3 General Functions of Spinal Cord

• Sensory input and Motor output for neck, trunk, limbs, and tail
• Reflex control of patterned mov't and adjustment of glandular activity in response to local afferent input without voluntary control
  
• Transmission of information to and from the brain

4 Branches of the Spinal Nerve

1. Dorsal branch: Sensory and motor nerves innervating deep muscles and skin dorsal to transverse process
2. Ventral branch: sensory and motor nerves innervating muscles and skin of limbs (contributes to lumbosacral and brachial plexuses) and lateral/ventral portions of trunk
3. Communicating branch: general visceral efferents (sympathetic) and afferent fibers innervating visceral structures
4. Meningeal branch: mainly sensory
   

Grey matter of spinal cord

• Dorsal horn: receive, process, and relay afferent information from dorsal root fibers
• Intermediate zone interneurons: grouped into networks that generate patterns of discharge within pools of motor neurons. These networks can be recruited for production of reflexes, antigravity support, balance, and voluntary mov't
• Ventral horn: motor neurons that send axons through ventral root to innervate muscle fibers
• Lateral horn (T1-L3 or L4): cell bodies of preganglionic sympathetic neurons.
  

White Matter of Spinal Cord

• Dorsal funiculi: contain fibers that carry info  concerning position and mov't of limbs in space
• Lareal and ventral funiculi contain both ascending sensory and descending motor pathways.

Surface features of Spinal Cord

• Dorsal median sulcus: dorsal cleft along midsaggital plane
  
• Ventral median fissure: ventral celft along midsaggital plane, through which the central branches of the ventral spinal artery projects
• Dorsolateral sulcus: cleft along the lines of attachment of the dorsal rootlets (separates dorsal funiculi from lateral funiculi)
• Ventrolateral sulcus: cleft along the lines of attachment of the ventral rootlets (separates lateral funiculi from ventral funiculi)
  

Spinal-cord-segment-to-vertebra Relationships

• C1 spinal cord seg lies within C1 (atlas) and spinal nerve exits via lateral vertebral foramen of atlas
• cd cervical and cr thoracic spinal cord seg are found cranial to corresponding vertebrae
• C2-C8 spinal nerves exit via intervertebral foramina cr to corresponding vertebrae
• T1 and remaining spinal nerves exit via intervertebral foramia cd to corresponding vertebrae
• Cd thoracic and cr lumbar spinal cor seg found within corresponding vertebrae
• Anything cd to that are found way cr to the corresponding vertebrae
  

Spinal Cord Regions

• Cr Cervical region (C1-C5): supplies axial mm and skin of Cr. cervical region
• Cervicothoracic region (C6-T2): supplies regional axial mm and skin, thoracic limb (brachial plexus), and preganglionic symp. to the eye (T1-T3)
• Thoracolumbar region (T3-L3): innervates regional mm and skin and contains preganglionic sympathetic neurons (T1-L5)
• Lumbosacral region (L4-S2): supplies regional axial mm and skin, pelvic limb (lumbosacral plexus), and partial supply of bladder and perineum
• Sacral region (S1-S3) innervates perineum, external anal sphincter, and supplies parasympathetic preganglionic to viscera of pelvis and bladder. Sciatic nerve receives fibers from S1-S2
• Cd region (Cd1-5) supplies tail
  

Cervical enlargement

• C6-T2. Enlarged in cd part of cervical region and cr part of thoracic region where dorsal and ventral roots of spinal nerves that make up brachial plexus attaches to the spinal cord Enlargement supplies sensory and motor innervation to thoracic limb

Lumbar engargement

• L4-S2: represents the region where the dorsal and ventral roots of spinal nerves making up lumbosacral plexus attach to the spinal cord. The enlargement supplies sensory/motor innervation to pelvic limb.

