Physiology - Neuro Flashcards
Describe the synthesis, release and action of acetylcholine at a nerve synapse and how is its action terminated
-synthesised from choline and acetyl-coA by the enzyme choline acetyltransferase in the cytoplasm of the cell
choline is directly synthesised in neurons and taken up at synaptic cleft
-acetylcholine then stored in small clear vesicles at cholinergic terminals (via VAT)
-action potential causes influx of calcium at terminal and exocytosis of acetylcholine into synaptic cleft
-acetylcholine then binds the the post-synaptic receptor
-acetylcholine is then broken down via hydrolysis by acetylcholinesterase into choline and acetate
broken down in blood and liver by pseudocholinesterase
-choline is taken back into the cell (there is no acetylcholine reuptake)
Describe the 2 types of acetylcholine receptors
Nicotinic = ion channels, 2 types, present in neuromuscular junction and brain
mimicked by nicotine and blocked by paralytics
Muscarinic = g proteins, 5 types, present in smooth muscle, glands and brain
mimicked by muscarine and blocked by atropine
Describe the synthesis, release and action of adrenaline at a nerve synapse and how its action is terminated
-tyrosine (mostly dietary) enters cells, where it is converted to dopa by tyrosine hydroxylase
-dopa is then converted to dopamine by dopa decarboxylase
-dopamine enter small dense-cored vesicles, where it is converted to noradrenaline by dopamine beta-hydroxylase
-noradrenaline is converted to adrenaline in adrenal medulla and parts of the brain
-noradrenaline is released by exocytosis and acts on postsynaptic and to a lesser extent pre-synaptic terminals
-noradrenaline is removed from the synaptic junction by:
binding to post-synaptic receptors
binding to pre-synaptic receptors
reuptake into presynaptic neurons by NET
catabolism to inactive form by COMT (in synaptic cleft) or MAO (inside cell)
What types of noradrenaline receptors are there
alpha (g proteins, 2 types)
beta (g proteins, 3 types)
dopamine (g proteins, 5 types)
What catecholamines act as neurotransmitters
noradrenaline, adrenaline, dopamine
How is serotonin synthesised, what are its functions and how is it broken down
- serotonin is synthesised by hydroxylation and decarboxylation of the amino acid tryptophan
- it is found in platelets, GI tract and brain
functions:
- platelet aggregation, smooth muscle contraction, nausea/vomiting, mood regulation, vasoconstriction after injury
- facilitate GI secretions and peristalsis, cerebral and meningeal vasoconstriction
catabolism:
- recaptured from synaptic cleft by SERT, inactivated by MAO, excreted in urine
Draw the components of a muscle spindle
-muscle spindle is made of 3 parts and is purely sensory, does not contribute to overall strength
1) intrafusal fibers: consists of dynamic nuclear bag, static nuclear bag and nuclear chain fibers
2) myelinated afferent fibers: from central non-contractile part to synapse on spinal cord dorsal root
3) myelinated gamma efferent fibers: from spinal cord ventral root to polar contractile part of intrafusal fiber
Describe the sequence of events in a stretch reflex
- monosynaptic reflex: muscle stretched, causing contraction of the same muscle as a response
- sense organ intrafusal fiber sends afferent impulse to synapse on motor neurone in spinal cord (NT = glutamate)
- efferent fibers travel to extrafusal fibers to cause contraction
- inhibitor interneuron projects to antagonistic muscle
Describe the withdrawal reflex
- polysynaptic reflex occurring in response to a painful stimulus where the sense organ is a nociceptor
- crossed response = flexion and withdrawal of ipsilateral limb and extension of contralateral limb
What is meant by the term polysynaptic reflex and what are the effects
- one or more interneurons interposed between the afferent and efferent neurons
- prolonged effect due to different time for stimulus to reach the effector
- may have reverberation circuit as some interneurons turn back on themselves and prolong the effect
What is the inverse stretch reflex
- reflex that occurs when too much tension is generated and relaxation occurs instead of contraction via inhibition
- modulated by the golgi tendon organ located at the origin and insertion sites of a muscle (NT is glycine)
What are upper motor neurons and lower motor neurons
-upper motor neuron: in cerebral cortex and brainstem, carry information down to activate lower motor neuron
lesion = initially weak and flaccid, then spastic and hypertonic and hyper-reflexic and upward plantars
-lower motor neuron: spinal and cranial motor neurons that directly innervate skeletal muscle
lesion = flaccid paralysis, muscular atrophy, fasiculations, hypo-reflexic
What is the physiological basis of clonus
- clonus is the regular, repetitive, rhythmic contractions of a muscle subjected to sudden, sustained stretch
- due to loss of descending cortical input to inhibitor neurons called Renshaw cells
- causes loss of inhibition of antagonist muscle, causing repetitive contractions of ankle flexors and extensors
What are long term complications of spinal cord injury
ulcers
muscle atrophy
hypercalcaemia - renal stones
uti
How is pain transmitted and what are the different types of fibers
-primary afferent fiber:
sense organ = naked nerve ending peripherally, cell body in dorsal root ganglion
2 types of fibers = A delta and C, terminate in dorsal horn
A delta = myelinated, neurotransmitter is glutamate, fast pain
C = unmyelinated, neurotransmitter is substance p, slow pain
