Exam I Study Guide

What are the mechanisms that protect the brain?

• bone

• meninges

• ventricular system

• Circle of Willis

• blood-brain barrier

• redundancy

bone

• mechanical protection: brain is encased in a bony skullcap

• periosteal membrane: outer layer of dura mater that adheres to the inner surface of skull (structural support and protection)

• when force comes in contact with the skull, it is distributed laterally by the trabeculae of spongey bone

  • distributing force over a large area = less effect/risk of damage

periosteal membrane

cranial cavity:

• dura has two fused layers: periosteal (attached to the skull) and meningeal (covers brain and spinal cord)

• dura is tightly attached

• layers form venous sinuses

spinal cord:

• no periosteal layer

• dura not attached (allows movement)

Why the brain needs extra protection?

• neurons are amitotic

• plasticy –> learned info and connection can be erased by injury

• emergence –> small localized damage can cause widespread cognitive effects

• modern forces (cars, sports, impacts) exceed what evolution prepared us for

meninges

• fluid-filled sac surround brain and spinal cord

• 3 layers, superficial to deep

1. dura mater

2. arachnoid

3. pia mater

dura mater

• outermost layer

• dense irregular C.T. (collagen)

• two layers in skull (periosteal and meningeal)

• projects inward to help anchor the brain

• attached to the periosteum of bone through cohesion (fluid filled gap between dura and periosteum)

• forms dural venous sinuses

dural venous sinuses

• between periosteum and dura mater

• collects venous blood (impure blood that returns to the heart after passing through the capillaries) flow from brain —> low BP

• directs blood flow back to jugular veins (major blood vessels that stretch from the head to the upper chest)

arachnoid

• middle layer

• web-like fibroblasts; loose cover over brain

• does not go into the sulci, but dips into the longitudinal fissure (divides the brain in half)

• subdural space - small space between dura and arachnoid

• creates subarachnoid space below it

• no blood vessels

• arachnoid villi

subarachnoid space

• between arachnoid and pia

• filled with blood CSF that cushions and protects the brain

• contains blood vessels

• secured to pia by weblike extensions of the arachnoid

arachnoid villi

• acts like one-way valves

• projection of arachnoid through dura into dural sinuses, allowing CSF to re-enter bloodstream

• allows CSF to exit the subarachnoid space and drain into venous blood of dural sinus while preventing blood from flowing back to the brain

  • CSF BP > venous blood BP —> CSF can push through into the sinus thus becoming part of blood

pia

• thin, innermost layer

• translucent C.T.

• directly touches brain and spinal cord

• supplies blood to neural tissue via blood vessels

• follows sulci and gyri

ventricular system: ventricles

1. right and right lateral ventricles: located within each cerebral hemisphere

2. third ventricle: between the thalamus, forming a bridge to the fourth ventricle

3. fourth ventricle: between the brainstem and cerebellum, continuous with the central canal of spinal cord

• all provide and circulate CSF

ventricular system

• chambers are continuous with each other and with the central canal of the spinal cord

• interventricular foramen connect lateral with third

• third is connected with fourth via cerebral aqueduct

• fourth is continuous with central canal

• fourths has openings to subarachnoid space: lateral and medial apertures

• ventricles are lined with simple epithelial cells called ependymal

ventricular system: flow path

lateral ventricles –> interventricular foramina –> third ventricle –> cerebral aqueduct –> fourth ventricle –> apertures –> subarachnoid space –> arachnoid villi –> dural sinus –> jugular vein

choroid plexus

specialized ependymal tissue lining each ventricle that continually synthesizes CSF

CSF composition

• clear, plasma-like fluid rich in glucose, ions, and low protein

• provides buoyancy, nutrient delivery, and waste clearance

cerebral aqueduct

• narrow channel connecting 3rd and 4th ventricle

• allows CSF to leave the ventricles to the subarachnoid space

• acts as a canal that passes through the midbrain

• common blockage site

lateral and medial apertures

• openings in 4th ventricle that allow CSF to enter the subarachnoid space

• circulation and drainage of CSF

Circle of Willis

• circle of arteries at the base of the brain that supply blood to the brain

• forms around the pituitary gland

• redundant blood supply

• allows blood to reach all brain regions from multiple paths

• if one vessels is blocked –> others compensate

arteries of the Circle of Willis

1. internal carotid: brings blood to the brain, ascend through the neck; branch from middle and ant. cerebral

2. vertebral arteries: brings blood to the brain, ascend through neck to the transverse foramina of cervical vertebrae; supply post. brain

