3 Groups of Somatosensory Pathways

• dorsal colum/medial lemniscus
• anterolateral pathways
• spinocerebellar

Dorsal Column/Medial Lemniscus system

(DCMLS)

• discriminative touch (2 point discrimination)
• pressure
• vibration
• conscious proprioception (position sense)

Neurons in the

Dorsal Column/Medial Lemniscus system

• 1st order cells in dorsal root ganglion(DRG): ascend via dorsal columns (DRG are outside spinal cord)
• 2nd order cells in dorsal column nuclei(n gracilis, n cuneatus): cross midline and ascend in medial lemniscus
• 3rd order cells in ventral posterior lateral(VPL) nucleus of the thalamus

Sensory Homunculus and blood supply

• supplied by the ACA and MCA
• ACA supplies medially and tends to deal with the leg foot and toes
• MCA supplies the lateral surface of brain and affects face hands and upper body

Higher order processing

(somatosensory)

• cortex combines information from different receptor types to form higher order perceptions(size, texture, shape)
• exaples of deficits: difficulty discriminating texter, or size/shape of objects
• agraphesthesia: inability to identify letters drawn on the skin
• astereognosia: inability to name objects held against skin

Projections of somatosensory cortex

(to other cortical areas)

• primary somatosensory cortex(postcentral gyrus): brodmanns areas 3-1-2
• projects of somatosens ctx to:
• parietal cortex: contributes to internal representation of the body and outside world
• motor cortex: provides essential information about body position, muscle status

Projections of Somatosens ctx

to subcortical areas

• to lower levels of somatosens pathways: spinal cord dorsal horn, dorsal column nuclei, VPL
• modiefies the processing and flow of sensory information

Phantom Limb

• phantom limb= perception of a body part that is not present
• likely because of activity within an intact cortex is perceived as origination within the missing limb
• phantom pain is bad 
• may help with use of prosthetic

Assessing DC/ML function

• two-point discrimination (calipers)
• vibration (tunning fork)
• proprioception: report passive limb movement(with closed eyes), and romberg test

Romberg Test

• patient stands, feet together, eyes open
• assess sway
• patient closes eyes
• re-assess sway
• postivie: increased swaying with eyes closed. indicates loss of proprioceptive info carried in DC/ML
• swaing is less with eyes open because visual system compensates for loss of position sense

Tabes dorsalis

• tabes means decay
• major symptom is ataxia (uncoordinated movts) due to degeneration of the dorsal columns
• occurs during tertiary stage of syphilis

Antereolateral System

(spinothalamic)

information

• pain
• temperature
• crude touch-poorly localized or identified
• damage causes loss of pain and temperature sensation

Pain/Nociceptive information

• fast pain: sharp, well localized, carried by A delta fibers in periphery
• slow pain: dull aching or burning, carried by c fibers in periphery

Primary Afferents for

Fast Pain

• A delta fibers
• enter cord and asc or desc a few levels
• terminate in laminae I and V
• substance P neurotransmitter
• ascending pathway is spinothalamic

Primary Afferents for 

Slow Pain

• C fibers
• enter cord and asc or desc a few levels
• terminate in laminae I and II (substantia gelatinosa)
• substance P neurotransmitter
• ascending pathways are spinoreticular (spinoreticulothalamic) or spinotectal (spinomesencephalic)
• these hava a lot more synapses (contributes to slow)

Spinothalamic pathway

Fast pain

• 1st order neuron: Adelta fibers with cell body in DRG (ascend/descend a few segments in dorsolateral tract)
• 2nd order neuron: cell bodies in lamina I, V. Axon crosses in ventral white commissure
• 3rd order neuron: VPL of thalamus. Axon projects to postcentral gyrus via posterior limb of internal capsule
• responsible for localization of pain

