pnb block 4

first-order neuron

cell body location: dorsal root ganglion (body) or sensory cranial nerve ganglion

function: carries information from receptor into the CNS

second-order neurons

location: spinal cord or brainstem

function: receives the first neuron’s signal and relays it, often crossing the midline.

third-order neurons

location: usually thalamus

function: relays information to the cerebral cortex

tracts

• begin and end at the same place

• the precise path may have slight anatomical variation because fetal environment is unique

• no apparent advantage of one tract over another, perhaps a more direct path may be faster

• all tracts develop in utero

  • clinically, the path taken is critical

dorsal column-medial lemniscus (DCML)

pathway that carries: fine touch, vibration, conscious proprioception, information important for two-point discrimination

step-by-step:

1. a sensory receptor detects a stimulus and first order neuron has its cell body in the dorsal root ganglion

2. its axon enters the spinal cord and ascends ipsilaterally

3. travels through dorsal columns to medulla, where it synapses in either the gracile nucleus or cuneate nucleus.

4. the second-order neuron crosses the midline in the medulla, where it ascends as the medial lemniscus.

5. it synapses with the third order neuron in the thalamus.

6. the third-order neuron projects to the primary somatosensory cortex.

fasciculus gracilis

carries information from lower body, especially legs and lower trunk. position in dorsal columns: medial.

fasciculus cuneatus

carries information from upper body, especially arms and upper trunk. position in dorsal columns: lateral.

spinothalamic pathway

important for pain, temperature, some crude touch and related sensations.

step-by-step pathway:

1. a nociceptor or thermoreceptor detects a stimulus

2. the first-order sensory neuron has its cell body in the dorsal root ganglion, where its axon enters the spinal cord.

3. it synapses with a second-order neuron in the dorsal horn, which crosses to the opposite side WITHIN the spinal cord.

4. it ascends through the contralateral anterolateral region.

5. it reaches the thalamus, where it synapses with a third-order neuron, which then projects to the somatosensory cortex.

olivary tracts

aid in muscle control of balance

tectal tracts

aid in head movement in response to cutaneous stimulation

reticular tract

conscious response to cutaneous stimulation, pain.

somatosensory cortex

site of the synapse of the 3rd neuron.

topographically mapped: spatial discrimination, sensation is physically mapped. subdivided into different stimulus modalities.

provided feedback from skin, muscles, tendons, and joints.

volume of cortex proportional to sensory input, and more stimulus collected sent to a larger area of cortex, proportional.

representation is drawn as homunculus

homunculus

a distorted representation of the human body across the primary somatosensory cortex. you will notice that the hands, lips, and face are disproportionately large compared with the trunk and legs because the amount of cortical representation devoted to a body region is related to its sensory input and processing requirements, rather than its physical size.

cortical plasticity

the brain’s ability to reorganize its sensory maps in response to changes in sensory input. this explains phantom limb pain, as the brain’s representation of the limb does not necessarily disappear immediately after amputation. changes in cortical organization, ongoing neural activity, and altered sensory processing can contribute to phantom sensations and pain.

haptic feedback

information provided through touch or physical sensation. examples:

• raised bumps at the top of stairs

• textured sidewalk edges

• vibrations from iPhone

• textures that communicate information without requiring vision.

nociceptors

• free nerve endings detect cell damage

  • cell damage causes release of intracellular chemicals that act as a stimulus.

• experienced pain depends on type of damage and type of nociceptor

  • fast, localized, and sharp pain - myelinated Adelta fibers

  • Full, slow, ache - unmyelinated C fibers

  • both impossible to ignore.

nociceptors are most abundant in skin, and may occur in many internal tissues. however, the brain tissue itself does not contain pain receptors in the same way that skin does. the brain, during headaches, dilates the cerebral and dural blood vessels, as well as inflames mediators.

CGRP

a migraine trigger. can promote vasodilation, participate in inflammatory signaling, and contribute to transmission of nociceptive information from intracranial structures.

acute pain

rapid onset, often related to an identifiable injury or stimulus; may resolve after the stimulus is removed

example: touching a hot surface

chronic

persistent or recurrent pain that continues beyond the expected period of healing

example: persistent neuropathic or musculoskeletal pain

visceral pain

originates from internal organs. generally, have less precise sensory localization than the skin, so it is harder to localize.

example: abdominal pain

referred pain

occurs when pain is perceived at a location different from the actual source of the problem. happens through a mechanism called convergence, which is when sensory fibers from different body regions can synapse on the same neurons in the spinal cord or brainstem.

examples:

• myocardial infarction (heart attack): pain may be felt in the neck, shoulders, back, or arms rather than only in the chest

• ice-cream headache: cold stimulation in the throat or palate can be associated with pain perceived in the head.

special senses

anatomically complex and process large amounts of information. requires substantial neural resources and considerable cortical processing.

