Reflexes Mrs Sinkala PDF

Reflexes Mrs Sinkala Reflex Reflexes are automatic, subconscious response to changes within or outside the body. A reflex involves a reflex arch ( sense organ, afferent neuron, intergration center, efferent neuron and effector organ). Function of reflex Reflexes maintain homeostasis (a...

Reflexes Mrs Sinkala Reflex Reflexes are automatic, subconscious response to changes within or outside the body. A reflex involves a reflex arch ( sense organ, afferent neuron, intergration center, efferent neuron and effector organ). Function of reflex Reflexes maintain homeostasis (autonomic reflexes) – heart rate, breathing rate, blood pressure, and digestion. Reflexes also carry out the automatic action of swallowing, sneezing, coughing, and vomiting. Reflexes maintain balance & posture, e.g Spinal reflexes – control trunk and limb muscles. Brain reflexes – involve reflex center in brain stem, e.g Reflexes for eye movement. Reflex Arc Nerve impulses follow nerve pathways as they travel through the nervous system. The simplest of these pathways, including a few neurons, constitutes a reflex arc. Reflexes whose arc pass through the spinal cord are called spinal reflexes. Parts of Reflex Arc 1. Receptor – detects the stimulus. a) Description: the receptor end of a particular dendrite or a specialized receptor cell in a sensory organ. b) function: sensitive to a specific type of internal or external change.  2. sensory neuron – conveys the sensory information to brain or spinal cord. a. Description: Dendrite, cell body, and axon of a sensory neuron. b. Function: transmit nerve impulses from the receptor into the brain or spinal cord. 3. Interneuron: relay neurons. a. Description: dendrite, cell body, and axon of a neuron within the brain or spinal cord. b. function: serves as processing center, conducts nerve impulses from the sensory neuron to a motor neuron. 4. Motor neuron: conduct motor output to the periphery. a. Description: Dendrite, cell body, and axon of a motor neuron. b. function: transmits nerve impulse from the brain or spinal cord out to an effecter. 5. Effector: a. Description: a muscle or gland. b. function: Response to stimulation by the motor neuron and produces the reflex or behavioral action. Properties of a reflex Adequate stimulation- the stimulation that triggers a reflex is precise Habituation and sensitization –the reflex responses are stereotyped and modified by experience. Final common path- all neural influences affecting muscular contraction ultimately funnel through them to the muscles. Central excitatory and inhibitory states- excitatory influences overbalance the inhibitory influences and vice versa. Properties of reflexes ONE WAY CONDUCTION : REACTION TIME -It depends upon the length of afferent and efferent nerve fibers, velocity of impulse through these fibers and central delay. REBOUND PHENOMENON - reflex activities can be forcefully inhibited for some time. Summation - both temporal and spatial summation occur in a reflex Classification of reflexes Depending upon development, (inborn or acquired)- Unconditioned Reflexes or conditioned Reflexes. Depending upon processing site- spinal cord or cranial. Depending upon number of synapse or complexity- monsynaptic and polysynaptic Depending upon response- somatic (superficial or deep) , visceral (automatic) Monosynaptic and polysynaptic reflexes There are two types of synapses on the basis of the number of synaptic connections involved. 1. Monosynaptic reflexes- sometimes also referred to as the muscle stretch reflex. E.g patellar reflex Only one synapse is involved between afferent and efferent neurons. It begins inside the muscle spindle of the muscle. 1. Polysynaptic reflexes- it involves a single sensory stimulus that synapses on interneurons ,e.g withdrawal reflex (flexor reflex) Stretch Reflex (monosynaptic reflex) It is used in the maintenance of posture. Stretching the muscle activates the muscle spindle Excited gamma motor neurons of the spindle cause the stretched muscle to contract Afferent impulses from the spindle result in inhibition of the antagonist muscle. Examples – Biceps, Triceps, quadriceps(Patellar, knee jerk), ankle(Achilles) jerk. patellar reflex Tapping the patellar tendon stretches the quadriceps and starts the reflex action The quadriceps contract and the antagonistic hamstrings relax Reciprocal innervation of stretch muscle Afferent nerve excites the motor neurons, which supply the flexors. Simultaneously, it also inhibits the motor neurons supplying extensors through an interneuron. Reciprocal inhibition and reciprocal innervation are very important in spinal reflexes, which are involved in locomotion. It helps in the forward movement of one limb while causing the backward movement of the opposite limb (like in withdrwal reflex). Stretch Reflex/ deep tendon reflex Events in monosynaptic stretch reflex 1. Passive stretch of a muscle (produced by tapping its tendon) stretches the spindle (intrafusal) fibers. 