Neurobiology Theme 3 - Part 1

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  • Created by: Splodge97
  • Created on: 23-05-17 14:21
What do skin receptors supply?
Low threshold mechanoreceptors/nociceptors which supply somatic general senstion (tactile, thermal and pain reception)
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Describe hair follices
Unencapsulated A-beta (type II) neurones wrapped around the base of hairs. Sense motion and direction (which cause mechanical changes in their membrane proteins)
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Describe free nerve endings
A-delta (encapsulated) or C (unencapsulated) nociceptive fibres
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Describe Messiner's corpuscles
Encapsulated nerve endings, shape change to proteins of A-beta axon in response to fluttering/tapping (5-40Hz). Small receptive field, >100/cm^2 in finger.
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Describe Merkel cells
Single cells around end of A-beta axons, respond to chemicals binding to them to give touch/pressure sensation. Unencapsulated, small receptive field, 70/cm^2 fingertip.
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Describe pacinian corpuscles
A-beta fibres surrounded by coiled connective tissue. Skin deformation by vibrations (60-300Hz) causes deformation of connective tissue layers, opening mechnoreceptive Na+ channels (cause a generator potential). Large receptive field, 20/cm^2 finger.
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Describe Ruffini endings
Largely encapsulated A-beta fibres which sense stretch. Large receptive fields, only 5/cm^2 fingertip.
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Give examples of slow adapting receptors. How to they act?
Merkel cells, Ruffini endings, some hair follicles. Fire constant tonic AP's, stimulus causes increase in firing rate (lasts for stimulus duration). May undergo temporal summation so AP in 2nd order neurone.
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Give eamples of rapid acting receptors. How do they act?
Messiner's corpuscles, Pacinian corpuscles, some hair follicles. Respond to transient/phasic/vibratory stimuli, don't fire tonic AP's but produce AP's on initial stimulus (cease on continual stimulation). High AP frequency may cause AP in 2nd order.
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Which area of the body has the lowest discriminatory ability by the CNS?
Legs and back as fewer 1st order neurones, have larger receptive fields (unlike in fingertip)
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What is convergence?
Many 1st order neurones synapse to same 2nd order; can't determine which 1st order was initially stimulated, but allows multisensory processing (modalities to modulate each other) and detection of smaller stimuli (via spatial summation)
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What is divergence?
Amplifies signals as many 2nd order neurones branch from 1st. Causes large response across many modalities but larger stimulus required
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What is lateral inhibition?
Receptor closest to stimulus has greatest increase in AP frequency; leads it to have greater inhibitiory effect (via interneurones from its synapse with 2nd order). Central neurone inhibited less so its 2nd order forms (larger) AP.
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What are the benefits of lateral inhibition?
Sharpens sensory inputs by creating contrast between inputs from the same receptive field (so the stimulus can be pin-pointed); occurs in eye to create contrast
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What is the difference between conscious and subconscious proprioception?
Conscious = delivered to CNS via dorsal column medial lemniscuc from mucsle spindles/Golgi tendon organs/joint receptors. Subconscious = performed by spinal cord and cerebellum.
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What is dismitria?
Misunderstanding of distance between objects
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How are motorneurones sub-divided?
In ventral horn as tensors/extensors. May be apha (initiate skeletal muscle contraction) or gamma (activate intrafusal fibres to muscle spindles, change length so tension constant). Alpha and gamma co-located (travel together to same muscle).
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What are the different classifications of musculature in the ventral column of the spinal cord?
Distal = controls fine movements (as in fingers). Proximal = controls body position (so controls shoulders). Axial = controls head and neck position.
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What is a myotactic (stretch) reflex?
Ia afferents from muscle spindles synapse in ventral horn and activate alpha-motorneurones and y-motorneurones.
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What is an inverse stretch (Ib tendon) reflex?
Ib afferents from Golgi tendon organs synapse in ventral horn with interneourones, release IPSP's to alpha-motorneurones. Occurs after strech reflex (since slower as two synapses). Prevents continual contraction whilst activating antagonist muscle.
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Why can neither the myotactic or inverse stretch reflex be singular?
Singular myotactic reflex = overresponse or dangerous (sometimes better to resist reflex). Singular inverse stretch = under-response to stimuli.
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What is the crossed-extensor reflex?
When one leg subject to pain; pained lifts, other to extends. Type III fibres to dorsal horn at knee, branch (one into interneurones, other to hip). One interneurone gives EPSP's (to flexor of pained/extensor of unharmed), other opposite EPSP's.
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What are central pattern generators?
Involved in programmed movements (walking, chewing, swallowing); coordinates them without constant sensory feedback (reflexes enter subconscious pattern). Two processes interact so each sequentially increases/decreases, makes cyclic.
