Assafa College Motor System & Reflexes PDF

Assafa College Department of Physiology NERUROPHYSIOLOGY Dr. Rayan Khalid, MBBS, M.Sc, MD Lecture No (5) MOTOR SYSTEM  MOVEMENT ▪ Defined as displacement of the body parts, that results in change in position of the body (whole or part) ▪ Occurs...

Assafa College Department of Physiology NERUROPHYSIOLOGY Dr. Rayan Khalid, MBBS, M.Sc, MD Lecture No (5) MOTOR SYSTEM  MOVEMENT ▪ Defined as displacement of the body parts, that results in change in position of the body (whole or part) ▪ Occurs due to the motor signals generated in the motor neurons in the CNS. ▪ Produced by the contraction and relaxation of muscles. ▪ Muscles that facilitate a particular movement = agonists ▪ Muscles that oppose the movement = antagonists  TYPES OF MOVEMENTS Automatic Movements Intentional Movements Reflexive in nature. Under voluntary control E.g. rapid response to E.g. painting, threading a a nociceptive stimulus. needle. Characteristics Characteristics : Stereotyped in nature Slow in execution Executed rapidly Can be modified easily Usually, cannot be Under voluntary control modified Rarely triggered by a Not under voluntary specific sensory stimulus control May be affected by factors Triggered by a specific like attention, emotion and sensory stimulus motivation  ORGANIZATION OF MOTOR SYSTEM Motor physiology deals with the study of control of movement and posture together. The components of motor systems are: Muscle and its efferent connections The spinal cord The brainstem controlling centers Basal ganglia Cerebellum Cerebral cortex MOTOR CORTEX  Directly controls the spinal cord motor neurons through the corticospinal tracts Strongly influences the brainstem nuclei (via corticobulbar projections) from where the extrapyramidal tracts originate. Directly and indirectly regulates the peripheral motor activities. Sensory cortex projects to the motor cortex and also contributes to the corticospinal fibers o The inputs from somatosensory cortex to the motor cortex provide feedback information to descending motor signals for alteration and improvement of motor performance o The motor system learn by doing and performance improves with repetition. This involves cortical plasticity o Some areas of motor control found in specialized the motor cortex  BASAL GANGLIA Important subcortical structures that strongly influence motor activities Involved in initiation, smoothening and coordination of the movement Project to motor cortex via the thalamus and strongly influence the motor output to spinal cord. Do not receive any direct somatosensory input from the spinal cord  THE CEREBELLUM ▪ Little brain, plays an important role in motor control & involved in some cognitive functions. ▪ It receives inputs from almost all sensory modalities and projects heavily to the brainstem motor nuclei, and motor cortex. ▪ Cerebellum plays a crucial role in the regulation of posture and movement.  Functions of the cerebellum ▪ Coordinate of voluntary movements ( rate , range , direction ) ▪ Increase the muscle tone ▪ Control posture & balance (information from the vestibular apparatus) ▪ Makes adjustments & predictions  THALAMUS major sensory relay station in the brain. Thalamus also receives inputs from cerebellum and basal ganglia. Thalamus plays an important role in sensory-motor coordination.  DESCENDING TRACTS Somatic motor activity depends on pattern & rate of discharge of spinal cord & Cranial nerves motor nuclei & impulses from descending pathways.  THE PYRAMIDAL (CORTICOSPINAL TRACT) Fibers from the cortex > internal capsule> midbrain & Pons In the medulla : 80% of fibers cross to the opposite side(decussation), Descend to spinal cord as lateral corticospinal tracts 20% of fibers descend to spinal cord on the same side, as direct or anterior corticospinal tract Some of these fibers cross to the opposite side at the segments. the rest pass ipsilateral.  Function of the pyramidal system Execution of complex, fine, skilled, voluntary movements (e.g. of finger & toes They are facilitator to muscle tone & deep reflexes. Modulate the intensity of incoming sensory signals  EXTRAPYRAMIDAL SYSTEM Made up of all those parts in the CNS that are concerned with motor control, other than pyramidal system & cerebellum These consist of: Cortical motor areas, especially the premotor area & parietal cortex The basal ganglia The reticular formation, the red nuclei, the tectum of the brain, & the vestibular nuclei  Functions of the extrapyramidal system concerned with planning, programming & initiation of movement. subconscious gross movement occurring in groups of muscles e.g. swinging of arms while walking provides the necessary postural background for performance of skilled movement by pyramidal system Some tract in this system are facilitator while