- Extrapyramidal – mechanisms involving injury to the basal ganglia motor pathway outside of the corticospinal “pyramidal” system. Consists of:
- Caudate nucleus
- Putamen
- Globus pallidus (pallidum)
- Associate areas include substantia nigra, subthalamic, nucleus and projections from Basal ganglia to thalamic nuclei
- Divisions:
- Neostriatum or Striatum -> caudate -> putamen
- Corpus Striatum -> striatum -> globus pallidus
- Lentiform nucleus -> putamen -> globus pallidus
- Symptoms of Impairment
- Dyskinesia – involuntary movements
- Bradykinesias – slowness in initiating or changing without significant weakness
- Abnormal fixation – equilibrium and righting
- Parkinson’s Disease – rigidity, bradykinesia, resting tremor, loss of postural reflexes
- Ballism – sudden, forceful, flinging dyskinesia involving whole limb (contralateral arm to lesion in subthalamic nucleus)
- Athetosis – slow, wormlike, writhing movements, usually in extremities
- Lesion in striatum (usually putamen)
- Torsion Dystonia – disorder of trunk and limb involving writhing movements which produce severe sustained contortion of neck, shoulder girdle, and pelvic girdle
- Lesion in striatum (usually putamen)
- Huntington’s Chorea – involuntary movements of limbs which are brisk and graceful and have appearance of fractions of purposeful movement
- caudate
- Choreoathetosis – combination of chorea and athetosis
Cerebellum
- Coordination of movement and posture, particular in adaption of movement to changing external and internal conditions
- Lesions cause:
- Ataxia
- Hypotonia
- Ipsilateral malfunctioning
- Dysdiadochokinesia – failure of rapid alternating movements
- Past pointing – inability to touch finger to nose or heel to shin
Gait Abnormalities
- Spastic hemiparesis – the arm on the affected side is held flexed and immobile against the body, instead of swinging freely by the side. The affected leg is moved forward stiffly and in a semi-circle, sometimes with the toe dragging on the floor as the leg is moved forward
- Scissors gait – steps are abnormally short and appear effortful. The knees remain in contact as if the patient were trying to hold an imaginary orange between the thighs as he walks – seen in MS
- Ataxia – patient has difficulty in keeping his balance. Walks with feed wide apart
- Parkinsonian – stooped posture, flexion at the hips, elbows, and knees. General mobility is decreased; steps are short and shuffling. Has difficulty both initiating and stopping
Muscle Tone
- Flaccidity – when muscle tone is less than normal and the limb feels limp to the examiner
- Posture retention – when a limb tends to remain in the position into which you move it or in which it was before you moved it
- Rigidity – limb is abnormally resistant to movement in all directions
- Cogwheel motion – limb yields in your attempt to move it, but with jerking rather than smooth movements (seen in Parkinson’s)
- Spasticity – impairment of UMN – increase in muscle tension characterized by certain postural changes involving flexion of the finger, hand, arm, and legs