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Skeletal Reflexes Lanny Shulman, O.D., Ph.D. University of Houston College of Optometry Afferent • • • 10 million Connect sensory receptor in PNS with spinal cord or brain Receptors can be grouped into 4 categories: 1. somatic sensory receptors 2. external receptors 3. proprioceptors 4. visceral or internal receptors Efferent • • System: Sensory Neurons System: Motor Neurons 0.5 million Carries instructions from the CNS to other tissues, organs Motor System Refers to neural pathways that control contractions of the skeletal muscles Skeletal muscle contractions result in: Posture Reflexes Rhythmic Activity (e.g. locomotion, respiration) Voluntary movements Corticospinal Pathway • Conscious control of skeletal muscles throughout the body Other Pathways: Medial and Lateral Pathways Subconscious regulation of skeletal muscle tone, controls reflexive skeletal muscle responses to equilibrium sensations and to sudden or strong visual and auditory stimuli (Dorsal horn) (Ventral horn) Sensory System Afferent Pathways Sensory Posterior Column Pathway Spinothalamic Pathway Spinocerebellar Pathway Posterior Column Pathway •Fine touch, pressure, vibration, and proprioception Other Pathways: • Spinothalamic Pathway touch, pressure, pain, temperature • Spinocerebellar Pathway proprioceptive information Muscle Fibers and Innervation Extrafusal Fibers: contractile muscle fibers Intrafusal Fibers: sensory muscle fibers (muscle spindle) Muscle Spindle Nuclear Bag Fibers: central nuclear area is dilated Nuclear Chain Fibers: No dilation; nuclei arranged in single row Report Muscle Length Golgi Tendon Organs • Found in tendons, connective tissue, around joint capsules • Supplied by Group 1B fibers • Report muscle tension Simple Reflexes Sensory fibers deliver information to CNS Motor fibers carry motor commands to peripheral effectors “Wiring” of a single reflex: REFLEX ARC Reflex Arc • • • • • • Sensory information is processed at the level of the spinal cord Resulting in action without involvement of the brain If the hand is burned by a candle, the hand is rapidly removed from the flame At the same time, the brain is receiving information that a painful event occurred in the hand Although the brain may perceive the painful stimulus, the reflex arc acts to retract the hand without direct input from the brain. The best known example of a reflex arc is the Stretch Reflex. Stretch Reflex • • • • • • • Stretch reflex provides automatic regulation of skeletal muscle length. The sensory receptors in the stretch reflex are muscle spindles. Stretch reflexes are important in maintaining normal posture and balance. Makes automatic adjustments in muscle tone. Stimulus is increasing muscle length Activates a sensory neuron that triggers a motor response which is contraction of the stretched muscle. This counteracts the stimulus. Can use stretch reflex to test general condition of the spinal cord, nerves and muscle The Stretch Reflex Withdrawal Reflex or Flexor Reflex • • • • • • • Have at least one interneuron Therefore, they are polysynaptic reflexes Designed to avoid harmful stimuli Move body parts away from the source of stimulation Strongest reflexes are triggered by pain (also touch or pressure) These are ipsilateral reflexes (same muscle contracts on the same side as the stimulus) These are intersegmental reflexes (stimulation of motor neurons come from different segments of the spinal cord) Withdrawal Reflex Tendon Reflex or the Inverse Stretch Reflex • Opposite of the Stretch Reflex • Instead of responding to excessive stretching, this reflex responds to excessive contraction. • The tendon reflex is a protective feedback mechanism to control the tension of a muscle by causing relaxation. • The stretch reflex is a feedback mechanism to control muscle length by causing muscle contraction. • The tendon reflex can over-ride the stretch reflex when tension is too great. • Affects rate of discharge of motor neuron Tendon Reflex Spinal Cord Deficits • Babinski’s Sign: • Damage to lateral corticospinal tract • Withdrawal reflex that is held in check by the lateral corticospinal system Sensory Endings in Extraocular Muscles and Tendons Human extraocular muscles have anatomically degenerated sensory organs (muscle spindles and Golgi tendon organs) Small Capsules poorly defined Fewer intrafusal fibers No obvious distinction between bag and chain fibers Human extraocular muscles have no stretch reflexes What is the role of sensory organs in extraocular muscle? No large or sudden change in load in extraocular muscles no reason for having mechanism to compensate for loading changes Could explain the degeneracy of sensory organs in number and structure They’re present because they are a natural constituent of striated muscle Oculocardiac Reflex Passive stretch could cause bradycardia Anatomical pathway is unknown No functional role for the reflex Probably aberrant neural wiring in the brainstem Presumably a developmental anomaly in some individuals Sensory endings in extraocular muscles probably do not provide information about eye position Experiments show that sensory endings of extraocular muscles only provide a small effect on perception of visual direction and ocular alignment. Why are there sensory signals? Sensory signals may be involved in motor learning, motor plasticity, and development This role could be important during growth and development Is the anatomical degeneracy related to age?