3. basilar artery: formed by the joining of the vertebral arteries, supply brain stem and cerebellum before branching into post. cerebral

4. ant, cerebral arteries: exit point, supply parietal and frontal lobes

5. middle cerebral arteries: exit point, supplies lateral surfaces of each brain lobe

6. post. cerebral arteries: exit point, supply occipital and temporal lobes

Circle of Willis: anterior and posterior communicating arteries

• anterior: connect right and left ant. cerebral

• posterior: connect post. cerebral to the internal carotid

• redundancy —> prevents ischemia

Circle of Willis: anterior, middle, and posterior cerebral arteries

• anterior: supplies medial surface of frontal and parietal lobes, primary motor and sensory cortices responsible for lower limbs

• middle: supply lateral surfaces of cerebral hemispheres (frontal, parietal, and temporal), primary motor and sensory areas for face and upper limbs

• posterior: supply occipital lobe, inferior and medial parts of temporal

hematoma

caused by a hemorrhage (bleeding), an accumulation of blood/bruising

blood-brain barrier

• regulates movement of materials from the blood into brain

• brain arteries quickly divide into highly selective capillaries

• tight junctions between capillary endothelial cells allow minimal substances to cross BBB

• protect brain from toxins

• maintain stable environment

blood brain barrier: characteristics

• endothelial cells (single cell layer that lines all blood vessels and regulates exchanges between the bloodstream and surrounding tissues) with tight junctions

• astrocyte foot processes (aid in the maintenance of the blood-brain barrier)

• basement membrane

• regulates passage of substances between the blood stream and brain

• protects against toxins and pathogens while allowing for essential nutrients pass

How does the blood-brain barrier work?

1. tight junctions between the brain’s capillaries (endothelial cells) prevents most substances from leaking out of the blood to the brain

2. astrocytes (glial cell) sends signals that help maintain tight junctions and control what passes

3. selective transport system allow necessary molecules to cross into the brain while blocking toxins, pathogens, and most drugs

peripheral nervous system (PNS)

divided into somatic and autonomic nervous systems

somatic nervous system

• voluntary movement

• sends motor commands from CNS to skeletal muscles

• sensory (afferent) neurons relay info from skin, muscle, and joints to CNS

• pathway: cell body in CNS –> peripheral axon signal to skeletal muscle; no peripheral cell bodies

autonomic nervous system

• regulates involuntary physiological functions (heart rate, digestion, and respiration)

• sympathetic and parasympathetic systems

• pathway:

  • preganglionic: cell body in CNS peripheral axon –> axon projects to peripheral ganglion

  • postganglionic: cell body in ganglion –> axon innervates target organs

sympathetic nervous system

• stress or emergencies –> increase heart rate, dilates pupils, inhibits digestion

• neurotransmitters (NE) prepare the body for action

parasympathetic nervous system

• promotes relaxation –> slows heart rate, constricts pupils, stimulates digestion

• uses ACh to maintain homeostasis during restful states

4 ways sympathetic and parasympathetic systems differ

1. function

• sympathetic: prepares body for stressful situation

• parasympathetic: promotes relaxation and energy conservation

2. NT

• sympathetic: uses ACh at preganglionic synapse and NE at postganglionic synapses

• parasympathetic: uses ACh at both pre and post-ganglionic synapses

3. ganglionic location

• sympathetic: ganglia are close to spinal cord

• parasympathetic: ganglia near or w/i target organs (localized control)

4. axon length

• sympathetic: short pre-ganglionic (because close to spinal cord) and long post-ganglionic (reach distant organs)

• parasympathetic: long pre-ganglionic (because near/within target organ) and short post-ganglionic

refraction

• the bending of light as it passes through different medias (cornea, aqueous humor, lens, vitreous humor) to focus on retina

• cornea provides most refraction, lens fine-tunes focus

accommodation

• ability of lens to change shape to focus on near or far objects

• controlled by ciliary muscles (controls the movement of the lens and pupil) and zonular fibers (tiny thread-like fibers that hold the lens in place)

• near vision: ciliary muscles contract, zonular fibers loosen, lens become rounder

• far vision: ciliary muscles relax, zonular fibers tighten, lens flattens

pupillary response

• pupil adjusts its size in response to light intensity and focus needs

• bright light: parasympathetic activation contracts sphincter papillae, constricting the pupil (miosis)

• low light: sympathetic activation contracts dilator papillae, dilating the pupil (mydrosis)

image characteristics of the eye

• inverted:

