Chapter 11: The Sensory System

Sensory Receptors

• Part of nervous system that detects sensory stimulation
• Free dendrites, end- organs, specialized cells
• Chemoreceptor – chemical
• Photoreceptor – light
• Thermoreceptor – heat/cold
• Mechanoreceptor – movement

Threshold stimulus

stimulus of adequate intensity in order to respond and generate a nerve impulse

Special Senses

• Receptors are localized in a special sense organ, such as eyes
• Vision
• Hearing
• Taste
• Smell
• Equilibrium

General Senses

• Pain
• Touch
• Pressure
• Temperature
• Position (proprioception)

Sensory Adaptation

• Sensory receptors are exposed to a constant stimulus, become less sensitive to that stimulus over time
• Temperatures become less acute when exposed over time, such as washing dishes in warm water or wearing a watch

Part of the nervous system that detects a stimulus.

What is a sensory receptor?

Chemoreceptors, photoreceptors, thermoreceptors, mechanoreceptors

What are some categories of sensory receptors based on type of stimulus?

Special senses are localized in a special sense organ. General sense is widely distributed throughout the body.

How do the special and general senses differ in location?

Receptor fails to respond to the stimulus

What happens when a sensory receptor adapts to a stimulus?

Orbit

skull eye socket, Upper and lower eyelids and eyelashes

Levator palpebrae

muscle in upper lid that keeps eye open when it contracts; ptosis when not working (drooping eyelid)

Conjunctiva

thin membrane that covers the whites of the eye; produces mucus; fold onto itself to form a sac that will hold eyedrops

Lacrimal glands

tear glands

Nasolacrimal ducts

tear ducts

Lacrimal apparatus

glands and ducts

Extrinsic Muscles

• Voluntary
• Outer surface of the eye
• Origin: orbital bones
• Insertion: surface of the sclera
• Superior rectus, Superior oblique, Inferior rectus, Inferior oblique, Lateral rectus, Medial rectus

Convergence

both eyes center on one visual field

Optic nerve (CN II)

from rods and cones to the brain (through the thalamus first)

Ophthalmic branch of Trigeminal nerve (CN V)-

pain, touch and temperature from eye and surrounding parts to the brain

Oculomotor nerve (CN III)

all but 2 extrinsic muscles

Trochlear nerve (CN IV)

Superior obliques

Abducens (CN VI)

Lateral rectus

Structure of the Eyeball
(Fibrous Tunic)

• Sclera
• Outer coat made of connective tissue
• Collagen & fibroblasts
• White of the eye
• No blood vessels

Cornea

anterior portion, forward curving, colorless, covers the iris

Structure of the Eyeball
(Vascular Tunic)

• Choroid
• Network of connective
tissue and blood vessels
• Reveals diseases when examined with an ophthalmoscope

• Contains the lens, ciliary muscles, suspensory ligaments, and iris

Suspensory ligaments

controls lens movement

Structure of the Eyeball
(Nervous Tunic)

• Retina
• Inner tunic
• Eye’s receptor layer
• Rods and cones located here
• Connects with optic nerve

Refraction

bending of light rays as they pass from one substance to another of different densit

Cornea

anterior continuation of sclera, transparent and colorless, main refractor of the eye

Aqueous Humor

watery (aqua) fluid that fills the eye up to the lens

Lens

clear, circular, firm, elastic; 2 bulging surfaces (biconvex); adjusts to light

Vitreous Body

jellylike substance that fills the eye behind the lens

Iris

colored pigment, 2 sets of circular muscles that open (dilates) and close (constricts) according to the amount of light

Pupil

iris’s central opening

Ciliary muscle

flattened ring with a central hole on the outer edge of the iris; controls the lens with suspensory ligaments

Accomodation

movement of the lens to focus on an objec

Movement of the Lens- Close objects:

ciliary muscles contract, suspensory ligaments relax, lens becomes rounded

Movement of the Lens- Far vision:

ciliary muscles relax, suspensory ligaments tense, lens flattens

Function of Retina- Rods

light sensitive, dim light, no sharp image (dark adaptation)

Cones

bright light and colors, localized at retinal center; color blindness when missing

Fovea Centralis

point of sharpest vision

Macula Lutea

yellowish {lute/o (yellow)} spot in the retina, fovea is located here; may show degenerative changes with age

Optic Disk

optic nerve exits the eyeball (blind spot)

Rhodopsin

• Pigment that help rods and cones function properly
• Vitamin A is needed to produce this
• Without this cannot see in dim light
• Night blindness