Disorders of Motor Function

Components of the Neuromuscular System
Neuromuscular unit containing motoneurons
Myoneural junction
Muscle fibers
Spinal cord
Descending pathways from the brain stem circuits

Classifications of Muscles
Extensors
Muscles that increase the angle of a joint
Flexors
Muscles that decrease the angle of a joint
Mechanisms Controlling Coordinated Movement
Agonists
Promote movement
Antagonists
Oppose movement
Synergists
Assist the agonist muscles by stabilizing a joint or contributing additional force to the movement
Requirements of Motor Systems
Upper motoneurons project from the motor cortex to the brain stem or spinal cord
Directly or indirectly innervate the lower motoneurons or contracting muscles
Sensory feedback from the involved muscles
Continuously relayed to the cerebellum basal ganglia and sensory cortex
Functioning neuromuscular junction that links nervous system activity with muscle contraction
Motor Systems
Pyramidal motor system
Originates in the motor cortex
Provides control of delicate muscle movement
Extrapyramidal system
Originates in the basal ganglia
Provides background for the more crude, supportive movement patterns
Categories of Skeletal Muscle Groups
Slow-twitch fibers
Have many mitochondria
Depend on blood-borne oxygen for energy
Are slow to fatigue
Fast-twitch fibers
Depend on muscle glycogen stores that can be rapidly depleted
Are further divided into fast-twitch fatigable and fast-twitch fatigue-resistant fibers
Disorders of Motor Function
Upper motoneuron lesions
Can involve the motor cortex, the internal capsule, or other brain structures through which the corticospinal or corticobulbar tracts descend, or the spinal cord
Lower motoneuron lesions
Disrupt communication between the muscle and all neural input from spinal cord reflexes, including the stretch reflex, which maintains muscle tone

Types of Paralysis
Monoplegia
Hemiplegia
Tetraplegia or quadriplegia
Paraplegia
Disorders of Skeletal Muscle Groups
Muscular Atrophy
If a normally innervated muscle is not used for long periods, the muscle cells shrink in diameter, lose much of their contractile protein, and weaken
Muscular dystrophy
Genetic disorders that produce progressive deterioration of skeletal muscles because of mixed muscle cell hypertrophy, atrophy, and necrosis
Neuromuscular Junction
Serves as a synapse between a motor neuron and a skeletal muscle fiber
Consists of the axon terminals of a motor neuron and a specialized region of the muscle membrane called the endplate
The transmission of impulses is mediated by the release of the neurotransmitter acetylcholine from the axon terminals
Acetylcholine binds to receptors in the endplate region of the muscle fiber surface to cause muscle contraction
Alterations of Neuromuscular Function
Drugs and agents can alter neuromuscular function by changing the release, inactivation, or receptor binding of acetylcholine
Curare acts on the postjunctional membrane of the motor endplate to prevent the depolarizing effect of the neurotransmitter
Used during many types of surgical procedures
Myasthenia Gravis
Definition
Disorder of transmission at the neuromuscular junction that affects communication between the motoneuron and the innervated muscle cell.
Cause
Autoimmune disease caused by antibody-mediated loss of acetylcholine receptors in the neuromuscular junction
Components of the Peripheral Nervous System
Motor and sensory branches of the cranial and spinal nerves
The peripheral parts of the autonomic nervous system
Peripheral ganglia
Peripheral Neuropathy
Definition
Any primary disorder of the peripheral nerves
Results
Muscle weakness, with or without atrophy and sensory changes
Involvement
Can involve a single nerve (mononeuropathy) or multiple nerves (polyneuropathy)


Mononeuropathies
Caused by localized conditions such as trauma, compression, or infections that affect a single spinal nerve, plexus, or peripheral nerve trunk
Fractured bones may lacerate or compress nerves
Excessively tight tourniquets may injure nerves directly or produce ischemic injury
Infections such as herpes zoster may affect a single segmental afferent nerve distribution
Polyneuropathy
Involve demyelination or axonal degeneration of multiple peripheral nerves that leads to symmetric sensory, motor, or mixed sensorimotor deficits
Typically, the longest axons are involved first, with symptoms beginning in the distal part of the extremities
Causes of Polyneuropathies
Immune mechanisms (Guillain-Barré syndrome)
Toxic agents (arsenic polyneuropathy, lead polyneuropathy, alcoholic polyneuropathy)
Metabolic diseases (diabetes mellitus, uremia)




Basal Ganglia
A group of deep, interrelated subcortical nuclei that play an essential role in control of movement
They receive indirect input from the cerebellum and from all sensory systems, including vision, and direct input from the motor cortex
They function in the organization of inherited and highly learned and rather automatic movement programs
They also are involved in cognitive and perception functions
Characteristics of Disorders of the Basal Ganglia
Involuntary movements
Alterations in muscle tone
Disturbances in body posture
Types of Involuntary Movement Disorders
Tremor
Tics
Chorea
Athetosis
Ballismus
Dystonia
Dyskinesias
Structural Components of the Basal Ganglia
Caudate nucleus
Putamen
Globus pallidus in the forebrain
Four Functional Pathways Involving Basal Ganglia
A dopamine pathway from the substantia nigra to the striatum
A γ-aminobutyric acid (GABA) pathway from the striatum to the globus pallidus and substantia nigra
Acetylcholine-secreting neurons, which are important in networks within the neostriatum
Multiple general pathways from the brain stem that secrete norepinephrine, serotonin, enkephalin, and several other neurotransmitters in the basal ganglia and the cerebral cortex
Parkinson Disease
Definition
A degenerative disorder of basal ganglia function that results in variable combinations of tremor, rigidity, and bradykinesia
Characteristics
Progressive destruction of the nigrostriatal pathway, with subsequent reduction in striatal concentrations of dopamine
Clinical Syndrome
Parkinsonism
Amyotrophic Lateral Sclerosis (ALS)
Definition
A devastating neurologic disorder that selectively affects motor function
The disease typically follows a progressive course, with a mean survival period of 2 to 5 years from the onset of symptoms
Locations of Motoneurons Affected by ALS
The anterior horn cells of the spinal cord
The motor nuclei of the brain stem, particularly the hypoglossal nuclei
The UMNs of the cerebral cortex
Multiple Sclerosis (MS)
A demyelinating disease of the CNS
Most common non-traumatic cause of neurologic disability among young and middle-aged adults
Characterized by exacerbations and remissions over many years in several different sites in the CNS
Initially, there is normal or near-normal neurologic function between exacerbations
As the disease progresses, there is less improvement between exacerbations and increasing neurologic dysfunction

Categories of Pharmacologic Agents Used to Treat MS
Those used to treat acute symptoms of the disease
Those used to modify the course of the disease
Those used to interrupt progressive disease
Those used to treat the symptoms of the disorder
Spinal Cord Injury (SCI)
Definition
Damage to the neural elements of the spinal cord.
Causes
Motor vehicle crashes, falls, violence and sporting activities
Involvement
Most SCIs involve damage to the vertebral column and or supporting ligaments as well as the spinal cord
Commonly involve both sensory and motor function

Types of Injuries to the Vertebral Column
Fractures
Dislocations
Subluxations
Types of Incomplete Spinal Cord Injuries
Central cord syndrome
Anterior cord syndrome
Brown-Séquard Syndrome
Conus medullaris syndrome
Areas Affected by SCI
Spinal reflexes
Ventilation and communication
Autonomic nervous system
Temperature regulation
Edema and deep vein thrombosis
Sensorimotor function

Areas Affected by SCI (cont.)
Skin integrity
Pain reception
Bladder and bowel function
Sexual function