Control of Posture and Movement Part-II

Learning Outcomes At the end of lecture, students should be able to: Describe in detail the role of spinal reflexes in regulation of posture and movement. Describe the acute and chronic effects of complete transection of spinal cord at mid-thoracic level; describe mechanism postulated to account for these features, therapeutic strategies to foster recovery. Explain the mechanisms postulated to account for exaggeration of stretch reflexes in chronic spinal animals or human.

Role of Reflexes Reflexes provide a window into the integrity of the motor system! Provide status of the motoneuron, the sensory fibers, and the interneuronal activity

Role of Spinal Reflex Posture and movement depend on appropriately controlled and monitored tone in the large postural muscles. Majority of spinal reflex actions unseen and unnoticed and yet vitally important to normal function . Often -taken-for-granted as activities of standing erect, walking, and running. Responsible for the smooth functioning of skilled movements of the trunk and limbs.

Role of Spinal Reflex Completely relaxed muscle (no resting tone), would overlengthen, require too much time, when a contraction was called for. On the other hand, too much tone would not allow for sufficient rest and recovery. Muscle Tone ~ essential for effective, appropriate & desired movement. Principal regulator of muscle tone is simple myotatic or stretch reflex

Spinal Reflexes ~ Posture and Movement Neuronal circuits of the spinal cord required to execute the commands to control of the muscles. Brain generate “analytical” and “command” signals to gives directions to these circuit to regulate the sequential activities of neurons in the cord, e.g. to promote turning movements when they are required, to lean the body forward during acceleration, To change the movements from walking to jumping as needed, and to monitor continuously and control equilibrium.

Postural reflexes Number of reflexes play role in regulation of static or dynamic posture. Can be categorized in to TWO; Tonic or static reflexes- integrated in to medulla Dynamic or Phasic Reflexes- integrated in Mid brain Other reflexes integrated in cerebral cortex are Placing reaction Hopping reaction

Tonic or static reflexes 1. Tonic labyrinthic reflexes Receptors: Otolith organs of vestibular apparatus via vestibulospinal tract Stimulus: Gravity, by change in body position Response: Supine position – maximum rigidity in extensor ; Prone position – minimum rigidity 2. Tonic neck reflexes Receptors: neck proprioceptors Stimulus: Head turned to side Response: Extension of limbs on side to which head is turned, up-hind leg flex, down-foreleg flex.

Dynamic or Phasic Reflexes 1. Labyrinthine righting reflexes Receptors: Otholithic organs Stimulus: Gravity Response: Head is kept at level 2. Body on head righting reflexes Stimulus: Pressure on side of body exteroceptors Response: Righting to head

Dynamic or Phasic Reflexes 3. Body on body righting reflexes Stimulus: Pressure on side of body exteroceptors Response: Righting of body even when head is prevented to right 4. Body on neck righting reflexes Stimulus: Stretch of neck muscle Response: Contraction of neck muscles rights thorax and abdomen

Other reflexes Hopping reactions If a standing animal is pushed laterally, it hops (jumps) to maintain equilibrium. Placing reaction Stimulus: Visual, exteroceptive and proprioceptive receptors Response: Foot placed on supporting surface in position to support body.

Acute and Chronic effects of complete transection of spinal cord at mid-thoracic level

Common causes of complete transection Gun shot injuries Dislocation of spine due to RTA Occlusion of blood pressure Infections like transverse meelitis

Effects of complete transection Occur in three stages Stage of Spinal Shock-Spinal shock refers to the temporary loss or depression of spinal reflex activity that occurs below a complete or incomplete spinal cord injury. Stage of reflex activity- phase of recovery Stage of reflex failure

Stage of Spinal Shock Duration of about 2 weeks in human but varies from species to species Acute effects, occur immediately following injury or transection Severity of the effects based on the site of injury and severity of trauma

Stage of Spinal Shock ; Characteristics Loss of muscle tone Complete flaccid paralyzed muscle below the level of injury Paralyzed bladder, rectum,& flaccid penis Loss of all sensation below the level of injury Sharp fall in blood pressure due to lack of sympathetic innervation, Loss of almost all somatic and visceral reflexes Skin becomes cold, blue, scaly and dry. Higher functions hardly affected except transient behavioral disturbances.

Underlying mechanism Cessation of tonic neuronal discharge from supraspinal centers leading to Decrease in neuronal excitability of cord neuron Reduced excitability incapable to trigger reflex activity Areflexia or poor reflex activity

Therapeutic strategies to foster recovery. Extensive, vigilant nursing care to prevent bed sores Adequate nutrition Continuous bladder drainage to prevent infection Constant monitoring of vitals and fluid intake and output Symptomatic medical management

Stage of reflex activity 2nd stage of complete transection of cord Occur after 3-6 weeks, individual variations Also called as stage of recovery During this stage, reflex activity, muscle tone of skeletal muscle and mass reflex begin to appear slowly in phased manner. Called as parts of chronic effects of complete transection

Stage of reflex activity- Characteristics Smooth muscle activity appears first Gradually defecation reflex and micturition reflex established After some time, sympathetic tone of blood vessels restore due to activity of spinal nerve, leading to restoration of blood pressure and sweating mechanism. Skin becomes healthy

Stage of reflex activity- Characteristics Recovery of muscle tone first appear in flexor than extensor-leading to more flexion of limbs make the individual to support the weight of body After about 4 weeks, tone of extensor appear Mass reflex appears, used by the patients to provide better bladder and bowl control.

Underlying mechanism Muscle tone appears due to denervation hypersensitivity

Stage of reflex failure Main features are;- Reflexes become difficult to eleicit Increased threshold for stimulus Loss of mass reflex Flaccid muscles undergoing wasting Hypercalcemia and hypercalciuria leading to formation of renal stones Septicemia & uremia

Underlying mechanism due to detoriating general health condition attributed to malnutrition, infection and toxemia. Pressure on bony prominences, pressures on blood vessels leading to break down of bony tissue and negative nitrogen balance.

Therapeutic strategies Administration of large doses of glucocorticoids has been shown to foster recovery of damages. Administration of neurotrophins shows some promise in experimental animals, and so does implantation of embryonic stem cells at the site of injury. Mobilization of patients prevent development of negative nitrogen balance and catabolism of large amounts of body protein~ thus prevent decubitus ulcers.

Exaggeration of Stretch Reflex in Spinal animal primarily due to hypertonia due to Denervation hypersensitivity,possible secondary to increase in number postsynaptic receptors Sprouting of new additional connections between sensory neurons, which may take over facilitatory function of supra-spinal centers.