Location of Conus Medullaris

• At junction of L6 and L7

Information received by afferent pathways

• exteroception: external environ
• interoception: internal environ
• proprioception: position and movement of the body

General somatic afferent (GSA)

Somatosensation: concerned with innervation of skin and somatic structures (e.g. skeletal muscle, periosteum)

General visceral afferent (GVA)

Sensory compotent of the autonomic nervous system and is concerned with afferent information from the viscera

4 ways that info received by sensory system is employed

1. Conscious awareness
2. Unconscious mediation of refle activity
3. by cerebellum to shape voluntary mov't
4. maintenance of arousal and motivation

Divergence

Same sensory info simultaneously sent to separate destinations for different purposes

Parallel processing

Different aspects fo the same sensory experience are perceived and evaluated in different parts of the brain

Transduction

The changing of some sort of environ energy (heat, pressure, light, etc...) into electrical activity of neurons

Peripheral process

Neuronal process of the primary afferent neuron that extends out into the periphery to the receptor (cell body also in the periphery)

Central process

Neuronal process of hte primary afferent that extends from the ganglion into the spinal cord (or brainstem for cranial nerves)

Primary afferent neuron

• Neuron that initiates AP when stimulation of a peripheral receptor reaches a threshold intensity, and conducts sensory info from the periphery into the CNS. 
• All but one primary afferent has cell body outside CNS ina sensory ganglion (DRG and cranial nerve sensory ganglia)

Second order neurons

• Neuron that the central process of the primary afferent establishes an excitatory synapses (GLUTAMATE) to in the ganglion
  
• May itself be excitatory or inhibitory to respectie targets, make reflex connections with local neurons, and also relay info to suprasegmental structures (brainstem, cerebellum, cerebrum)
• In most conscious perception of somatosensation, some or all second order neurons decussate before reaching hte thalamus

Tracts

Discrete bundles of sensory neuron axons that ascend the spinal cord toward the brain. Are segregated from one another by anatomical location and sensory modality.

Primary relay nuclei

• nuclei within the thalamus whose neurons deal with a specific type of sensation (all except olfaction relayed in thalamus)
• axons of neurons in primary relay nuclei project to the primary sensory cortex, appropriate to the sensation
• these axons constitue large part of the internal capsule
• Due to decussation of second order sensory neurons, somatosensory experiences on one side of body projected to contralateral thalamus and cortex.
  

Topographic arrangement

The tendency for info travelling in the nervous system to retain the spatial relationships of the receptors in the periphery at any point in the pathway

• retinotopic: visual system
• tonotopic: auditory system
• somatotopic: somatic sensory system

Sensory tracts with high degree of somatotopy (e.g. proprioceptive tracts) are also highly discriminative, and the source of stimulus can be preceisely localized.

Nonencapsulated receptors

• Free naked nerve endings, which are sensitive primarily to noxious stimuli and are the termini of small diameter, lightly- (A delta) or non-myelinated (C fibers) primary afferent fibers

Encapsulated receptors

• large receptors that are primarily concerned with touch sensations
• invested with specialized CT capsules that are responsible for imparting modality specificity and the dynamic responses
• Primary afferents conducting non-noxious tactile info are typically more highly myelinated and larger diameter than those conduction pain info (i.e. Aalpha and Abeta)

5 general types of receptors based on stimulation type

1. mechanoreceptor: physical deformation
2. thermoreceptors: both head and cold varieties
3. nociceptors: noxious stimuli
4. photoreceptors: rods and cones of retina
5. chemoreceptors: chemical changes associated with taste, smell, and O2 and CO2 concentrations in blood

adequate stimulus

• unique stimulus to which a given receptor is most sensitive

Modality

• The experience of a given sensation as it is perceived at the cortical level with qualities that make it distinct from other types of sensations (e.g. light as a stimulus results in the sensory modality that is experienced as vision)

Receptor potential/generator potential

• local change in membrane potential of the receptor resulting from the adequate stimulus.
  
• Usually depolarizing by opening cation channesl permeable to Na and K.
  
• Opening is usually brought on by second messenger systems, though mechanoreceptors mediated by mechanical deformation of the receptor membrane.
• GRADED event: amplitude of potential change and idstance along the membrane that hte potential travels are proportional to the strength of simulus
• Once potential change reaches threshold, and AP is initiated in the trigger zone of the primary afferent's peripheral process
  

Labeled Line code

• determines modality. starts at receptor that has an adequate stimulus, and subsequent neurons that always project to the same cortical areas resulting in the same conscious perception of the stimulus

How is the intesity of a stimulus encoded?

