Neurobiology Theme 3 - Part 2

  • Created by: Splodge97
  • Created on: 23-05-17 18:04
Describe the layers of the retina
Outermost is epithelia with a black pigment to absorb light (preventing reflection/double vision). Layer of photocells above this, connected to bipolar cells which project to retinal ganglion cells (transmit to optic tract).
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What are the modulating cells of the retina?
Amacrine cells (activate/deactivate bipolar cells dependant upon light patterns) and horizontal cells (modulate the signals recieved by photoreceptors); both allow complex sight
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How is light transduced?
In dark membranous discs have cGMP opening Na+/Ca2+ channels (some K+ hyperpolarising); depolarisation so IPSP's to bipolar cells. In light pigments change; transducin hydrolyses cGMP, Na+/Ca2+ channels close (K+ hyperpolarisation stops IPSP's).
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Describe the optic pathway
Opric nerve from eye passes through optic canal and optic chiasm to form the optic tract. Synapses in lateral geniculate nucleus and reaches to superior colliculus (subconscious processing) then primary visual cortex.
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What are the different parts of the retina?
Temporal = where light hits lateral side, travels ipsilateral. Nasal = where light hits medial side, decussates in optic chiasm. Means light from right only activates left cortex. Also macula processing central vision.
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Whar visual field deficits may occur?
Bilateral temporal hemianopia = damage to both nasal retina leads to loss of 1/2 visual field on lateral sides (when lesion in optic chiasm). Homonymous hemianopia = 1/2 visual field lost as sight only in one eye (as damage to opposite optic tract).
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What causes blind sight?
Subconscious visual processing in superior colliculus and passage of the dorsal/ventral streams through visual association areas means those with damaged primary motor cortex can still avoid objects/turn to objects at the corner of their eye.
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What is the pupillary light reflex?
Light into one eye causes both irises to contract (via direct and consensual light reflexes). Retinal fibres from stimulated eye to pre-tectal area then Erdinger-Westphal nucleus (activates both ciliary body muscles).
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What is the accomodation reflex?
Causes bilateral eye convergence and pupillary constriction to focus on objects moving to us. Retinal fibres to the primary motor cortex then back to pre-tectal area; gives branches to Erdinger-Westphal nuclues and oculomotor nucleus.
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How do vibrations of the basilar membrane differ?
Vibrate more at base in high frequency, more at apex in low frequency (so frequency can be mapped by primary auditory cortex's tonotopic organisation).
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What occurs in the Organ of Corti during sound transduction?
Four rows of specialised sterocilia are moved forwards and back by the tectorial membrane (as the basilar membrane vibrates). Causes mechanically gated K+ channels to open in all stereocilia layers; Ca2+VGC's open, influx causing dopamine to VIII.
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What are the different types of hair cells in the organ of Corti?
Inner hair cells (95%) have sensory function, outer hair cells (5%) modulate the movement of the basilar membrane by causing it to loosen/tighten (adapting its sensitivity)
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Describe the auditory pathway
Unilateral activation of VIII, extends to dorsal/ventral cochlear nuclei; then bilateral (localises - furthest hemisphere last). To superior olivary nuclei, inferior colliculi (subconscious), medial geniculate nuclei then primary auditory cortices.
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What occurs between the hair cells of the Organ of Corti and VIII?
Ganglia of the auditory sensory organs
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What is the auditory stream?
Passes from the primary auditory cortex to the association auditory cortex (more complex processing), Wernicke's then white matter arcuate fasciculus to Broca's, then extends to motor cortices.
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Describe the structure of the ampullae
Hair cells with a cupula (connective tissue layer) atop, move in direction of endolymph flow (in opposite way to body's rotation). As hair cells move K+ channels open, opening Ca2+ VGC's so domamine released from vesicles to activate VIII.
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What are the otolith organs? How are they arranged?
Utricle and saccule; modulate head movements (sense linear head movements in relation to gravity) and acceleration/decelleration. Sensory epithelium (macula) has hair cells covered by gelatinous cap (othliths atop), moves minutely with position.
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Describe the vestibular pathway
Ampullae send fibres via VIII to vestibular nucleus; synapse before projecting to nuclei of III, IV and VI via medial longitudianl fasciculus. Saccule projects to vestibular nucleus, then cerebellum and spinal cord (utricle only spinal cord).
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What are the requirements for voluntary movement?
