General Senses + Special Senses

Categories of Senses

- General senses

- Special senses

General Senses

- Refers to temp., pain, touch, pressure, vibration, and proprioception

- Receptors are distributed throughout the body

- Can be both autonomic + somatic

Special Senses

- Unique to the head

- Refers to smell, taste, balance, hearing, and vision

- Receptors are located in the complex sense organs (ears, eyes, + taste buds)

Sensory Receptors

- Specialized to respond to changes in environment (stimuli)

- Are the interface between the nervous system + the internal/external environment

- Work as transducers

- Picks up stimuli and convert it into action potentials

Stimuli

Packets of info. that can be detected by receptors throughout the body

Receptor Specifity

Each sensory receptor responds primarily to one particular type of stimulus

Receptor Specifity Example

- Photoreceptors in retina of eye primarily detects photons (packets of light)

- Photoreceptors can also respond to pressure such as when you see flashes of light due to pressing the eye

Transducers

Something that can convert one stimulus into another

Sensation

Sensory info. arriving at the CNS

Perception

- Conscious awareness of a sensation

- Happens in cortical areas of brain

Receptive Fields

- Area monitored by a single receptor

- Can be both large or small

Large Receptive Fields

Can be difficult to localize a specific stimulus to a specific area

Small Receptive Fields

- Several receptors are close together which makes it easy to localize a stimulus

- Found in hands

Touching Two Points Inside the Same Receptive Field

- The stimulus will activate the same receptor

- Brain cannot differentiate between the different stimuli in that area since both stimuli were picked up by the same receptor

Touching Two Points in Different Receptive Fields

- Two different receptors will be stimulated

- Brain will be able to differentiate between the two different points of touch

Tonic Receptors

- Constant tone

- Adapt very slowly or not at all

- Responds to every single stimulus with the same magnitude

- Always active

Tonic Receptors Example

- Photoreceptors of the eye

- Nociceptors

> Nociceptors is constantly letting the brain know of any pain that's happening

Phasic Receptors

- Fast adapting

- Receptors become less sensitive to a particular stimulus over the course of time

> That particular stimulus is eventually ignored

- Provides information about the intensity and rate of change of a stimulus

Phasic Receptors Example

- Touch + pressure receptors of the skin

> When you have glasses on

- Olfactory receptors of the nose

> When you put perfume on

Sensory Adaptation

Occurs when a receptor becomes so accustomed to the stimulus that it stops generating impulses

Two Ways to Classify Receptors

- Body location

- Nature of stimulus

Receptor Location

- Exteroceptors

- Proprioceptors

- Interoceptors

Exteroceptors

- Provide info. about external environment

- Generally found more superficially (skin)

- We are aware of these sensations

Proprioceptors

- Provide info. about the position + stretch of joints/tendons of the body

- Responds to stimulus within our body

- "Proprio" = self

- Let's us know location of our limbs if our eyes are closed

- Integration with sensory info. from the inner ear conveys info. about body position

Proprioceptors Example

- Moving our arms around while our eyes are closed (we know where they are located)

- Provides info about joint position + stretch

Interoceptors

- Provide info. about the inside of the body

- Found in walls of digestive + respiratory organs

- Involved in detecting O2 levels in blood + pressure within the walls of the organs

- Not always aware of what is being detected

- Even if you feel pain inside your body, you are not able to pinpoint the exact location that pain is coming from

Nature of Stimulus

- Mechanoreceptors

- Thermoreceptors

- Nociceptors

- Chemoreceptors

Mechanoreceptors

- Sensitive to physical distortion of cell membranes

- Can detect pressure, vibration, stretch, + twisting

Types of Mechanoreceptors

- Tactile receptors

- Baroreceptors

- Proprioceptors

Tactile Receptors

- Receptors related to touch, pressure, + vibration

- Most abundant type of receptor

- Found in skin

Types of Tactile Receptors

- Unencapsulated tactile receptors

- Encapsulated tactile receptors

Unencapsulated Tactile Receptors

- The nerve endings are not wrapped around in any connective tissue or glial cells

- Free nerve endings

- Tactile discs

- Root hair plexus

Free Nerve Endigs

- Basically dendrites

- Mostly found in papillary layer of the dermis

- Can also be found going up towards the epidermis

- Can detect light contact, temp. (thermoreceptors), + pain (nociceptors)

Tactile Discs

- Aka Merkle's discs

- Found in association with Merkle cells in the stratum Basale of epidermis

Merkle Cells

- Specialized epithelial cells that, in response to light touch, release neurotransmitter containing vesicles

