C&M Neuro final

what is the only imput the brain receives from the “real” world

a series of action potentials passed along the neurons of various sensory pathways

what are sensory receptors

specialized cells that convert sensory energy (like light) into neural activity

What are the sensory system receptors and what are the band of energy they respond to

1. vision: light energy and chemical energy

2. auditory: air pressure and mechanical energy

3. somatosensory: mechanical energy

4. taste and olfaction: chemical molecules

what is sensation

the registration of physical stimuli from the environment by sensory organs/receptors

are our visual experiences objective representations of the world or subjective constructions of reality manufactured by the brain

subjective constructions of reality manufactured by the brain

what are the main sturctures of the eye

• cornea: clear outer covering of the eye

• lens: focused light; bends to accommodate near and far objects

• retina: where light energy initiates neural activity. light-sensitive surface at the back of the eye locating neurons and photoreceptor cells

• fovea: region at the center of the retina that is specialized for high acuity. recptive field at the center of the eye’s visual field

which eye structure had the largest visual process

Fovea (focus area)

what are retina ganglion cells

types of neurons located near the inner surface (ganglion cell layer) of the retina that recives info from photorecptors via bipolar cells and retina amacrine cells 

what happens when there is less excitatory neurotransmitters onto ganglion cells

they become more active

what is the retina ganglion cells responsible for

transmit image-forming and non-image forming visual information from the retina in the form of action potentials

what brain regions do retian ganglion cells transmit  action potentials to?

several regions in thalamus, hippothalamus and midbrain

How is light handled in the eyes

1. light enters optic nerve

2. follows down the ganglion cell, inner plexiform layer

3. then goes to amacrine and bipolar cells, then down to outer plexiform layer

4. finally it reaches the cones and rods

What do the retinal ganglion cells in the retinohypothalamic tract do?

A small subset of retinal ganglion cells send axons through the retinohypothalamic tract, helping regulate circadian rhythms and control the pupillary light reflex (pupil resizing).

differenciate between rods and cones

• rods are more numerous than cones, more sensitive to low levels of light (dim), so they are mainly used for night vision. Only one opsin protein [G-coupled protein receptors] (Rhodopsin)

• cones are more responsive to bright light and are specilaized for color and high visiual acuity, located in the favoa and has 3 types of opsin protein [G-coupled protein receptors] (S-opsin, M-opsin, L-opsin)

what are the opsin(s) proteins for rods

Rhodopsin

what are the opsin(s) proteins for cones

S-opsin, M-opsin, L-opsin

what are the 2 types of photoreceptors

1. Rod: generic photorecptor

2. cone: respond to different frequencies of light but the same function and structure as rods

What are the three main parts of photoreceptors and what happens in each?

• Outer segment: Contains visual pigment molecules (e.g., rhodopsin).

• Inner segment: Holds the cell body, nucleus, mitochondria, ribosomes; this is where opsin molecules are assembled.

• Synaptic terminal: Releases neurotransmitters to the next neurons in the visual pathway.

What happens in a rod in the dark

• Outer segment:

  • cGMP levels are high

  • cGMP-gated channels are open, letting Na⁺ and Ca²⁺ flow into the cell

  • This creates the dark current

• Inner segment:

  • The influx of positive ions (Na+)  keeps the rod depolarized (around –40 mV)

  • The inner segment continuously pumps K⁺ out to balance the dark current

  • Because the rod is depolarized, it releases a tonic (continuous) small amount of glutamate at the synaptic terminal

What happens in a rod when there is light

• Outer segment:

  • cGMP levels are reduced

  • cGMP-gated Na⁺/Ca²⁺ channels CLOSE

  • Na⁺ and Ca²⁺ stop entering the cell

• Inner segment:

  • With less Na⁺ coming in, the rod becomes more negative inside → hyperpolarized (drops from about –40 mV down to around –70 mV)

  • The inner segment keeps pumping K⁺ out, but now there's no big Na⁺ influx to balance it

  • Because the rod is hyperpolarized, it reduces its glutamate release at the synaptic terminal

How do visual pigments in the outer segment absorb light?

• Rhodopsin is reduced to Meta II

• Meta II interacts with G-protein transducin (alpha,beta and gamma subunits)

• GDP that is attached on Galpha is reduced to GTP

• G-alpha dissociates from beta-gamma and activates phosphodiesterase

• activation causes cGMP to be hydrolysed to GMP which causes hyperpolorization

what is rhodopsin

• A opsin protein with light-absorbing retinal.

