Section 3: Sensory Systems Preview Bellringer Key Ideas Perception of Stimuli Sensory Receptors Processing Sensory Information Summary

Bellringer Select a spot in the room to look at, such as a poster or bookshelf. Point to a specific place on the object with your index finger held out in front of you. Focus on that spot, first with both eyes open, then with just the left eye closed, and then with just the right eye closed. Write down what happens to the image you see each time. Explain why you think this occurs.

Key Ideas How is sensory information detected? What are the five types of sensory receptors? Where are the sites of sensory processing in the brain?

Perception of Stimuli The body’s ability to perceive things depends on sensory receptors and the regions of the brain that decode sensory stimuli. Specialized neurons called sensory receptors detect sensory stimuli and convert them to electrical signals. These signals then can be interpreted by the brain. The sensory division of the peripheral nervous system collects information about sensory stimuli in and around the body. This information is then sent to the brain.

Visual Concept: Sense Organs

Sensory Receptors Sensory receptors are most concentrated in the sense organs—the eyes, ears, nose, mouth, and skin. Sensory receptors detect all forms of energy, including heat, light, pressure, and chemicals. The major classes of sensory receptors are photoreceptors, mechanoreceptors, chemoreceptors, pain receptors, and thermoreceptors.

Visual Concept: Types of Sensory Receptors

Types of Sensory Receptors Transparency # J69

Sensory Receptors, continued Vision The eyes enable the perception of color, movement, and the fine details of objects. Light enters the eye through the pupil and passes through the lens, a transparent disk that focuses light on the retina. The retina is a layer of specialized light-sensitive cells that line the inner surface of the eye.

Visual Concept: Human Eyes

Sensory Receptors, continued Vision The retina contains two types of photoreceptors called rods and cones which convert light energy to electrical signals that can be interpreted by the brain. Rods respond best to dim light and transmit our “black and white” vision. Cones respond best to bright light and transmit our “color” vision. The brain combines the slightly different images received from both eyes to produce three-dimensional vision.

Visual Concept: Comparing Rods and Cones

Sensory Receptors, continued Hearing and Balance The mechanoreceptors responsible for hearing and balance are hair cells located in fluid-filled chambers found in the inner ear. The hair cells bend when the surrounding fluid moves, triggering signals that are sent to the brain. When sound waves enter the ear, they strike the tympanic membrane, or eardrum, and cause the eardrum to vibrate.

Structure of the Ear

Sensory Receptors, continued Hearing and Balance Behind the eardrum, three small bones of the middle ear transfer sound vibrations to the cochlea located in the inner ear. The cochlea is a coiled tube that resembles a snail’s shell with hair cells lining its fluid-filled chamber. When the hair cells in the cochlea are stimulated, they generate impulses which travel along the auditory nerve to the temporal lobe of the brain to be processed.

Visual Concept: Human Hearing

Sensory Receptors, continued Hearing and Balance Changes in body position and movement are detected by hair cells found in the semicircular canals. The semicircular canals are three fluid-filled looped rings in the inner ear lined with hair cells that respond to changes in head position relative to gravity. These hair cells send electrical signals which eventually arrive at the cerebellum for processing in order to determine the orientation of the head.

Sensory Receptors, continued Taste and Smell Chemoreceptors are sensory receptors that respond to chemicals. Taste buds, which contain chemoreceptors, are found on the tongue, palate, and pharynx. Taste buds can detect at least five basic chemical substances: sugars (sweet), acids (sour), alkaloids (bitter), salts (salty), and proteins (savory). When food molecules dissolve in saliva, they bind to taste cells which send impulses to the brain for processing.

Sensory Receptors, continued Taste and Smell Chemoreceptors responsible for the sense of smell are located in the nostrils and are called olfactory receptors. Chemicals in the air stimulate the olfactory receptors which send impulses to the brain in order to detect odors. The ability to taste food depends on the olfactory receptors in the nostrils as well as the taste buds in the mouth.

Visual Concept: Comparing Taste and Smell

Sensory Receptors, continued Touch and Other Senses The skin is responsible for the sense of touch and the ability to sense changes in the surrounding temperature. Touch and other body-related senses are collectively referred to as somatosensation. Pain receptors in the skin respond to potentially harmful stimuli, such as intense heat or cold. Many self-protective responses, such as reflexes, are started by pain receptors.

Sensory Receptors, continued Touch and Other Senses Thermoreceptors are nerve endings that detect changes in temperature and play an important role in homeostasis. These receptors are found in the skin and in the hypothalamus. Mechanoreceptors are found throughout the body and respond to physical stimuli such as pressure and tension that cause the bending of tissue. Mechanoreceptors are concentrated in sensitive areas, including the face, hands, fingertips, and neck.

Processing Sensory Information Most sense organs send signals through the PNS to the CNS where the thalamus in the brain relays this information to specific regions of the cortex for processing. Specialized regions of the cerebral cortex detect different sensory information. Grooves divide the cerebral hemispheres into four general areas, or lobes, called the parietal lobe, the frontal lobe, the occipital lobe, and the temporal lobe.

Processing Sensory Information, continued The parietal lobe of the cortex receives somatosensory (“body sense”) stimuli. The frontal lobe regulates skeletal muscle function, intellectual function, and aspects of our personality. The occipital lobe processes visual information. The temporal lobe processes sound information.

Processing Sites and Lobes of the Cerebral Cortex Transparency #J63

Summary Specialized neurons called sensory receptors detect sensory stimuli and convert them to electrical signals. These signals then can be interpreted by the brain. The major classes of sensory receptors are photoreceptors, mechanoreceptors, chemoreceptors, pain receptors, and thermoreceptors. Specialized regions of the cerebral cortex detect different sensory information.