BIO-375 Exam #2

Unit 2- Sensory Systems

Dorsal Column/Medial Lemniscus (Epicritic) Tract

The sensation of vibration, fine touch, and tactile

• Starts with mechanoreceptors

Spinothalamic/Anterolateral (Protopathic) Tract

The sensation of temperature, pain, itch, and tickle.

• Starts with free nerve endings

Proprioceptive

The body’s internal sensation of spatial awareness, position, and movement.

• Directly communicates with cerebellum

Mechanoreceptors

Specialized sensory receptors that convert mechanical stimuli (touch, stretch, pressure, vibration, etc.) into nerve impulses.

• Found in different parts of the body

Receptor Neuron

• The primary afferent neurons

• The cell body is located in the dorsal root ganglia

• Pseudo unipolar neuron (two branches- peripheral and central branches)

• Heavily myelinated - fast conduction velocity (↑R)

High Resolution

Is the resolution higher or lower when there is a large density of receptors with small receptive fields?

Transduction

The conversion of mechanical stimuli into electrical impulses.

• Stretch receptors cause ion channels to open, causing depolarization

Receptor Potential

The initial electrical response to a mechanical stimulus.

• Graded potential - amplitude dependent on the strength of the stimulus

• If the stimulus is strong enough to reach the threshold, it will generate an action potential

Fast Adapting Receptors

Receptors that respond to changes in stimuli and are essential for detecting dynamic environmental changes.

Slow Adapting Receptors

Receptors that respond continuously to a stimulus and provide sustained information about a static stimulus.

Merkel Discs/Receptors

Slow-adapting mechanoreceptors that are important for detecting form, texture, edges, and light touch.

• Only receptor cells located in the epidermis

• Highest spatial resolution

• Highest density in glabrous tissue

Meissner (Tactile) Corpuscles

Rapid adapting mechanoreceptors that are important for detecting movement across the skin, slippage, light touch, and high-frequency vibration

• 4x more sensitive than Merkel discs

• Small receptive fields, but larger than Merkel discs

Pacinian (Lamellar) Corpuscle

A fast-adapting and extremely sensitive mechanoreceptor that is important for detecting gross pressure changes and vibration.

• Very large receptive fields

• Very common throughout the body

Ruffini (Bulbous) Corpuscle

A slow-adapting mechanoreceptor that is responsible for detecting skin stretch and finger position/movement.

• Connected with proprioception

Circumferential Lanceolate Endings

A type of hairy skin mechanoreceptor that is slow-adapting and detects light touch and hair movement.

• Light purple

Longitudinal Lanceolate Endings

A type of hairy skin mechanoreceptor that is rapidly-adapting and detects hair deflection, hair movement, and directional discrimination.

• Blue, green, and red

Proprioceptors

Mechanoreceptors that are responsible for detailed continuous information essential for complex movements and to prevent injury (reflexes)

• Unconscious of it majority of the time

• Very low threshold

• Very wide and heavily myelinated axons

• Types:

  • Muscle Spindles

  • Golgi Tendon Organs

  • Modified Ruffini

Muscle Spindle

A type of proprioceptor that detects the length and stretch of muscles

• Moves with the muscle

• Has its own motor neuron

• Has 2-3 afferent (sensory) neurons that detect dynamic info (changes in length) and static info (constant length)

• Run parallel to muscle fibers

Golgi Tendon Organs

A type of proprioceptor that monitors both muscle tension and force and is made of collagen and sensory fibers.

• Runs in series with the muscle and is located in the musculotendinous junction

Modified Ruffini

A type of proprioceptor that detects the position/angles of a joint

• Joint receptor

Area 3a

What area(s) of the primary somatosensory cortex does position sense/proprioception?

Areas 3b and 1

What area(s) of the primary somatosensory cortex is concerned with cutaneous stimuli?

Area 2

What area(s) of the primary somatosensory cortex is responsible for tactile and proprioceptive information?

Area 1 and 2

Where does Area 3b send its information to?

General Body

What part of the body does sensory information get sent to the VPL of the thalamus?

Face/Head

What part of the body does sensory information get sent to the VPM of the thalamus?

Dermatomes

A spinal root exists on each side of the spinal cord at every level of the spinal cord and each represents a _________.

• Sensory!

Plasticity

The brain’s ability to rewire itself and try to recover from injury/illness.

• Allows us to learn new skills

• Study with primates where finger usage was mapped out on the brain, the third digit was removed, and the other fingers took over that area of the brain

Pain

Physical suffering or discomfort caused by illness or injury

• Our perception of a stimulus

• Experienced differently for everyone

• Exists to remove oneself from harmful situations, to protect damaged tissues, and to learn to avoid similar occurrences.