Spinoreticular pathway

Slow Pain

• 1st order neurons: C fibers with cell body in DRG (can ascend/descend in dorsolateral tract of lissauer)
• Higher order: 2nd order cell bodies in lamina I and II. Axons of these and other 2nd order cells(in other sp cord laminae) project bilaterally to reticular formation at several levels of brainstem
• projections to intralaminar nuclei of thalamus. then axons project via posterior limb of internal capsule too: postcentral gyrus, insula and anterior cingulate gyrus
• insula and anterior cingulate gyrus are particularly important for affective (suffering) aspects of pain

Spinotectal tract

• aka spinomesencephalic tract
• similar to spinoreticular tract, except axons end in superior colliculus and periaqueductal gray
• part of slow pain

Random Pain Pathway facts

• spinoreticular esp important for attention, awareness, and levels of consciousness
• access to limbic system contributes to suffering component of pain
• note: redundancy of pathways limits the effectiveness of surgical intervention to alleviate pain by cutting tracts

Unilateral Spinal Cord Lesion

• ipsilateral loss of discrim touch
• contralateral loss of pain/temp
• these are at levels lower than lesion

Spinal Cord Lesion

• lesion of anterolateral tract produces contralateral analgesia(loss of pain sensation)
• lesion of ventral white commissure produces bilateral analgesia, in a ring around the body in the dermatomes corresponding to the lesion
• discriminative touch can be spared

Lateral Medullary Lesion

• produces contralateral analgesia
• no loss of discriminative touch

Medial medullary lesion

• no loss of pain and temperature
• contralateral loss of discriminative touch 

Pons or Midbrain lesions

• ALS and DC/ML fibers run close to one another as they ascend to the thalamus
• lesions of both patways lead to contralateral anesthesia (loss of all sensations)

Thalamic Pain Syndrome

• lesions in the thalamus can cause analgesia but this can change to chronic pain after a few months
• this is a form of central pain, meaning pain that arises in the CNS

Somatosensory Cortex Lesion

• somatosensory cortex is important for localizing pain
• however , suffering aspects of pain are still felt after loss of somatosensory cortex

Visceral Pain

• also known as referred pain
• pain that originates in viscera but is percieved in somatic areas
• conveyed within anterolateral system by neurons concerned with cutaneous pain
• example: pain arising from hypoxia in heart arises in upper chest and left arm
• pathway runs along DC/ML and projects to insula for suffering aspect
• in spinal cord, axon ascends along midline of fasciculus gracilis
• surgical transection of medial part of dorsal columns can offer relief of visceral pain

Spinocerebellar pathways

• non-conscious proprioception
• different from conscious proprioception conveyed by DC/ML system
• main pathways are dorsal spinocerebellar and cuneocerebellar
• other ones are ventral and rostral spinocerebellar pathways but are smaller then main ones

Dorsal Spinocerebellar Path

• trunk, leg (C8 and below) lower body
• 1st order cell body in DRG: C8-L2 axon enters Clarkes nu directly, below L2 axon ascends in F. gracillis to clarkes nu
• 2nd order cell body in Clarkes Nuc(C8-L2): axon ascends ipsilaterally in dorsal spinocerebellar tract and enters cerebellum via inferior cerebellar peduncle

Cuneocerebellar Pathway

• arm, neck(upper limb, upper trunk)
• 1st order cell body in DRG: axon enters cord above C8 and ascends ipsilaterally in fasciculus cuneatus
• 2nd order: cell body in external cuneate nu(caudal medulla): axon ascends ipsilaterally in cuneocerebellar tract and enters cerebellum via inferior cerebellar peduncle

Ventral Spinocerebellar path

(other spinocerebellar)

• lower trunk, leg
• 1st order axon enters cord and ends on 2nd order cell around ventral horn
• 2nd order axon: crosses midline in ventral white commisure and ascends contralaterally, then entrers via super cerebellar peduncle and recrosses within cerebellum
• double crossed; enters via SCP

Rostral Spinocerebellar pathway

• upper trunk, arm
• 1st order axon enters cord and ends on 2nd order cells in cord scattered rostral to clarkes nucleus
• 2nd order axon ascends ipsilaterally: enters via inferior cerebellar peduncle

