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1 Physiology Week 4 – Reflexes

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1 Physiology Week 4 – Reflexes Physiology week 4 – Reflexes Reflexes Reflex arc Sensory organ Afferent neuron (enters cord via dorsal roots or cranial nerves) One or more synapses Efferent neuron (leaves the cord via ventral roots or cranial nerves) Effector Bell-Magendie law Spinal cord dorsal roots are sensory Spinal cord ventral roots are motor Monosynaptic reflexes Stretch reflex eg. knee jerk, ankle jerk Sense organ (muscle spindle) Ia fibre, afferent neuron (same as efferent nerve supply) One synapse (neurotransmitter is glutamate) Efferent neuron (motor neurone) Effector (muscle) Reciprocal innervation of muscle antagonist Muscle spindle fxn - parallel intrafusal fibres respond to stretch with different dynamic/static responses. Inverse stretch reflex Stretch can increase to a maximum, but once this point is reached, the muscle relaxes (following prolonged stretch or muscle contraction the contracted muscles suddenly relax) Sense organ (golgi apparatus) Afferent neuron (same as efferent nerve supply) One synapses (neurotransmitter is glutamate) Efferent neuron Effector (muscle) Clonus ‘regular rhythmic contraction of a muscle that is subjected to sudden maintained stretch’ Caused by increased gamma efferent discharge in response to hyperactive muscle spindles Polysynaptic reflexes Polysynaptic reflexes branch in a complex fashion One or more interneurons, interposed between the afferent and efferent neurones Number of synapses is variable Withdrawal reflex Sense organ (noxious stimulus skin or subcut) Afferent neuron (same as efferent nerve supply) Multiple synapses (neurotransmitter is glutamate) 1 Efferent neuron Effector (ipsilateral flexor contraction/extensor relaxation + crossed extensor response of opposite limb) Results in withdrawal of limb from stimulus Cross extensor response opposite limb Prepotency of the withdrawal reflex the reflex pre-empts spinal pathways from any other reflex activity occurring at the same time. Effects of a polysynaptic reflex Prolonged effect as different time for stimulus to reach effector The reflex effect becomes stronger and more prolonged the greater the stimulus Due to: - impulse arriving at effectors at different times due to interneurones - irradiation of the impulse up and down the spinal cord - recruitment of motor units - reverberation of the circuit as some interneurons turn back on themselves Above effects result in after-discharge due to continued bombardment of motor neurones by impulses arriving by complicated and circuitous paths Cutaneous, deep and visceral sensation Sensory pathways Ascending pathways Divided into posterior, lateral and anterior columns Lateral and anterior columns Fibres entering the cord form the dorsolateral tract that runs at the tip of the posterior horn and ascends or descends for 1 or 2 segments before synapsing. Fibres then decussate to enter the anterolateral tract (anterior to the denticulate ligament) 90% then synapse with the brainstem reticular formation Anterolateral tracts composed of Lateral spinothalamic (pain, temperature) Ventral spinothalamic (crude touch, pressure) Pain Temperature Crude touch, itch, tickle Tracts to cerebellum for muscular coordination 2 Posterior columns Fibres from lower body carried medially in the gracile tract, upper body laterally in cuneate tract Both tracts synapse in the medulla (gracile and cuneate nuclei) then decussate to form the medial lemniscus, pass to the thalamus, synapse again then pass to the cortex Light (discriminative) touch Vibration Proprioception (position sense) Bladder and rectum fullness Pain Sense organ Naked nerve endings Found in most areas of the body Respond to multiple stimuli including chemical, heat, electrical, mechanical Afferent fibres A delta (myelinated) – 20m/s (fast, sharp pain); Large diameter (2-5 microm) C (unmyelinated) – 1m/s (slow, dull pain); Small diameter (0.4-1.2 microm) Synapse A delta - lamina I and IV of the dorsal horn C (unmyelinated) – lamina I and II of the dorsal horn Considerable plasticity results in dorsal horn cells acting as a gate where impulses can be modified by presynaptic inhibition Transmitters include glutamate and substance P Pathway Some axons end in the spinal cord Most ascend in the anterolateral system (lateral spinothalamic tract) and ascend to the ventricular posterior nuclei of the thalamus, reticular system and cortex 3 Sensory Homunculus Vision Visual pathways Optic nerve Nasal hemiretina fibres decussate in the optic chiasm Ipsilateral temporal fibres and contralateral nasal fibres in optic tract Lateral geniculate body of thalamus Upper half of retina projects to medial side Lower half of retina projects to lateral side Geniculocalcarine tract passes to the occipital cortex (brodman’s