Neurobiology Exam 4

What is the pathway that the visual system follows?

Photoreceptors → Bipolar Cells → Retinal Ganglion Cells (RGCs) → Lateral Geniculate Nucleus (LGN) → Primary Visual Cortex (V1) → Extrastriate Cortical Areas (V2, Dorsal/Ventral Streams)

What do are photoreceptors and what do they do?

• They are light-senstive cells in the retina that releases glutamate.

• They transduce light energy into electrical signals (graded changes in membrane potential)

What is the role of bipolar cells?

• They act as a relay station within the retina, receiving direct synaptic input from photoreceptors and passing it to retinal ganglion cells.

• Modified by horizontal cells to provide lateral inhibition to create surround center receptive fields

What is the role of retinal ganglion cells?

• They fire all-or-none action potentials and are the source of visual output leaving the eye

• Their axons form the optic nerve, optic chiasm, and optic tract

What is the role of lateral geniculate nucleus?

A structure in the dorsal thalmus that segregates visual input from the two eyes and different ganglion cell types into distinct layers

What is the role of primary visual cortex?

• The first cortical area to receive visual information, primarily arriving in layer 4C.

• V1 integrates information from multiple pathways to compute features such as stereoscopic depth (binocularity), object shape (orientation selectivity), and color

What is the role of Extrastriate areas?

They further processes specific visual features through divergent, higher-order cortical pathways

What is the role of the cornea in vision?

• Its the glassy, transparent external surface of the eye that performs the majority of the eye’s refractive (focusing) power

• Its role is to bend parallel light rays to the back of the eye

What is the role of the lens in vision?

• Located at back of iris to contribute to additional refractive power

• Its primary role is accommodation, a process where the ciliary muscles change the shape of the lens (making it rounder and thicker) to form crisp, focused images of near objects

What is the role of the retina in vision?

• Contains photoreceptors, bipolar cells, and ganglion cells

• Its role is to collect focused light, convert (transduce) that light energy into neural activity, and begin processing the image (such as encoding contrast and color) before transmitting the signals to the brain

What is the function of the ventral extrastriate pathways?

• Projects from V1 ventrally towards the temporal lobe

• The function of this stream is to analyze visual attributes, object recognition, and visual memory.

• The areas involved in this stream (V4 & IT) process object shape, depth, color, and complex biological stimuli like faces

What is the function of the dorsal extrastriate pathways?

• Projects from V1 dorsally toward the parietal lobe.

• The function of this stream is to process spatial vision, object motion, and visually guided action.

• Areas in this stream (MT) are highly direction-selective and compute the movement of objects in the environment

What is the first step involved in transduction of light in rods?

Light energy causes a conformational change in a prebound agonist called retinal, which activates the receptor protein opsin

What is the second step involved in transduction of light in rods?

The activated rhodopsin stimulates a G-protein in the disk membrane called transducin

What is the third step involved in transduction of light in rods?

The activated alpha subunit of transducin turns on the effector enzyme PDE.

What is the fourth step involved in transduction of light in rods?

PDE rapidly breaks down (hydrolyzes) cGMP molecules that are normally present in the cytoplasm in the dark.

What is the fifth step involved in transduction of light in rods?

Since cGMP is required to keep specialized sodium channels open, its destruction causes these channels to close. The reduction in the inward flow of Na + (the dark current) causes the cell membrane to hyperpolarize.

What are some consequences if these steps are manipulated?

Because this transduction cascade relies on sequential activation to reduce cGMP, manipulating the steps will radically alter the rod's resting potential.

• If PDE is inhibited or missing: cGMP would not be broken down when light hits the rod. Consequently, the cGMP-gated Na + channels would remain open, the dark current would persist, and the photoreceptor would remain permanently depolarized, rendering the rod completely "blind" to light.

• If transducin cannot exchange GDP for GTP: The G-protein could never activate PDE. Light would bleach rhodopsin, but the signal would halt there, failing to close the Na + channels and preventing hyperpolarization.

How is information encoded in photoreceptors?