- Activates nerve fibers that send action potentials to the brain to feel the light touch

Root Hair Plexus

- Located in the dermis

- Associated with hair follicles throughout the skin

- Senses movement of hair in the surface

> Light touch receptors that detect bending of hair

Root Hair Plexus Example

Allows us to feel when a mosquito lands on our skin

Encapsulated Tactile Receptors

- The nerve endings are wrapped around in connective tissue or glial cells

- Gives receptors an enlarged area at the end of the nerve

- Tactile corpuscle

- Bulbous corpuscle

- Lamellar corpuscle

Tactile Corpuscle

- Aka Meissner's corpuscles

- Found in papillary layer of dermis

- Found in higher concentrations in finger nails, lips, nipples, + genitals

- Involved with fine sensation in sensitive parts of the body

- Helps in sensing if something is rough or smooth in texture

- Able to respond to low frequency vibrations that happens when objects are moved across the skin in parts that there are no hair

Bulbous Corpuscle

- Aka Ruffini corpuscles

- Found deep in the dermis

- Responds to deep + continuous pressure

- Detects stretch in the skin

Lamellar Corpuscle

- Aka Pacinian corpuscle

- Located deep in the dermis

- Sensitive to deep pressure + high frequency vibrations

Baroreceptors

- Detects change in pressure

- Found in the digestive tract, lungs, urinary bladder, + carotid/aortic sinus

Baroreceptors Example

Detects when the urinary bladder is full + informs us

Thermoreceptors

- Responds to changes in temp.

- Found in the papillary layer of dermis

- Exists as free nerve endings

- Phasic

Nociceptors

- Responds to sensation of pain

- Consists of free nerve endings

- Large receptive fields

- Tonic

Chemoreceptors

- Monitors the chemical composition of body fluids

- Found near respiratory centers of medulla oblongata

- Found in carotid + aortic bodies

Chemoreceptors Example

Detects when O2 levels are low due to being at a higher elevation and tells brain to make us breathe faster

Referred Pain

- Painful sensation from visceral organs that is perceived as coming from another region

- Happens because nerve fibers from a certain part of the body shares the same pathway of another organ

> Brain is not able to differentiate the two different locations

Referred Pain Example 1

During heart attack, you can feel pain radiating from the left arm which is not close to the heart

Referred Pain Example 2

Pain from liver + gallbladder can be felt in the shoulder

Referred Pain Example 3

Pain from stomach can be felt in abdominal area

Gustation

- Sensation of taste

- Mouth + tongue

- Has transitional (support) cells

- Has gustatory epithelial cells

- Has basal cells

- Has molecules that goes towards nasal cavity (why you're not able to taste well when you are sick)

Gustatory Cells

- Specialized cells that help with gustation

- Found within the taste buds in the tongue

- Has gustatory receptors that allows us to be sensitive to different types of chemical stimuli

- Has microvilli (taste hairs) that open up into the taste pore

Taste Hairs

Chemicals on saliva can activate receptors on taste hairs which allows gustatory cells to activate the nerve fibers that allow us to be conscious of taste

Taste Buds

- Specialized epithelial tissue that has gustatory cells

- Includes papilla that contains taste buds which lines a cleft

> Taste buds will open to a taste pore

- Located on the dorsal surface of the tongue (top surface)

Primary Taste Sensations

- Sweet

- Salty

- Sour

- Bitter

- Umami

- Associated with different types of chemoreceptors

Sweet

Detects sugars in food

Salty

Detects salts such as NaCl, KCl, and NaI

Sour

Detects acids such as lactic acid (sour cream)

Bitter

Detects complex organic molecules that may be toxic

Umami

- Detects specific amino acids + MSG

- Meaty taste

Gustatory Pathway

- Taste info. is sent to a solitary nucleus in the brainstem

- Taste info. goes to the thalamus

- Taste info. goes to the gustatory cortex located in the insula

- Anterior 2/3 of tongue is innervated by facial nerve (VII)

- Posterior 1/3 of tongue is innervated by glossopharyngeal nerve (IX)

Solitary Nucleus

Axons that travel towards the thalamus

Olfaction

- Sensation of smell

- Has nerve fibers located in superior part of nasal cavity

> Capable of detecting odor molecules

- Info. is transduced into action potentials

Odor Molecules

Chemical molecules that binds receptors found on the olfactory nerve (I) fibers that are on the superior part of nasal cavity