• Frequency code: Once trigger zone of primary afferent has reached threshold, APPs are generated at a frequency proportional to the intensity of the stimulus
• Population code: the number of receptors (and consequently primary afferents) that are activated by a given stimulus.

How is stimulus duration encoded?

Duration determined by onset and termination of stimulus (rapidly-adapting receptors) and by the persistent response of slowly-adapting receptors

• Rapidly adapting receptors: receptor potential present only at onset and at termination of the stimulus; APs associated only briefly with chagnes in stimulation
• Slowly adapting receptors: more persistent receptor potential that decays more slowly.

Receptive field

• portion of the environment in which an adequate stimulus can be expected to elicit a response in that neuron
• for primary afferents, it's the region that activates the receptor.
• in higher order neurons, there is an inhibitory region surrounding the excitatory region, which helps enhance contrast

cutaneous zone

• the combined receptive fields of all the primary afferents whose axons make the cutaneous sensory nerve

dermatomes

area of skin that is provided sensory innervation by a particular spinal nerve segment

Proprioception

Sensibility comprising static position sense and kinesthesia. carried by conscious pathways that terminate in the cortex, and by unconscious ones that project to the cerebellum

• static position sense: joint angle, muscle length, tendon tension, and tactile sensations
• kinesthesia (detection of actively moving joints and mm, and how theese mov'ts produce change in the orientation of body parts relative to one another and to the surrounding world

What is the principle pathway bringing proprioceptive info to the cerebral cortex?

Dorsal Column-Medial Lemniscus (DCML)

Receptors contributing info about proprioception to DCML

Proprioceptors:

• Skin mechanoreceptors
• Joint receptors (pressure and stretch in jt capsules)
• Muscle spindles
• Golgi tendon organs

Only exception to the rule that primary afferent cell bodies reside in ganglia outside the CNS

• Primary afferents carrying proprioceptive info in CNV
• Somata of primary afferents located in the mesencephalic nucleus (deep narrow nucleus that runs rostrad from pons through midbrain) and contains only somata of pseudounipolar sensory neurons dedicated to transmission of proprioceptive info from structures of the head.

Circumduction

Sign of ataxia, where limbs may stray abnormally wide in th eswing phase of the gait

Sensory ataxia

Ataxia of proprioceptive deficits that results from the inability of the motor cortex to make ongoing corrections in the execution of motor commands, corrections normally based on continuous feedback about hte moment-to-moment position of limbs, trunk, and head.

2 anatomical features of the DCML that renders it especially vulnerable to injury

• Size of the fibers: heavily myelinated Aalpha fibers, which are vulnerable to compression due to their high metabolic activity and the fact that damage to myelin may produce a failure of transmission
• DCML are superficially located on the cord.

Ways to test proprioception

Postural reactions:

1. Proprioceptive positioning
2. Wheelbarrowing
3. Hopping
4. Hemistanding and Hemiwalking
5. Placing (both tactile and visual)

Nociceptors

• Receptors that detect tissue-damaging stimuli
• naked nerve endings, widely distributed in skin and dep tissues which are peripheral termini of nociceptive primary afferent neurons (A delta and C fibers)
• Activation of A delta fibers are associated with sharp, pricking pain
• Activation of C fibers are associated with slower burning type of pain

Silent nociceptor

nociceptor with high threshold that ensure sthat under normal circumstances it is relatively insensitive to any stimuli, but the threshold is markedly reduced following release of tissue inflammatory mediators.

hypalgesia

Decreased pain perception

analgesia

complete absence of pain perception

anesthesia

absence of all sensory perceptions

Ascending reticular activating system (ARAS)

• Orchestrates the relationship between sensation and consciousness
• Component of the reticular formation (loose aggregation of nuclei in the central core of the brainstem, extending from diencephalon through medulla
• Critical to regulation of level of consciousness through its rostral projections to the diencephalon, which in turn diffusely excites cerebral cortex

Functions of the reticular formation

• Regulation of heart and respiratory rates
• Control of certain visceral functions and some postural motor systems,
• Maintaining wakefulness

Coma

Loss of consciousness from which the patient cannot e roused (may be caused by damage to the ARAS)