vision

one of the most complex sensory systems in the human body. involves: the eyeball, eyelids and brows, lacrimal glands and ducts, cornea, lens, retina, optic nerve, muscles that move the eyes, and extensive processing in the brain. some facts to know:

• only 1/6th of eye is exposed

• image formed on retina is inverted

• the retina contains a large number of photoreceptors

• the fovea has the highest density of cones

• the optic disc contains no rods or cones

• the cornea is avascular (the only tissue without blood vessels in body)

fibrous tunic

structures: schlera and cornea

functions: protection, shape, and light entry

vascular tunic

structure: choroid, ciliary body, iris

function: blood supply, lens adjustment, pupil control

nervous tunic

structures: retina

function: detects light and initiates visual signaling.

sclera

part of the fibrous tunic.

sclera is the white, tough connective tissue covering that helps protect the eye and maintain its shape.

cornea

part of the fibrous tunic. the cornea is the transparent front surface of the eye. it provides a major portion of the eye’s refractive power by bending incoming light. the cornea is avascular, and fed by tears. it is transparent due to keratan sulfate proteoglycans.

choroid

part of the vascular tunic. the choroid is part of the vascular layer that supplies nutrients to the retina and absorbs stray light.

ciliary body

part of the vascular tunic. the ciliary body contains the ciliary muscle, which changes the shape of the lens during accommodation

iris

part of the vascular tunic. the iris is the colored part of the eyes, and it controls pupil diameter.

retina

part of the nervous tunic. the retina contains photoreceptors and other neurons that convert light into electrical signals and begin visual processing.

pupil

the opening in the center of the iris. it is not a structure made of black tissue, but appears black because light entering the eye is largely absorbed rather than reflected back out. the iris changes pupil size: can dilate or constrict, which controls how much light enters the eye.

lens

biconvex, meaning it bulges outward on both sides. function is to refract light so that incoming rays are focused onto the retina.

what lens do to light:

• a convex lens converges light rays

• the light rays are brough toward a focal point

• the eye must adjust its lens to focus light from different distances onto the retina.

accomodation

the process of hcanging lens shape to focus on objects at different distances. the ciliary muscle controls the tension on the suspensory ligaments (zonular fibers), which attach to the lens.

when looking at a nearby object…

1. the ciliary muscle contracts

2. tension on the zonular fibers decreases

3. the lens rebounds into a more rounded shape

4. the lens becomes more convex

5. its refractive power increases

6. light from the nearby object can be focused on the retina

when looking at a distant object…

1. the ciliary muscle relaxes

2. zonular fiber tension increases

3. the lens becomes flatter

4. its refractive power decreases

5. light from distant objects can be focused on the retina.

aqueous humor

fluid in the anterior portion of the eyes, in front of the lens.

vitreous humor

gel-like substance filling the large posterior cavity behind the lens.

schlemm’s canal

Aqueous humor is produced by the ciliary processes. It flows through the posterior and anterior chambers, then drains through the trabecular meshwork into Schlemm's canal.

This drainage helps regulate intraocular pressure

cataracts

clouding of the lens. when the lens becomes cloudy: light is scattered, images become blurry, glare can increase, color perception may become less vivid, and vision can become impaired.

differences between glaucoma and cataracts

fovea

region of the highest cone density; responsible for the sharpest central vision

peripheral retina

contains many rods, which are especially useful in low-light conditions

optic disc

where a ganglion cell axons leave the eye to form the optic nerve; contains no rods or cones.

rods

a photoreceptor in the retina. excels in low-light vision, as rods are extremely sensitive to light. they allow you to see in dim environments. Multiple rods can synapse onto shared downstream neurons. This means that even if each rod responds to a very small amount of light, their combined input can help activate the next neuron.

This increases sensitivity.

Trade-off: When multiple rods converge onto shared neurons, the brain has less information about exactly which rod was activated. Spatial acuity decreases.

cones

bright-light vision, best function in brighter conditions. they provide high visual acuity, color discrimination, and detailed central vision. In the fovea, cone pathways have very little convergence, allowing the brain to preserve detailed spatial information.

Trade-off: Less convergence improves acuity but means that cones are less sensitive to very dim light.

rods and cones tradeoff

scotopic vision

lighting conditions: dark

main photoreceptors: rods

visual experience: grayscale

photopic vision

lighting conditions: bright light

main photoreceptors: cones

visual experience: color vision

mesopic vision

lighting conditions: intermediate lighting

main photoreceptors: both rods and cones contribute

visual experience: mixed characteristics.

phototransduction

unlike many sensory receptors that depolarize when stimulated, rods and cones actually hyperpolarize in response to light. in darkness:

1. cGMP levels remain relatively high.

2. cGMP-gated (CNG) channels remain open.