• Encode visual information using graded membrane potential

• They release glutamate in the dark and decrease their glutamate release proportionally in response to light intensity (dim vs. bright light changes the depth of hyperpolarization)

• Cones encode color via population coding, as the brain interprets color based on the relative, overlapping activation ratios of three different broadly tuned cone types (red, green, and blue)

How is information encoded in off-center bipolar cells?

• Contain ionotropic glutamate receptors.

• In the dark, high glutamate release from photoreceptors causes Na + influx, depolarizing the cell.

• In the light, less glutamate means less Na + enters, and the cell hyperpolarizes.

How is information encoded in on-center bipolar cells?

• Contain metabotropic (G-protein-coupled) glutamate receptors.

• In the dark, glutamate activates these receptors to close cation channels, hyperpolarizing the cell.

• In the light, reduced glutamate relieves this inhibition, causing the cell to depolarize.

How is information encoded in retinal ganglion cells?

• First cells in the pathway to fire action potentials and they encode contrast through rate coding (changes in the frequency of action potentials)

• Firing rate is highly modulated by spatial differences in light because they have antagonistic center-surround receptive fields

• Example:

  • An ON-center ganglion cell will fire a rapid burst of action potentials (high rate) if light hits the center of its receptive field, but its firing rate will be suppressed below baseline if light hits the surround.

What is spatial coding (retinotopy) in the CNS?

Visual information is encoded by maintaining precise spatial maps. Neighboring cells in the retina project to neighboring cells in the LGN and primary visual cortex, ensuring that a discrete point of light on the retina corresponds to a specific, mapped location of neural activity in the visual cortex.

What is population coding in the CNS?

• The CNS does not rely on single highly specialized "grandmother cells" to perceive complex stimuli. Instead, perception is based on the distributed pattern of activity across large populations of broadly tuned neurons.

  • For example, the precise direction of a moving object is determined by tallying and averaging the "votes" (discharge rates) of a vast population of cells in motor or extrastriate areas, each tuned to a broad range of directions. Similarly, color is decoded by comparing the combined activity of various red/green and blue/yellow color-opponent cells

What is the neural pathway from the tongue the gustatory cortex?

The sensation of taste primarily begins on the tongue, which is covered in small projections called papillae (foliate, vallate, and fungiform)

• While the tip of the tongue is most sensitive to sweetness, the back to bitterness, and the sides to saltiness and sourness, most of the tongue is sensitive to all five basic tastes.

• Taste receptor cells are classified into two groups based on their tuning: specialists, which respond primarily or exclusively to just one basic taste (salt, sour, sweet, bitter, or umami), and generalists, which respond strongly to two or more basic tastes.

• The neural pathway for gustation begins with the taste cells forming synapses with primary gustatory axons found in three cranial nerves: the facial (cranial nerve 7), glossopharyngeal (cranial nerve 9), and vagus (cranial nerve 10) nerves.

• These axons enter the brain stem and synapse within the gustatory nucleus of the medulla.

• From there, the neurons project to the ventral posterior medial (VPM) nucleus in the thalamus

• Finally, the VPM taste neurons send their axons to the primary gustatory cortex

How is rate coding used in gustation?

• Rate coding involves measuring the number of action potentials (spikes) produced by a specific receptor cell or afferent axon in response to an optimally preferred chemical stimulus

How is population coding used in gustation?

• The brain primarily relies on population coding to convey complex combinations of tastes. Because many taste axons and central neurons receive convergent input from generalist taste cells, their individual responses are broadly tuned and ambiguous.

• By utilizing population coding, the brain compares the combined firing rates across a large population of broadly tuned neurons—where some fire strongly, some moderately, and some not at all—to clearly specify the unique properties of a particular flavor

Describe the anatomy of the olfactory epithelium and olfactory bulb.

• The organ of smell is a thin sheet of cells high in the nasal cavity called the olfactory epithelium, which consists of three main cell types: olfactory receptor cells (which are genuine neurons), supporting cells (which produce mucus), and basal stem cells.