Spinocervicothalamic tract

• Concerned with the transmission of superficial pain and tactile sensations and is regarded as the primary conscious pain pathway in carnivores.
• Somatotopy is well defined, and sensations brought to consciousness by the spinocervicothalamic tract are very well localized

Spinoreticular tract

• primarily concerned with transmission of deep pain and visceral sensations
• Primary afferents enter cord and immediately diverge within dorsolateral fasciculus to send collatearls several segments rostral and caudal to the segment of entry. axons of projection neurons present diffusely in lateral and ventral funiculi, close to the grey matter of the cord. Info entering cord on one side will project craniad bilaterally. Most ascending projections terminate in the reticular formation, form which fibers project to multiple cortical and limbic destinations

axon reflex

• the ability of stimulated nociceptors to release substances directly into the surrounding tissue.
• One of those chejmicals released is substance P, which produces local dilation of blood vessels and degraulation of mast cells, both of which contribute to inflammation and increased sensitization of local nociceptors

primary hyperalgesia

when a noxious stimulus produces greater than normla activity in nociceptive pathways

allodynia

when normally non-stimuli can activate nociceptors

Wide-dynamic-range (WDR) neurons

Neurons that respond to both noxious and non-noxious stimuli, and are recruited in pathways that exhibit less somatotopy. Relatively insensitive to tactile info, while responding more vigorously to noxious stimulation

Referred pain

• noxious stimuli originating in viscera are perceived as originating instead from a somatic region.
• Also neurons that are probably most important in expression of wind-up

spinal facilitation of pain/wind-up

• Rapid prolonged firing of primary nociceptive afferents result in the increased release of NTs onto second order neurons resulting in a sustained, augmented post-synaptic depolarization, which activates NMDA receptors.
• NMDA receptors admit calcium ions along with sodium, and brings the second order neuron closer to threshold, so that subsequent NT release is more likely to produce APs in the postsynaptic neuron
• Ca also causes intracellular signaling, which produces long-lasting changes in the postsynaptic cell that decreases its threshold for excitation over longer periods of time.
  

Preemptive analgesia

nerve blocks and other analgesics being use pre-or intra-operatiely, as a preemptive strike against the development of wind-up during surgery

Gate control theory

• both nociceptive specific C fibers and non-nociceptive A alpha and A beta fibers converge on WDR neurons of the dorsal horn
• The non-nociceptive fibers also make connections with inhibitory interneurons
• When the A alpha and A beta fibers are actiated by tactile stimuli, they weakly stimulate the WDR projection neurons and simultaneously stimualte the inhibitory interneurons, which reduces transmission in the nociceptive pathways by inhibiting release of NTs from the C fiber terminals in an axoaxonic connection
• Result is decreased nociception
  

Brainstem modulation of pain

• Collaterals from ascending nociceptive pathways, esp those conveying deep pain (C fibers of rspinoreticular tract) activate th eperiaqueductal grey and the nucleus raphe magnus.
• PAG receives input from ascending nociceptive pathways along with higher centers, and project to he nucleus raphe magnus, other reticular nuclei, etc...
• PAG axons release NTs (mostly endorphins)
• The nucleus raphe magnus releases serotonin and NE onto the dorsal column, which will stimulate dorsal horn interneurons to release NTs (enkephalin and dynorphin), to inhibit transmission in spinal cord pain pathways.

How to reduce pain

1. Modulate or prevent transduction at the nociceptor with NSAIDs to prevent lowering of pain threshold and primary hyperalgesia that acoompanies inflammation
2. Interfere with conduction to the CNS by applying local anesthetics by blocking sodium channels.
3. Recruit the neuroanatomic substrate of the gate control theory with cool soaks or warmth, gentle massage, or acupuncture (reduce nociception at dorsal horn)
4. Prevent spinal cord sensitization. Use NMDA antagonists to prevent wind-up
5. Augment descending modulatory mechanisms with opioids
6. Reduce cortical perception of pain with sedatives
   

Three major classes of opioid receptors

G-protein linked receptors that are inhibitory on target neurons through 1.) inhibition of v-gated Ca channels (less NT release), and 2.) indirect activation of K+ channels