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What is the termination thalamocortical pathway?
Nuclei pass from the thalamus to the cortex through the white matter tract of the internal capsule (forming part of the corona radiata)
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Where do superior parietal areas recieve from?
Head, neck and upper arm more laterally, legs feet and genitals around the central sulcus
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Where do the posterior occipital and parietal lobes recieve from (present laterally in the horizontal brain cross-section)?
The hands, eye, face and tongue
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What is columnar organisation of the cortex? Why is it beneficial?
Different columns deal with different areas/modalities/functions. Innervated by alternating rapid and slow adapting neurones so different receptors in the same field are processed in the same part. Tightly packed (lots of info in small area).
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Describe the lateral corticospinal tract
Descending pathway providing voluntary control to distal musculature; formed when 80% of fibres from corticospinal tract (from frontal and motor cortices - primarily primary motor) decussate at the pyramids
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Describe the anterior corticospinal tract
Descending pathway providing voluntary control to the proximal musculature, formed by 20% of corticospinal fibres remaining ipsilateral after the pyramids (decussate at relevant level of spinal cord)
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Describe the corticobulbar tract
Form the corticospinal tract takes before reaching the medullary pyramids; however, it passes through the internal capsule to the motor nuclei of V, VII, XI or XII (where they synapse directly with LMN's) or the reticular formation.
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How does the corticobulbar tract supply the cranial nerves?
Bilateral innervation to V and branches of VII to the upper face (to cause bilateral paralysis if damaged). Contralateral innervation to VII branches to lower face. Ipsilateral branches to IX. Contralateral to XII except genioglossus.
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Where do III, IV and VI (innervate extraocular muscles) recieve their contralateral innervation from?
Frontal and parietal eye fields
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How can the ipsilateral innervation of genioglossus by XII be used in diagnosis?
UMN lesions cause tongue to project contralaterally, LMN lesions cause ipsilateral projections
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Describe the reticulospinal tract
Regulates flexor reflexes and initiates patterned activity. Present in both the lateral and ventral funiculi as some fibres decussate higher (shift from ventral to lateral). Formed from efferents of sensory, pre-motor and supplementary cortices.
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Describe the pontine (medial) reticulospinal tract
Originates from caudal and oral pontine nuclei of the reticular formation. Enhances antigravity reflexes (maintaining upright posture).
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Describe the medullary (lateral) reticulospinal tract
Originates from gigantocellular nucleus in the medullary portion of the reticular formation. Liberates the anti-gravity muscles from reflex control (allowing cortical control).
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Describe the rubrospinal tract
Descending pathway from the red nucleus of the midbrain (specifically the tectum). Excites flexors and inhibits extensors. Gains most inputs from cerebellum; these are involved in the regulation of learned movement.
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Describe the vestibulospinal tract
Descending pathway from the vestibular nucleus (which recieves from VII and the cerebellum). Lateral controls the antigravity muscles, medial coordinates head movements and fixes gaze. Also moves body towards sound. Most input from spinal reflexes.
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Describe the tectospinal tract
Descending pathway from the superior colliculus (at roof of brainstem above cerebral aqueduct). Decussates in brainstem to terminate in cervical areas. Orientates the body according to visual stimuli.
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What is the olivospinal tract?
Supposed descending pathway (may be integrated with another). Originates from the olives to give proprioceptive input to the cerebellum.
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How is the white matter arranged in the spinal cord?
Split into dorsal, lateral and ventral funiculi by the dorsal and ventral horns of the grey matter
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What is present in the dorsal funiculus?
Dorsal column (as cuneate/gracile funiculus)
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What is present in the lateral funiculus?
Ascending = posterior and anterior spinocerebellar tracts, anterolateral system. Descending = lateral corticospinal tract, rubrospinal tract, 1/2 reticulospinal tract.
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What is present in the ventral funiculus?
Descending = 1/2 reticulospinal tract, anterior corticospinal tract, vestibulospinal tract, olivospinal tract. Ascending = small part of anterolateral system.
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What is the internal capsule?
White matter tract bordered by grey matter (thalamus and caudate nucleus). Its ascending/descending pathways form the corona radiata. Stroke likely as supplied by small arteries easily blocked (causes LACS). Central so impacts rare.
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What occurs upon damage to UMN's which synapse directly with LMN's?
Babinski's sign (toes outwards on tickling), paralysis and pariesis (muscle weakening)