• The olfactory receptor neurons (ORNs) have long, thin cilia waving in the mucus layer where odorants bind

• The unmyelinated axons of these ORNs bundle together to form the olfactory nerve, penetrate the bony cribriform plate, and enter the olfactory bulb. The input layer of the olfactory bulb contains spherical structures called glomeruli

  • A massive convergence occurs within each glomerulus: the endings of about 25,000 primary olfactory axons terminate onto the dendrites of only about 100 second-order neurons, known as mitral cells. This mapping is incredibly precise, as all ORNs expressing a specific receptor gene converge onto the exact same glomeruli

What is the first step involved in the transduction of olfactants?

Odorants dissolve in the mucus layer and bind to odorant receptor proteins on the cilium membrane to activate a G-protein Golf

What is the second step involved in the transduction of olfactants?

Golf stimulates the enzyme adenylyl cyclase, which produces the intracellular second messenger cAMP.

What is the third step involved in the transduction of olfactants?

cAMP directly binds to and opens cyclic nucleotide-gated (CNG) cation channels, allowing an influx of Na+ and Ca2+.

What is the fourth step involved in the transduction of olfactants?

Increase in intracellular Ca2+ triggers the opening of Ca2+-activated Cl- channels.

What is the fifth step involved in the transduction of olfactants?

Because the internal Cl- concentration is unusually high in olfactory cells, Cl- flows outward, causing a depolarizing receptor potential. If this potential is large enough, it triggers action potentials.

How can altering transduction of olfactants affect perceptions of odor?

Altering transduction can severely impair the perception of odors. For instance, if genetic engineering is used to knock out critical transduction proteins such as Golf, adenylyl cyclase, or the CNG channel, mice are rendered anosmic (unable to smell) for a wide variety of odors

What are the neural projection pathways from the olfactory epithelium to the orbitofrontal cortex?

olfactory receptor cells sending their axons into the olfactory bulb → axons course through the olfactory tract → To reach the orbitofrontal cortex, which is responsible for the conscious perception of smell, the pathway travels from the olfactory tract to the olfactory tubercle → then to the medial dorsal nucleus of the thalamus → projecting into the orbitofrontal cortex

How is population coding used in the gustatory system?

The gustatory system identifies tastes by evaluating the unique overall pattern of discharge rates across a massive population of broadly tuned neurons, allowing the brain to distinguish specific foods from thousands of alternatives.

How is population coding used by the olfactory system to identify tastes/odors?

Individual olfactory receptors are broadly tuned to many different chemicals, and single chemicals activate multiple receptor types. The olfactory system uses the combined pattern of activity from this large population of neurons to unambiguously identify specific odors.

How is spatial mapping used by the olfactory system to identify tastes/odors?

An odor is converted into a sensory map, which is a specific and reproducible spatial pattern of activated glomeruli in the olfactory bulb and active neurons in the olfactory cortex. The brain may decode the identity of an odor by tracking these topological patterns of neural activation across "neural space".

How is temporal coding used by the olfactory system to identify tastes/odors?

The olfactory system also relies on the precise timing of action potentials to encode the quality of an odor. The brain analyzes an odor by tracking when olfactory neurons fire, their rhythmic patterns, and whether they fire synchronously. Disruption of rhythmic synchronous spiking has been shown to eliminate the ability to discriminate between similar odors

How does mechnoreceptors explain how somatosensation conveys complex signals through different receptor types?

• These receptors detect physical distortion such as touch, vibration, and pressure, sending signals via fast Aβ axons. The specific sensation is related to the receptor's structure and location:

  • Pacinian corpuscles are large, deep receptors that detect high-frequency, rapid vibrations (200–300 Hz).

  • Meissner’s corpuscles are located in the ridges of hairless skin and detect moderate vibrations (40–60 Hz), giving us the sensation of flutter and rough textures.

  • Merkel’s disks lie close to the skin's surface and detect the edges, corners, and points of objects pressed against the skin.

  • Ruffini’s corpuscles lie deep in the dermis and detect the stretching of the ski

How do nociceptors explain how somatosensation conveys complex signals through different receptor types?