• mu
• delta
• kappa

mu opioid receptors

• u1 receptors: associated with Periaqueductal grey, and associated with brainstem intrinsic analgesic centers
• u2 receptors: strongly mediates respiratory derpession and dorsal horn analgesia

kappa and delta receptors

• endogenous ligands ofr k receptors are dynorphins (NTs of inhibitory dorsal horn interneurons that inhibit transmission in spinal cord pain pathways)
• delta receptors bind enkephalins (also NTs of inhibitory dorsal horn interneurons)

pruritis

• particular sensory modality that provokes desire to scratch (confined to skin and mucous membranes)
• C fibers are the primary afferents conveying pruritus, and pathways seems similar to pain
• Activity in nociceptive pathways blocks perception of itch, so mildly noxious stimuli like scratching relieves itch.

Neuropathic pain

• pain that arises as a result of injury to the nervous system
• Hyperalgesia and allodynia are both commonly associated with neuropathic pain.
• dysesthesia can also be present: an unpleasant abnormal sensation that is often characterized as tingling or "electric"

10 Neural features taht characterize acupuncture points

1.) bigger nerve trunk size     2.) Less nerve trunk depth

3.) Nerves that penetrate deep fascia

4.) Nerves that exit bone foramina

5.) Neuromuscular attachement sites

6.) Prominent neurovascular pairing

7.) Nerve fiber composition

8.) Neural bifurcations

9.) Sensitive dense connective tissue structures

10) Cranial sutures

Tonic Stretch Reflex

Sets the basic tone of a muscle, and is tested by palpating mm and manipulating limbs to determine resistance to passive movement

Phasic Stretch REflex

Causes a rapid increase in muscle tone in response to rapid stretching of that muscle, and is tested by striking its tendon with a reflex hammer

Spinal cord segments that mediate panniculus reflex

T1 and C8 --> lateral thoracic n

Reaction and Reflex to Cutaneous Stimulation

• Damage between L3 and C7 will block both reaction and reflex
• Damage cranial to C8 will only block reaction

special proprioception

• Vestibular senses of balance and acceleration (NOT limb position and mov't)

Bony labyrinth

Elaborately sculpted set of chambers within the dense petrous bone, filled with perilymph

Membranous labyrinth

• Soft tissue set of ducts and sacs which follow the convoluted of the bony labyrinth filled with endolymph
• Contains 2 enlargements (utriculus and sacculus), and three loops attached to the utriculus (semicircular ducts) and the spiraled cochlear duct.

Vestibular apparatus

Receptor organs of the vestibular system

• utriculus
• sacculus
• semicircular ducts

Macula

• thickened oval plaque of neuroepithelium within the utriculus and sacculus with an embedded population of hair cells in its surface

Otolithic membrane

• Gelatinous sheet overlaying the macula, onto which the cilia of the hair cells project.
• Surface of the otolithic membrane is studded with otoliths (crystals of Calcium carbonate) which increase the inertial mass of the otolithic membrane
• Under linear acceleration, the otolithic membrane bends the cilia of the underlying hair cells in the direction opposite that of acceleration

Cilia of hair cells

• Each hair cell is topped by a single large kinocilium at one edge of the cell, and upto 100 small sterocilia
• If stereocilia bends towards the kinocilium, the hair cell is depolarized
• If stereocilia bend away from the kinocilium, hair cell hyperpolarized
• The kinocilia of the macula are oriented with respect to a functional line arcing across the middle of the macula (striola)
• Maculae of the uriculus and sacculus oriented perpendicular, so there's a distintive discharge pattern for any head position
  

semicircular ducts

• semi-circular extensions of the membranous labyrinth
• ampula: dilated end of each duct which houses receptor organs of the semicircular ducts
• crista ampullaris: transverse ridge of CT on one wall of the ampulla
• cupula: attached to the crista, and extends across the ampula from the crista ampullaris to the opposite wall and forms a flexible but complete barrier to the flow of endolymph
• cilia of hair cells are embedded in the cupula and are bent by the mov'ts of it
• hair cells on crista are oriented so othat their kinocilia face the side of the ampulla adjacent to the utriculus.
  

vestibular ganglion

• Where somata of the primary vestibular afferent neurons reside within the petrous temporal bone.  Central processes of these neurons form the vestibulocochlear nerve, and a majority terminate in the vestibular nuclei.