• Can become maladaptive/pathological- Chronic

Nociception

The detection of the stimulus that causes pain

• Neural pathway = Spinothalamic/Anterolateral (Protopathic)

Nociceptors

Receptors made up of free nerve endings that are used to detect potentially damaging thermal, chemical, or mechanical stimuli

• In epidermis

• Peripheral pain receptors

• Very high threshold (damaging heat detected around 45 °C

• Axons are lightly myelinated or not myelinated

  • Very slow

  • Tightly wired to reflexes- signal only has to travel to the spinal cord

Aδ Fibers

Type of nociceptor fiber that corresponds to the first pain, which is tingling or sharp pain, and is more rapid than the other (has a thin layer of myelin)

• Type 1- sensitive to mechanical and chemical stimuli

• Type 2- sensitive to thermal (noxious) stimuli

C Fibers

Type of nociceptor fiber that corresponds to the second pain which is delayed/diffuse (no myelin), characterized as a dull ache or a burning sensation

• Some of these fibers are polymodal - detect mechanical, chemical, and thermal stimuli

• Some are specific - detect only one type of stimuli

Superficial Somatic

A type of pain/nociception that is the easiest to localize and treat.

• Ex. Scrape on the skin

Deep Somatic

A type of pain/nociception that is more difficult to determine the exact location and more difficult to treat.

• Ex. Pulled muscle

Visceral

A type of pain/nociception that is always diffuse, the most difficult to locate and treat, and causes referred pain

• Low resolution

• Need imaging/testing to determine

1. Main Pain Pathway

2. Emotional/Memory Pathway

3. Descending Pathway (can +/- pain signals)

What are the three components to the pain pathway?

Insular Cortex

Part of the pain pathway that is located in the cortex and is responsible for the perception of the quality of the pain.

• Hint: IC

Anterior Cingulate Cortex

Part of the pain pathway that is responsible for the emotional status when experiencing pain.

• Hint: ACC

Hippocampus

Part of the pain pathway that is responsible for creating new memories based on a pain experience.

• Hint: HC

Amygdala

Part of the pain pathway that is responsible for plasticity and emotions such as fear and anxiety.

Periaqueductal Grey

Part of the pain pathway that is located in the midbrain and is responsible for inhibitory output (try to lessen nociceptor signal)

• Hint: PAG

Medulla

Part of the pain signal that works to either increase of decrease the nociceptor signal

• Hint: NRM

• Spinal Input

• thalamus

• Nociceptor Signal

What components of the pain pathway make up the MAIN pain pathway?

• Amygdala

• Insular Cortex

• Anterior Cingulate Cortex

What components of the pain pathway make up the Affective-Motivational Part?

• Periaqueductal Gray

• RVM (Medulla)

What components of the pain pathway make up the descending part?

Gate Theory of Pain

The idea that stimulation of other parallel mechanoreceptors will make the pain feel somewhat better

• Ex. rubbing or squeezing the toe after stubbing it

Hyperalgesia

When something that wouldn’t normally cause a significant amount of pain does because of sensitization.

• Increase in both stimulus intensity and pain intensity

Allodynia

When something that normally is non-noxious (doesn’t cause pain) does cause pain because of sensitization.

• Increase in both stimulus intensity and pain intensity

Sensitization

An inflammatory response/signal after the first pain/initial injury lowers the threshold of the nociceptors, causing the stimulus intensity and pain intensity to increase.

Use-Dependent Plasticity

What type of sensation of pain without stimulus is this?

• Caused by a severe traumatic injury that took a long time to heal

• Synapses along the pain pathway were strengthened (central sensitization)

Neuropathic Pain

What type of sensation of pain without stimulus is this?

• Caused by a lesion or disease of the somatosensory system (damage to neurons along the pathway)

• Ex. Central Neuropathy- stroke, neurodegenerative diseases, spinal cord injury, syringomyelia, MS

• Ex. Peripheral Neuropathy- trigeminal neuralgia, diabetic neuropathy, HIV infection, chemotherapy

Autoimmune Disorders

What type of sensation of pain without stimulus is this?

• Chronic inflammation

• Ex. Rheumatoid Arthritis, Lupus, etc.

Acute Pain

Which pain is the easiest to treat?

• Chronic vs. Acute vs. Neuropathic

Neuropathic Pain

Which pain is the most difficult to treat?

• Chronic vs. Acute vs. Neuropathic

Breakthrough Pain

Activity-related pain that is common in cancer patients

• Some level of pain was already being managed, but then it gets worse with changing activity

NSAIDs

Drugs used for pain relief and to try to stop the inflammatory response after injury that target the enzyme that produces prostaglandins (part of the inflammatory response)

• Ex. Ibuprofen, Aspirin, Naproxen, Celebrex, Meloxicam

• Enzyme = Cox (Cyclooxygenase)

• Cox also found in other places (Cox1), so taking a lot of this medication can be damaging (Ex. cause ulcers)

• Some prescription medications made to only target Cox2

Opioids/Opiates

Drugs used for pain relief that bind to specific receptors that are found all throughout our body/pain pathway

• μ (mu) receptors

• Ex. Morphine, Codeine, Oxycodone

• Receptors found in both the pre-synaptic terminal and post-synaptic cell

  • Difficult for pain signal to go through

Congenital Insensitivity to Pain

The inability to feel pain

• Rare genetic disorder

• Very dangerous because you won’t know if you are injured or have an illness like appendicitis

Saccades

Rapid, jerky, ballistic movements of both eyes in the same direction.

