Chapter 50 Disorders of Motor Function

Chapter 50Disorders of Motor Function

Spinal Cord Somatosensory • Dorsal column-medial lemniscus tract • Touch/proprioception/vibration sensory pathway • Anterolateral system • Pain/temperature sensory pathway

Spinal Cord • Motor • Corticospinal tract • Motor pathway for upper motor neuronal signals coming from cerebral cortex and brainstem motor nuclei

Spinal Cord Trauma • Often leads to paraplegia or quadriplegia depending on the location and extent of the injury • Hypertension Injury • When the forehead is struck and driven posteriorly • Diving impact in shallow water • May tear the anterior spinal ligament and spinal cord may contact the vertebral body

Trauma to the Spinal Cord • Hyperflexion Injury • When the head of shoulders are struck from behind by an object of considerable weight or when this region of a fall

Spinal Cord Trauma • Concussion • Mild injury, transient and reversible • Contusion • Severe trauma with hemorrhagic necrosis, edema and softening of the cord – Myelomalacia, or blood in the cord – Hematomyelia • Laceration or Tansection

Brain Stem Midbrain • Associated with vision, hearing, motor control, sleep/wake, arousal (alertness), and temperature regulation Pons • Nuclei that deal primarily with sleep, respiration, swallowing, bladder control, hearing, equilibrium, taste, eye movement, facial expressions, facial sensation, and posture

Brain Stem Medulla • Contains the cardiac, respiratory, vomiting and vasomotor centers dealing with autonomic, involuntary functions • Breathing, heart rate and blood pressure

The cerebellum receives continuous information about the sequence of muscle contractions from the brain • Receives sensory information from the peripheral parts of the body • Proprioception • sequential changes in the status of each body part

The Cerebellum • Coordination of motor movement • proprioception • Cerebellum-associated movement disorders • Causes • Congenital defect, vascular accident, or growing tumor • Types • Vestibulocerebellarataxia • Not smooth movement • Decomposition of movement • Cerebellartremor • Rhythmic back-and-forth movement of a finger or toe • Cannot maintain a fix on the body part

Thalamus • It relays to the cerebral cortex information received from other regions of the brain and spinal cord. • Sends information down spinal cord to the body • a brain “switching station”

Thalamus • The cerebral cortex is interconnected with the Thalamus • Excitatory circuit • If unmodulated would cause hyperactivity = stiffness and rigidity with a continuous tremor (tremor at rest)

Structural Components of the Basal Ganglia • Caudate nucleus • Putamen • Globuspallidus in the forebrain • SubstantiaNigra (midbrain) • Subthalamic nucleus

Basal Ganglia • A group of deep, interrelated subcortical nuclei that play an essential role in control of movement • Receive indirect input from the cerebellum and from all sensory systems, including vision, and direct input from the motor cortex • Function in the organization of inherited and highly learned and rather automatic movement programs • Also involved in cognitive and perception functions

Structural Components of the Basal Ganglia • Caudate + Putamen = Striatum • Putamen + GlobusPallidus = Lentiform nucleus

Basal Ganglia • Basal Ganglia modulates the Thalamic excitability by inhibitory loop • Basal Ganglia monitors sensory information coming into the brain • sends it to the right place to be stored as a memory

Four Functional Pathways Involving Basal Ganglia • A dopamine pathway from the substantianigra to the striatum • A γ-aminobutyric acid (GABA) pathway from the striatum to the globuspallidus and substantianigra • 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

Characteristics of Disorders of the Basal Ganglia • Involuntary movements • Alterations in muscle tone • Disturbances in body posture

Motor Cortex • Highest level of motor function • Precise, skillful, intentional movements • Speech, flexor muscles of limbs, etc. • Controlled by the primary, premotor and supplementary motor cortices in the frontal lobe • Receives information from the Thalamus, cerebellum and basal ganglia

Motor Cortex Primary motor cortex Responsible for execution of a movement. Adjacent to central sulcus Motor Humunculus Premotor cortex (areas 6 and 8) Generates intricate plan of movement. Throwing a ball or picking up a fork

Homunculus

Motor Cortex Supplementary motor cortex • Involved in the performance of complex, skillful movements • (areas 6 and 8)

Pyramidal motor system • Originates in the motor cortex • Controls all of our voluntary movements • Consists of upper motor neurons in the Primary Motor Cortex and lower motor neurons in the anterior horn of the spinal cord • The Ventral Corticospinal tract • Damage to LMN's causes flaccid paralysis Extrapyramidal motor system • Originates in the basal ganglia • Provides background for the more crude, supportive movement patterns • Includes the substantianigra, caudate, putamen, globuspallidus, thalamus, and subthalamic nucleus. • All of these nuclei are synaptically connected to one another, the brainstem, cerebellum and the pyramidal system.

