sensory system ( afferent )

sensory system

this includes sensory receptors, neural pathway from receptors to CNS & portion of the brain that processes sensory information

afferent pathways carry sensory information from the …

PNS to the CNS

sensory receptors

Peripheral ends of afferent neurons that generate graded potentials ( graded potentials are at the receptor, aka receptor potentials)

visceral afferent pathways

carry internal information we are not aware of

• Visceral receptors detect stimuli from the internal organs and structures (chemoreceptors and mechanoreceptors)

adequate stimulus

The type of energy to which a certain type of receptor responds best; however, the receptor may also respond to other types of energy (e.g photoreceptors respond best to light but also respond to pressure. If you get hit in the eye, you will see a flash of light)

sensation

conscious awareness of a stimulus

• being aware or becoming aware

• we are limited to what we are aware off bc of our receptors ( e.g., animals & insects can see more colors than us)

perception

conscious interpretation of sensation

• Understanding that the pain is throbbing due to a cut

• Perception of stimuli can vary among individuals based on their past experiences

mechanoreceptors

a type of receptor that respond to pressure or stretch

thermoreceptors

respond to diff temp

photoreceptors ( cones)

responds to light of different wavelengths

chemoreceptors

specific chemicals

nociceptors

receptors that respond to pain ( and detect it)

sensory transduction

Sensory receptors convert a physical or chemical stimulus from the environment into an electrical signal

receptor potential

the change we see in a membrane potential in a sensory receptor, resulting in an action potential

• The stronger the stimulus, the more action potentials a neuron is going to generate

• Weaker stimulus, the fewer action potentials, or less likely to make an action potential

Explain the specialized afferent ending of sensor neurons

Specialized dendrites, or a cell that acts like a receptor and responds to stimuli

1) A stimulus is applied to the specialized ending of an afferent neuron

2) causing a cation channel to open, which depolarizes the receptor

• When a sensory neuron acts like the receptor, they open Na channel, causing a change in the MP ( aka RP bc it technically occurs at the receptor now )

Explain when a cell acts like a receptor

1) the cell responds to the stimulus

2) causing the cation to open, changing the MP in a receptor cell ( aka RP)

3) The receptor cell depolarizes to all Ca2+ channels to open

• Signals vesicles within the neuron to release NT

4) NT binds to the afferent neuron, causing an ion channel to open and convey a signal ( either inhibitory or excitatory )

5) The stronger the stimulus → NT release by receptor cell is more frequent, and action potentials are generated by the afferent neuron

for both cells that have a receptor and specialized afferent neuron endings, a stimulus is causing a change in the permeability of the receptor ions → causing a generation of an AP

Explain this photo

• The greater the stimulus → more APs

• stimulus must be transduced into an electrical signal that the neuron can understand & carry into the CNS

receptor adaptation

a decrease in the strength of the RP over time in the presence of a continuous stimulus

Phasic receptors

a stimulus occurs on an afferent neuron, causing an action potential. It adapts quickly, meaning that even though the stimulus remains, the neuron stops firing APs.

What happens?

• It sends one big AP to signal the start of a stimulus and one small AP to mark the end of a stimulus (when you feel the weight of the ring after you take it off)

tonic receptor

1) a stimulus begins flipping the on switch, causing a rise in the receptor potential, which eventually causes an AP to fire

2) Compared to phasic receptors, this one doesn’t adapt. it has a starting stimulus and keeps a high RP for AP

e.g., photoreceptors; they don’t stop sending signals even if you have been looking at something for a long time

• proprioceptors: they must always be aware of where your body is in the environment

the brain is often bombarded with irrelevant and continuous info and responds by ignoring that info bc it needs to focus on new info

neuron habituation

neurons reduce their response by depressing activity at the synapse; basically, they close some Ca channels, causing there to be less NT to be released

• Change at the axon terminal, affecting voltage-gated Ca channels

neuron sensitivity

( aka sensitization ) A neuron increases its responsiveness to a strong stimulus, the opposite of neuronal adaptation; the neuron sends more AP to bc sensitive to an out-of-place stimulus

e.g in a loud area, you will still be able to hear a loud bang bc the neurons will be sensitive to out-of-place sounds ( construction site, hypersensitive to danger)

sensory coding

how the brain codes a stimulus to understand it, the brain codes it based on location, type, and strength

peripheral ending of sensory neurons can have many branches acting as receptors innervating the same region of the skin

sensory unit

a afferent neuron and all of its receptors

motor unit

a motor neuron and all the muscle fibers it innervates

receptive field

a region of our skin that is activated by a sensory unit and produces a response

dense receptive fields =

more receptive units, which means high acuity ( e.g finger tips)

sensory unit with smaller receptive fields have more acuity compared to sensory unit with larger fields bc

small fields are easier to “search” compared to larger ones

all the afferent neurons in a receptive field can resolve when a stimulus is strong enough

cerebral cortex

the outer 4 mm of the brain

All sensory receptors have a path to the cerebral cortex

explain the sensory receptor pathway to the cortex

1^st order neuron: the afferent neuron that first detects a stimulus and sends it to the spinal cord