Lesions of Spinocerebellar paths

• loss of spinal input to the cerebellum leads to clumsy, uncoordinated movts, called ataxia
• clinically, we'll consider only dorsal spinocerebellar and cuneocerebellar tracts, so: a lesion causes an ispilateral deficit (ataxia)

Trigeminal Pathways

• discriminate touch, pain and temperature, and other
• chief(principal or main) sens nu: discriminative touch, etc (pressure, vibration, conscious proprioception)
• spinal V nu (caudal part): pain and temp
• mesencephalic nu: some reflexes

Internal Capsule Topography

• axons from DC/ML and trigeminal pathways maintain topography through posterior limb of internal capsule
• trigeminal(face) axons near genu
• axons from bady travel increasingly caudally(posterior) along posterior limb (Arm, trunk, leg)

Jaw Jerk Reflex

• tap jaw
• reflex closing of jaw
• peripherally, tests only CN V
• centrally, tests Mesencephalic V and motor V

Corneal Reflex

• touching cornea produces direct response(close touched eye), and consensual response(close opposite eye)
• pain fibers in ophthalmic branch of CN V to
• spinal V nu to
• reticular formation to
• bilateral proj to VII nucleus to
• VII nerve

Lateral Medulla Lesions

CN V pathways

• interrupts spinal V tract and nucleus
• ipsilateral analgesia for face 

Lateral Pons Lesion 

CN V pathways

• interrupts all incoming V fibers
• ipsilateral anesthesia for face 

Alternating hemianalgesia

• alternating means a deficit on one side of face and opposite side of body thus
• example is ipsilateral face and contralateral body
• example is laterally medullary lesion: spinal V tract and nu and ascending fibers of anterolateral system

Sensory Transduction mechanisms

• direct transduction (mechanoreceptor): stimulus directly opens channels (fast)
• indirect transduction (olfactory receptor): stimulus activates a secondary messenger pathway that then opens channels (slow)

Adaption of Receptors and Neurons

• slow adaption: a raise in the constant stimulus causes a constant spike potential
• rapid adaption the raise in stimulus constant causes a bunch of spike potentials at the raise but then no more 

Receptive fields and perception

• receptive field is region of skin that causes firing of neurons
• differ in size and location
• for primary somatosensory neurons, this appears to be based on branching pattern of receptor neurons underneath the skin
• receptive field size is basis for two-point discrimination

Somatic sensibility

• somatic sensation arises from information provided by a variety of receptors distributed throughout the body (4 major sub-modalities)
• dicriminative touch: recognition of size, shape and texture of objects and their movt across skin
• proprioception:the sense of static position and movnt of the limbs and body
• nociception: signaling tissue damage or chemical irritation, percieved as pain or itch
• temerature sense: warmth and cold

pseudo-unipolar neurons

• false unipolar cause it has two branches but they are connected
• bypass the cell body (straight into spinal cord from stimulus)
• peripheral processes ramify within the skin or muscle and central processes synapse with neurons in spinal cord and higher levels

somatosensory receptor types

• mechanoreceptors
• proprioceptors
• thermoceptors
• nociceptors

Cutaneous (skin) mechanoreceptors

• specialized to receive tactile information
• respond to mechanical deformation of tissue (stretch, touch, pressure, vibration)
• best studied in glabrous (hairless) portion of the hand (palm and fingertips)
• these regions of the skin surface are specialized for providing a high-def neural image of manipulated objects
• first layer in ridges has merkel cell-neurite complex, then meissner corpuscle that is closest to skin, but in different layer
• then ruffini ending and lastly pacinian corpuscle

Merkel Cell Afferents

• slowly adapting
• 25% of mechanoreceptors
• fingertips, info from epidermis
• highest spatial resolution (because they are small)
• points, ridges and curvature
• info about form and texture

Meissner Afferents

• rapidly adapting, make up 40% mechanoreceptors
• tips of dermal papillae adjeacent to the primary ridges
• closest to skin surface
• elongated receptors, formed by connective tissue capsule comprising schwann cell lamellae
• more sensitive than merkels to skin deformation
• has larger receptive fields then merkel