  • light from the top of an object hits the bottom of the retina

  • light from the bottom of an object hits the top of the retina

  • cornea and lens are convex

  • convex lenses causes light rays to converge and rays to cross the focal point —> image flips vertically

• reversed:

  • light from the right visual field hits the left side of the retina

  • light from the left visual fields hits the right side of the retina

  • light rays cross the midline as they focus

gross anatomy of the eye: superficial structures

• sclera

• cornea

• conjunctiva

• eyelids and eyelashes

sclera

• white, fibrous outer layer

• provides protections and structure

cornea

• transparent

• curved front part that refracts light

conjunctiva

thin membrane covering the front of the eye and inner eyelids

eyelids and eyelashes

protects eye from debris and excessive light

cross-sectional anatomy of the eye: structures

• iris

• lens

• ciliary body

• zonulae fibers

• aqueous and vitreous humors

• optic disc

• macula and fovea

iris

• controls pupil via size

• parasympathetic: contracts sphincter pupillae

• sympathetic: contracts dilator pupillae

lens

transparent, flexible structure that adjusts shape to focus light on retina

ciliary body

• contains ciliary muscle

• controls lens shape and produces aqueous humor

zonulae fibers

suspensory ligaments that connect the lens to the ciliary body, adjusting tension for focusing

aqueous humor

• fluid in anterior chamber

• provides nutrients

• maintains intraocular pressure (measurement of the fluid pressure inside the eye)

vitreous humor

• gel-like substance in the posterior chamber

• maintains eye shape

• supports retina

optic disc

• “blind spot” where optic nerve exits the eye

• lacks photoreceptors

macula

• central area of retina

• responsible for sharpness and focus

fovea

central point of the macula with the highest concentration of cones for detailed color vision

ophthalmoscopic

an exam that uses a magnifying lens and light to check the fundus of the eye (back of the eye, including the retina and optic nerve)

macula and fovea

• visible as it is a slightly darker region in retina

• critical for detailed vision

optic disc

• appears pale

• circular region where optic nerve exits

• with no photoreceptors (blind spot)

layers of the retina and each of their cell types

1. ganglion cell (ganglionic cells)

2. inner plexiform (amacrine cells)

3. inner nuclear (bipolar cells)

4. outer plexiform (horizontal cells)

5. outer nuclear (photoreceptors)

6. outer segment (pigmented epithelial)

ganglionic cell

• only retinal cells that fire AP

• axons form the optic nerve

• neurons receive visual information from bipolar and amacrine cells

• sends signals to the brain via optic nerve

amacrine cells

• modulate signals between bipolar and ganglion cells

• helps refine visual processing and increases motion detection and contrast

bipolar cells

• acts as intermediaries

• transmits signals from photoreceptors to ganglion cells

horizontal cells

• integrate and regulate input from multiple photoreceptors

• helps with contrast enhancement and lateral inhibition

photoreceptors

• light sensitive cells in the retina

• contains rods (for low-light) and cones (for color and sharp vision)

outer segment

contains light-sensitive portions of rods and cones, where phototransduction (conversion of light into neural signals) occurs

pigmented epithelial

• beneath retina

• absorbs excess light to prevent reflection

• provides nutrients to photoreceptors, supporting their function and maintenance

scotopic retinal systems

• type of photoreceptor: rods (highly sensitive to dim light – does not detect color)

• photopigments: rhodopsin (highly sensitive to light, does not distinguish color)

• circuitry: many rods converge onto a single bipolar, increase sensitivity but reduces detail

• distribution and density of photoreceptors: rods are highly concentrated in peripheral retina, absent in fovea (allowing better motion detection)

photopic retinal system

• types of photoreceptors: uses cones (provide high-activity vision and color perception)

• photopigments: 3 types of opsin –> s-cones (short, blue light), m-cones (medium, green light), l-cones (long, red light)

• circuitry: one cone converges onto a single bipolar, maintains high resolution but reduces low light

• distribution and density of photoreceptors: highly concentrated in the fovea

phototransduction: response to light

1. light hyperpolarizes photoreceptors by activating rods or cones

2. decrease in cGMP, closure of Na+ channels

3. cell becomes more negative (hyperpolarized) and reduces glutamate onto bipolar cells

phototransduction: response to absence of light

1. photoreceptors are depolarized, maintain high cGMP

2. Na+ channels remain open, allowing steady influx of Na+, keeping the cell depolarized

3. glutamate is continuously released, affecting bipolar and horizontal cells

phototransduction: which cells release NT and generate AP

1. NT

• photoreceptors: release glutamate in darkness, decreases release response in light (does not generate AP)

