N245: Alterations in Motor Function

Motor system function

Plans and executes smooth/purposeful/coordinated body movements, maintains posture

Motor cortex

Primary motor cortex, premotor cortex, and supplementary motor cortex --> controls precise/skillful/and intentional movements

Cerebellum

"Little brain"; coordinates timing/accuracy of movement and balance

Basal ganglia

Structures in the brain that help to control graceful movement

Brainstem

Relays information from the brain down to the spinal cord; responsible for automatic survival functions

Spinal cord

Nerves that run up and down the length of the back --> transmit messages between the body and brain

Upper motor neurons

Extend from the motor cortex --> brainstem or the ventral horns of the spinal cord (cell bodies and axons located in the CNS)

Lower motor neurons

Extend from the brainstem or the ventral horns of the spinal cord --> skeletal muscles (cell bodies in CNS, axons in PNS)

Descending pathways

The systems of two neurons that bring motor information from the brain down to the correct spinal level

Pyramidal tracts

Location: Motor cortex --> medullary pyramids

Function : Voluntary control of the head, face, and body muscles

Corticobulbar tracts

Location: Carry motor fibers from the cortex to the brainstem

Function: Control head and face movements

Corticospinal tracts

Location: Carry motor fibers from the cortex to the spinal cord

Function: Control limb and trunk movements

Lateral corticospinal tract

Fibers cross --> control limb movements (related to ALS)

Anterior corticospinal tract

Fibers do not cross --> control trunk movements

Extrapyramidal tracts

Location: Brainstem

Function: Involuntary and automatic control of body muscles (regulate balance and posture)

Carry motor fibers from different parts of the brainstem to the spinal cord

Where do UMNs and LMNs synapse?

At the termination of the pyramidal and extrapyramidal tracts

Motor unit

A single motor neuron plus the skeletal muscle fibers it innervates

Spinal reflexes

What: Simple involuntary responses that rapidly generate muscle contractions

Function: Buys time for the brain to plan more intentional movements

Muscle spindle reflex

Proprioceptors in skeletal muscles monitor length changes --> muscle spindle is stretched --> reflex contraction of muscle (monosynaptic)

Golgi tendon organs

Proprioceptors in tendons monitor length changes --> Golgi tendon is is stretched when muscle contracts --> reflex relaxation of tendon --> protects against excessive tension

Sequence of events in spinal reflex

Sensory (afferent) neurons relay information about muscle length and tone to the dorsal horns of the spinal cord--> Info is relayed from the dorsal horns to the ventral horns --> LMNs initiate reflex

Paresis

Weakness in muscles

Paralysis

Loss of movement in muscles

Hypotonia/Atonia

Reduced/absent muscle tone

Hyporeflexia/Areflexia

Reduced/absent spinal reflexes

Myopathies

Diseases of muscle

Endplate disorders

Affect neuromuscular junctions

Peripheral neuropathies

Affect axons of LMNs

Ventral horn cell disorders

Affect cell bodies of LMNs

Muscle Atrophy

Reduction in the diameter of muscle fibers due to a loss of actin and myosin filaments

Disuse atrophy

Muscle wasting in people experiencing temporary disabling circumstances (ex: prolonged bed rest)

Denervation atrophy

Muscle wasting in disorders that deprive muscles of their normal innervation (ex: Poliovirus damages the LMNs)

Muscular Dystrophy

Group of genetic disorders --> progressive degeneration and necrosis of skeletal muscle fibers

Pseudohypertrophy

Affected muscle fibers are replaced with fat and connective tissue --> increased size of corresponding muscles

Two forms of muscular dystrophy

Duchenne Muscle dystrophy (most severe)

Becker Muscular Dystrophy (less severe)

Etiology of Muscular Dystrophy

Mutations in the dystrophin gene on X chromosome; normally gene codes for dystrophin protein (maintain the structural integrity of skeletal/cardiac muscle cells)

S+S of Muscular Dystrophy

Asymptomatic at birth --> muscle weakness and falls --> hypertrophy of calf muscles --> wheelchair --> scoliosis --> death?

