Physiology week 4 – Reflexes

Reflexes Reflex arc Sensory organ Afferent neuron (enters cord via dorsal roots or ) One or more synapses Efferent neuron (leaves the cord via ventral roots or cranial nerves) Effector

Bell-Magendie law dorsal roots are sensory Spinal cord ventral roots are motor

Monosynaptic reflexes Stretch reflex eg. knee jerk, ankle jerk Sense organ () 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 Anterolateral tracts composed of Lateral spinothalamic (, temperature) Ventral spinothalamic (crude touch, pressure)

Pain Temperature Crude touch, itch, tickle Tracts to 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 , pass to the , 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 ) 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 of 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 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 Neocortex Six layers

Reticular formation Old reticular core of brain in midventral portion of medulla and midbrain Contains cell bodies of many of serotonergic, noradrenergic and adrenergic systems Contains areas regulating HR, BP, respiration Some fibres inhibit transmission of sensory pathways from spinal cord Some areas involved in spasticity and stretch reflexes Contains network of reticular activating system (RAS)

Reticular activating system Functions Regulate resp, CVS, vegetative and endocrine functions Nonspecific activation from any modality (neurons activated equally well by different stimuli) Sends signals mostly to thalamus’ midline and intralaminar nuclei – then signals sent to neocortex Increases cortical electrical activity - on EEG as desynchronisation –resting alpha waves to alpha block Increased consciousness, alert state, heightening sensory perception General anaesthetics exert greatest effect on RAS Location and structure Complex polysynaptic network with cell bodies in mid ventral portion of medulla + midbrain Converging sensory fibres from long tracts and cranial nerves

Electroencephalogram (EEG) Record of electrical activity of cortical neural units in a volume conductor Positive if net current flow toward electrode, negative if net flow away EEG records unipolar or bipolar (between 2 electrodes) Alpha rhythm – awake, at rest, eyes closed – regular waves with frequency 8-12 Hz, esp in parietal-occipital area Beta rhythm – harmonic to alpha, 18-30 Hz but lower amplitude, esp frontal regions Gamma oscillations – 30-80 Hz, when aroused and focuses attention on something Theta rhythm – in children, large amplitude, regular 4-7 Hz Delta waves – large, slow waves frequency <4 Hz

Frequency of dominant rhythm varies with age: Fast, beta-like in infants, speeds up in childhood, adult alpha pattern appears in adolescence Alpha rhythm decr by low blood sugar, low body temp, low level glucocorticoids, high arterial partial pressure CO2 Hyperventilation (lowers CO2) can bring out latent EEG abnormalities Focusing on something - alpha replaced by fast, irregular low voltage activity = alpha block/arousal/alerting response

Sleep patterns 2 kinds Rapid eye movement (REM) sleep High amplitude slow waves replaced by rapid, low voltage activity resembling stage 1 Threshold for arousal by sensory stimuli and by stimulation of reticular formation elevated Also called paradoxical sleep (since EEG rapid) Rapid, roving movements of eyes Occurance of large phasic potentials in groups of 3-5, originating in , pass rapidly to lateral geniculate body and from there to occipital cortex = ponto-geniculo-occipital (PGO) spikes Marked reduction in tone of skeletal muscles of neck Relative paralysis of voluntary activity

7 Non-REM (NREM) or slow-wave sleep 4 stages 1 – falling asleep: low-amplitude, high-frequency EEG activity 2 – appearance of sleep spindles (bursts of alpha-like, 10-14 Hz waves 3 – lower frequency, increased amplitude 4 – maximal slowing with large waves Deep sleep = rhythmic slow waves, indicating marked synchronisation

Control of Posture and Movement

Voluntary movement via corticospinal and corticobulbar systems Posture continually adjusted by posture-regulating systems Movement smoothed and coordinated by medial and intermediate portions of cerebellus (spinocerebellum) Planning and organization of voluntary movement by cortex, , lateral portions of cerebellum (neocerebellum) 2 types motor output: voluntary and involuntary/reflexive Motor systems learn by doing – synaptic plasticity Lateral pathways supply distal limb muscles Medial pathways supply axial and proximal limb muscles Fibres of lateral form pyramids in medulla = pyramidal system Rest of pathways = (do not pass through pyramids)

8 Motor system also divided into upper and lower motor neurons LMN lesions – flaccid paralysis, muscle atrophy, absent reflexes UMN lesions – spastic paralysis, incr reflexes, no atrophy

Corticobulbar tract – from to cranial nerve nuclei Lateral corticospinal tract – cross midline in medullary pyramids (80% of fibres of corticospinal pathways) Skilled voluntary movement Damage causes Babinski sign (a flexor withdrawal reflex usually held in check by lateral corticospinal tract) Ventral/anterior corticospinal tract (20%) – doesn’t cross midline until reaches muscles it controls

Cortical Motor Areas Motor cortex (M1) in on cingulate sulcus on medial side of hemisphere Premotor cortex on lateral surface of brain Plasticity – motor cortex has similar plasticity to sensory cortex

Cerebellum Connected to brainstem on each side by superior, middle and inferior peduncles Medial vermis and lateral cerebellar hemispheres

Medulla Oblongata Autonomic reflex control of circulation, heat and lungs = vital centres Swallowing controlled by central program generator Vomiting centre in reticular formation Chemoreceptor trigger zone in wall of fourth ventricle

Hypothalamus Neural connections with posterior pituitary and vascular connections with anterior pituitary Connections to midbrain, pons and Functions: Temperature regulation Neuroendocrine control of Catecholamines, Vasopressin, Oxytocin, TRH, ACRH, GnRH, PRH, somatostatin/GRH Appetitive behaviour Thirst Hunger/satiety via feeding center and satiety center Neuropeptide Y, Leptin, gut peptides (glucagons, somatostatin, CCK), glucose Sexual behaviour Control of some autonomic function Sleep/wakefulness Circadian rhythms, by suprachiasmatic nuclei (SCN)

Thirst Thirst regulated by osmolality and ECF volume Osmolality/hypertonicity acts on osmoreceptors in anterior hypothalamus Hypovolaemia acts on barorecptors and angiotensin II

9 Posterior pituitary Secretes neural hormones (hormones secreted into circulation by nerve cells) Vasopressin (ADP) Oxytocin

Synthesized in cell bodies in supraoptic and paraventricular nuclei, transported down axons to endings in posterior lobe, secreted in response to electrical activity in endings Some neurons make oxytocin, some make vasopressin

Vasopressin

Oxytocin Actions Contraction of myoepithelial cells in breath – milk ejection Contraction of smooth muscle of uterus Via G-protein coupled receptor triggering increase in intracellular Ca2+

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