• Opsin is a G-protein-coupled receptor (GPCR), initiating phototransduction when light activates retinal.

how many ransmembrane (hydrophobic) helices does opsin (rhodopsin) have

7

what is the process of bleaching rhodopsin

1. photon of light absorbed by Rhodopsion 11-cis retinal changes its 3D shape to all-trans shape in picoseconds

2. within 10-12 seconds All-trans retinal undergoes photoisomerization (changes configuration) - only light sensitive step - to make Metarhodopsin II

3. All-trans retinal + opsin is precursor
   for synthesis of Rhodopsin 11-cis retinal

what is Metarhodopsin II

crucial for phototransduction

• Activates 2nd messenger systems

can all trans retinal be synthesised naturally by humans

no, Brought in from diet (vitamin A). Vitamin A deficiencies can lead to night blindness

What happens when rhodopsin becomes Meta II?

• Inactive rhodopsin: opsin + 11-cis-retinal.

• Meta II: light converts 11-cis-retinal → all-trans-retinal.

• Meta II adopts a G-protein-interacting conformation, allowing Gα binding to start the phototransduction cascade.

what is the correlation between light intensity and hyperpolarization

brighter lights = more hyperpolarization

How do photoreceptors respond electrically to light in vertebrates vs. invertebrates?

• Invertebrates: photorecptors respond to light with depolarization

• Vertebrates: photoreceptors respond to light with a graded hyperpolarisation response (the magnitude of which depends on light intensity).

How does Meta II rhodopsin trigger phototransduction?

• Photoactivated rhodopsin (Meta II) interacts with rod proteins via its cytoplasmic surface.

• It binds and activates the G-protein transducin, initiating the phototransduction cascade in the rod cell.

What are the steps of transduction

1. Meta II acts on G-protein transducin (Gα, Gβ, Gγ).

2. GDP bound to Gα is exchanged for GTP 

3. Gα dissociates from Gβγ and activates membrane-bound phosphodiesterase (PDE).

What happens after phosphodiesterase (PDE) is activated in phototransduction?

• PDE hydrolyzes cGMP → GMP, lowering intracellular cGMP levels.

• Fewer cGMP-gated Na⁺/Ca²⁺ channels remain open.

• Rod hyperpolarizes, reducing neurotransmitter release.

What is sound

pressure wave of different frequencies, important for speech, music and other natural sounds

How is sound perceived

changes in air pressure and brain interprets them. Perception of sound requires a brain, without it sound doesn’t exist.

what are the properties of sound

• Pure tone = sine wave

• pitch = frequency

• loudness/intensity = amplitude.

• Most sounds are complex, not pure tones.

How does the auditory system convert air pressure into neural signals?

1. Tympanic membrane vibrates in response to air pressure changes.

2. Vibrations are amplified by ossicles and transmitted to the cochlea.

3. Basilar membrane vibrates at different frequencies; contains hair cells (auditory receptors).

4. Hair cells move against tectorial membrane, opening K⁺ channels.

5. Ion influx generates neural signals that are sent to the brain.

How are sounds encoded?

1. sound waves are converted to fluid waves in cochlea

2. Fluid waves cause basilar membrane vibration.

3. Hair cells on basilar membrane transduce mechanical movement.

4. K⁺, not Na⁺, flows into hair cells due to high extracellular K⁺, generating receptor potentials.

where does Sensory transduction occurs in Audition

in the hair cells located on the basilar membrane within the cochlea

What is the cochlear environment and hair cell arrangement?

• Scala media is extracellular fluid high in K⁺, low in Na⁺ (like neuron cytoplasm).

• Inner and outer hair cells sit on the basilar membrane.

• Hair cell sensory cilia project into the tectorial membrane to transduce sound vibrations.

where are the hair cells are stuck in the basilar

membrane

Scala media

How do hair cell cilia transduce sound?

• Sensory cilia bend against the tectorial membrane.

• Bending opens K⁺ channels, allowing K⁺ influx.

• This mechanical depolarization (not voltage-gated or chemical) triggers receptor potentials.

How does the basilar membrane encode sound frequency?

air pressure produce a traveling (water) wave that moves along the basilar membrane

• Fast wave frequencies: cause maximum displacement near the base of the membrane

• Slower wave frequencies: cause maximum displacement near the membrane’s apex

How does basilar membrane location affect frequency detection?

• Base (near oval window): responds to high frequencies.

• Apex (far from oval window): responds to low frequencies.

• Hair cell loss or stiffening at base → loss of high-frequency hearing.

how does aging affect the basilar membrane

• Basilar membrane becomes less flexible, reducing frequency sensitivity.

How do auditory hair cells transduce sound into neural signals?