Photoreceptors

Specialized cells located in the retina of the eye are crucial for detecting light, and they play a critical role in phototransduction

• Two Types

  • Rods

  • Cones

Vertical Pathway of Retina

A series of connections in the retina

• Photoreceptors (GP) connect to →

• Bipolar Cells (GP) connect to →

• Ganglion Cells (AP)

Refraction

Visual information appears in the neural retina as upside down and backwards

Phototransduction

The conversion of photons into electrical signals

• Occurs in the outer segment

  • The outer segment of photoreceptors contains discs with 100s of opsins that detect photons

1) Depolarized

2) More Neurotransmitter

If a room is super dark, will the cell be

1) Depolarized OR Hyperpolarized

2) Release More OR Less Neurotransmitter

***Dark = Max influx of Na⁺/Ca²⁺ into the cell

1) Hyperpolarized

2) Less Neurotransmitter

If a room is super bright, will the cell be

1) Depolarized OR Hyperpolarized

2) Release More OR Less Neurotransmitter

***Dark = Max influx of Na⁺/Ca²⁺ into the cell

11-cis retinal → 11-trans retinal

When a photon strikes the opsin, what gets converted?

• Opsin activated

Transducin

After the activation of opsin in the transduction pathway, what gets activated next via phosphorylation?

Transducin α-Subunit activates PDE

After the activation of transducin via phosphorylation in the phototransduction pathway, what subunit of transducin activates what?

Hydrolyze cGMP → GMP

In the phototransuction process, what does the activated PDE do?

Na⁺/Ca²⁺ Channel Closes

What happens when cGMP is hydrolyzed into GMP during the phototransduction pathway?

Hyperpolarization and Less Neurotransmitter Released

When the Na⁺/Ca²⁺ channel closes, there is decreased Na⁺/Ca²⁺ entering the cell, which leads to what?

Light Adaptation

The rapid adjustment of the human visual system that allows us to see a wider variety of luminescence.

1. Rhodopsin Kinase phosphorylates Rhodopsin

2. Arrestin binds to phosphorylated Rhodopsin which shuts it down

How is phototransduction stopped?

1. Guanylate Cyclase

2. Rhodopsin Kinase

3. Binding affinity for cGMP and Na⁺/Ca⁺ Channel

What three things is calcium inhibitory towards according to light adaptation?

More Difficult

If there is a decrease in [Ca²⁺] in the photoreceptor, is it easier or more difficult for the Na⁺/Ca⁺ channels to close per photon?

1. Increase in Guanylate Cyclase activity - increase cGMP

2. Increase in Rhodopsin Kinase activity - more arrestin binding

3. Increase in cGMP-Na⁺/Ca⁺ channel affinity

4. Decrease in the rate of closing Na⁺/Ca⁺ channels

A decrease in [Ca²⁺] in the photoreceptor leads to what four things occurring in the photoreceptor?

Scotopic Phase

The phase of vision where only rods are used, at the lowest levels of light intensity.

Mesopic Phase

The phase of vision that uses both cones and rods and is during medium light intensity.

Photopic Phase

The phase of vision where only cones are being used because the rods have fully saturated. This is at very high levels of light intensity.

Rods

A type of photoreceptor that has:

• Higher sensitivity

• Larger receptive field size and lower spatial resolution

• Less effective at light adaptation

• Slower recovery from saturation

• More in number in the retina

• Dominate monochrome and peripheral vision

Cones

A type of photoreceptor that has:

• Lower sensitivity

• Smaller receptive fields and higher spatial resolution

• More effective at light adaptation

• Faster recovery from saturation

• Less in number in the retina

• Concentrated in the fovea

• Responsible for color vision and fine detail

Color Vision

The type of vision produced by the three types of cones

• Short cones (blue)

• Medium cones (green)

• Long cones (red)

What are the three types of cones and their perspective colors?

Information from at least 2 different cones is compared

How is color perceived?

Color Opponency

The visual system’s mechanism of processing colors in opposing pairs, such as red vs. green and blue vs. yellow, enhancing color contrast and discrimination.

Retinoid Cycle

The retinal pigmented epithelium (RPE) absorbs stray light and is critical for the survival of photoreceptors. It also takes 11-trans retinal and recycles it back into 11-cis retinal to be used by the photoreceptors again.

Off-Center Bipolar and Ganglion Cells

In bright lighting, photoreceptors are hyperpolarized, and smaller amounts of neurotransmitter will be released and will go to the bipolar cells. This bipolar cell pathway will respond the same way the photoreceptor did and hyperpolarize, therefore releasing smaller amounts of neurotransmitter to the ganglion cell, which will then create more less action potentials.

On-Center Bipolar and Ganglion Cells

In bright lighting, photoreceptors are hyperpolarized, and smaller amounts of neurotransmitter will be released and will go to the bipolar cells. This bipolar cell pathway will respond opposite than the photoreceptor did and depolarize, therefore releasing larger amounts of neurotransmitter to the ganglion cell, which will then create more more action potentials.