area 17) Medial fibres pass to the superior lip of the calcarine fissure Lateral fibres pass to the inferior lip of the calcarine fissure Macular fibres separate from other fibres and project more posteriorly Optic pathway lesions Optic nerve – monocular blindness Optic tract – homonymous hemianopia Optic chiasm – bitemporal hemianopia Occipital lesions – macular sparing common Pupillary reflexes Near response Accommodation due to contraction of ciliary body Convergence of the visual axes due to contraction of medial rectus Papillary constriction Involve cortical and subcortical pathways via the edinger-westphal nucleus, ciliary ganglion and ciliary Pupillary light reflex/consensual light reflex 1st order neurons bypass lateral geniculate body, project to superior colliculus of midbrain via optic nerve Second order neurons project to ipsilateral and contralateral Edinger-Westphal nucleus Third order neurons project to ciliary ganglion of oculomotor nerve Fourth order neuron project via short ciliary nerves to the pupillary sphincter Argyll-Robertson pupil – reflex lost (CNS syphilis, midbrain lesions) while accommodation intact 4 Eye movements Hearing and equilibrium Nystagmus - characteristic jerky movement of the eyes seen at the start and end of period of rotation - different types: o horizontal (eyes move horizontal plane) o vertical (head tipped sidewise in rotation) o rotatory (head tipped forward) - direction of eye movements is identified by the direction of the quick component - why does nystagmus occur: o reflex that maintains visual fixation on stationary points while the body rotates, although not initiated by visual impulses o when rotation starts the eyes move slowly in a direction opposite to the direction of rotation, maintaining visual fixation (vestibular-ocular reflex, VOR) o when the limit of this movement is reached, the eyes quickly snap back to a new fixation point and then again move slowly in the other direction - how is nystagmus mediated: o slow component is initiated by impulses from the labyrinths o quick component is initiated by a centre in the brain stem Caloric Stimulation Semicircular canals stimulated by instilling water hotter or colder than body temp into external auditory meatus Temp difference sets of convection currents in endolymph, causing motion of cupula Causes nystagmus, vertigo, nausea Temperature regulation Balance between heat loss and heat production determines body temperature. Constant body temperature is essential to the normal functioning of enzymatic and other body processes. Normal oral body temperature 36.3-37.1 oC Oral temperature is 0.5 oC lower than core temperature Circadian fluctuation 0.5 oC, also ovulatory variations 5 Heat production Muscular exercise Assimilation of food Thyroid Processes contributing to metabolic rate Heat loss Radiation and conduction 70% Radiation Transfer of heat by infrared electromagnetic radiation from one object to another at different temperature with which it is not in contact. Conduction Heat exchange between objects or substances in contact. Temperature of the skin largely determines the degree to which heat is lost or gained Convection Movement of molecules away from the area of contact Vapourisation of sweat 27% 1g of water removes 0.6kcal of heat. In humid environments sweat production can reach 1600ml/hr Respiration 2% Urine and faeces 1% Temperature regulation mechanisms Mechanisms activated by cold Increase heat production Shivering Hunger Increased voluntary activity Increased secretion of catecholamines Decreased heat loss Cutaneous vasoconstriction Reducing body surface area – curling up. Horripilation (pilo erection) Mechanisms activated by heat Increase heat loss Cutaneous vasodilation Sweating Increased respiration Decreased heat production Anorexia Apathy and inertia Central control of temperature Temperature sense organs are naked nerve endings. Cold receptors respond from 10-38 o and utilise Adelta and C fibres Warm receptors respond from 30-45 o and utilise C fibres Afferents project via the lateral spinothalamic tract to the postcentral gyrus Afferents come from sensory receptors in skin, deep tissue, spinal cord, extrahypothalamic parts of brain and hypothalamus itself Hypothalamus receives afferents mainly from cold receptors in the skin, spinal cord, and hypothalamus itself. Controls range, circadian and ovulatory variations. Posterior responds to cold, anterior responds to heat. Pyrogens “reset the thermostat” causing fever. Cytokines are endogenous pyrogens. Temperature thresholds exist for the main regulating responses Shivering – 35.5 o Sweating – 37 o 6 Alert behaviour, sleep, electrical activity of the brain Thalamic Nuclei 3 parts Epithalamus Connections to olfactory system Dorsal thalamus Connections to auditory, visual and motor systems Ventral thalamus
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