Task identified by frontal/limbic plus auditory/visual (from posterior parietal). Then descision making in pre-frontal, integration in pre-motor, planning/processing in supplementary/basal ganglia and execution in primary motor (via corticospinal).
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What are the inputs/outputs of the pre-motor cortex?
Recieves from ventral anterior and ventral lateral nuclei of the thalamus (via striatum). Projects to many areas of cortex (partially giving corticospinal tract).
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What are the inputs/outputs of the supplementary motor cortex?
Recieves from ventral anterior and ventral lateral nuclei of the thalamus (via striatum). Gives integrated outputs to the corticospinal/corticobulbar tract, reticular formation and primary motor cortex.
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What are the inputs/outputs of the posterior parietal cortex?
Recives from the visual (primary and association), auditory and somatosensory cortices. Feeds internal model of required movement to the motor system.
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Describe the course of the dorsal (magnocellular) stream
From primary visual cortex through visual association areas V2 and V3 into posterior parietal cortex; allows visuospatial recognition of stimulus (where it is). Also accepts from Broca's, passing impulses to the motor areas for speech coordination.
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Describe the course of the ventral (parvocellular) stream
Passes from primary visual cortex through visual association area V4 to the posterior parietal cortex (allowing recognition of stimulus features/what it is). Also accepts from primary auditory cortex and Wernicke's for semantics/speech understanding.
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What are the different elements of the basal ganglia?
Striatum (putamen and caudate nuclei - recieves from cortex and pars compacta), substantia nigra (pars compacta and pars reticularis, contains black pigment (dopamine precursor)), globus pallidus (internal and external, pale) and subthalamic nucleus
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What is the direct pathway of the basal ganglia?
Pars compacta gives EPSP's to striatum; passes IPSP's to interior globus pallidus and pars reticularis (inhibit the spinal cord and thalamus) so motor function is increased.
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What is the indirect pathway of the basal ganglia?
IPSP's from pars compacta to striatum, gives IPSP's to exterior globus pallidus. Reduces IPSP's to subthalamic nucleus, activating it to give EPSP's to interior globus pallidus and pars reticularis (increasing motor function after a time as longer).
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What occurs in Parkinson's to reduce movement?
Fibres of pars compacta undergo cell death (so black pigment fades). No IPSP's at start of indirect, so greater inhibition of EGP and then lesser inhibition of SN; more EPSP's to IGP/PR. No EPSP's at start of direct, reducing IGP/PR inhibition.
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What occurs in Huntingtons to facilitate excessive movement?
Dominant genetic disease, causes cholinergic neurone death between striatum and EGP. Inhibition of SN increased, decreasing EPSP's to IGP and PR; as they aren't activated to inhibit spinal cord/thalamus, movement excessive/hard to stop.
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What are the functions of the cerebellum?
Functions in posture and balance, updating ongoing movements, initiation/planning of movement and motor learning. May also predict movement (as recieves from posterior parietal cortex and inferior olivary nucleus).
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Describe the anatomy of the cerebellum
Has an anterior lobe and a much larger posterior lobe (separated by primary fissure), small flocculonodular lobe (superior to posterolateral fissure) and vermis down centre (contains deep cerebellar nuclei). Folia show rostro-caudal organisation.
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Why are cerebellar pathways always ipsilateral overall?
Decussate towards and away from the cerebellum (so cancels)
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Describe the corticopontine tract
Runs similarly to corticospinal/corticobulbar tracts (from the frontal lobe, pre-motor and posterior parietal cortices). Descends to ipsilaterally synapse in the pontine nucleus.
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Describe the pontocerebellar tract
From corticopontine tract; formed in pontine nucleus, projects to purkinje cells of contralateral cerebellum. Allows precision and efficiency of movement.
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Where are the projections from the cerebellar purkinje cells?
Project to the deep cerebellar nuclei and then the contralateral thalamus (cortex then sends efferents down corticospinal tract); this allows planning/prediction. Or, pass fibres to the contralateral red nucleus (then thalamus) to signal movement.
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How do the purkinje cells function in memory of repeated movement?
Cause long term depression/excitation of the impulses involved in the movements
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Describe the spinocerebellar tract
Involved in modulating ongoing movement. Projects from spinal cord to purkinje cells (providing proprioceptive/exteroceptive input). Direct it to the contralateral red nucleus via the deep cerebellar nuclei. Synapses, then sent back down spinal cord.