Paccinian Afferents

• rapidly adapting, 10-15% of mechanoreceptors
• deep in dermis or subcutaneous tissue
• onion-concentric layers of membrane surrounding a single afferent fiber
• very sensitive-respond to 10 nanometers skin displacement
• large rceptive fields
• detect vibrations transmitted through objects that contact the hand or are grasped

Ruffini afferents

• slowly adapting, 20% of mechanoreceptors
• elongated, spindle shaped capsulated, deep in skin
• ligaments and tendons
• long axis of the corpuscle parallel to stretch lines of the skin
• sensitive to the cutaneous stretching 
• info about finger position and hand conformation

muscle spindles

• signal changes in muscle length (proprioceptions)
• activate motor neuron to extrafusal muscle fibers
• muscle stretch
• tension on intrafusal fibers
• activates nerve endings
• activate mechanically gated ion channels
• action potentails
• group Ia and II afferent fibers
• spindle recieves motor input too

Golgi Tendon Organ

• proprioceptor that is a mechanoreceptor
• signal changes in muscle tension
• are formed by branches of group 1b afferents distributed among collagen fibers that form the tendons
• each GTO is arranged in series with 10-20 extrafusal muscle fibers (in series btw muscle and tendon)

Descriptive Features of pain

• pain threshold: minumum stimulus that elicits pain-indicator of the health of the CNS and PNS
• pain tolerance: degree of pain a subject can tolerate before experiencing physical or emotional impairment
• pain hypersensitivity: pathological condition

Separate pain

• noxious stimulus-Adelta fibers activated first(larger diameter)-sharp pain first
• increase stimulus intens-c fibers(small d) get activated-slower, delayed second pain(dull)
• distinct mechanisms of pain perception; independent pathways(proven by selective anesthetized fibers)

Capsaicin

• activates TRP channel on C fibers (heat and acid also open it)
• these are polymodal nociceptors(activated by other agents)
• inject capsaicin into skin-burning sensation
• elicits hyperalgesia to thermal and mechanical stimuli
• paradox-repeaded applications of capsaicin act as analgesic-due to desensitization of the TRP receptor
• desensitization allows you to use it to treat pain

Vanilloid Receptor (VR)

• member of the TRP family(transient receptor)
• same family as somatosensory receptors
• occurs on the free nerve endings of C and Adelta fibes
• activated by heat+capsaicin;heat only;acid
• activated by different heat levels: 45 is uncomfortable;52 is higher heat threshold
• transduction mechanisms are similar to other somatosensory receptors-but trigger is a noxious stimulus

Classification and distribution of nociceptors

• mechanonociceptors-mechanical tissue damage
• thermo-nociceptors-heat or cold tissue damage
• chemo-nociceptors-chemical tissue damage
• nociceptors found in muscles, joints, visceral organs ect-activated by ischemia
• nociceptors can also transduce non-painful stimuli-low levels of heat or cold that dont cause tissue damage and therefore dont cause pain
• not all nociceptor activation produces pain

Catagories of pain

• physiological pain-nociception
• inflammatory pain-inflammation
• neuropathic pain-pathological
• neuroimmune pain

Nociceptive Pain, transient ouch

• direct activation of nociceptors
• tissue damage-release of bradykinin ect-leads to activation of nociceptors to release subs P which causes inflammatory response
• inflam response activates more nociceptors
• peripheral sensitization of pain-hyperalgesia

Neuropathic Pain

• chronic
• initiated by lesion in PNS or CNS
• nerve sprouting and rewiring from hyperactivity
• allodynia-pain evoked by a normally nonpainful stimulus
• hyperalgesia-exaggerated pain evoked by normally moderately painful stimulus

Neuroimmune pain

• immune signaling remote from injury site
• migroglial induced inflam
• affects nociceptive relay neurons(second order) in spinal cord
• hyperactivity in VPL neurons, increased microglial based inflam in thalamus
• increased pain perception in cortex

