Lecture 21 Major Senses

Sensory Perception The sensory nervous system tells the central nervous system what’s happenin’! Sensory receptors Specialized sensory cells that detect changes inside and outside the body Sensory organs Complex sensory receptors Eyes, ears, taste buds

The path of sensory information Stimulation Physical stimulus activates a sensory receptor Transduction Converting the stimulus into an action potential Stimulus-gated ion channels in sensory neuron are opened or closed An action potential is generated Transmission Nerve impulse is conducted to the CNS Two main types of sensory receptors Extroreceptors sense stimuli in external environment Introreceptors sense stimuli in internal environment

Sensing the Internal Environment Vertebrates use many different sensory receptors to respond to changes in internal environment Temperature Change Two nerve endings in the skin One stimulated by cold, the other by warmth Blood chemistry Receptors in arteries sense blood CO2 levels Pain Special nerve endings within tissues near the surface

Sensing Pressure & Strectch Muscle contraction Sensory receptors called proprioceptors embedded within muscle & tendons sense stretch of muscle Touch Pressure receptors buried below skin Blood pressure Neurons called baroreceptors in major arteries

Sensing Chemicals: Taste Taste buds are located in raised areas called papillae Food chemicals dissolve in saliva and contact the taste cells

Sensing Chemicals: Smell Olfactory receptor cells are embedded in the epithelium of the nasal passage These are far more sensitive in dogs than in humans

Evolution of Balance & Hearing Lateral Line and the fish’s sense of hearing Fish are able to sense objects that reflect pressure waves and low-frequency vibrations The system consists of canals running the length of the fish’s body under the skin Canals have sensory structures containing hair cells projecting into a gelatinous cupula Vibrations produce movements of the cupula Hair cells bend and depolarize associated sensory neurons

Human Sensation of Gravity and Motion Receptors in the ear inform the brain where the body is in three dimensions Balance Gravity is detected by shifting of otolith sensory receptors These are located in a gelatin-like matrix in the utricle and saccule chambers of the inner ear Motion Motion is detected by the deflection of hair cells by fluid in a direction opposite to that of motion These hair cells are found in the cupula, tent-like assemblies in the three semicircular canals

Properties of Sound Sound is: A pressure disturbance (alternating areas of high and low pressure) originating from a vibrating object Composed of areas of rarefaction and compression Represented by a sine wave in wavelength, frequency, and amplitude Frequency – the number of waves that pass a given point in a given time Pitch – perception of different frequencies (we hear from 20–20,000 Hz) Amplitude – intensity of a sound measured in decibels (dB) Loudness – subjective interpretation of sound intensity

Sensing Sounds: Hearing When a sound is heard, air vibration is detected Eardrum membrane is pushed in and out by waves of air pressure Three small bones (ossicles) located on other side of eardrum increase the vibration force Amplified vibration is transferred to fluid within the inner ear Inner ear chamber is shaped like a tightly coiled snail shell and is called cochlea

Sensing Sounds: Hearing Cochlea are hair cells that rest on a membrane running up and down the chamber They are covered by another membrane Sound waves entering the cochlea cause this membrane “sandwich” to vibrate Bent hair cells send nerve impulses to brain Pitch is determined by different frequencies causing different parts of the membrane to vibrate Different sensory neurons are fired Sound intensity is determined by how often the neurons fire PLAY Transduction of Sound Waves

The Evolution of Vision Vision begins with the capture of light energy by photoreceptors Many invertebrates have simple visual systems Photoreceptors are clustered in an eyespot Perceive light direction but not a visual image Members of four phyla have evolved well-developed, image-forming eyes Annelids Mollusks Arthropods Vertebrates The eyes are strikingly similar in structure but are believed to have evolved independently

Eyes in Three Phyla of Animals

Structure of the Vertebrate Eye The vertebrate eye works like a lens-focused camera Cornea – Transparent covering that focuses light Lens – Completes the focusing Ciliary muscles – Change the shape of the lens Iris – Shutter that controls amount of light Pupil – Transparent zone Retina – The back surface of the eye Contains two types of photoreceptors: rods and cones Fovea – Center of retina Produces the sharpest image

How Rods and Cones Work Rods are extremely sensitive to dim light Cannot distinguish colors Do not detect edges Produce poorly defined images Cones can detect color Detect edges well Produce sharp images

How Light is Converted to a Nerve Impulse Pigment in rods and cones are made from carotenoids cis-retinal is attached to a protein called opsin This light-gathering complex is called rhodopsin When light is absorbed by cis-retinal, it changes shape to trans-retinal This induces a change in the shape of the opsin protein A signal-transduction pathway is initiated leading to generation of a nerve impulse

Color Vision Three kinds of cone cells exist, each with its own opsin type Differences in opsin shape, affect the flexibility of the attached cis-retinal This shifts the wavelength at which it absorbs light 420 nm – Blue 530 nm – Green 560 nm – Red

Colorblindness Colorblindness is a condition in which a person cannot see all three colors Caused by a lack of one or more types of cones It is inherited as a sex-linked trait and is more likely to affect males

Conveying the Light Information to the Brain Rods and cones are at the rear of the retina, not front! Light passes through four types of cells before it reaches them Photoreceptor activation stimulates bipolar cells, and then ganglion cells Nerve impulse travels through the optic nerve to the cerebral cortex

Focusing the Eye Focusing for Distant Vision: Light from a distance needs little adjustment for proper focusing Far point of vision – the distance beyond which the lens does not need to change shape to focus (20 ft.) Focusing for Close Vision: Accommodation – changing the lens shape by ciliary muscles to increase refractory power Constriction – the pupillary reflex constricts the pupils to prevent divergent light rays from entering the eye Convergence – medial rotation of the eyeballs toward the object being viewed

Problems of Refraction Normal eye (Emmetropic) – with light focused properly Nearsighted (Myopic) – the focal point is in front of the retina Corrected with a concave lens Farsighted (Hyperopic) – the focal point is behind the retina Corrected with a convex lens

Muscles That Move the Eye Six strap-like extrinsic eye muscles Enable the eye to follow moving objects Maintain the shape of the eyeball Four rectus muscles originate from the annular ring Two oblique muscles move the eye in the vertical plane

Binocular Vision Primates and most predators have eyes on front of the head The two fields of vision overlap allowing the perception of 3-D images and depth Prey animals generally have eyes located on sides of the head This prevents binocular vision but enlarges the perceptive field

Lacrimal Apparatus Consists of the lacrimal gland and associated ducts Lacrimal glands secrete tears Tears Contain mucus, antibodies, and lysozyme Enter the eye via lacrimal excretory ducts Exit the eye medially via the lacrimal punctum & lacrimal canal Drain into the nasolacrimal duct

Other Types of Sensory Reception Heat Pit vipers can locate warm prey, using infrared radiation Heat-detecting pit organs Electricity Used by aquatic vertebrates to locate prey and mates Magnetism Eels, sharks and many birds orient themselves in relation to the Earth’s magnetic field