vestibuloocular reflex

• coordinates extraocular eye muscle mov'ts with the mov't of the head, and thereby fix the visual image in one place on the retina

Nystagmus

• Slow movement of the eyes in the direction opposite the head turn, followed by a rapid readjustment in the same direction as the head turn

Medial longitudinal fasciculus (MLF)

• bundle of fibers near the midline of the brainstem that mediates the motor limb of the vestibuloocular reflex.
• axons in MLF take origin from vestibular nuclei and ascend bilaterad to the motor nuclei of cranial nerves III, IV, and VI to permit coordination of conjugate eye movements in response to vestibular info

Vestibulocollic reflex

• Some neurons in the vestibular nuclei send axons caudad in the descending portion of the MLF (aka Medial vestibulospinal tract)and descend mostly iplilaterad in the ventral funiculus and terminate on interneurons in cervical and cranial thoracic spinal segments to activate neck musculature and forelimb extensors
• Purpose of these reflex connections is to produce neck mov'ts and forelimb extension that counteract the tilt of hte head that elicits them
• Designed to keep head level with respect to gravity and mov't
  

Vestibulospinal reflex

• Some efferent fibers form the vestibular nuclei form an ipsilateral descending tract in the ventral funiculus that extends the length of hte spinal called the lateral vestibulospinal tract
• Its activity recruits interneurons that facilitate ipsilateral extensors, inhibit ipsilateral flexors, and inhibit contralateral extensors
• reflex uses vestibular info to produce limb extension that counteracts the displacement of the head that elicits it to prevent falling with shifts in head position

emetic center

• vomiting center in the reticular formation, to which some axons from the vestibular nuclei project

Nystagmus is named for the ________

_______ of the nystagmus is to the side of lesion

fast phase

slow phase

post-rotatory nystagmus

• nystagmus that occurs after head has been rotating for a long time and all of the sudden stops
• The inertial mov't of the endolymph after the head's stopped moving causes the vestibular system to perceive it as the head is turning the opposite direction

Strabismus

• deviation of the eye position
• sometimes occurs with vestibular disease, but usually positional

Peripheral vs. Central vestibular dz

• Peripheral lesions will not affect ascending or descending tracts that are not specifically part of the vestibular system (normal propioception and normal strength)
• Nystagmus of peripheral lesions is either horizontal or rotatory and does not change with position
• Nystagmus of central lesions may be horizontal, rotatory, or vertical, and can be positional

Paradoxical vestibular disease

• results from lesions of the flocculonodular lobe of the cerebellum or the cerebellar peduncles
• exhibit vestibular signs with laterality opposite that expected with vestibular lesions

Audition

• special somatic sense for hearing
• primary afferents in the cochlear division of the vestibulocochlear nerve
• adequate stimulus is air pressure waves produced by vibration of objects

Auricle/pinna

• portion of auditory apparatus visible on the outside of the head

aural hematoma

traumatic rupture of the blood vessels in the auricular cartilage due to head-shaking or vigorous ear-scratching

cerumen

fancy word for earwax

tympanic cavity

• air-filled space lined by mucous membrane and contained within the temporal bone (the middle ear)
• Communicates with the nasopharynx via the auditory tube
• most dorsal part contains the auditory ossicles and their muscles

Auditory ossicles

• 3 bones (malleus, incus, stapes) that trasmit movemtns of the tympanic membrane to the vestibular (oval) window of the inner ear.
• Manubrium mallei is embedded in the tympanic membrane
• associated with the m. tensor tympani (attached to malleus and CNV) and the m. stapedius (attached to te stapes and CN VII). both dampen the mov'ents of the tympanic membrane/stapes to protect from excessively loud damaging noise

cochlea

• spiraled portion of the bony labyrinth that containsn the cochlear duct, and spirals up before terminating at is cupula (apex)
• Cochlear duct within closes off a middle section with 2 membranes (vestibular membrane on top and basilar membrane on the bottom) and is filled with endolymph. The space above is called the scala vestibuli, and the one below, the scala tympani, and both contain perilymph.
  
• air pressure moves through the scala vestibuli and the scala tympani, and then is dissipated at the round (cochlear) window.