2^nd order neuron: an interneuron that sends info from the spinal cord → thalamus

3^rd order neuron: synapses with the second, where it will take the signal to the right region of the cortex to determine: type, strength, and location

all sensory info goes through the thalamus, then the cortex, except olfaction (smell), which goes directly to the temporal lobe, where the olfactory cortex is located

vestibular cortex

balance

gustatory cortex is in the

temporal lobe

when sensory info reaches the cortex, that is when you become aware of it

stimulus modalities

The ability of the brain to distinguish the modalities it receives, e.g heat, cold, pressure, light

• Sensory units will only respond to their specific modality

How does the brain ID stimulus modality

• the receptor it activates

• the pathway it takes to the brain ( there are two )

dorsal column lemniscal pathway

involves mostly mechanoreceptors, proprioceptors, touch, vibration, two-point discrimination

1st-order neuron: enters the spinal cord in the dorsal horn on the same side of the stimulus and synapses with the 2nd-order neuron

2nd-order neuron: crosses over to the other side ( decussation) to the thalamus to synapse with the 3rd-order neuron

3rd-order neuron: goes to the specific region of the brain where that stimulus belongs to

decussation

the crossing over of signal in the brain stem

spinothalamic pathway ( aka anterolateral pathway)

carry info: nociceptors & thermoreceptors ( pain and temp )

1st order: pain or temp receptors carry info to the spinal cord and synapse with the 2nd neuron, right as it enters the spinal cord, bc of the reflex involved ( pulling your hand off the stove )

2nd order: descends, then goes all the way up to the thalamus, then the 3rd order neuron

3rd order neuron: takes it into the part of the cortex that associates with that part of the body

Pain also activates the limbic system, which deals with emotion, meaning pain will not just cause a physical response but an emotional response ( making you angry or cry )

e.g chronic pain can cause depression

the brain codes the strength of a stimulus based on

• how many sensory units are activated

• How many APs are sent

stimulus strength is also coded by the recruitment of more sensory neurons or more afferent neurons

• more neurons are needed to send one signal

• more neurons = more AP

population coding

the recruitment of more receptors

acuity

How precisely a stimulus is perceived

depends :

• size of receptor field

• how many of the receptor units (density) → two-point discrimination

the lips have more smaller sensory units, making you more likely to hit the two different fields compared to the back, where you have a larger field, so you are more likely to hit one field with two points

lateral inhibition

occurs in the field that overlaps with each other, increasing the density and acuity of receptive fields

• increasing sensory discrimination & loc

• works by exciting one receptive field and inhibiting others at the 2nd-order neuron, causing them to send fewer APs ( eliminating the excess APs to be more precise)

explain the process of lateral inhibition

1) a group of sensory units activates

2) Out of that group, the sensory unit that receives the strongest stimulus uses its lateral inhibitory neuron to weaken the AP of the other units lateral to it

3) They make the lateral receptive fields send weak enough APs to the brain that it doesn’t notice, making the strong receptive field stand out, without changing the intensity or AP of the original neuron

somatic sesation

sensation associated with the skin

mechanoreceptors

… response to pressure touch, sound ( vibration), any mechanical distortion

thermoreceptors

temp receptors specifically on the skin

nocireceptors

pain through chemical detection

proprioceptors

a type of mechanoreceptor found in muscles, tendons, & ligament tell the brain where your body is in space and how stretched your muscles & tendons are, and what angle your limbs are in

photoreceptors

respond to diffent wavelengths of light

chemoreceptors

detects specific chemical associated with taste and smell

meissner’s corpuscle

tonic mechanoreceptors detect distortion in the skin, when someone move the skin these structures do too and send AP to the brain about the sensation

merkel’s corpuscle

like meissner corpuscle but slow reacting, detect distortion in the skin, when someone move the skin these structures do too and send AP to the brain about the sensation

free nerve endings

basically Merkel’s corpuscle but included nociceptors, itch receptors, thermoreceptors, and mechanoreceptors

pacinian corpuscles

loc in the dermis ( deeper than the Meissner’s), not covered in connective tissue, but will rapidly adapt. Important for vibration & deep pressure

Ruffini corpuscles

slow adapting mechanoreceptors, that detect stretch

thermoreceptors & nocireceptor are all free nerve endings

warm receptors

adapt rapidly ( phasic receptors) send frequent AP to let you know about the extreme temp (response to temps between 30 - 45 °C)

cold receptors

adapt rapidly ( phasic receptors ) send frequent APs to let you know about the extreme temp ( responds to temp around 35 - 20^o C)

nociceptor

respond to chemicals released by damaged tissue, they don’t adapt, respond to noxious stimuli ( dangerous stimuli), any chemical that is related to tissue damage