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What occurs upon damage to UMN's which synapse in brainstem nuclei (then project to LMN's)?
******* paralysis (muscles remain fixed and tense) and hyperreflexia (overresponse to stimuli) - may also be loss of balance/posture/equilibrium/gait.
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What occurs upon damage to LMN's?
Paralysis, pariesis, muscle wasting, loss of reflexes, fibrillations (muscle spasms) and fasciculations (small involuntary/uncontrolled twitches).
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What is the axial lamina?
Develops the sensory parts of the CNS (dorsal horn in spinal cord, migrates laterally in the brain (why sensory nuclei lateral))
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What is the basal lamina?
Develops into the motor parts of the CNS (ventral horn in spinal cord, migrates medially in brain (why motor nuclei central))
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What are the motor cranial nerve nuclei calssifications?
Somatic = innervates structures from somites (eye and tongue). Branchiometric = innervates structures from brachial arches (pharynx/larynx). Autonomic = sends parasympathetic/sympathetic fibres.
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What are the sensory cranial nerve nuclei classifications?
Visceral = recieves from the viscera. Somatic = recieves pain, touch, proprioception and temperature. Special = recieves from the special senses (hearing and balance).
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What are the afferent cranial nerve nuclei fibre types?
General somatic = GSA, recieves from skin, skeletal muscle, joints and bones. Special somatic = SSA, recieves vision, hearing and balance. General visceral = from viscera. Special visceral = taste plus touch, temp and proprioception from viscera.
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What are the efferent cranial nerve nuclei fibre types?
Special visceral = SVE, innervates skeletal muscle from the brachial arches. General somatic = GSE, innervates skeletal muscle from somites. General visceral = innervates smooth muscle and glands.
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Which cranial nerves carry parasympathetic fibres?
Oculomotor (to eye), facial (to sublingual and submandibular via the corda tympani), glossopharyngeal (to parotid) and vagus (main parasympathetic to the body viscera)
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What is the role of the nucleus of the trigeminal nerve?
Integrates all trigeminal nerve pathways via its spinal root (recives pain, continuous with Rexed's lamina II and III), chief nucleus (recieves fine touch, all nuclei except proprioceptive) and mesencephalic root (recieves proprioception).
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Where does VII give pain sensation from?
External auditory meatus
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Where does X give pain sensation from?
Meninges towards the back of the cranium (where V not present), ear canal and mucosa of the larynx
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Where does V give pain sensation from?
Face, oral cavity and nasal cavities
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Where does IX give pain sensation from?
Posterior 1/3 of tongue and back of the pharynx
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Where does cervical plexus give pain sensation from?
Neck and the meninges of the posterior cranial fossa
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Where do the main pain projections (from the spinal root of V) go to?
Primary somatosensory cortex (where pain awareness processed) and the insular cortex (deep in lateral fissure) and anterior cingulate cortex (coordinate emotional response to pain and pain empathy)
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Why do pain afferents give collateral branches to the reticular formation (from spinal root of V to the cortex)?
Integrates pain afferents to allow CPG generation as well as produce smaller pain pathways (such as fast trigemino-reticular pathway, delivers pain to cortex after processing)
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What occurs in the corneal reflex?
Cornea touched with a wisp of cotton. V1 signals spinal root of V, leads to impulses from motor mucleus of VII to orbicularis oculi and lavator palpebrae superioris (causing bilateral blink).
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What occurs in the gag reflex?
Back of oropharynx touched. IX signals the nucleus of the solitary tract, impulses sent down efferents from the nucleus ambiguus along X. Palate raised and pharynx contracted.
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What occurs during the jaw jerk reflex?
Rapid closure of mouth upon tapping of ramus of mandible. Exaggerated upon stroke/UMN lesion. Fibres from muscle spindles of masticatory muscles enter mesencephalic root of V. Signals efferent fibres from motor nucleus of V so masseter contracts.
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What occurs during the jaw unloading reflex?
Prevents teeth clashing. Proprioception info from pressure receptors of PDL upon loss of pressure signals mesencephalic root of V then motor nucleus of V to cause masseter contraction so jaws don't collide.
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What is the masseter-hypoglossal reflex?
Prevents damage to tongue - inhibits tongue muscles when masseter active and contraction of masseter when tongue protruded
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Card 2

Front

Describe hair follices

Back

Unencapsulated A-beta (type II) neurones wrapped around the base of hairs. Sense motion and direction (which cause mechanical changes in their membrane proteins)

Card 3

Front

Describe free nerve endings

Back

Preview of the front of card 3

Card 4

Front

Describe Messiner's corpuscles

Back

Preview of the front of card 4

Card 5

Front

Describe Merkel cells

Back

Preview of the front of card 5
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