Spiral organ

• Contains the receptor cells (hair cells), with cilia which are embedded in a rigid overlying membrane, the tectorial membrane.
• hair cells synapse with peripheral porcesses of primary afferents whose cell bodies lie within the spiral ganglion, which is housed in the modiolus, the bony core of the cochlea. The central processes gather to form the cochlear nerve.

Stria vascularis

• vascularlized epithelium on the wall of the cochlear duct that lies against the ouside of the spiral organ.
• Produces teh endolymph ithin the membranous labyrinth, and is also the source of neurotrophic substances that allow for the survival of hair cells.

3 bases of pitch discrimination

1. basilar membrane varies in width from the base of the cochlear duct where it is the narrowest, to the apex where it si the widest, and therefore each portion has a different resonant frequency at which it preferentially vibrates
2. Cilia of of the hair cells shorter and stiffer in the base and more flexible towards the apex
3. each hair cell undergoes small spontaneous oscillations in membrane potential that vary in different hair cells according to the position along the basilar membrane

Central processing of auditory info

• Cell bodies of primary afferents are in the spiral ganglion, and central projections enter the brainstem at the junction of the medulla and pons to terminate in dorsal and ventral cochlear nuclei. Seocnd order neurons can either decussate in the trapezoid body to join fibers in the contralateral lemniscus or ascend the ipsilateral lateral lemniscus, which project to the caudal colliculi
• From the caudal colliculi, neurons travel up the brachium of hte caudal colliculus to the medial geniculate nucleus, and then onto the primary auditory cortex

Sound localization

based on interaural intensity and time differences.

Conduction deafness

• disease processes that affect the ability of the tympanic membrane or auditory ossicles to transmit vibrations ot the vestibular window

Sesorineural deafness

• Disease processes that affect the spiral organ or more proximal components of the auditory system (cochlear nerves, brainstem, auditory cortices)

Presbycusis

• reduced hearing or deafness of advancing age
• due to loss of receptor cells, loss of cells in the spiral ganglion, and/or degenerative changes in the auditory ossicles (combination of conduction and sensorineural deafness)

ototoxicity

• toxicity to the neural elements of hte inner ear
• aminoglycoside antibiotics (streptomycin, gentamicin, neomycin, etc...) most common

Brainstem auditory evoked potentials

• earphones are used to produce a series of discrete sound stimuli unilaterally, and skin electrodes on the patient's head is placed to see electrical activity.

Deep cerebellar nuclei

• three paris of nuclei deep in the cerebellar medulla (fastigial, interposed, and lateral nuclei), from which the vast majority of info leaving the cerebellum emanates

Three layers of the cerebellar cortex

• molecular layer (unmyelinated axons of granule cells and the dendritic arbors of purkinje cells)
• Purkinje cell layer (monolayer of large, GABAergic neurons)
• Granule cell layer (thick layer of smnall, densely0staining excitatory neurons)

Afferents to the cerebellum

• mossy fibers
• climbing fibers: originates from the excitatory neurons in the medulla, and enters cerebellum through cd peduncle, and then to the neurons of the deep cerebellar nuclei, then synapse in an extensive arborization on the dendritic trees of the purkinje cells. cause depolarization of the purkinje cells with an inward calcium current (results in second messenger systems that allow for motor learning)

The output from the cerebellar cortex issues strictly from the _______________

Purkinje cells!

Vast majority of their GABAergic axonal projections terminate in the deep cerebellar nuclei, and reduce the spontaneous activity of the deep cerebellar nuclei

ALL OUTPUTS FROM THE CEREBELLUM ORIGINATES FROM THE DEEP CEREBELLAR NUCLEI and leaves thru the rostral cerebella peduncle

3 basic functions of the cerebellum

1. regulation of equilibrium
2. control of posture and muscle tone
3. coordination of voluntary mov't

Flocculonodular lobe

• Oldest part of the cerebellum, and receibes input from the vestibular system
• Afferents enter the ipsilateral floculonodular lobe via the cd cerebellar  peduncle
• Efferents from the purkinje cells of the flocculonodular lobe project to the ipsilateral vestibular nuclei either directly or indirectly via the fastigial nucleus
• Influence activity in the vestibulospinal tracts and MLF