2. Stretching of a spindle distorts its central (chain) region, which stimulates dendritic endings of sensory nerves. 3. Action potentials are conducted by afferent (sensory) fibers into the spinal cord on the dorsal roots of spinal nerves.  4. Axons of sensory neurons synapse with dendrites and cell bodies of somatic motor neurons located in the ventral horn gray matter of the spinal cord. 5. Efferent impulses in the axons of somatic motor neurons (which form the ventral roots of the spinal nerves) are conducted to the ordinary (extrafusal) muscle fibers. These neurons are (alpha) motor neurons.  6. Release of Ach from the endings of alpha motor neuron stimulates the contraction of extrafusal fibers, and thus the whole muscle. 7. Contraction of the muscle relieves the stretch of its spindles, thus decreasing electrical activity in the afferent nerve fibers, and relaxes the spindle fiber and terminates the stretch reflex and muscle contraction. Note: By sending command to the motor neurons, the brain set a muscle’s length. The stretch reflex makes sure the muscle stay at that length. The stretch reflex is therefore important for maintaining muscle tone and upright posture. The deep tendon Reflex (monosynaptic reflex) If you tap on the tendon of a muscle, it contracts. Its synergists contract and its antagonists are inhibited. A tap on the patellar tendon stretches the extensor muscle and its spindles. The deep tendon Reflex (stretch reflex)  The spindle discharges and excites the associated sensory fibers that excite the motor neurons to the extensor muscle. Contraction of the extensor muscle extends the lower leg (knee - jerk). Ipsilateral flexor muscle relax for extensors to function. Branches of the sensory fibers from muscle spindle activate inhibitory interneuron, which in turn inhibit the motor neuron to the flexor Golgi Tendon Reflex (polysynaptic reflex) The opposite of the stretch reflex. Contracting the muscle activates the Golgi tendon organs. Afferent Golgi tendon neurons are stimulated, neurons inhibit the contracting muscle, and the antagonistic muscle is activated. As a result, the contracting muscle relaxes and the antagonist muscle contracts. Golgi Tendon Reflex The withdrawal reflex (flexor reflex) polysynaptic reflex The automatic withdrawal of an extremity from a painful stimulus. A polysynaptic reflex. Sensory pain signals excite motor neurons to the flexor muscles, eliciting flexion and withdrawal of the leg. Motor neurons to the extensor muscles are inhibited via inhibitory interneurons. This would relax the extensors of the same leg. The withdrawal reflex The crossed extensor reflex A polysynaptic reflex. Opposite of the withdrawal reflex. It is the reflex on the contralateral leg where the pain stimuli was applied. E.g. In flexor reflex, Painful stimulation of one foot causes flexion (withdrawal) of the same (ipsilateral) leg. In crossed extensor reflex, the extension of the opposite leg, without the stimuli occurs to stabilize the posture; thus the contralateral leg flexors are inhibited and they relax while the extensors are activated and they contract allowing balance and body posture to be maintained. Abnormalities of Muscle stretch reflex Hyporeflexia- decrease in muscle stretch reflex Areflexia –Absent of muscle stretch reflex (polyneuropathy). Hypereflexia – abnormally increased muscle stretch reflex (upper motor neuron lessions). Asymmetrica hyperreflexia – (upper motor neuron, corticospinal tract, lesion) Babinski reflex-(indicates upper motor neuron, corticopinal tract lession) in adult. Pathological reflexes Babinski sign Clonus Pendular movement Plantar reflex (Babinski) Superficial mucous membrane reflexes Superficial cutaneous reflexes Deep reflexes