Chapter 10

Sensory receptors sense changes in their surroundings.

Each type of receptor is most sensitive to a distinct type of stimulus.

The major types of receptors are: 1. Chemoreceptors- Stimulated by changes in the

chemical concentration of substances

2. Pain Receptors- Tissue damage

3. Thermoreceptors- Changes in temperature

4. Mechanoreceptors- Changes in pressure and

movement

5. Photoreceptors- Light energy

Sensations are feelings resulting from sensory stimulations.

A particular part of the sensory cortex interprets every impulse reaching it in the same way.

The cerebral cortex projects a sensation back to the region of stimulation.

Sensory adaptations are adjustments of sensory receptors to continuous stimulation.

Impulses are triggered at decreasing rates.

Somatic senses are associated with receptors in the skin, muscles, joints, and viscera.

Sensory Nerve Fibers- Receptors for the sensations of touch and pressure and their free ends are between epithelial cells.

Meissner’s Corpuscles- Receptors for the sensation of light touch and they are abundant in the hairless portions of the skin (connective tissue) such as fingertips, palms, soles, ect.

Pacinian Corpuscles- Receptors for the sensation of heavy pressure; composed of connective tissue fibers and cells. Commonly found in the deeper subcutaneous tissue and in muscle tendons and joint ligaments.

Include two sets of free nerve endings heat and cold receptors both of which adapt rapidly.

Heat Receptors- Most sensitive to temperatures above 25℃ (77℉) and become unresponsive at temperatures above 45℃ (113℉). Temperatures near or above 45℃ produce a burning sensation.

Cold Receptors- Most sensitive to temperatures between 10℃ (50℉) and 20℃ (68℉). Temperatures below 10℃ produce a freezing sensation.

Pain receptors are free nerve endings that tissue damage stimulates.

Pain receptors are widely distributed throughout the skin and internal tissue. However the brain lacks pain receptors.

Visceral Pain

Only receptors in viscera that whose stimulations produce sensations.

Sensations produced from the visceral receptors feel as if they are coming from some other body part.

Pain may be referred because sensory impulses from the skin & viscera travel on common nerve pathways.

Sense of Pain Cont…Pain Nerve Fibers

Two main types of pain fibers that conduct impulses away from receptors are acute pain fibers and chronic pain fibers.

Acute pain fibers conduct nerve impulses rapidly and are associated with sharp pain. Chronic pain fibers are conduct impulses more slowly and produce a dull, aching pain.

Sense of Pain Cont…

Pain impulses are processed in the gray matter of the spinal cord and ascend to the brain.

Once in the brain, pain impulses pass through the reticular formation before conduction to the cerebral cortex.

Regulation of Pain Impulses

Awareness of pain occurs when pain impulses reach the thalamus.

The cerebral cortex determines pain intensity and locates its source.

Impulse descending from the brain stimulate neurons to release pain-relieving neuropeptides, such as enkephalins and endorphins.

Sense of Pain Cont…

Special Senses

Special Senses

Are those whose receptors are within relatively large, complex sensory organs of the head.

Sense of Smell

Olfactory Receptors

Smell and taste receptors are chemoreceptors that are stimulated when chemicals are dissolved in liquids.

They function together with taste and aid in food selection.

Sense of Smell Cont…

Olfactory Organs

Contain olfactory receptors and supporting cells in the nasal cavity.

Cells are bipolar neurons with hairlike cilia.

Sense of Smell Cont…

Olfactory Nerve Pathway

Nerve impulses travel from the olfactory receptors cells through the olfactory nerves, olfactory bulbs, and olfactory tracts to interpreting centers in the temporal and frontal lobes of the cerebrum.

Sense of Smell Cont…

Olfactory Stimulation

Impulses may result when gaseous molecules combine with specific sites on cilia of receptor cells.

Olfactory receptor cells adapt rapidly.

Sense of TasteTaste Receptors

Taste buds consist of taste (receptors) cells and supporting cells.

Each taste bud includes a group of modified epithelial cells.

Taste cells have taste hairs, with receptors sites to which chemicals combine.

Sense of Taste Cont…

Taste Sensations

The four primary taste sensations are sweet, sour, salty, and bitter.

Various taste sensations result from the stimulation of one or more sets of taste receptors.

Sense of Taste Cont…

Taste Nerve Pathways

Sensory impulses from taste receptors travel on fibers of the facial, glossopharyngeal, and vagus nerves.

These impulses are carried out to the medulla oblongata and then ascend to the thalamus, from which they travel to the gustatory cortex in the parietal lobes.

Sense of Hearing External ear- Collects sound waves of vibrating objects.

Middle ear- Auditory ossicles of the middle ear conduct sound waves from the eardrum to the oval window of the inner ear.

Auditory tube- Connect the middle ear to the throat and help maintain equal air pressure on both sides of the eardrums.

Inner ear- A complex system of connected tubes and chambers--

osseous and membranous labyrinths Organ of Corti contains the hearing receptors that

vibrations in the fluids of the inner ear stimulate.

Sense of Hearing Cont…

Auditory Nerve Pathways

Auditory nerves carry impulses to the auditory cortices of the temporal lobes.

Some auditory nerve fibers cross over, so that impulses arising from each ear are interpreted on both sides of the brain.

Sense of Equilibrium Static Equilibrium- Maintains the stability

of the head and body when they are motionless.

The organs responsible for this stability are located in the vestibule.

Dynamic Equilibrium- Balances the head and body when they are moved or rotated suddenly.

Other structures that help maintain equilibrium include the eyes and mechanoreceptors associated with certain joints,

Sense of Sight Visual accessory organs- include the eyelids, lacrimal

apparatus,(protect eye) and extrinsic muscles (move eye) of the eyes.

Structure of the eye- the wall of the eye has an outer, middle and inner layer (tunic)

Outer tunic (Sclera)- is protective and its transparent anterior portion (cornea) refracts light entering the eye.

Middle tunic (Choroids coat)- is vascular and contains pigments that keep the inside of the eye dark.

Sense of Sight Cont… Inner coat (Retina)- contains the visual

receptor cells.A. Lens is a transparent, elastic structure. Ciliary

muscles control its shape.

B. Lens must thicken to focus on close objects.

C. Iris is a muscular diaphragm that controls the amount of light entering the eye.

D. Spaces within the eye are filled with fluids that help maintain its shape.

Light Refraction- the cornea and lens refract light waves to focus an image on the retina.