Paradoxical hot /cold

when the temp receptors of one temp stop firing, and nociceptors and the opp start firing due to recruitment bc there is so much stimulation

capsaicin

a chemical found in hot peppers that warm receptors response to

methanol

bind to receptors and give the sensation of cold

NT released by damaged cells

prostaglandin, cytokines, bradykinins, histamine, potassium

pain perception

Helps to prevent further damage; makes us aware that something is wrong

• Causes sweating, an increase in blood pressure, and emotional responses of anxiety or even fear

• two types: fast and slow pain

fast pain

Carried by A-delta fibers, transmitted very quickly because these fibers are myelinated

• Initial pain, aka acute pain, is localized (sharp pain)

slow pain

the pain that comes after fast pain is going to last longer because it is carried on C-fibers, which are unmyelinated, making the propagation of pain slower

• Slow pain is fast in transmission but slow compared to fast pain

• Localized and poorly defined region

• guarantees you are aware that your body needs to heal ( throbbing)

3 types of nociceptors bc there is more than one way to get hurt

mechanical nociceptors

travel on A-delta fibers, making them fast-responding

Detects crushing, pinching, cutting, and tearing; any intense mechanical stimulation

thermal nociceptors

travel on A -delta fibers, making them fast, responding to extreme temperatures

polymodal nociceptors

travels on c fibers, making them slow reacting, they respond to all types of damage stimuli

endogenous analgesic system

the brains pain blocking system

how does the endogenous analgesic system work?

1) 1^st order neurons: nociceptors will receive the pain signal and start releasing substance P, which will bind to the 2nd order neuron

2) Afferent neurons (substance P) from the brainstem are going to synapse with inhibitory neurons (interneurons) to release endogenous opioids, which are NTs (enkephalin & endorphins, substances similar to morphine; they are powerful analgesics )

3) Instead of substance P binding to the 2nd-order neuron to create a damping effect by preventing the second-order neuron from sending an action potential

4) Analgesics are painkillers, including ibuprofen, aspirin, narcotics, and NSAIDs

referred pain

visceral pain in the organ you can’t actually feel it is instead felt on the surface of the body

• This happens bc the nociceptors on the skin & nociceptors on the organ both synapse on the same second-order neuron in the spinal cord; they also share the same neural pathway

• Someone experiencing a heart attack will feel pain in their chest but also in their shoulder and arm

reflection

when light waves bounce off an object and enter our eyes and become the color we see

fibrous tunic ( outermost layer)

sclera + cornea; extraocular muscles insert into the sclera, making the eyeball

• has collagen fibers

• cornea is the clear part of the eye that allows light into the eye

vascular tunic ( middle layer)

includes the choroid, ciliary body, iris, and contains a lot of blood vessels

neural tunic ( inner tunic)

a layer that includes the retina

• Contains the blind spot, where there are no photoreceptors

iris

colored part of the eye

anterior chamber

the part of the eye that is filled with a “ fluidity” humor

• loc in front of the lens → to the cornea

• aqueous humor is nourishing the lens

posterior chamber

Everything posterior to the lens is much larger and filled with a gel-like humor called the vitreous humor

• Vitreous humor functions to keep the retina in place and support the eye as well

how light enter the eye

1) goes through the cornea, then the pupil, which is loc in the center of the iris

2) hits the lens, then passes through the vitreous humor to eventually hit the retina, where the photoreceptors are

the functions of tears

to cleanse the eye, also contains a lysosome to prevent bacteria from growing over the eye

The iris is the part of the eye that contains color and is unique to every person, more unique than fingerprints

Has 2 sets of smooth muscles: circular and radial muscles

circular muscles

muscles that surround the pupil of the eye

• When these muscles contract, they cause the pupil to constrict

• The parasympathetic NS innervates here; this is why doctors shine a light in your eye ( parascope)

radial muscles

the outermost ring of muscles around the pupil; when they contract , they cause the iris to dilate

• The sympathetic NS innervates here; it dilates if there is not enough light

I am sympathetic when people fall in love

refraction

the bending of light

when light enters the eye …

• It hits curved surfaces: the cornea & the lens, changing the light waves to refract

• The lens can change shape, changing the refractive power of the light waves. Light hits your retina upside down, but when it hits your brain, it’s right side up again ( the visual cortex reverses it back to the original image)

concave surfaces

surface curves in ( caves in ); light spreads out

• When light passes through this surface, the edges of the refracted light move away from each other ( aka divergence of light )

convex surfaces

surface curves out; light spreads in

• The light bends inward toward a single point ( focal point) that must be projected onto the retina to see the image clearly

zonular fibers ( aka suspensory fibers)

attach and circle the lens. The contraction of the ciliary muscle will either tighten or loosen the suspensory fibers, causing the thickness of the lens to change

looking at distant objects

1) light rays come into the eye at most parallel, close to each other

2) When they hit the lens, they refract

looking at object close up

1) light will come in at a wider angle

2) Light will hit the lens and refract into a focal point behind the retina

3) the lens will accommodate and move the focal point onto the retina