Mechanisms of phantom pain

• part of the axon of a nociceptor still exists after amputation; when you sever an axon it sometimes becomes hyperexcitatory (keeps firing)
• another way is getting rid of inhibitory nociceptors, so without stimulus, the pain keeps firing

Visceral Pain

• via the DC/ML system
• afferents from the pelvic/abdominal viscera-> spinal cord->2ndorder neuron in dorsal horn of the lumbar sacral spinal cord -> give rise to anterolateral systems that contribute to visceral pain
• other neurons are the intermediate gray region of spinal cord near the central canal; send axons through dorsal columsn near midline and through arcuate fibers that form contralateral medial lemniscus that synapse with Ventral-posterior thalamus

local circuit neuron

• makes connections only inside the dorsal horn
• contains enkephalin (inhibits)

Sensitization to pain

• prolonged hyper-responsiveness to pain-pain sensation increases with repeated stimulation, even though stimulation intensity does not increase
• transcription-independent (early onset)
• transcription-dependent (late onset)

Transcription-independent

• early onset
• windup: homosynaptic: repeated stimulus that compounds: lasts 500ms
• activity dependent CS: heterosynaptic (must have two synapses interacting):Abeta fiber and C fiber: last 10 mins to hours
• Long Term Potentiation (LTP): homosynaptic: last several minutes to hours

Transcription-dependent

• late onset and lasts much longer
• activity dependent: get localized changes in transcription at synapse
• ativity independent: start off with inflammation and you end up releasing enzymes(COXs) that migrate along spinal cord

Papilledema

• choked or swollen disk
• increased intracranial pressure(through subarachnoid space) leads to swollen head of optic nerve 
• constricts optic nerve and central retinal artery

Refractive Power

• focusing?
• measured in diopters
• total refractive power of eye is around 60 diopters
• cornea is 42 and lens is 18 (lens can accomadate)

Lens Curvature control

• used for accomadation
• edinger-westphal nucleus (part of CN III group)
• ciliary ganglion and ciliary muscle
• to focus on nearby objects you contract ciliary muscles; it reduces tension on zonule fibers and increases curvature of lens
• to focus on distant objects: relax ciliary muscle; increases tension on zonule fibers, decreases curvature of lens

Depth of Field

• narrow depth of field comes from an increase in aperture (dilation of pupil from iris); subject is in focus, foreground and background are blurry 
• Wide depth of field comes from reduced aperture (pupil constriction); focus is good extending into foreground and background
• dilation done by ppilary dilator(radial) muscles contracting (sympathetic)
• constriction done by sphincter muscles contract

Focusing eye (problems)

• emmetropia: normal vision
• myopia (nearsighted): eyeball elongated: object focused in front of retina
• hyperopia(farsighted): eyeball shortened: object focused behind the retina
• presbyopia: age related loss of lens focusing ability with age; due to thickening of lens, reducing its ability to round, NOT due to any age related changes in ciliary muscles (start to need reading glasses eventually)

Light Hyperpolarization

• several biochemical steps first happen when light hits a photoreceptor
• decreases cGMP concentration
• closes Na/Ca channels(gated by cGMP
• hyperpolarizes cell because the positive ions cannot come in
• less neurotransmitter released onto next cell in pathway (bipolar cell)

Phototransduction in rods

• photon absorbed by photopigment
• photopigment is a GPCR:lightabsorb chromophore retianl(Vit A aldehyd) coupled to an opsin protein
• type of opsin tunes the wavelength sensitivity of the photopigment; different opsins across rods and cones
• photopigment in rods is rhodopsin
• 11 cis retinal converts to all trans retinal when light
• activates Gprotein transducin that activates a phosphodiesterase
• the PDE hydrolyzes cGMP leading to lower concentrations and eventually hyperpolarization
• provides huge signal amplification

Light and Dark Adaptation

• to adapt to ongoing light stimuli a dissociation of retinal from opsin as well as other biochemical changes
• to adapt to reduced light, a reassociation of retianl and opsin
• rods more sensitive, but cones have more acuity 

Color Blindness

• normally red green color blind
• protanopia is loss of L cone function (red) (long wavelengths)
• deuteranopia is loss of m cone function (green) medium wavelength