3. Sodium and calcium enter the photoreceptor.

4. The photoreceptor remains relatively depolarized.

5. It continuously releases glutamate.

This ongoing release of glutamate helps inhibit certain bipolar cells.

phototransduction sequence

1. A photon is absorbed by retinal.

2. 11-cis-retinal changes into all-trans-retinal.

3. The activated opsin changes conformation.

4. Opsin activates a G protein called transducin.

5. Transducin activates phosphodiesterase (PDE).

6. PDE breaks down cGMP.

7. Intracellular cGMP concentration decreases.

8. cGMP-gated sodium/calcium channels close.

9. Positive ion entry decreases.

10. The photoreceptor hyperpolarizes.

11. Glutamate release decreases.

how does information travel from the retina to the brain

rods/cones —> bipolar cells —> ganglion cells —> optic nerve. more specifically:

What happens in darkness?

photoreceptors continuously release glutamate in darkness.

This glutamate inhibits certain bipolar cells, depending on the bipolar cell's receptor type.

What happens in light?

1. Photoreceptors hyperpolarize.

2. Glutamate release decreases.

3. The inhibition of certain bipolar cells is relieved.

4. Bipolar cell activity changes.

5. Bipolar cells influence ganglion cells.

6. Ganglion cells generate action potentials.

7. Ganglion cell axons form the optic nerve.

horizontal cells

modify communication between photoreceptors and bipolar cells. contribute to lateral interactions that help the retina detect contrast and edges. important because the visual system needs to distinguish changes in brightness across neighboring areas, not merely detect the total amount of light.

amacrine cells

modify communication between bipolar cells and ganglion cells. help regulate visual information, including temporal changes and movement-related processing.

cell types of vision

binocular vision

the brain processes two images together to create depth perception

stereopsis

1. Each eye views an object from a slightly different angle.

2. The two retinas receive slightly different images.

3. The visual cortex compares the images.

4. The brain uses the differences to estimate depth.

helps you estimate how far away an object is, how far you need to reach, and where an object is located in 3D space.

myopia - nearsightedness

occurs when light from distant objects focuses in front of the retina.a too-long eyeball may be the cause.

as a result of this disease, nearby object appear relatively clear, but distant objects appear blurry. correction involves a concave lens

hyperopia - farsightedness

.occurs when light would focus behind the retina. a too-short eyeball may the cause. as a result, nearby objects may be difficult to focus on. correction involves a convex lens

presbyopia

Presbyopia is the age-related loss of accommodation.

As the lens becomes less flexible, it cannot round up as effectively when focusing on nearby objects.

This makes reading and other close-up activities increasingly difficult.

It is different from myopia and hyperopia because it primarily involves reduced ability to change lens shape.

astigmatism

Astigmatism occurs when the cornea or lens has an irregular curvature.

Instead of focusing light evenly onto one point, the optical system produces uneven focusing.

This can cause:

• Blurry vision.

• Distorted images.

• Difficulty seeing fine details at different distances.

emphasis on malformed corneal shape

age-related macular degeneration (AMD)

The macula is the central region of the retina responsible for detailed central vision.

AMD affects this region and can cause progressive loss of central vision.

The lecture distinguishes two forms:

Dry AMD

• Associated with thinning and degeneration of retinal tissues.

• Generally progresses more gradually.

Wet AMD

• Associated with abnormal growth of blood vessels beneath or within the retina.

• These vessels can leak fluid or blood and damage retinal structures.

tunnel vision

The lecture describes tunnel vision as potentially resulting from a tumor pressing on the optic nerve.

The general idea is that damage or compression affecting visual pathways can cause loss of part of the visual field.

However, tunnel vision has multiple possible causes, and optic nerve compression is only one possibility.

sound

produced by air or water molecules. vibrations travel as pressure waves that can be detected by the ear. frequency and amplitude are two major properties that determine how we perceive sound.

frequency

physical measurement: Hertz (Hz) // we perceive pitch (high vs. low)

the number of wave cycles per second. A high frequency sounds high-pitched; a low frequency sounds low-pitched.

amplitude

physical measurement: size/intensity of pressure wave. we perceive loudness.

related to the strength of the pressure wave. greater amplitude is generally perceived as louder.

decibels

measure sound level on a logarithmic scale.

Outer ear

contains: pinna, auditory canal

function: collects and directs sound

middle ear

contains: tympanic membrane, maellus, incus, stapes, Estuachian tube

function: collects and directs sound

inner ear

contains: cochlea, vestibule, semicircular canals, hair cells

function: hearing and balance

pinna

the visible external part of the ear, it funnels sound waves into the auditory canal

auditory canal (auditory meatus)

directs sound toward the tympanic membrane.

tympanic membrane (eardrum)

vibrates when sound waves reach it

ossicles

three small bones in the middle ear

malleus (hammer): attached to the tympanic membrane

incus (anvil): receives vibrations from malleus

stapes (stirrup): transfers vibrations to the oval window.

sequence: tympanic membrane —> malleus —> incus —> stapes —> oval window

eustachian tube

connects the middle ear to the throat region and helps equalize air pressure on both sides of the tympanic membrane. this is why your ears may pop when you change altitude.

inner ear

The inner ear contains structures involved in both hearing and balance:

• Cochlea: Hearing.