Lateral Hemisphere of cerebellum

• newest part of cerebellum
• derives all of its input from the motor cortices int eh cerebrum by way of the PONTINE NUCLEI
• Fibers from motor corties descend through the internal capsule and crura cerebri and pass through pons on their way to the LMN pools of brainstem and spinal cord. However, they also send collateral axons to synapse in the pontine nuclei, and fibers from it decussate as the TRANSVERSE FIVERS OF THE PONS and project via the middle cerebellar peduncles into the lateral hemisphere
• Efferents emanate from the lateral nucleus, leaving the cerebellum via the rostral cerebellar peduncles and synapsing in the thalamus before projecting back to he motor cortices
• In effect, initiate the planning and coordinate the timing of complex, skilled limb movements
  

Vermis and paravermal regions of cerebellum

• receive most of their afferents from the spinal cord and regulates posture and coordination of ongoing movements.

Spinocerebellar Tracts

• Mediates unconscious proprioception with collaterals from the DCML
• Dorsal spinocerebellar tract (DSCT) and Ventral spinocerebellar tract (VSCT) carry info from the hindlimbs and caudal half of the trunk
• Cuneocerebellar tract (CCT) and rostral spinocerebellar tract (RSCT) serve the neck, forelimbs and cranial thoracic segments
• Trigeminocerebellar tract: carry proprioception from head structures

Dorsal spinocerebellar tract

• Located superficially in the dorsal part of the lateral funiculus
• All spinal cord segs cd to and including T1
• Proprioceptive fibers enter at each dorsal root and synapse in the nucleus thoracis (cell bodies in the medial side of the base of the dorsal horn)
• Second order neurons project into the lateral funiculus to form the ipsilateral DSCTand enter the vermal and paravermal  parts of the cerebellum via the cd cerebellar peduncle

Ventral spinocerebellar tract

• All spinal cord segs cd to and including T1 contribute to VSCT
• Majority of the afferents in the dorsal horn ducussates and ascend the cord in the superficial art of the lateral funiculus, ventral to the DSCT
• The tract enters the cerebellum via the rostral cerebellar peduncle
• Majority of fibers decussate again so that they terminate in the vermis and paravermal parts of the cerebellum ipsilateral to the input.

Cuneocerebellar tract

• Receives proprioceptive info from spinal cord seg C1-T8
• Primary afferents enter dorsal roots and joins fasciculus cuneatus without synapsing until they reach the lateral cuneate nucleus in the cd brainstem
• Synapse on second order neurons whose axons become the CCT, and enter ipsilateral vermis and paravermal parts of cerebellum via cd cerebellar peduncle

rostral spinocerebellar tract

• Priary proprioceptive afferents from the forelimbs enter spinal cord rostral to T1 and synapse on neurons in the base of hte dorsal horn
• Axons of second order neurons form the RSCT and ascend in the ipsilateral lateral funiculus
• RSCT fibers enter the ipsilateral vermis and paravermal parts of the cerebellum via both rostral and cd cerebellar peduncles

Types of fibers in the spinocerebellar tracts

A alpha

Damage susceptibility of spinocerebellar tracts

• Big, heavily myelinated fibers and superficial location of tracts makes it extremely susceptible to compression

motor ataxia

The uncoordinated mov'ts of cerebellar dz

Paradoxical vestibular syndrome

• vestibular signs are contralateal to the lesion and fast phase of nystagmus directed towards the side of lesion.
• Occurs because by removing the inhibitory input of the flocculonodular Purkinje cells will disinhibit the ipsilateral vestitibular nuclei and produce an increase in their activity relative to the contralateral side

Truncal sway

Tremors of the epaxial musculature that produces a sinuous oscillation in the body's long axis

Pendular nystagmus

• fluttering eyemovemtns back and forth with no discernible slow or fast phase that occurs with diffuse cerebellar dz

Cerebellar Development

• The external germinal layer (which produce granule cells) are mitotically active until around 10 weeks of age in carnivores
• Feline panleukopenia shows predilection of rapidly dividing cells of the external germinal layer, which results in cerebellar hypoplasia --> cerebellar ataxia