Olfaction Pathway

- Info travels up through cribriform plate of ethmoid bone

- Info reaches olfactory bulb

- Info travels through olfactory tract

- Info extends away from olfactory bulb and goes towards brain to reach olfactory cortex in temporal lobe

- Results in awareness of specific smell

Olfaction + Hippocampus + Amygdala

- Smell info. also reaches hippocampus which is involved in long term memory

- Smell info also reaches amygdala which is involved in emotion

- Why you associate certain smells with specific memories

Dementia

Losing sense of smell can be an early sign of dementia

Olfactory Epithelium

- Is embedded with specialized cells such as olfactory sensory neurons, supporting cells, and basal cells

- Has receptors for smelling

Olfactory Sensory Neurons

- Each olfactory sensory neuron has its own type of receptor

- Is sensitive to a selective variety of chemical molecules

Basal Cells

Stem cells that replace olfactory receptor cells + supporting cells

Olfactory Receptor Cells

- Needs to be replaced every 2 months because they get damaged

- Replacement declines with age (why our ability to smell declines with age)

Olfactory Glands

Secrete mucous

Mucous Layer

- Located in superior part of nasal cavity

- Traps chemical molecules in the mucous

- Chemical molecules will bind to specific receptors on the olfactory sensory neurons and transduce stimulus into action potentials

Photoreceptors

- Sensitive to photons

- Located on posterior part of eyeball

- Capable of detecting different wavelengths of light

- Allows us to see color

Eyelashes

- Involved with helping to prevent foreign objects from hitting the eyeball

- Contains root hair plexus

> Helps with blinking reflex

Eyelid

- Aka palpebrae

- Helps to wash the debris across the surface of the eye

- Has palpebral fissure

Palpebral Fissure

Fissure in-between the superior + inferior palpebrae

Levator Palpebrae Superioris

Muscle that helps to raise the upper eyelid

Orbicularis Oculi

Muscle responsible for closing the superior + inferior eyelids

Tarsal Plates

- Located in the inner side of the upper lower eyelids

- Has tarsal glands

Tarsal Glands

Sebaceous glands that produce oily secretion to prevent eyelid from sticking together

Orbital Fat

Functions as an adipose cushion for eyeballs

Conjunctiva

- A stratified squamous epithelium that lines the sclera and goes back + curves and connects to the eyelid

- Forms a protective sheath that prevents debris from getting behind the eyeball

Bulbar Conjunctiva

- Aka ocular conjunctiva

- Covers the anterior surface of the eyeball

Palpebral Conjunctiva

Covers the inner side of the eyelid

Fornix

- A fold between the bulbar + palpebral conjunctiva

- Where the lacrimal ducts open up and allows lacrimal fluid (tears) to be released

Lacrimal Glands

- Located in the lateral, superior corner of eye

- Produces tears

- Helps to wash anterior surface of eyeball towards the medial side of eye

Lacrimal Punctum

- On medial corner of eye

- An opening that drains tears from eyes into the lacrimal canaliculi ---> lacrimal sac ---> nasolacrimal duct (opens up inside the nose)

> Reason for runny nose when crying

> This path forms the lacrimal apparatus

Lacrimal Apparatus

- Always active to some extent

- Even if we are not crying, the lacrimal apparatus will produce tears to lubricate + protect the eyeball

Tears

- Move lateral ---> medial

> Exception: Crying a lot causes tears to also run down the cheek

- Are basic

- Contains lysozymes + antibodies which are capable of breaking down bacterial cell wall

> Prevents bacterial infection in eyes

Lacrimal Caruncle

A mass of soft tissue that has glands that produces the thick secretion sometimes found on the inner edge of eyes

Iris

- A pigmented smooth muscle

- Controls the pupil

Pupil

- A hole

- A space that can change size when the iris contracts (shrinks) or relaxes (dilate)

- Pupil will be more constricted when there is a lot of light and vice versa

Lens

Separates anterior + posterior cavities of the eye

Layers of the Eye

- Fibrous layer

- Vascular layer

- Inner layer

Fibrous Layer

- Has sclera

- Has cornea (covers anterior part of eyeball)

- These structures are made of fibrous connective tissue

- Provides a degree of protection

- Point of attachment for intrinsic eye muscle

Corneoscleral Junction

Cornea + sclera

Intrinsic Eye Muscles

Muscles that move the eyeball

Vascular Layer

- Aka uvea

- Has iris

- Has ciliary body

- Has choroid