Fovea

• means pit, actual indentation
• slightly towards the temporal side
• photoreceptors clustered in fovea
• macula lutea surrounds fovea and also dense in photoreceptors
• both have better resolution then rest of the eye
• in this area, the cell bodies and bipolar and galngion cells are moved out of the way so that theres a direct path to the photoreceptors
• only cones in the fovea (very densly packed)
• high rods density surrounding the fovea

off center receptive fields

• work by using a different glutamate receptor on the bipolar cell
• its glutamate receptor is excitatory instead of inhibatory like in on center 

Color and brightness contrast

• there are ON-center and OFF-center cells
• can encode brightness contrast (if surround has same color sensitivity)
• can encode color contrast (if surround has different color sensitivity)
• examples are ON green off red or vice versa for both on and off center fields

M-type Ganglion cells

• magnocellular= large cells
• large receptive field
• prefer brief/moving stimuli
• little if any color information
• provides information about motion and spatial location
• project to lateral geniculae nucleus layers 1 and 2(IM channel)

P-Type Ganglion Cells

• parvicellular=small cells
• smaller receptive field
• respond well to sustained stimuli
• mediate color and form vision
• project to lateral geniculate nucleus layers 3-6 (IP channel)

Functional Properties of 

Lateral Geniculate Nucleus

• separate layers for ipsi and contra inputs
• separate layers for M and P inputs
• receptive fields similar to retinal ganglion cells
• systematic representation of retina(retinotopy) and visual field (visuotopy)

Inferior and superior retina

• inferior retina is the top visual field
• superior retina is the bottom visual field
• superior retina goes back to the cuneus in the occipital lobe
• inferior retina goes to the lingual gyrus in the occipital lobe
• these are seperated by the calcarine sulcus

Myers loop

• part of the optic radiations
• carry some of the information from the inferior retina to the lingual gyrus 
• the myers loop is when some of the optic radiations dip down into the temporal lobe
• important for lesions and blood supply questions

Pathways for V1

• regardless of which pathway, first step is down V2
• then the dorsal (spatial vision) pathway goes into MT and up into the parietal lobe (location and direction of motion)
• the ventral (object recognition) pathway flows into V4 and beyond into the temporal lobe (form and color) 

Scotoma

• a part of the retina with reduced or non-functional vision
• deficit 
• part of retina responding to a certain visual field

Anopia

• lack of vision in a portion of the visual field

Homonymous and heteronymous

(visual deficits)

• homo is a deficit in the same part of the visual field viewed by both eyes (they are congruent)
• heter are deficits that are not in the same part of the visual field for each eye

visual field lesions

• hemianopia: lesions may affect half of the visual field
• quadrantanopia: lessions may affect one quarter of the visual field

Suprachiasmatic Nucleus

• found in the hypothalamus
• recieves direct input from the retina
• deals with circardian rhythyms (24hour light/dark cycle)

Pretectal Nucleus

• recieves direct retinal stimulation from a branch of the fibers running to the LGN
• deals with pupillary light reflex
• pretectal nucleus projects intralaterally to the edinger-westphal nucleus (CN III)
• also projects contralaterally to the other pretectal nucleus
• shine light in one eye and see both pupils constrict
• lesioning optic tract causes a weaker pupillary reflex (because one tract carries info from both eyes)

Pupillary Dilation

• hypothalamic projections
• intermediolateral horn of spinal cord to superior cervical ganglion to pupillary dilator muscles in iris
• sympathetic NS

Horner's Syndrome

• loss of sympathetic inpute to the eye/face
• pupillary constriction (miosis)
• partial ptosis(drooping eyelid)
• anhidrosis (loss of facial sweating)

Argyll-Robertson pupil

• result of late stage syphilis infection of CNS
• small pupils with weak pupillary light reflex; but visual acuity is good and pupil can constrict during accomodation
• lesion in pretectum or pathway to edinger-Westphal nucleus, rather than in sensory pathway or within Edinger-westphat nucleus