Gower's maneuver

MD --> lack of hip/thigh muscle strength --> use arms to push on legs to stand up

Scoliosis

An abnormal lateral curvature of the spine

Treatment of Muscular Dystrophy

1. Splints: Provide support, prevent deformities

2. Steroids: Reduce fibrosis, improve muscle regeneration

Myasthenia Gravis

Disorder of the neuromuscular junctions; affects the transmission of impulses between the LMNs and innervated skeletal muscle fibers

Pathophysiology of Myasthenia Gravis

Antibodies directed against acetylcholine receptors on the muscle surface --> receptor destruction --> weakness

Acetylcholine function

Binds to ACh receptors --> allows muscles to contract

Acetylcholineesterase (AChE)

An enzyme that breaks down acetylcholine

S+S of myasthenia gravis

Ptosis, diplopia, difficulty chewing and swallowing, speech impairment, difficulty walking

Myasthenia crisis

Complication of myasthenia gravis --> worsening of S+S due to stress, difficulty breathing

Tensilon test

Used to diagnose MG --> injection of tensilon --> inhibits acetylcholinesterase --> more acetylcholine in NMJ --> improves muscle strength in MG patients

Peripheral neuropathy

Disorder of the peripheral nerves that carry information to and from spinal cord

Mononeuropathies

Local disorders that affect only a single peripheral nerve --> caused by compression/trauma/tumor

Pure sensory neuropathy

Sensory disturbances (pain); function remains intact

Pure motor neuropathy

Function (muscle movement) impaired; no pain

Carpal Tunnel Syndrome

Overuse --> swelling nd inflammation of the flexor tendon sheaths --> compression of the median nerve within the carpal tunnel --> pain/tingling/numbness of hand and fingers

Polyneuropathies

Immune system disturbances/toxins/diabetes --> systemic degeneration of axon and myelin of many peripheral nerves

Guillain-Barré Syndrome (demyelinating polyneuropathy)

Flu-like illness --> inflammation of the myelin sheath of peripheral nerves --> rapidly worsening muscle weakness --> temporary paralysis

Spinal Muscular Atrophy

Low levels of survival motor neuron (SMN) protein --> progressive degeneration of LMN cell bodies in spinal ventral horns --> hypertonia (floppy baby syndrome)

Cerebellar Disorders

Cerebral ataxia

Damage to cerebellum or excess alcohol --> loss of coordination --> wide/unsteady gait

Cerebellar tremor

Slow tremor of the extremities that occurs while performing a purposeful movement

Dysmetria

The inability to estimate distances and control the range of a movement

Basal Ganglia Disorders

Drug overdose, head injury, or infection --> will impair ability to move gracefully during slow and sustained movements

Dystonia

Repetitive muscle contractions --> abnormal body movements

Chorea

Jerky, unpredictable movements in shoulders and hips

Hemiballismus

Rapid, flinging movements of the limbs on one side of the body

Parkinson's Disease

Degeneration of neurons that synthesize dopamine in substantia nigra (basal ganglia) --> tremors, rigidity, bradykinesia, stooped posture

Lewy bodies

Abnormal protein collections in cytoplasm of affected neurons

Spasticity

Stiffness of muscles

Clonus

involuntary muscle contractions and relaxations

Hypertonia

Increased muscle tone

Hypereflexia

Exaggerated spinal reflexes

Babinski sign

Used to test for brain damage or upper motor neuron damage --> positive if the toes flare up upon stroking the plantar aspect of the foot

Amyotrophic Lateral Sclerosis (ALS)

Degeneration of LMN cell bodies in ventral spinal horns --> atrophy of muscle fibers --> degeneration/sclerosis of axons in lateral corticospinal tract --> weakness/stiffness of muscles, involuntary contractions, dysphagia, slurred speech

Multiple Sclerosis (MS)

Low Vit D, stress? --> destruction of myelin in UMNs in brain and spinal cord --> slow nerve impulses --> weakness, stiffness of muscles, abnormal gait, slurred speech, diplopia, sharp/tingling sensation down back/legs when flexing neck (Lhermitte's sign)

Spinal Cord Injuries

Fracture/dislocation/compression of vertebrae, hyperflexion, hyperextension --> damage to the UMNs in the spinal cord --> loss of sensory or motor function

Tetraplegia (quadriplegia)

Damage to cervical UMNs --> Paralysis of all four limbs

Paraplegia

Damage to thoracic/lumbar/sacral UMNs --> Paralysis from the waist down

Spinal shock

Spinal injury --> complete but temporary loss of motor, sensory, reflex, and autonomic function