1. Basilar membrane moves → bends hair cell cilia (mechanical shearing).

2. K⁺ channels open in the cilia → depolarization of the hair cell.

3. Depolarization opens voltage-gated Ca²⁺ channels at the hair cell’s base.

4. Ca²⁺ influx triggers neurotransmitter release onto auditory nerve fibers.

How do hair bundles enable mechanoelectrical depolorization in hair cells?

Displacement of hair bundle to taller side increases tension to promote channel opening

• Influx of K+

• Depolarization

How does hair bundle displacement lead to depolarization?

1. Displacement of the hair bundle toward the tallest cilium opens K+ channels

2. causing a massive K+ influx due to the high extracellular potassium concentration in the scala media

3. resulting in depolarization and subsequent Ca++ entry and neurotransmitter release

What happens during hair cell depolarization leading to neurotransmitter release?

Hair bundle displacement (toward the tallest cilium) opens K+ channels, causing a massive K+ influx because the surrounding fluid (scala media) is uniquely high in potassium. The K+ entry depolarizes the cell, which opens voltage-sensitive Ca++ channels2. The resulting Ca++ influx triggers the release of neurotransmitter

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What happens at the hair cell’s active zone?

• The active zone is always loaded with NT-filled vesicles ready for release.

• When Ca²⁺ enters, vesicles fuse and release neurotransmitter.

• NT binds to receptors on the postsynaptic auditory nerve fiber, sending the signal toward the brain.

What are inner cochlear hair cells

• 1 row on basilar membrane, inside organ of Corti, spanningthe entire lenght of the cochlea

• true sensory receptors

• Carry ~95% of afferent auditory nerve fibers

What are outer cochlear hair cells

• 3 rows in the brain lateral side of the organ of Corti

• receive efferent output from the brain

• involved in modulating basilar membrane movement.

differenciate between the inner and outer hair cells

inner = the bougie VIP “I actually send signals to the brain” cells

outer = the hype squad that amplifies and sharpens the sound

What does the superior olivary complex detect?

Sound localization — it figures out where a sound is coming from.

• Receives input from both ears via left and right projections to compare timing and intensity differences.

What is the main role of the inferior colliculus?

Orientation — letting you turn toward and focus on a sound.

• Helps orient the head and eyes toward a sound.

• Receives crossed input from the superior olivary complex for spatial orientation.

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What happens at the medial geniculate nucleus (MGN) of the thalamus?

Frequencies get separated (tonotopic organization continues). Acts as a relay to the auditory cortex.

From the auditory cortex onward, what can you determine about a sound?

The identity — “what is that sound?”

• Pattern separation begins earlier at hair cells on the basilar membrane.

Where does pattern separation in the auditory system begin?

At the hair cells on the basilar membrane.

How does the auditory system localize sound (WHERE)?

• Medial Superior Olive (MSO) computes binaural time differences.

• Strength of MSO response depends on path lengths from each ear → arrival timing differences.

• Systematic differences in timing of 2 inputs (ears) create a map of sound location in space.

what is the Medial Superior Olive (MSO)

An optimal circuit for optimizing binaural differences

How does the superior olivary complex use timing to localize sound?

compares sound arival from one ear and the other. if the left ear has a longer path than the right ear then it creates a timing difference. the time differece allows the olivary complex to detect sound location

What principle allows the MSO to map sound location?

• Systematic variation in input delays from each ear.

• Neurons in the MSO fire maximally when inputs from both ears arrive simultaneously, encoding sound direction.

What is the role of the inferior colliculus in auditory processing?

• Integrates and routes multi-modal sensory information.

• Important for startle response and vestibulo-ocular reflex (orientation to sound).

• Responds to specific amplitude modulation frequencies, contributing to pitch detection.

Where is the inferior colliculus located?

• Part of the midbrain

• Positioned above the pons and medulla oblongata, near the fourth ventricle.

What key structures surround the inferior colliculus?

• Superior colliculus (L) – above, visual orientation.

• Flocculus (G) – part of cerebellum, balance.

• Fourth ventricle (E) – nearby fluid space.

Which thalamic nucleus processes auditory information (“WHAT”)?

medial geniculate nucleus of the thalamus

Where is the Primary Auditory Cortex (A1) located?

• Found in Heschl’s gyrus within the temporal lobes.

• Structures are asymmetrical between hemispheres.

what is proprioception

perception of the location and movement of the body. sensitive to stretch of muscles and tendons and the movement of joins

what is the characteristic of free nerve endings

• high threshold

• can detect pain

• located in the epidermis of the skin

what are proprioreceptors

deep receptors in muscle fibers

how are mechanical forces on the skin conveyed to the CNS

via an array of somatosensory afferent neurons

what is the main Somatosensory afferent neuron

mechanoreceptors

what are the 2 types of mechanoreceptors

1. slowly adapting afferents

2. rapidly adapting afferents

What do slowly adapting mechanoreceptors do?

