THE SENSESBiology 14

Sensory Reception Sensory receptors

- specialized neurons or other cells that are tuned to the conditions of the external world or internal organs.- It sweeps its wide

head back and forth, like a beachcomber scanning the shore with a metal detector.

- Electroreception, the ability to sense electric field.

Hammerhead shark (Sphyrna mokarran)

Sensory receptors convert stimulus energy to action potentials

In stimulus detection, the receptor cell converts one type of signal (the stimulus) to another type, an electrical signal.

This conversion, called sensory transduction, produces a change in the cell’s membrane potential (the potential energy stored by the plasma membrane of the receptor cell.

• Changes in the flow of ions create a graded change in membrane potential called a receptor potential.

Sensory adaptation, the tendency of some sensory receptors to become less sensitive when they are stimulated repeatedly.

Five Categories of Stimuli

1. Pain Receptorsrespond to excess

heat or pressure or to chemicals released from damaged or inflamed tissues.2. Thermoreceptors

detect either heat or cold.

3. Mechanoreceptorsare stimulated by

various forms of mechanical energy, such as touch and pressure, stretching, motion, and sound.

Five Categories of Stimuli

• light touch- transduce very slight inputs of mechanical energy into action potentials.

• pressure sensor- lying deeper in the skin, is stimulated by strong pressure.

• touch receptor around the base of the hair, detects hair movements.

TYPES

Another type of mechanoreceptor (not shown) is found in our skeletal muscles. Sensitive to changes in muscle length, stretch receptors monitor the position ofbody parts.

4. Chemoreceptorsinclude the sensory

receptors in our nose and taste buds, which are attuned to chemicals in the external environment, as well as some receptors that detect chemicals in the body’s internal environment. Internal chemoreceptors

include sensors in some of our arteries that can detect changes in the amount of O2 in the blood.

Five Categories of Stimuli

5. Electromagnetic Receptors

detected as Energy occurring as electricity, magnetism, or various wavelengths of light. Photoreceptors,

including eyes, are probably the most common type of electromagnetic receptor. Photoreceptors detect the electromagnetic energy of light, which may be in the visible or ultraviolet part of the electromagnetic spectrum

Five Categories of Stimuli

For each of the following senses in humans, identify the type of receptor: seeing, tasting, hearing, smelling. Photoreceptors Chemoreceptors Mechanoreceptors Chemoreceptors

Hearing and BalanceBasic principle: the stimulation of long projections on hair cells (mechanoreceptors) in fluid-filled canals.

Three regions:

1. Outer ear 2. Middle ear3. Inner ear

1. outer ear consists of the flap-like pinna— the fleshy structure we commonly refer to as our “ear”—and the auditory canal.

2. The pinna and the auditory canal collect sound waves and channel them to the eardrum, a sheet of tissue that separates the outer ear from the middle ear.

When sound pressure waves strike the eardrum, the eardrum vibrates and passes the vibrations to three small bones: the hammer (more formally, the

malleus), anvil (incus), stirrup (stapes)

The middle ear also opens into a passage called the Eustachian tube, which connects with the pharynx (back of the throat), allowing air pressure to stay equal on either side of the eardrum.

The inner ear consists of fluid-filled channels in the bones of the skull. Sound vibrations or movements of the head set the fluid in motion. One of the channels, the cochlea (Latin for “snail”), is a long, coiled tube.

• Our actual hearing organ, the organ of Corti, is located within the middle canal.

• The organ of Corti consists of an array of hair cells embedded in a basilar membrane, the floor of the middle canal.

Hearing

Sound waves, which move as pressure waves in the air, are collected by the pinna and auditory canal of the outer ear.

Volume and Pitch The brain senses a sound as an increase in

the frequency of action potentials from the auditory nerve.

How is the quality of the sound determined?

“The higher the volume (loudness) of sound, the higher the amplitude (height) of the

pressure wave it generates.”

The pitch of a sound depends on the frequency of the sound waves.

Deafness

Deafness can be caused by the inability of the ear to conduct sounds, resulting from middle-ear infections, a ruptured eardrum, or stiffening of the middle-ear bones (a common age-related problem).

Can also result from damage to sensory receptors or neurons.

In recent years, however, many deaf people have received cochlear implants, electronic devices that convert sounds to electrical impulses that stimulate the auditory nerve directly.

These fluid-filled structures lie next to the cochlea (Figure 29.5) and include three semicircular canals and two chambers, the utricle and the saccule.

The ear contains three semicircular canals that detect changes in the head’s rate of rotation or angular movement.

The inner ear houses our organs of balance

What causes motion sickness?Motion sickness is believed to be

caused when the brain receives signals from equilibrium receptors in the inner ear that conflict with visual signals from the eyes.

Symptoms maybe relieved by closing the eyes, limiting head movements, or focusing on a stable horizon.

Vision

2 types of image-forming eyes:1. compound eye

consists of up to several thousand light- etectors called ommatidia.

2. Single-lens eye evolved independently in vertebrates.

The human eye has a small opening at the center of the eye, the pupil, through which light enters.

An adjustable doughnut shaped iris changes the diameter of the pupil to let in more or less light.

After going through the pupil, light passes through a single disklike lens.

The lens focuses light onto the retina, which consists of many photoreceptor cells.

Photoreceptor cells are highly concentrated at the retina’s center of focus, called the fovea.

The outer surface of the human eyeball is a tough, whitish layer of connective tissue called the sclera.

At the front of the eye, the sclera becomes the transparent cornea, which lets light into the eye and also helps focus light.

The sclera surrounds a pigmented layer called the choroid. The anterior choroid forms the iris, which gives the eye its color.

The lens and ciliary body divide the eye into two fluid-filled chambers.

The large chamber behind the lens is filled with jellylike vitreous humor.

The much smaller chamber in front of the lens contains the thinner aqueous humor.

• A thin mucous membrane helps keep the outside of the eye moist. This membrane, called the conjunctiva

Arrange the following eye parts into the correct sequence encountered by photons of light traveling into the eye: pupil, retina, cornea, lens, vitreous humor, aqueous humor.

Cornea aqueous humor pupil

vitreous humor lens

Artificial lenses or surgery can correct focusing problems

Reading from an eye chart measures your visual acuity, the ability of your eyes to distinguish fine detail.

People with nearsightedness cannot focus well on distant objects, although they can see well at short distances (the condition is named for the type of vision that is unimpaired).

Farsightedness (also known as hyperopia) is the opposite of nearsightedness. It occurs when the eyeball is shorter than normal, causing the lens to focus images behind the retina.

2 types of photoreceptors:1. Cones- are stimulated by bright light

and can distinguish color, but they contribute little to night vision.

• Cones contain visual pigments called photopsins, which absorb bright, colored light.

2. Rods are extremely sensitive to light and enable us to see in dim light, though only in shades of gray.

Rods contain a visual pigment called rhodopsin, which can absorb dim light.

Taste and Smell

Taste and odor receptors detect chemicals present in solution or air

Olfactory (smell) receptors are sensory neurons that line the upper portion of the nasal cavity and send impulses along their axons directly to the olfactory bulb of the brain

Many animals rely heavily on their sense of smell for survival. Most other mammals have a much more discriminating sense of smell than humans.

Odors often provide more information than visual images about food, the presence of mates, or danger.four familiar taste

perceptions:sweet, sour, salty, and bitter

and fifth,UMAMI (Japanese for

“delicious”)