These are free nerve endings that signal tissue damage or the threat of it. While many are polymodal (responding to mechanical, thermal, and chemical stimuli), others are selectively tuned to exclusively detect extreme pressure, burning heat, freezing cold, or specific chemical irritants.

How does thermoreceptors explain how somatosensation conveys complex signals through different receptor types?

These free nerve endings are highly sensitive to temperature. Distinct populations of receptors are specifically tuned to sense mild warmth, mild coolness, painfully hot temperatures, or noxiously cold temperatures.

How does proptioreceptors explain how somatosensation conveys complex signals through different receptor types?

These specialized receptors inform the brain about body position and movement. Muscle spindles (innervated by Ia axons) detect changes in muscle length (stretch), Golgi tendon organs (innervated by Ib axons) detect muscle tension, and joint receptors respond to changes in the angle, direction, and velocity of joint movement

What is the circuit trajectory of touch receptors? (Dorsal Column–Medial Lemniscal Pathway)

touch travel via fast, myelinated Aβ axons that enter the spinal cord and immediately ascend the ipsilateral dorsal column → axons synapse in the dorsal column nuclei at the junction of the spinal cord and medulla → second-order axons cross to the opposite side of the brain → ascend through the medial lemniscus to synapse in the ventral posterior (VP) nucleus of the thalamus → Thalamic neurons then project to the primary somatosensory cortex (S1)

What is the circuit trajectory of pain receptors?

pain travels via slower Aδ and C fibers that enter the spinal cord and branch in the zone of Lissauer → fibers synapse onto second-order neurons in the substantia gelatinosa of the dorsal horn → second-order pain axons cross the opposite side of the brain immediately within the spinal cord and ascend via the contralateral anterolateral tract → axons synapse over broader regions of the thalamus, including the VP nucleus and intralaminar nuclei, before projecting to a wide expanse of the cerebral cortex

How does transduction manifest in touch receptors?

• Touch receptors detect physical forces (stretching, bending, pressure) using mechanosensitive ion channels (such as PIEZO2). These channels are forced open directly by the stretching of the lipid membrane or by tension from linked cytoskeletal or extracellular proteins.

• For both touch and pain, the opening of these specialized ion channels allows an influx of cations, primarily Na + and Ca 2+. This influx causes the cell membrane to depolarize, generating an excitatory receptor potential. If the depolarization is sufficiently large, it triggers action potentials.

• Touch signals propagate rapidly to the central nervous system via heavily myelinated, fast-conducting Aβ axons (up to 75 m/sec).

How does transduction manifest in pain receptors?

• Pain receptors detect stimuli that damage tissue; they have ion channels that open in response to extreme temperatures (e.g., TRPV1 opens above 43°C), intense mechanical stretching, or inflammatory chemicals released from damaged cells, such as ATP, K +, bradykinin, and histamine.

• For both touch and pain, the opening of these specialized ion channels allows an influx of cations, primarily Na + and Ca 2+. This influx causes the cell membrane to depolarize, generating an excitatory receptor potential. If the depolarization is sufficiently large, it triggers action potentials.

• Pain signals are propagated much more slowly via lightly myelinated Aδ fibers (causing a fast, sharp "first pain") and unmyelinated, thin C fibers (causing a delayed, dull, and longer-lasting "second pain" at speeds of just 0.5–2 m/sec)

How does mechanosensory responses depend on receptive fields?

A receptor's response depends on the size of its receptive field on the skin. Meissner’s corpuscles and Merkel’s disks have very small receptive fields (just a few millimeters wide), making them highly sensitive to localized, fine details. Conversely, Pacinian and Ruffini’s corpuscles have large receptive fields that can cover an entire finger or half the palm, detecting broader patterns of stimulation.

How does mechanosensory responses depend on adaption?

Responses heavily depend on how receptors process continuous stimuli over time. Rapidly adapting receptors (Meissner’s and Pacinian corpuscles) fire quickly at the onset of a stimulus but stop firing if the pressure is maintained; this makes them highly sensitive to vibrations and changes in stimulus. Slowly adapting receptors (Merkel’s disks and Ruffini’s corpuscles) provide a sustained, continuous action potential discharge as long as the stimulus is applied.