Disorders of Motor Function Upper motoneuron (UMN’s) • Originate in the motor region of the cerebral cortex or brain stem • Carries motor information down spinal cord to stimulate target muscle • 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

1. Paralysis or weakness of movements of the affected side but gross movements may be produced. • No muscle atrophy is seen initially 2. Babinski sign is present: 3. Loss of performance of fine-skilled voluntary movements especially at the distal end of the limbs 4. Superficial abdominal reflexes and cremasteric reflex are absent. 5. Spasticity or hypertonicity of the muscles. 6. Clasp-knife reaction: initial higher resistance to movement is followed by a lesser resistance 7. Exaggerated deep tendon reflexes and clonus may be

Disorders of Motor Function Lower motoneurons (LMN’s) • Connects the brainstem and spinal cord to muscle cells • Brings nerve impulses from upper motor neuron to the muscles • Lesions disrupt communication between the muscle and all neural input from spinal cord reflexes, including the stretch reflex, which maintains muscle tone

Signs of Lower Motor Neuron Lesions (LMNL) 1. Flaccid paralysis of muscles supplied. 2. Atrophy of muscles supplied. 3. Loss of reflexes of muscles supplied. 4. Muscles fasciculation (contraction of a group of fibers) due to irritation of the motor neurons – seen with naked eye

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

Muscular Dystrophy • Involves the motor neuron • Probably do not involve the nervous system • Slow progressive onset of muscle weakness

Duchenne Muscular Dystrophy • 1:3500 male births • Inherited recessive single-gene defect • On short arm of X chromosome • Gene codes for dystrophin • Connects Z-lines to connective tissue surrounding muscle • Break down of sarcolemma = necrosis of muscle fibers

Duchenne Muscular Dystrophy Symptoms usually appear before age 6 and may appear as early as infancy. They may include: Fatigue, mental retardation, muscle weakness (begins in legs and pelvis), difficulty with motor skills (running, jumping hopping), frequent falls May be confined to wheelchair by age of 12

Signs and Tests • A complete nervous system (neurological), heart, lung, and muscle exam may show: • Abnormal heart muscle • Congestive heart failure • Arrhythmia • Scoliosis • Respiratory disorders • Muscle wasting

Tests • Electromyography (EMG) • Genetic tests • Muscle biopsy • Serum CPK

Treatments • There is no known cure for Duchenne muscular dystrophy. • Treatment aims to control symptoms to maximize quality of life. • Gene therapy may become available in the future.

Becker Muscular Dystrophy • Very similar to Duchenne muscular dystrophy • Becker muscular dystrophy gets worse much more slowly • 3 - 6 out of every 100,000 males • X-linked • Manifests later in childhood of adolescence

Question • Which motor system is responsible for crude muscle movements? • Pyramidal motor system • Extrapyramidal motor system

Answer b. Extrapyramidalmotor system: This system originates in the basal ganglia and provides background for the more crude, supportive movement patterns.

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 Toxins can alter neuromuscular function by changing the release, inactivation, or receptor binding of acetylcholine. • Curare acts on the post-junctionalmembrane of the motor endplate to prevent the depolarizing effect of the neurotransmitter. • Used during many types of surgical procedures • Clostridium botulinumblocks acetylcholine release and results in paralysis • Botox • Organophosphates block acetylcholinesterase • Nerve gases and pesticides

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 • Sensitized Helper T Cells • Antibody directed attack on receptors

Myasthenia Gravis • Muscle weakness and fatigability with sustained effort • Ptosis due to eyelid weakness • Diplopia • Progresses to generalized weakness • Myasthenic crisis • Compromised ventilation • Usually during a period of stress

Chapter 50 Disorders of Motor Function