Reflexes 2024 PDF

REFLEXES NG SOOK LUAN, PhD Jabatan Diagnostik Kraniofasial & Biosains Fakulti Pergigian UKM 012-9305208 [email protected] LEARNING OBJECTIVES 1. describe the overall functions of the nervous system. 2. describe the components of a reflex arc. 3. explain th...

REFLEXES NG SOOK LUAN, PhD Jabatan Diagnostik Kraniofasial & Biosains Fakulti Pergigian UKM 012-9305208 [email protected] LEARNING OBJECTIVES 1. describe the overall functions of the nervous system. 2. describe the components of a reflex arc. 3. explain the functions and mechanism of a stretch reflex. 4. explain the importance of gamma motor neuron. 5. explain the functions and mechanism of inverse stretch reflex. 6. explain the function and mechanism of polysynaptic reflex.  FUNCTIONS OF THE NERVOUS SYSTEM Sensory input Integration Gather information Processes, regarding internal analyses and and external interprets sensory changes (stimuli) input and makes decision Motor output A response by activating muscles or glands to integrated stimuli REFLEX A rapid, involuntary sequence of actions occurring in response to a stimulus. Functions: To maintain muscle length To protect from injury To maintain body balance and posture Integrating centre involved: Spinal cord (e.g. knee jerk, inverse stretch reflex, withdrawal reflex) Brainstem (e.g. pupillary light reflex, gag reflex, coughing reflex) Clinical importance To determine the location of nerve damage/lesion  Components of the Stretch Reflex Arc r Stretch No interneuron; direct reflex is a deep monosynaptic reflex and its - components are: 1. Sensory receptor (muscle spindles) 2. Sensory neuron (group Ia and group II afferents) 3. Integrating center (spinal cord) 4. Motor neurons (α- and γ- spinal motor neurons) 5. Effector (the same muscle (homonymous) of muscle spindles) This reflex is the simplest; it involves only 2 neurons & one synapse,  MUSCLE SPINDLES Are found in almost all skeletal muscles esp. in those that exert fine motor control Functions: to maintain muscle length and muscle tone and posture 3 Main components: Intrafusal fibres Provide regulation of muscle spindle stiffness – variable sensitivity to stretch Primary sensory axons Ia and II fibres send impulses to spinal cord Type Ia detects muscle length and rate of change in muscle length Type II detects muscle length Gamma motor neurones Innervate the contractile end of intrafusal fibres Regulates muscle spindle tension (stiffness)  GAMMA MOTOR NEURON (γMN ) Stimulation of γMN causes the contractile ends of intrafusal fibres to shorten Therefore, nuclear bag portion stretches. This leads to reflex contraction of the skeletal muscle. Increased γMN activity increases spindle sensitivity during stretch. Input from various brain regions to γMN influences the sensitivity of the muscle spindles  Extrafusal skeletal muscle fiber Intrafusal Spinal cord Muscle spindle fiber Contractile end Afferent input from sensory endings of muscle spindle fiber Alpha motor neuron output to regular skeletal-muscle fiber Stretch reflex pathway (Arc) γ-motorneuron output to the contractile end of spindle fiber ye Descending pathways co -ac t iv ate α- and γ- motor neurons ??? A man iE lA mi n s Co-activation of α- and γ- Motor Neurons Muscle spindles A. Relaxed B. Contracted muscle in C. Contracted muscle in muscle: spindle hypothetical situation of normal situation of fiber sensitive to no spindle coactivation; spindle coactivation; stretch of muscle slackened spindle fiber contracted spindle fiber not sensitive to stretch sensitive to stretch of of muscle muscle What is the significance of this coactivation? ▪ Regulate the sensitivity of the spindle by keeping its length constant ▪ Oppose sudden changes in muscle length  STRETCH REFLEX summary(simplest reflex arc) 3 Monosynaptic reflex 2 patellar-tendon reflex / knee-jerk stretch reflex 1 1. Stimulus: stretching of the skeletal muscle 4 2. Stimulation of intrafusal fibre/muscle spindle (stretch rec.) 3. Impulses sent via Ia/II fibre 5 4. Integration centre: spinal cord(L2-4), monosynapse. 6 5. Impulse sent to effector via α-MN fibre 6. Effector & response: extrafusal fibres (the same Note that reciprocal innervation causes the flexor muscles to relax muscle) contract Collateral of Ia fibre to the inhibitory interneuron causes relaxation of antagonist muscle Monosynaptic Reflex What is the significance of stretch reflexes? ▪ They help maintain a normal posture ▪ They function to oppose sudden changes in muscle length  POLYSYNAPTIC REFLEX (1 or more interneurons interposed between the afferent and efferent fibres) Withdrawal Reflex Cross Extensor Function: Flexor - to allow rapid withdrawal of appendage from a dangerous stimuli Cross extensor – to stabilise the body  - Golgi Tendon Organ (GTO) Function: to detect muscle tension, prevent muscle damage Aligned in series with the muscle fibre Can be stimulated by passive stretching and active contraction of the muscle.  INVERSE STRETCH REFLEX (POLYSYNAPTIC REFLEX) Inverse stretch reflex 1. Stimulus: excessive muscle contraction detected by 2. Receptor: Golgi tendon organ (GTO) 3. Afferent nerve: Ib fibre (myelinated) 4. Integration centre: spinal cord Ib synapses with inhibitory interneuron to αMN supplying agonist muscle and excitatory interneuron to antagonist muscle 5. Efferent nerve: αMN (inhibited) 6. Effector/response: muscle (agonist) relaxation GTO i s a reflex, seatmons o n e  Practicum Plantar reflex Function – protective Normal response Plantar flexion Clinical importance In UMNL – extension of toes (Babinski sign)  Babinski sign - plantar extensor response: extensors contract, dorsiflexion (toes curl up) due to upper motor neuron lesion / pyramidal lesion - normal for infants below 2 yo due to incomplete myelination of UMN - pyramidal tract still developing Normal plantar reflex - plantar flexion (toes curl down) - upper motor neuron inhibits contraction of extensors  Clasp knife response Loss of inhibitory commands from UMN Receptor involved is GTO JENDRASSIK MANEUVER  Factors that increase γMN discharge Increased γMN discharge causes hyperactive of stretch reflex Physiological factors: Anxiety Stimulation of skin esp. by noxious agents Jendrassik’s maneuver Pathological condition: Upper motor neuron lesion (UMNL) due to damage to descending fibres, cortex and brain stem  Different features of UMN and LMN lesions upper motor honan lower Features UMN LMN Site of the lesion Cerebral hemispheres, Anterior horn cell, nerve cerebellum, brainstem, roots, peripheral nerves, spinal cord NMJ Muscle Disuse atrophy Denervation atrophy Muscle tone Hypertonia Hypotonia Tendo reflex Exaggerate reflex Hyperreflexia (spasticity) Hyporeflexia Fasciculation Absent Present Paralysis Spastic paralysis Flaccid paralysis RGC – retinal ganglion cell LGN – lateral geniculate nucleus N.III – oculomotor nerve / CNIII watch youbolak THANK YOU