others inhibitory to muscle to tone PATHWAYS FOR TRANSMISSION OF CORTICAL MOTOR SIGNALS VOLUNTARY MOVEMENT EXECUTION  REFLEX The nervous system has three main functions, sensory input, integration of data and motor output. A reflex is an involuntary response to a stimulus. Reflex arc : The basic functional unit of the nervous system Smallest, simplest portion capable of receiving a stimulus and producing a response  Receptor Special structures located at the peripheral end of the afferent neurons According to site of receptor s the reflexes are classified into : Superficial reflex: receptors in the skin e.g. abdominal reflex Deep reflexes : receptors found deep within a muscle e.g. stretch reflex Visceral reflexes : receptors in the viscera e.g. vomiting reflex  Afferent neurons Enters the spinal cord through the dorsal root carries impulses from receptors to CNS Efferent neurons the motor neurons that leaves the spinal cord through the ventral root Effector The muscle or the myoepithelial cell in the gland that perform the effect. REFLEX CLASSIFICATION  STRETCH REFLEX Simple ,deep, monosynaptic reflex. The stimulus that initiates the reflex is stretch of the muscle. The response is contraction of the muscle being stretched. E.g. knee jerk.  Receptor: Intrafusal fibers (Encapsulated muscle fibers) in muscle spindle Afferent: 1a fibers. The center: spinal cord Efferents: Aα, Aγ  WITHDRAWAL REFLEX spinal polysynaptic reflex, intended to protect the body from damaging stimuli.  SUPERFICIAL REFLEXES Corneal reflex Nasal reflex (sneezing reflex) Plantar Reflex: a gentle scratch over the outer edge of the sole of foot causes plantar flexion and adduction of all toes.  PATHOLOGICAL REFLEXES ❖Babinski sign Abnormal plantar reflex is called Babinski sign. there is dorsiflexion of great toe and fanning of other toes. (positive ) Babinski sign is present in upper motor neuron lesion. Physiological conditions when Babinski sign is present are: infancy (non-myelination of pyramidal tracts) deep sleep. ❖Clonus a series of rapid and repeated involuntary jerky movements. clonus occurs when the deep reflexes are exaggerated due to hypertonicity of muscles in pyramidal tract lesion. Clonus is well seen in calf muscles producing ankle clonus quadriceps producing patella clonus.  SKELETAL MUSCLE TONE A state of continuous mild contraction of skeletal muscle during rest. Mechanism: it is static type of stretch reflex , is caused by the normal tonic discharge of the A gamma efferents. certain factors may affect the A gamma discharge resulting in either hypertonia and hypotonia.  Functions of muscle tone Maintaining the erect posture against gravity prevent the jerkiness of movement In abdomen, it holds the viscera back and prevent bulging. It is important in venous return. It maintains normal body temperature.  HYPOTONIA ▪ Occur when the A gamma discharge is decreased ▪ The affected muscles are flaccid , offering very low resistance to passive stretch. ▪ Can occur due to : Neural shock Lower motor neuron lesions Cerebellar lesion Interruption of the afferent limb of stretch reflex Myopathies  HYPERTONIA ▪ Occurs when A gamma discharge is increased The affected muscles are spastic or rigid ,offering high resistance to passive stretch Spasticity: is present when hypertonia occurs in one group of muscles Rigidity: is present when hypertonia occurs in both groups of muscle Can occur due to : Corticospinal lesions UMNL Some extrapyramidal lesions like parkinsonism  UMN VS LMN Performance of normal voluntary movement depends on the integrity of two sets of neurons : UMN: originating in the cerebral cortex & brain stem, synapse with anterior horn cells or with the motor neurons of the cranial nerves LMN: spinal & cranial motor neurons directly innervate skeletal muscles motor fibers of the cranial nerves (3, 4, 5, 6, 7, 9, 10, 11, and12)  UPPER MOTOR NEURON LESION The lesions do not occur at the tract but pyramidal fibers are most commonly involved due to hemorrhage, thrombosis or embolism characterized by : Spastic paralysis or weakness of voluntary movement on the opposite of the body if the lesion is at or above the pyramidal decussation Hypertonia. Exaggerated tendon jerks these seen on the affected side Loss of superfacial reflexes. Absence of significant muscle- wasting.  LOWER MOTOR NEURON LESION A lesion of the motor neuron or their axons. characterized by the following features: Flaccid paralysis: Loss of the voluntary movement occurs in the muscles supplied by the affected segments only on the same side Hypotonia or atonia. Absent deep reflexes Absent superfacial reflexes: Abdominal, cremasteric, & plantar reflexes are lost muscle-wasting (atrophy). Presence of fasciculation: There are visible, spontaneous contraction of bundles of fibers in the affected muscles