• Vestibule: Detects head position and linear acceleration.

• Semicircular canals: Detect rotational head movements.

The cochlea contains sensory hair cells that convert mechanical movement into electrical signals.

inner ear nested systems

The inner ear has two nested systems:

• Bony labyrinth: Cavities carved into the temporal bone. (LOW sodium, HIGH potassium)

• Membranous labyrinth: Membranous structures located inside the bony labyrinth. (HIGH sodium, LOW potassium)

The membranous labyrinth contains endolymph, a fluid with an unusual ionic composition.

The bony labyrinth surrounds the membranous labyrinth and contains perilymph in the spaces around it.

anatomy of cochlea

a spiral-shaped structure that contains three fluid-filled chambers

• scala vestibuli: begins at the oval window. contains perilymph.

• scala media: cochlear duct; contains endolymph

• scala tympani: leads toward the round window. contains perilymph.

vestibular membrane

Separates the scala vestibuli from the scala media.

basilar membrane

Separates the scala media from the scala tympani and supports the organ of Corti, which contains the auditory hair cells.

oval window

receives vibrations from the stapes

round window

allows pressure changes in the cochlear fluid to be released. we need the round window because the cochlea is fluid-filled. since fluid is difficult to compress, movement at the oval window needs a flexible region where pressure can be displaced. the round window provides that flexibility.

how sound becomes an electrical signal.

1. sound waves enter the auditory canal and the tympanic membrane vibrates.

2. the malleus, incus, and stapes transmit the vibration.

3. the stapes pushes against the oval window.

4. movement of the oval window generates pressure waves in the cochlear fluid, which causes the basilar membrane to vibrate.

5. movement bends the stereocilia on the hair cells.

6. bending opens mechanically gated cation channels.

7. K+ enters the hair cell from the potassium rich endolymph.

8. the hair cell depolarizes and releases NT onto cochlear neurons

9. cochlear neurons generate APs that travel through cochlear nerve.

stereocilia

tiny projections in hair cells. arranged in bundles and connected by tip links

tip links

arranges the stereocilia in bundles.

tympanic reflec

two muscles contract: tensor tympani and stapedius. their contraction stiffens the middle-ear structures and reduces transmission of vibrations to the oval window.

this helps reduce the amount of sound energy reaching the inner ear.

inner vs. outer hair cells

Inner hair cells are the main sensory receptors. They communicate with cochlear neurons, whose axons form the cochlear nerve.

Outer hair cells change their length in response to electrical stimulation. This movement helps amplify and sharpen vibrations of the basilar membrane.

This is called the cochlear amplifier.

A useful way to remember the distinction:

• Inner hair cells tell the brain what sound is present.

• Outer hair cells help make the cochlea more sensitive and selective.

cochlear frequencies

The basilar membrane varies in physical properties along its length:

• Base of cochlea: Narrower and stiffer; responds best to high-frequency sounds.

• Apex of cochlea: Wider and more flexible; responds best to low-frequency sounds.

This arrangement is called tonotopic organization.

auditory pathway

cochlear nerve —> dorsal and ventral cochlear nuclei —> superior olive —> inferior colliculi —> medial geniculate body —> auditory cortex

how does the brain locate sounds?

1. interaural intensity differences (ILDs): A sound coming from one side of your head is generally louder in the ear closest to the source.

   The lateral superior olive (LSO) compares sound intensity between the ears.

   The lecture emphasizes that this mechanism is particularly useful for frequencies above approximately 3,000 Hz.

2. interaural timing differences (ITDs): A sound reaches the closer ear slightly earlier than the farther ear.

   The medial superior olive (MSO) compares the timing of signals arriving from the two ears.

   The lecture emphasizes that this is particularly useful for frequencies below approximately 3,000 Hz.

ostosclerosis

abnormal bone growth/remodeling restricts movement of middle-ear bones, especially the stapes

meniere’s disease

inner ear disorder associated with hearing loss, dizziness, and balance symptoms

tinnitus

perception of buzzing, ringing, hissing without an external sound source

ototoxicity

certain medications damage inner-ear structures, including hair cells.

vestibular system

detects information about

• Head position.

• Gravity.

• Linear acceleration.

• Rotational movement.

• Orientation in three-dimensional space.

It helps you maintain balance and stabilize your gaze while moving.

The vestibular system uses hair cells similar to those in the cochlea, but the structures and mechanics differ.

static labyrinth

structures: urtricle and saccule

function: head position and linear accleration