• Fire continuously as long as the stimulus is present.

• Encode size and shape of the stimulus (e.g., holding a heavy object).

• tells the brain if a body part is in an odd position

What do rapidly adapting mechanoreceptors do?

• Fire only at stimulus onset and sometimes offset.

• Encode movement or change of a stimulus (e.g., feeling clothing brush against skin).

What is sensory transduction?

Conversion of stimulus energy into an electrical signal in a sensory neuron.

What is a receptor potential?

a depolarising current caused by stimulus opening ion channels in afferent nerve endings

Which ion channels do Merkel cells use for mechanotransduction?

the peizo2 ion channels which open in response to mechanical stimuli

What is proprioception and why is it important?

• Senses position of limbs and body in 3D space.

• Essential for complex movements and postural control.

What are muscle spindles and what do they detect?

• Composed of 4–8 intrafusal muscle fibers inside a connective tissue capsule.

• Detect changes in muscle stretch and length.

What are extrafusal muscle fibers?

• Skeletal muscle fibers that generate force for movement.

• Surround and work with intrafusal fibers in proprioception.

what is the purpose of Tendons

attach musles to bones, allowing for movement when the muscle contracts

What are the 3 types of joint mechanoreceptors?

1. Type I: Slowly adapting, located in outer layers of joint capsule.

2. Type II: Rapidly adapting, detect movement/change.

3. Type III: Slowly adapting, found in ligaments and tendon terminals.

Which joint receptors are slowly vs. rapidly adapting?

• Slowly adapting: Type I (joint capsule) & Type III (ligaments/tendons)

• Rapidly adapting: Type II

What is the structure of a mammalian muscle spindle?

• Intrafusal fibers (small) are embedded within the bulk of the muscle.

• Surrounded by extrafusal fibers (bulk of the muscle).

Which neurons innervate muscle fibers?

• α-motoneurons: innervate extrafusal fibers (generate force).

• γ-motoneurons: innervate intrafusal fibers (adjust spindle sensitivity).

What are the afferent fibers of the muscle spindle?

• Group Ia: large diameter, fast-conducting, form primary endings. myelinated by schwann cells

• Group II: smaller diameter, slower-conducting, form secondary endings.

Which muscle spindle afferents are fast vs. slow?

• Fast: Group Ia (primary endings)

• Slow: Group II (secondary endings)

What do primary endings of muscle spindles detect?

• Connected to Group Ia axons.

• Sensitive to rate of change of stretch (dynamic stretch).

• Fire maximally during the dynamic phase, then adapt if the stretch is maintained.

What do secondary endings of muscle spindles detect?

1. connected to group II axons

2. sensitive to static muscle tension level (slowly adapting)

3. firing gradually increases with tension and maintains discharge during steady stretch

How can mechanoreceptor ion channels be directly activated?

• A. Lipid tension: forces in the membrane open channels.

• B. Structural protein linkages: intracellular or extracellular proteins connected to channels transmit force to open them.

How can mechanoreceptors be activated indirectly?

• Indirect activation: a separate protein force sensor detects the stimulus.

• Signal is transmitted via a second messenger, which opens ion channels.

How do haptic-proprioceptive axons ascend to the brain?

• Travel ipsilaterally up the spinal cord. (on the same side of the body)

• Cross at the brainstem before reaching the brain.

How do nociceptive axons (pain, temperature, itch) ascend to the brain?

• Synapse in the spinal cord.

• Axons cross to the contralateral side immediately. before brain

• Then ascend to the brain.

Where do proprioceptive afferents for the lower body synapse?

On neurons in the dorsal and ventral horns of the spinal cord.

what is the main receptor on free nerve ending

TRP ion channels for Na+ and Ca2+ .

What are nociceptors and how do they detect stimuli?

• TRP (Transient Receptor Potential) ion channels are on sensory nerve endings.

• Function as the “front line” for detecting noxious, irritating, and inflammatory stimuli.

What stimuli do TRP channels directly sense?

• Endogenous and exogenous chemical, mechanical, and thermal stimuli.

• Present on sensory nerve endings (free nerve endings).

How are TRP channels linked to GPCR signaling?

• TRP channels act as downstream effectors of G-protein coupled receptor (GPCR) activity.

• This GPCR-TRP axis contributes to pain, itch, cough, and neurogenic inflammation.

What is the functional significance of TRP channel sensitization?

• High-threshold TRP channels can respond to subtle inputs like flutter or breeze.

• Sensitization increases responsiveness to noxious stimuli via GPCR activity.