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What is the cuneocerebellar tract?
Provides proprioceptive/exteroceptive input to the cerebellum from the head and neck
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Describe the olivocerebellar tract. What is its role?
Recieves proprioceptive inputs from collateral branches of the rubrospinal tract to the inferior olivary nucleus. This projects climbing fibres to the contalateral cerebellum so they may re-enter the pathway to the red nucleus (allows modification).
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What are the defined functions of the olivocerebellar tract?
Movement initiation, descision making and motor learning
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What is the vestibulocerebellar pathway?
Provides input to the cerebellum from ampullae via the vestibular nucleus to facilitate balance
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What is the tectocerebellar pathway?
Gives input to the cerebellum from the auditory and visual systems via the tectum for balance maintenance
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What are mossy fibres?
Fibres (other than climbing fibres of the olivocerebellar tract) which enter through the cerebellar peduncles to modulate movement
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What is the vestibulocerebellum?
Floculonodular lobe and part of vermis - functions in posture/balance. Recieves from vestibular nucleus and spinocerebellar tract, outputs via vestibular nuclei. Lesions cause staggered gait (fall ipsilaterally) and nystagmus (flickering gaze).
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What is the spinocerebellum?
Rest of vermis, deep nuclei (except dentate), intermediate hemispheres. Updates ongoing movement. Recieves from primary motor cortex, spinocerebellar tract and olivary nuclei. Outputs to red nucleus. Lesions cause ataxia (walking issues) and tremors.
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What is the neo(cortico/cerebro)cerebellum?
Lateral hemispheres and dentate nucleus - initiation/planning and motor learning. Recieves from posterior parietal cortex and inferior olivary nucleus, outputs to pre/supplementary motor cortices and thalamus.
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What results from lesions to the neocerebellum?
Slow movement onset, dysmetria (overshooting/undershooting), dysdiadochokinesis (inability to perform rapid alternating movements), rebound phenomenon (limbs move once pressure removed) and tremors.
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What comprises the control of mastication?
Normally controlled by CPG but non-essential cortical control may be stimulated. First the opening mastictory muscles, then the closing ones are activated (muscles of the tongue, hyoid and cheek also acting).
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How does the masticatory CPG operate?
Muscle spindles of masticatory muscles sense stretch when food voluntarily input; send impulses via mesencephalic root of V. Period of reflex control, then CPG in pontine reticular formation (signals premotor neurones/motor nucleus of V to start).
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How is mastication altered?
Once ongoing sensory input from pressure receptors of PDL and tongue/muscle spindles/receptors of TMJ feed into reticular formation to adjust (or signal swallowing). Also recieves from fibres of jaw-unloading reflex to adjust force to food type.
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How is swallowing controlled?
Receptors activate the frontal swallowing centre; switches off masticatory CPG and projects to descending brainstem pathways. Lateral CPG in solitary tract nucleus (and surrounding RF), then medial CPG in nucleus ambiguus (and surrounding RF).
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How does the medial swallowing CPG act?
Activates motor nuclei of V, VII and XII, nucleus ambiguus and dorsal motor nucleus of X. Send efferents to muscles then mucosa of oral cavity, oesphagus, pharynx and soft palate, initiating. Feedback to frontal swallowing centre/lateral CPG adjusts.
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What are the actions and components of the limbic system?
Comprised of hippocampus, hypothalamus, amygdala, thalamus and cingulate gyrus. Important in homeostasis, works to control emotional behavoiur and motivational drives (operate in extreme circumstances when physiological homeostasis insufficient).
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Describe the Papez circuit of the limbic system
Cingulate gyrus recieves from association cortices, signals the hippocampus. May signal the amygdala (innervates hypothalamus then prefrontal cortex) or mamillary bodies (stimulate anterior thalamic nuclei, feedback to cingulate gyrus).
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What are the roles of the hypothalamus?
Controls vegitative and endocrine functions; vital in homeostasis. May signal anterior thalamus or limbic system (modulating behaviour), the reticular system (influencing the ANS) or the pituitary gland (modorating endocrine function).
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What role does the hypothalamus play in sexual behaviour?
In females oestrogen/progesterone binds to the ventromedial hypothalamus; in males testosterone binds to preoptic region. Its sexual dimorphic nucleus influences preference (smaller in hetrosexual females and homosexual males).
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What role does the hypothalamus play in temperature regulation?
