Question

MCAT Tutorial: Sensory Reception & Processing

Answer 1

\({\bf{Touch:}}\) detection of pain, temperature, and proprioception
- free nerve endings: sensory receptors in the dermis/epidermis
- hair follicle receptors: around hair follicles, detect movement of hair
- tactile cells: lower epidermis, touch
- Meissner's/Pancinian corpuscules: deep layers of the epidermis, touch and pressure

- proprioception: detecting one's position relative to the body
- muscle spindles: subtype of proprioreceptors that detect muscle stretch
- stretch reflex: detection of muscle stretch --> transmission to spinal cord --> activating contraction
- golgi tendon organs: detect stretch in tendons, relax muscles
- joint kinesthetic receptors: detect stretch/pressure in joints

\({\bf{Olfaction:}}\)
- olfactory receptors in the nasal cavity, end in olfactory cilia, bind to odorants
- are part of the olfactory nerve
- olfactory epithelium: olfactory receptor cells + epithelial cells
- olfactory bulb: region of the brain that receives signals from the receptor cells
- olfactory tract: carries impulse to the thyalamus/hypothalamus --> frontal lobe/limbic system

\({\bf{Gustation:}}\)
- taste buds, located in mucous membrane projections
- gustatory cells: taste cells, stem cells in the epithelium of papillae
- gustatory hairs: long microvilli, chemoreceptive parts of the gustatory cells
- taste pore: opening in the tongue epithelium
tastants trvel along facial/glossopharngeal, vagus nerves to the medulla --> thalamus --> gustatory cortex

Answer 2

\({\bf{Structure~of~the~Ear:}}\)
three regions: external, middle, and inner

External: auricle (also called pinna) and external auditory canal
- pinna: captures sound waves
- external auditory canal: connects to the temporal bone, directs sound waves towards the eardrum
- eardrum (tympanic membrane) separates the external and middle ear

Middle:
- ear ossicles: small bones that amplify sound
subtypes of ossicles: malleus (hammer), incus (anvil) and stapes (stirrup)
- oval window: opening into inner ear
- round window: lies below the oval window
- pharyngotympanic/Eustachian tube: opens into pharynx, equalizes air pressure

Inner:
- fluid filled tubes inside bony chambers in the temporal bone
three sub-structures: vestibule (connects to middle ear), semicircular canals, and cochlea (detects sound)
Cochlea: two hair types (inner hair and outer hair cells)
inner: send signals to brain
outer: causes cells to expand/contract, amplifying sound

Answer 3

Subcomponents of the cochlea:
- central chamber (scala media/cochlear duct)
- organ of Corti: lies on teh basilar membrane, senses sound
- vestibular membrane: top part of the choclear duct and the bottom part of the scala vestibuli

Basic Mechanism of Sound Transduction:
1. sound waves hit the ear drum, causes vibration
2. vibration transfers to the ossicles
3. the stapes (part of the ossicles) transmit the vibration to the oval window, causing pressure waves
4. pressure waves transmitted to the inner ear, from the vestibule through the scala vestibuli
5. waves are transmitted into the cochlear duct into the scala tympani, to the round window
6. organ of Coti transmits the energy to an electrochemical signal, bending of its cilia open ion channels
7. hair cell receptors release glutamate and activate cochlear nerve cells
8. cochlear nerve cells + vestibular nerve cells join into vestibulochoclear nerve

Answer 4

Basic Mechanism of Auditory Processing:
- cochlear nerve transmits signal to the brain stem --> midbrain --> thalamus --> primary auditory cortex
- high frequency: vibrations at the base of cochlea
- low frequency: vibrations near the apex
- pitch: displacement of the basilar membrane
- loudness: pressure against the hair cell cilia

Detection of Equilibrium:
- vestibule has hair cells that detect position/movement
- when body moves/changes directions --> fluid inside the canals change --> receptors send signals to the vestibular nerve --> cerebellum --> balance adjusted/maintained

Answer 5

\({\bf{Structure~of~the~Eye:}}\)
- three layers around a fluid filled interior (fibrous tunic, vascular tunic, nervous tunic)
- lens divides the eye into anterior (aqueous humor) and posterior (vitreous humor)

lens: directs light towards the back of the eye, changes shape
> flattened shape: increases distance vision
> thick shape: close vision

fibrous tunic (outer):
- cornea, outer layer, protects the eye
- sclera: white layer, also protects the eye

vascular tunic: supplies nutrients to the tissues
- choroid: dark, absorbs light, prevents reflection inside the posterior
- ciliary muscle, ciliary zone, and ciliary body: control shape/position of lens, secrete the aqueous humor
- iris: colored band of muscle, controls size of pupils

nervous tunic:
- retina, outer layer of neurons and ganglial cells and inner layer of pigmented epithelium
- ganglial cells: connect retina w/ brain (optic nerve)
- bipolar neurons: transmit information between the ganglion and inner photoreceptor cells (ros and cones)

Answer 6

Photoreception:
- retinal: light sensitive, binds to opsin to form pigments
- rods: contain rhodopsin (sensitive to light), high sensitivity
- cones: carry pigments, located near the back of the eye (fovea centralis), color vision, low sensitivity

Basic Mechanism of Light Transduction:
1. rods/cones detect light --> retinal dissociates from opsin (bleaching of the pigment)
2. enzyme cascade, closing of ioin channels, hyperpolarization
3. photoreceptors release less glutamate
4. bipolar neurons release neurotransmitters and excite ganglion cells
5. ganglion cells send action potential to optic nerve --> brain

Basic Mechanism of Light Processing:
1. optic nerves connect to the optic chiasma
2. optic chiasma connects to the opposite eye
3. optic tracts transmit input to the midbrain
4. optic radiation: transmits input from thalamus to primary visual cortex in the brain
5. interpretation of input and relaying signals to the other regions of the brain

Answer 7

Anyway, that's the end of my tutorial, I hope it was a helpful resource. Source material is the 2nd Edition Barron's Prep book for the new MCAT