How does mechanosensory responses depend on lateral inhibition?

As somatosensory information travels through relays like the dorsal column nuclei, circuits utilize lateral inhibition to process and refine the stimulus. In this process, stimulated neurons synaptically excite inhibitory interneurons that suppress the firing of adjacent, less-active neurons. This creates contrast enhancement, amplifying the difference between stimulated and unstimulated areas of skin to sharply define the stimulus.

What is two-point discrimination?

the minimum distance required to differentiate between two points touching the body simultaneously that varies greatly across the body, with the highest resolution found at the fingertips

How does two-point discrimination arises from receptive field organization?

• This high-resolution acuity arises from four organizational factors:

  • There is a much higher density of mechanoreceptors in the skin of the fingertips compared to other areas

  • The fingertips are heavily enriched in receptor types that have small receptive fields, such as Merkel's disks, allowing for precise localization

  • The brain's somatotopic map devotes a disproportionately larger amount of cortical tissue (and thus computational processing power) to the sensory input from each square millimeter of the fingertips.

  • The system relies on specific neural mechanisms, such as lateral inhibition, to enhance the spatial contrast and discriminate between closely spaced stimuli

What is a motor unit and its role in muscle fiber innervation?

the fundamental, elementary component of motor control that consists of a single alpha motor neuron and all the individual skeletal muscle fibers that it innervates.

When the alpha motor neuron fires an action potential, all muscle fibers in that unit contract simultaneously.

What is a motor pool and its role in muscle fiber innervation?

the entire collection of alpha motor neurons that innervate a single, whole muscle (such as the biceps brachii)

How does the medial-lateral axis in the spinal cord relate to motor innervation patterns?

Motor neurons controlling axial (trunk) muscles lie medially, while those controlling distal muscles (hands, feet) lie laterally.

How does the dorsal-ventral axis in the spinal cord relate to motor innervation patterns?

Motor neurons controlling flexor muscles lie dorsal to those controlling extensor muscles.

How does the rostro-caudal axis in the spinal cord relate to motor innervation patterns?

Segments of the spinal cord containing the vast number of motor neurons needed to innervate the complex musculature of the arms and legs are significantly swollen, creating the cervical enlargement (arms) and lumbar enlargement (legs)

How are muscle fiber types determined?

They are intrinsically determined by the type of alpha motor neuron that innervates them; if a nerve from a fast muscle is surgically forced to innervate a slow muscle, the muscle switches its biochemical phenotype to become fast

How do motor units generate different levels of force based on type and recruitment?

• There are three distinct motor unit types: Slow (S) units use oxidative metabolism to resist fatigue for long periods, Fast fatigue-resistant (FR) units generate moderately fast/strong contractions, and Fast fatigable (FF) units generate the strongest, fastest contractions but tire quickly. The central nervous system generates different levels of force using two mechanisms:

  • Rate Coding (Firing Rate): An alpha motor neuron can vary its firing rate. High-frequency action potentials cause individual muscle twitches to summate, creating a smooth, sustained, and higher-tension tetanic contraction.

  • Recruitment (Size Principle): The brain grades force by recruiting synergistic motor units in a strict size order. Smallest, slower motor units are recruited first, while larger, fast fatigable units are recruited last only when maximal force is required.

What are the steps for the Patellar stretch reflex?

A load is added → muscle stretches → muscle spindle stretches → mechanosensitive ion channels in its group Ia sensory axons are activated →Ia axons fire rapidly and enter the spinal cord → alpha motor neurons are depolarized and excited to inervate that same muscle → The muscle reflexively contracts to shorten toward its original length.

What is the functional importance for the patellar stretch reflex?

This arc serves as an antigravity feedback loop to continuously regulate and maintain desired muscle length and posture.