Hot and cold temperature receptive neurones in the preoptic region. If cold neurones stimulated trigger TSH (so more metabolic heat via thyroxine), vasoconstriction and behaviour (put on a jumper).
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What role does the hypothalamus play in water homeostasis?
Creates thirst stimulation in lateral hypothalamic area and releases ADH via its supraoptic nuclei in response to high blood osmolarity.
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What role does the hypothalamus play in hunger?
Signals from stretch receptors along GI tract initiate long reflexes via the mammillary bodies which activate the satiety centre (in ventromedial nucleus) and switch off the hunger centre (in lateral hypothalamic area). Links with reward system.
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What hormones act upon the hypothalamus to regulate hunger?
Grehlin (activates hunger, inactivates satiety) and leptin (inactivates hunger, activates satiety)
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What role does the hypothalamus play in circadian rhythms?
Regulated via its suprachiasmic nucleus (which operates faster than circadian rhythms modulated by specific organs as quicker to adapt). Modulates cycles for sleep, food, temperature and hormone release.
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What is the amygdala? What role does it play?
Processes a large amount of new information; assesses these inputs for significance, initiating hypothalamus to generate emotional memory. Also involved in fear conditioning (fear memory to induce fear response without stimulus recognition).
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Describe the role and course of the reward pathway
Responsible for wanting/liking/positive reinforcement. Info from receptors sent to ventral tegmental area. Signals amygdala to initiate memory in the hippocampus (when signals premotor cortex and nucleus accumbens to modify motor so focus on reward).
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What is the reticular formation?
Net-like structure passing through pons and medulla at back of brainstem; various nuclei present as condensed neurones. Works with limbic system/hypothalamus in homeostasis, coordinates cranial nerve nuclei to generate CPG's and activates the cortex.
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Where does the reticular formation recieve inputs from/give inputs to?
Recieves from spinal cord, cranial nerves, cerebellum and forebrain. Outputs to other brainstem nuclei, the spinal cord and cerebral cortex.
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Describe the midline zone of the reticular formation
Contains raphe nuclei, gives ascending projections for arousal/attention and descending projections to control muscle tone/respiration/BP
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Describe the medial zone of the reticular formation
Contains long magnocellular neurones (projecting to the cortex). Gives ascending projections for arousal/attention and intrinsic connections controlling the eyes/swallowing/brainstem reflexes.
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Describe the lateral zone of the reticular formation
Contains short parbocellular neurones projecting to the brainstem (to control the cardiorespiratory centres). Gives intrinsic connections (control eyes/swallowing/brainstem reflexes) and descending projections (control muscle tone/respiration/BP).
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What is the reticular activating system?
All ascending fibres from the reticular formation to the cortex, operates to keep us awake; can influence consciousness this way as its nuclei project to all cortical areas
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How does the locus coeruleus act as part of the reticular activating system?
Releases nor-adrenaline (promotes wakefulness), giving fibres to the limbic system and prefrontal cortex
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How does the raphe nucleus (many raphe nuclei and nucleus raphe magnus) act as part of the reticular activating system?
Releases serotonin (inhibitory/promotes sleep) to frontal/cerebral cortices and limbic system. Also influences mood/cognition (low in depression). Gives descending fibres to cerebellum/spinal cord to modulate pain (activates IPSP interneurones).
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How does the ventral tegmental area act as part of the reticular activating system?
Releases dopamine (promotes sleep or wakefulness). Fibres projecting from it constitute the reward pathway (and are stimulted in addiction).
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How do the basal forebrain (nucleus of Mynert - not RS) and pontomesencephalic nucleus act as part of the reticular activating system?
Release cholinergic fibres to induce wakefulness (by projecting to whole area of cortex)
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What is REM (rapid eye movement) sleep?
Complete paralysis of all voluntary muscles except eyes (as pontine RS signals spinal cord) occurs so don't act out dreams. Observed as beta rhythms, indicating cortical activity. Paralysis makes vulnerable so woken by slight sounds (not shaking).
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What is NREM (non-REM) sleep?
Doesn't cause muscle paralysis, does reduce temp and HR. Delta rhythms occur characteristic of sleep. Can be woken easily by shaking but not by sound.
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How does the reticular system function in habituation?
Allows adaptation to your environment so not constantly alerted to regular/non-threatening stimuli. Reticular system prevents activation of the cortex in this instance.
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