Mechanoceptors

(names, adaption, receptive field)

• Merkel - pressure, slowly adapting (static stimulation), small receptive field
• Ruffini - stretch receptor, slow adapting, large receptive field
• Meissner - light touch, rapidly adapting, small receptive field
• Pacinian - high frequency vibration, rapidly adapting, large receptive field 
• Free nerve endings - detection of touch, pressure and stretch, can be slowly or rapidly adapting 

Nociceptors

(types and peripheral potentiation)

• Thermal - noxious heat or cold, TRPV1/2 for heat over 42C, TRPM8 - detection of cold stimuli
• Mechanical - free nerve endings detect high intensity mechanical stress 
• Polymodal - contains many receptors 
• Bradykinins - activation of G protein BK-Rs, tigger release of prostaglandins (potentiate), and histamine (pain potentiation), 
• IL and TNF increase NGF production to increase ion channel expression, and transcription of BDNF to potentiate dorsal horn neurons 

• All non-painful stimuli conveyed by myelinated Aβ (second fastest), except free endings - conveyed by myelinated Aδ
• Dedicated thermal and mechanical nociceptors carried by Aδ, polymodal fibres are non-myelinated C fibres

• Aδ fibres terminate on laminae I and V
• C fibres terminate on laminae I and II
• Aβ fibres terminate on laminae III and IV
• Secondary neurons extend from II spinothalamic tract (Aδ and C pain stimuli), as well as some secondary neurons with cell bodies in laminae V (Aδ) extending dendrites into IV and III - spinoreticular tract (Aδ)
• Interneurons in laminae I, II and III

• Head and neck - C2, C3
• Arms - C4/5/6/7/8/T1
• Trunk - T2-L1
• Legs - L2-S2
• Biceps - C5, triceps C7, Knee L3/4, ankle S1

• Photon excites rhodopsin, hyperpolarises cell through G-protein action of cGMP generation to close sodium channels to prevent depolarisation 

• In ganglion cells, P-type cells are smaller and receive information from small receptive fields and can transmit colour stimuli (R⁺G^-
• M-type cells have larger receptive fields and only sensitive to low-contrast stimuli
• NonM-NonP neurons can transmit B⁺Y^- stimuli

• Contralateral input from nasal retina, ipsilateral input from temporal retina to give contralateral visual field
• Layer I = contralateral M type (magno)
• Layer II = ipsilateral M type (magno)
• Layer III = ispilateral P type (parvo)
• Layer IV = contralateral P type (parvo)
• Layer V = ipsilateral P type (parvo)
• Layer Vi = contralateral P type (parvo)
• Interspersed are the koniocellular layers

• Magnocellular - synapses in IVCα, project to IVB, where left and right stimuli combine
• Parvocellular - project to IVCβ, project to layer II and III blobs, left and right combine
• Koniocellular - project to IVCβ or directly to layer II and III blobs

• Myopia - blurred distance vision as image focused in front of the retina, therefore concave lens reduces the power of the eye's optics, focusing on retina
• Hypermetropia - blurred close vision as image focused behind retina, therefore convex lens increases the power of the eye's optics, focusing on retina
• Astigmia - refractive error meaning light does not focus evenly on retina

• Ventral stream - V1-2-4-IT (form recognition and object representation)
• Dorsal stream - V1-2-DM-MT (motion, object location, eye control)

• Perilymph is much the same as extracellular fluid
• Endolymph - [sodium] = 1mM, [potassium] = 150mM, [calcium] = 0.02mM, [glucose] = 0.5mM, potential = 80mV

Auditory pathway

(nuclei and decussations)

• Cochlea --> cochlear nucleus (ipsilateral) --> superior olive (biaural) --> inferior colliculus (biaural) --> medial geniculate body (biaural) --> auditory cortex

• Mechanical stretch of tiplink opens cation channel, depolarisation of membrane, TMC1 and 2 channels open and potassium influx
• Potassium dependent calcium channels opened, allows glutamate release from basal membrane and opening of calcium gate potassium channels to allow potassium efflux 

• Otoliths - head position and linear acceleration 
• Posterior canal - senses tilt of the head towards shoulder
• Superior canal senses rotation of the head from front to back (nod), horizontal canal senses rotation to left or right (shake)

Vestibular pathways

(vestibulospinal, -cerebellar, -ocular, - cortical)

• Hair cell --> lateral, medial and inferior vestibular nuclei, the through lateral (limb and trunk) and medial (upper back and neck) vestibulospinal tract
• Inferior nucleus to flocculonodular lobe (inferior cerebellar peduncle) of cerebellum
• Inferior nucleus to (medial longitudinal fasciculus) oculomotor nucleus, abducens nucleus and trochlear nucleus
• Superior and lateral nucleus to the ventroposterior nucleus (medial lemniscus) then to vestibular cortex via internal capsule

• Stabilisation of gaze through the reciprocal inhibiton and excitation of the two nuclei vestibularis. This then activates the abducens nucleus on one side and the oculomotor nucleus on the other, and vice versa for inhibition
• This means that both eyes will move in a direction through activation of the rectus medialis on one eye and the rectus lateralis on the other
• Vestibulospinal reflex has a similar effect on the antagonistic pairs of muscles in response to changes in gravity 

• Olfactory receptors (GPCR) - only one type expressed per neuron. Binding of odorant activates Gα_olf, activation of ACIII, increased production of cAMP, opening of cyclic nucleotide gated channels, calcium influx and depolarisation, opening of calcium gated chloride channels, further depolarisation and action potential propagation 
• Taste buds contain taste receptor cell with afferent nerve to ST complex
• Salt receptor - ENaC channel mediated, sour receptor - H⁺ stimulus through PKD2L1/3 (TRPP) channel
• Sweet, bitter and salty receptors are G protein coupled - activation of PLC, increased calcium allows Na entry through TrpM5 and therefore neurotransmitter release. Sweet/umami used T1R, bitter uses T2R GPCRs

• Olfactory bulb --> olfactory tract 
• Medial olfactory tract -->septal nuclei and to contralateral olfactory bulb via anterior commissure
• Lateral olfactory tract --> 1y olfactory cortex --> limbic system (entorhinal cortex, hippocampus, amygdala, hypothalamus)
• Lateral olfactory tract also --> mediodorsal thalamus --> orbitofrontal cortex (2y olfactory cortex)
• Gustatory information carried through facial (VII) nerve (anterior 2/3) and glossopharyngeal (IX) nerve (posterior 1/3)
• Project to nucleus of solitary tract, second order neurons to ventral-posterior nucleus of thalamus, information passes to 1y taste cortex (insula/opercula) and 2y taste cortex (OFC)

• Inputs from 'where' streams from visual, somatosensory, auditory, motor and limbic cortical pathways
• Outputs to frontal cortex, basal ganglia and cerebellum
• Allows multimodal association for direction of attention, movement, speech and reading
• Prefrontal cortex - reciprocal inputs from limbic, visual and basal ganglia - functions in working memory, intention and decision