What are the steps for the Golgi tendon organ reflex?

muscle contracts → tension increases on the collagen fibrils in the Golgi tendon organ, squeezing the interwoven group Ib sensory axons → The Ib axons fire and enter the spinal cord → they synapse onto both inhibitory and excitatory interneurons → the interneurons then adjust the alpha motor neurons of the same muscle → this leads to inhibiting them to reduce force, or exciting them during specific tasks like walking

What is the functional importance for the Golgi tendon organ reflex?

This reflex acts as a highly sensitive strain gauge to regulate muscle tension and force, allowing for precise execution of fine motor acts and preventing muscle/joint overload

Compare and contrast the stretch reflex and GTO reflex.

• The stretch reflex relies on muscle spindles situated in parallel with extrafusal muscle fibers to monitor muscle length and operates via a monosynaptic loop for the agonist muscle

• The Golgi tendon reflex relies on tendon organs situated in series to monitor muscle force/tension and is strictly a polysynaptic loop mediated by spinal interneurons

Why is proprioceptive feedback important in movement control?

Proprioception is the "body sense" that continuously informs the brain about how the body is positioned and moving in space. Proprioceptors, including muscle spindles, Golgi tendon organs, and joint receptors, provide critical sensory feedback to the spinal cord and brain. Without this feedback (e.g., due to a genetic loss of PIEZO2 channels), individuals lose their natural sense of limb position, resulting in severely uncoordinated movement (ataxia), irregular gait, and absent reflexes

What is the roles of the lateral pathway of the descending motor tracts in movement control?

Responsible for the voluntary movement of the distal musculature (arms, hands, fingers) and are directly controlled by the cerebral cortex. The primary pathway is the corticospinal tract, which originates in the motor cortex (areas 4 and 6), passes through the midbrain, decussates (crosses) at the medullary pyramid, and descends the lateral spinal cord to control contralateral lower motor neurons.

What is the roles of the ventral pathway of the descending motor tracts in movement control?

Responsible for reflexive posture, balance, and locomotion, and originate in the brainstem. The vestibulospinal and tectospinal tracts keep the head and eyes stable and balanced as the body moves. The pontine and medullary reticulospinal tracts balance the reflex control of antigravity muscles in the trunk and legs to maintain an upright stance

What is the predicted behavioral consequence of damage in the major descending tracts?

Damage to the upper motor system (e.g., from stroke) causes paresis (weakness), paralysis (hemiplegia), hypotonia/areflexia (spinal shock), which often develops into spasticity (hypertonia), and the emergence of the Babinski sign.

What are the functions and inputs/outputs of the primary motor cortex?

• Functions to trigger and execute voluntary movements. It receives input from the premotor areas, somatosensory cortex, and VL thalamus. Its output primarily travels via Layer 5 Betz cells down the corticospinal tract to lower motor neurons.

• Disruption causes contralateral paralysis.

What are the functions and inputs/outputs of the premotor cortex?

Highly interconnected with M1, it functions to plan, select, and sequence willed movements. It fires in anticipation of a required movement.

What are the functions and inputs/outputs of the basal ganglia?

• Deep telencephalon structures that function to focus and initiate desired willed movements while filtering out unwanted ones. It receives input from the widespread cerebral cortex and outputs back to the SMA via the VL nucleus of the thalamus.

• Disruption causes movement initiation failure (Parkinson's disease) or uncontrollable, violent spontaneous movements (Huntington's disease, ballism).

What are the functions and inputs/outputs of the cerebellum?

• Functions to coordinate the precise timing, direction, and force of ongoing multi-joint movements based on learned predictions. It receives massive input representing motor intent from the sensorimotor cortex via the pontine nuclei, and outputs back to the motor cortex via the VL thalamus.

• Disruption leads to wildly inaccurate and uncoordinated movements (ataxia).

What is the direct pathway ("Go" signal) of the basal ganglia in movement regulation?

GPi inhibits the thalmus → cortical neurons release GPi by exciting the striatum → striatum releases GPi to allow thalmus to initate motor movement

What is the indirect pathway ("Stop" signal) of the basal ganglia in movement regulation?

cortical neurons signals striatum → striatum shuts down GPE to free STN → STN excites GPi → GPi inhibits thalmus to stop movement