Accomodation to 

Near Vision

• lens thickening: edinger-westphat path to ciliary muscles
• pupillary constriction: edinger-westphat path to sphincter muscles
• convergence: contract medial rectus-external eye muscles

Parts of the ear

• external ear: external auditory meatus, and tympanic membrane
• middle ear: malleus-> incus -> stapes, eustachian tube, round and oval window
• inner ear: cochlea, semicircular canals, otolith organ (saccule)
• cochlear nerve and vestibular nerve

Endolymph

• found in membranous labyrinth
• high potasium and low sodium
• 0 to +90mV potential
• requires Na/K ATPase
• modified epithelial cells or stria vascularis
• tight junctions

Vestibular receptor organs

• otolith organs: utricle and saccule (static balance)
• semicircular canals: anterior ventrical, posterior ventrical and horizontal (angular acelleration)
• utricle large space at confluence of semicircular canal and contains its receptor the macula

The Utricle

• located at confluence of semicircular canals
• hair cells surrounded by supporting cells
• hair cells (stereocilia) project into a gelatinous layer, the otolithic membrane
• otoconia (rocks) is sitting on top of the gelatinous membrane
• fluid is bathed in endolymph(membranous labyrinth
• has map of all different directions of movnt based on arangement of hair cells
• striola is dividing line 
• does forward and back while saccule does up and down

Vestibular Hair cell

• has 40 to 70 stereocilia that differ in length
• contain tight junctions to prevent endolymph leakage
• kinocillium is the largest stereocila 
• links between tips of stereocilia and contain potassium channels (open when you mechanically move the stereocilia)

Transduction in Hair Cells

• at rest, K leaks into the stereocilia from endolymph and gets pumped out into the extracellular space
• causes a little bit of NT to be released and spontaneous activity
• pushing from short to long stereocilia causes K to come in and excitation (depolarization) and release of more NT
• pushing in other direction closes channels into tips, causing less K in and hyperpolarization (less NT released)
• pushing them off axis (90 degrees) nothing happens

Semicircular Canals

• contain endolymp and at each end there is a swelling called an ampulla
• have ampulary crest with hair cells that project up into a gelatinous matrix called cupula
• endolymph pushes against cupula during angular acceleration causing displacement (hair cells work like the other ones)
• one hair cell can distinguish btw moving head to left or right
• all hair cells allighned in one direction for a specific ampulary crest 
• function as antagonistic pairs(right and left)

Lateral vestibulospinal pathway

• senses change in orientation with respect to gravity and have muscle contraction to counteract the tilt
• stimulated by angular acc or tilt in any direction (saccule)
• activates primarily axial muscles
• reflex doesnt develop immediatly
• unilateral pathway

Medial Vestibulospinal Pathway

• stabilizes position of the head in space
• involves semicircular cannals and medial vestibular nucleus
• descending bilateral projections travel in medial longitudinal fasciculus 
• if you rotate you body in one direction, reflex movnt of head in the other direction (continue looking at object that you were looking at before)
• pathway only goes down to cervical cord

Vestibular-occular reflex

• medial vestibular nucleus recieves info that head turned one direction and sends it to the abducens and occulomotor nuclei
• net affect is that in the first ms after a head turn the eyes move in oposite direction to stay focused on the visual field
• stimulated by the semicircular canals
• can addapt using cerebellum 
• nystagmus is slow eye mvt away and they fast eye movnt back to the way the head is going
• damage to the medial long fasc (multiple sclerosis) results in disconjugate movnts of left and right eye(internuclear opthalmoplegia)

benign paroxysmal positional vertigo

• perceived bodily motion, sensation of spinning
• brief periods, often after sleeping
• calcium carbonate crystals lodged in semicircular canal
• crystals cause stimulus

endolymphatic hydrops (menieres disease)

• debilitating
• severe, recurrent episodes
• progressive
• increased endolymphatic pressure (membranes can burst)
• vertigo, spontaneous nystagmus, auditory symptoms
• eventual loss of reeptor cells unless controled