• bipolar cells: receive input from photoreceptors–depolarizing (“on” cells) or hyperpolarizing (“off”) in response to glutamate changes (does not generate AP)

2. AP

• ganglion cells- first retinal cells to generate AP, sends signals through optic nerve to brain

visual pathway: correspondence between visual world and retina

• left visual field is processed by the right retina of both eyes

• right visual field processed by left retina of both eyes

• temporal retina sees nasal visual field

• nasal retina sees temporal visual field

• image on retina is inverted and reversed compared to visual field

visual pathway: pathway from retina and LGN (lateral geniculate nucleus - part of thalamus)

• photoreceptors –> bipolar cells –> ganglion cells

• ganglion cells’ axons form optic nerve (CNII)

• optic nerves partially cross at optic chasma

  • nasal retina fibers cross to opposite side

  • temporal retina fiber stays on the same side

visual pathway: organization of LGN

• has 6 layers, receiving input from either contralateral (opposite) or ipsilateral (same) eye

  • layers 1, 4, 6 –> receive signals from contralateral

  • layers 2, 3, 5 –> receive signals from ipsilateral

• preserves retinoptic mapping, meaning adjacent points

visual pathway: correspondence between LGN and retina

• each LGN layer receives input from only one eye, maintaining separate visual streams

• fovea has large representation in LGN due to high acuity

visual pathway: type of ganglion cells

• m-type (magno cellular): large, fast-conducting cells that detect motion and contrast

• p-type (parvo cellular): small, color sensitive cells that detect fine detail and form

• k-type (konio cellular): involved in color processing

visual pathway: correspondence between retinal cells and LGN

• magno: layer 1 and 2 (motion and contrast detection)

• parvo: layer 3-6 (fine detail and color processing)

• konio: intercalated between layers (color perception)

visual pathway: pathway from LGN to primary visual cortex

• LGN neurons project to layer 4 of PVC

  • upper visual field – travels via Meyer’s Loop (temporal lobes)

  • lower visual field – travels via parietal pathway

visual pathway: organization of PVC

• retinotopically organized: neighboring areas in retina correspond to neighboring area in PVC

• cortical magnification: fovea is large compared to peripheral retina

• orientation columns: neurons respond to a specific angke

• ocular dominance columns: alternating input from left and right eye

visual pathway: connection between layers of primary visual cortex and LGN (and the ganglion cells)

1. cortical organization

• arranged in 6 layers

• layer IV is subdivided into three separate layers (IV A, B, and C)

2. LGN projects primarily to layer layer IV C

3. layer IV C is divided into two tiers: Alpha and Beta

4. magnocellular LGN layers project to IV C Alpha (movement)

5. parvocellular LGN layers project to IV C Beta (color)

sound wave

• alternating compressed and rarefied air

• a mechanical wave moving through air

frequency

• number of waves per second

• higher frequency = higher pitch and more energy

amplitude

• height of the sound wave

• determines how much air moves

• greater amplitude = louder sound

anatomy of ear: outer

• pinna: visible cartilaginous part of the ear that helps collect and direct sound waves into the ear canal

• external auditory meatus: opening leading to auditory canal

• auditory canal: tube-like passage that channels sound waves toward the tympanic membrane and helps amplify certain frequencies

• tympanic membrane: thin membrane that vibrates in response to sound waves, transmitting energy to middle ear

• tensor tympani: small muscle attached to eardrum that dampens excessive vibrations to protect inner ear from loud sounds

anatomy of ear: middle

• ossicles: transmits vibrations from tympanic membrane to inner ear

  • malleus (hammer): connects tympanic membrane

  • incus (anvil): transfers vibrations from malleus to stapes

  • stapes (stirrup): transfer vibrations to oval window of cochlea

• eustachian tube: canal connecting middle ear to the throat (nasopharynx) that equalized pressure on both sides of eardrum

anatomy of ear: inner

• cochlea: spiral-shaped structure filled with fluid and hair cells that convert sound vibrations into neural signals

  • basilar membrane: base (near oval window - detects high-frequency sounds), apex (further inside - detects low-frequency sounds)

• vestibular apparatus: includes semicircular canals, utricle, and sacral (responsible for balance and spatial orientation)

surface area difference between tympanic membrane and oval window

• tympanic membrane = large S.A.

• oval window = small S.A.

• same force concentrated on smaller area –> increase in pressure

• why is it necessary?