-dorsal horn neurons: axons travel in lateral spinothalamic tract to thalamus
Differentiate between fast pain and slow pain fiber
- fast pain: A delta, myelinated, 2-5um diameter, conduction rate 12-30m/s, NT is glutamate
- slow pain: C, unmyelinated, 0.4-1.2um diameter, conduction rate 0.5-2m/s, NT is substance P
What is referred pain and what is the theory involved
-irritation of a visceral organ causing pain in a distant somatic structure
-example: diaphragm projects to the shoulder, heart projects to the arm, ureter projects to the testicle
-convergence projection theory:
visceral pain referred to a distant somatic structure that shares the same embryonic origin or dermatome
somatic and visceral pain fibers converge on the same 2nd order neuron in the dorsal horn
How is acute pain modulated
- gate theory: stimulation of large touch/pressure afferents causes inhibition of pain pathways
- stress-induced analgesia
- medication: opioids
What site do opioids act on
opioid receptors (mu, kappa, delta) in afferent nerve fibers, dorsal horn of spinal cord and brain
Describe the neural connections of the visual pathways and describe visual field defects on the picture
-retina to optic nerve to optic chiasm to optic tract to lateral geniculate body in the thalamus
lateral geniculate body projects to:
occipital lobe = primary visual cortex
pretectal nucleus in midbrain = synapses on edinger-westphal nuclei, controls pupillary responses
Why is the fovea important for visual acuity
- fovea is the area where visual acuity is the greatest, it is positioned at the center of the macula
- it is the area of the retina with the highest concentration of cones and no rods
What ocular factors influence visual acuity
- optical factors: cataracts, keratitis, astigmatism, myopia, hyperopia
- retinal factors: retinopathies, optic neuritis
- stimulus factors: illumination, brightness, contrast, length of time exposed to stimulus
How is visual acuity measured
- use of a Snellen chart viewed from a distance of 6 meters or 20 feet
- numerator is the distance at which the chart is read, denominator is the smallest line that can be read
What is nystagmus
-the vestibulo-ocular reflex is present to maintain eye position when the head moves
-nystagmus is the involuntary movement of eye during rotation and may be physiologic or pathologic
-physiological nystagmus is part of the VOR characterised by 2 phases
slow = smooth pursuit in the direction opposite of head rotation
fast = saccadic movement in direction of head rotation once limit of the slow component is reached
-pathologic nystagmus is characterised by excessive saccadic movement
causes: damage to a component of the vestibular system
-types: horizontal, vertical, rotational
-direction of eye movement is identified by the direction of the fast component
What are the 2 main mechanisms of deafness and how are they differentiated
types:
- conductive - due to impaired sound transmission in external or middle ear
ex. wax, foreign body, otitis media/externa, perforated eardrum
- sensorineural - due to loss of cochlear hair cells or problems with CN 8 or central auditory pathways
ex. aminoglycoside damage, CN 8 tumours, medulla CVA
diagnosis:
-weber - tuning fork on vertex
sound loudest in bad ear = conductive
sound loudest in good ear = sensorineural
-rinne - tuning fork over mastoid then air
sound not heard in air after bone in bad ear = conductive
sound persists in air after bone in bad ear = sensorineural
What is the pathogenesis of fever
- the hypothalamus controls temperature regulation
- fever occurs when the thermoregulatory mechanism is altered to maintain a body temperature above normal
1) bacterial toxin (endotoxin) cause monocytes/macrophages/kupffer cells to release cytokines (interleukins, TNF)
2) cytokines act as endogenous pyrogens and activate the pre-optic area of the hypothalamus
3) hypothalamus causes the release of PGE2, which increases the temperature set point
4) hypothalamus then regulates heat generating behaviours
What is the body’s response to hot and cold environments
mechanism activated by cold: posterior hypothalamus
-increased heat production = shivering, hunger, increased voluntary activity, increased A and NA release
-decreased heat loss = cutaneous vasoconstriction, curling up, horripilation
mechanism activated by heat: anterior hypothalamus
-increased heat loss = cutaneous vasodilation, sweating, increased respiration
-decreased heat production = anorexia, apathy, inertia
How is heat lost from the body
radiation and conduction (70%), vaporisation of sweat (27%), respiration (2%), urination and defecation (1%)
How is thirst regulated
-thirst is mainly controlled in the hypothalamus diencephalon
increased thirst:
- hypertonicity of plasma sensed by osmoreceptors in anterior hypothalamus
- hypovolaemia sensed by baroreceptors in heart and blood vessels
- physiological stress
decreased thirst:
- hypothalmic disease
- altered mental state
- damage to diencephalon
What is the purpose of the reticular activating system
- regulates respiratory, cardiovascular and endocrine functions
- increases consciousness and alert state and heightened sensory perception
- sends signals to the thalamus, which is a gateway to the cerebral cortex
- location: mid-ventral portion of the medulla, midbrain and 4th ventricle
A nerve fibre types
alpha - somatic motor and proprioception
beta - touch/pressure
gamma - motor to muscle spindle
delta - fast pain response, temp (cold)