Wallenberg Syndrome

• vestibular nuclei are supplied by posterior inferior cerebellar artery
• vertigo and nystagmus (short term) are effects of loss of PICA circulation

Middle Ear Muscles

• tensor tympani muscle: attached to malleus, CN V
• stapedius muscle: attached to stapes, CN VII
• dampen movement of middle ear ossicles to control vibrations during loud sounds
• reduce distortion of loud sounds, and can be protective 

Otosclerosis

• calcification of the foot plate of the stapes that locks it in place
• keeps it from vibrating to the extent that it should
• reduce sensitity to sound (by several decibles)

Cochlea Cross section

• Scala vestibuli (connected to oval window) on top, scala media, then scala tympani on bottom
• reissners membrane btw vestibuli and media
• vestibuli and tympani filled with perilymph 
• scala media contains endolymph (about +90 mV)
• basilar membrane seperating media from tympani 
• tectorial membrane in scala media
• stria vascularis in scala media wall creates endolymph

Organ of Corti

• sits on top of basilar membrane
• contains receptor cells (hair cells): inner and outer (more)
• afferent and efferent nerve fibers
• tectoral membrane sitting on top of organ of corti
• 95% of inner hair cells connected to NS, provide almost all of info of sound
• outer hair cells recieve input from the CNS, contracts at rate of sound, have contact with tectoral membrane

Transmission of Mechanical energy

to the cochlea

• stapes moves in and out of oval window(scala vestibuli) in response to sound vibration
• pressure waves move up sca vestibuli through the perilymph 
• pressure waves move across the scala media and basilar membrane on their way into scala tympani (basilar membrane vibrates)
• increased pressure in perilymph of scala tympani is relieve at round window
• high frequency sounds cause maximum vibration nearer to base of cochlea
• low frequency sounds cause maximum vibration nearer to apex of cochlea

Upward and downward deflection of

basilar membrane

• upward deflection causes stereocilia to be moved from short to tall, opening the potassium channels and depolarizing the receptor
• in the downward phase the stereocilia are moved from tall to short, causing K channels to close and hyperpolarization
• outer hair cells pivotal for supply full sensitivity and selectivity (one from another) for sound because they are where the vibrations are largest
• efferent inervation adjusts outer hair cells to protect them from sensitivity

Ascending auditory pathway

(brainstem)

• dorsal (vertical locali) and ventral (timing) cochlear nuclei
• acoustic stria are fiber pathways leaving cochlear nuclei that cross midline in pons-meddula junx in the trapezoid body and goes to the superior olive nuclei(lower pons)
• superior olive compares sound levels (loudness) from the two ears (1st point in aud system it occurs)
• superior olive also provides olivarcochlear fibers that are efferent to outer hair cells
• ascending tracts go from crossing over to join the lateral lemniscus in pons-midbrain
• next is inferior colliculus, to brachium of this to the medial geniculate body in thalamus

Ascending auditory pathway

(cerebrum)

• ascending tracts (contralateral) go through brachium of inferior colliculs to the medial geniculate of the thalamus 
• unilateral auditory radiation then projects up to the cerebral cortex (superior bank of temporal lobe) 
• cortical area 41; primary auditory cortex that recieves info from the medial geniculate nucleus
• low frequencies in more anterior parts and high frequencies more posterior part of primary auditory cortex
• can then project to cortical area 22, secondary auditory gyrus 

Wernicke's Area

• part of area 22(temporal lobe), supramarginal and angular gyrus in parietal lobe
• involved in speech perception on left side of brain
• supplied by left MCA

causes of hearing loss

• age
• exposure to loud sounds
• genetic syndromes
• toxic substances that can kill haircells (antitumor agents and a certain type of antibiotics)
• acoustic neuroma(vestibular schwannoma) swan cell tumor thats slow growing and effects both vestibular and auditory but auditory first because cerebellum can compensate

cochlear implant

• sound seperated electronically by a microphone
• a recieved inside the skull then processes this
• provides systematic stimulation pattern inside the auditory nerve.
• allows speech perception
• the electrode array goes into the cochlea