  • air is easy to move

  • cochlea contains fluid, which makes it harder to move (needs more force to move)

anatomy of the cochlea: fluid-filled chamber

• scala tympani: lower chamber, filled with perilymph (high Na+, low K+) , connects to round window

• scala media (cochlear duct): middle layer filled with endolymph (low Na+, high K+), houses organ of corti, where sound transduction occurs

• scala vestibuli: upper chamber, filled with perilymph, connects to oval window

anatomy of the cochlea: membranes that separate chambers

• basilar: separates scala media from scala tympani, supports organ of corti and is essential for frequency detection

• vestibular (Reissner’s): separates scala media from scala vestibuli, helps maintain fluid balance

oval window

• stapes transfers sound vibration to oval window

• initiating fluid movement in cochlea

round window

acts as pressure release valve, allowing fluid displacement when oval window vibrates

basilar membrane: difference between base and apex

• base (near oval window): narrow and stiff, detects high frequency sounds (high-pitched 20 kHz)

• apex (far end of cochlea): wide and flexible, detects low frequency sounds (low-pitched 20 Hz)

basilar membrane: correspondence with sound wave frequency

• different regions of basilar membrane vibrate in response to different frequencies – tonotopic organization

  • base: requires high energy and responds to high frequency

  • apex: requires less energy and responds to low frequency

• higher frequencies vibrate at base, lower frequencies vibrates apex; allows brain to distinguish pitch

anatomy of the Organ of Corti: basilar membrane

same shit

anatomy of the Organ of Corti: hair cells

• inner hair cells (IHCs): primary sensory receptors, sends auditory signals to brain

• outer hair cells (OHCs): amplifies sound and increase sensitivity of IHCs by modifying basilar membrane motion

• stereocilia: tiny hair-lke structure on top of hair cells that bend in response to sound induced movement

anatomy of the Organ of Corti: tectorial membrane

• gelatinous structure that overlies hair cells

• outer hair cells are embedded in it, while IHCs are stimulated by endolymph movement

how are mechanical forces are transduced into neural based signals

• shearing of hair cells

  • when the basilar membrane moves, hair cells are pushed against tectorial membrane

  • bending opens ion channels in stereo cilia

• mechanical gating of K+ channels

  • hair cells’ stereocilia are connected by tip links, which open K+ channels when stretched

  • because scala media has high K+ concentration (due to endolymph), K+ enters hair cells –> causes depolarization

• transduction that cause NT to release

  • depolarization of hair cells due to K+ influx

  • Ca2+ channels open, leading to an influx of Ca2+

  • NT (glutamate) is released onto auditory nerve fibers

  • auditory never sends AP to brain via cochlear nerve

how is amplitude encoded

• proportion of activated hair cells

  • louder sounds displace basilar membrane more, activates more hair cells

  • increased activation results in stronger signal sent to brain

• firing of individual hair cells

  • greater amplitude = greater hair cell depolarization; leads to more NT release and higher firing rates of auditory nerve

• brain intercepts rate of firing and number of active neurons determine loudness

sound localization

brain determines the location of a sound using 2 main mechanisms, depending on the frequency of the sound

sound localization: MSO (medial superior olive)

• coincidence detection for low-frequency sounds

• function: detects interaural time differences (ITD)- the tiny difference in when sound reaches the ear

• how it works:

  • if a sound arrives sooner at one ear, the neurons in the MSO compare timing differences between the ears to determine the sound’s direction

  • used for low-frequency sounds (<1,500 Hz) since they have longer wavelengths, making time differences easier to detect

sound localization: LSO (lateral superior olive)/MNTB (medial nucleus of the trapezoid body)

• level differences for high-frequency sounds

• function: detects interaural level differences (ILD)- the difference in sound intensity between the two ears

• how it works:

  • head creates a “sound shadow” for high-frequency sounds (>3,000 Hz), meaning one ear hears the sound more intensely than the other

  • LSO is excited by the stronger signal from the ear closer to the sound

  • MNTB inhibits the weaker signal from the opposite ear, helping pinpoint the sound’s direction

4 types of somatic receptors

1. proprioceptors

2. touch receptors

3. thermoreceptors

4. nociceptors

proprioceptors

• body position and movement

• detect: muscle stretch, joint position, body movement

• receptors: muscle spindles, Golgi tendon organs

• function: provide body awareness (proprioception) and help coordinate movement

touch receptors

• mechanoreceptors

• detect: light touch, vibration, pressure, texture

• types:

  • Meissner’s corpuscles- light touch, fast-adapting

  • Merkel cells- pressure, texture, slow-adapting

  • Pacinian corpuscles- deep vibration, fast-adapting

  • Ruffini endings- skin stretch, slow-adapting