Sensory systems

What happens to light as it passes into the eye? Which structures in the eye are involved at each step? How does the information get converted into an image?

1. To form an image, the cornea, a thin tissue located in the posterior of the eye, and lens bend incoming light rays, focusing them on the retina. 2. In doing so, the light rays cross, inverting the image on the retina and changing up to down and left to right. 3. Humans, other mammals, and birds focus light on the retina by changing the shape of the lens. 4. Two eyes provide a wider field of vision. 5. Binocular vision, which allows for depth and distance perception, is the ability to combine images from both eyes to produce a single visual image.

How do sensory cells distinguish between weak and strong stimuli?

Because nerve cells transmit action potentials in an all-or-nothing fashion, information about the strength of the signal is coded by the number of action potentials fired over a period of time or the firing rate. o A strong stimulus constantly gives off a high frequency, leading to an intense reading of the stimuli. A weak stimulus provides a low frequency of action potentials, leading to a weaker reading of the stimuli. ▪ Wrapping your hand around a warm mug is a weak stimulus, while putting your hand in a fire would be a strong stimulus.

? How are mechanoreceptors involved in sensing touch, balance, and hearing? What secondary structures are involved in activating these receptors?

Mechanoreceptors respond to physical deformations of their membrane produced by touch, stretch, pressure, motion, and sound. ▪ Deformation of the receptor membrane opens Na+ channels, causing a depolarization of the endings of the cell's dendrites. o Examples of mechanoreception include: ▪ bacteria that can detect physical forces and internal cell pressure ▪ whiskers of dogs, cats, rodents that detect touch ▪ stretch receptors in muscles that influence a muscle's motor activation

How does rhodopsin work in allowing detection of light? What happens to rhodopsin and retinol when a photon is absorbed and how does this result in a signal being passed on?

Rhodopsin is covalently bounded to retinal, a derivative of vitamin A that absorbs light • It undergoes a conformational change from a cis to a trans configuration, and that conformational change causes Na+ channels to close. o the cell membrane becomes hyperpolarized (rather than depolarized) and the cell's neurotransmitter release is reduced

How does stimulation of a sensory receptor cell result in an actual sensation? (Where does the information get processed and how does it get there?)

Sensory reception depends on converting the energy of a physical or chemical stimulus into an action potential, either in the sensory receptor itself or in the neuron with which it synapses. 2. Processed in the brain

How do sensory receptor cells pass information about sensory inputs to other cells?

Spatial Summation: Multiple receptors that receive a stimulus often converge onto a neighboring neuron that increases its firing rate proportionally to the number of signals received. • Temporal summation is the integration of sensory stimuli that are received repeatedly over time by the same sensory cell. o When a sensory cell is stimulated more frequently, the excitatory postsynaptic potentials sum, making it more likely that the cell is depolarized above threshold and fires an action potential.

What parts of the brain are responsible for processing sensory information?

The cerebellum coordinates complex motor tasks, such as catching a ball or learning to write and talk—it integrates both motor and sensory information. • The brainstem consists of the medulla, pons, and midbrain: It initiates and regulates motor functions such as walking and controlling posture and coordinates breathing and swallowing. • The brainstems activates the forebrain by relaying information from lower spinal levels. • High levels of activity within the brainstem maintain a wakeful state; low levels enable sleep. • If the midbrain is damaged, loss of consciousness and coma result.

How are chemoreceptors involved in smell, taste, and pheromone detection?

• In animals, chemoreceptors respond to molecules that bind to specific protein receptors on the cell membrane. o Luna moths (photo), which have chemoreceptors in their antennae to detect pheromones. • Chemoreception also underlies the sense of smell and taste. o Molecules bind to a protein receptor on a taste receptor, causing the protein receptor to change conformation. o That change in conformation in turn triggers the opening of Na+ channels through G protein signal transduction pathways. o The influx of Na+ ions depolarizes the receptor cell membrane. o In the case of taste receptors, no action potential fires, but the depolarization travels far enough down the receptor's short axon to trigger the release of neurotransmitters.

How do eyes differ between major phyla of animals?

• In flatworms, two simple eyecups on the dorsal, or back, surface of the head detect the direction and intensity of light sources. o Uses simple photoreceptors and a pigmented epithelium to sense the direction of light • Insects and crustaceans have compound eyes that consist of individual light-focusing elements called ommatidia. o The number of ommatidia in the eye determines the resolution, or sharpness, of the image. The image produced by compound eyes is a mosaic because individual light regions are sensed by separate ommatidia • Single-lens: Vertebrates and cephalopod mollusks like squid and octopus have a single-lens eye. o The eye works like a camera to produce a sharply defined image of the animal's visual field. • Vertebrate eyes are protected within the eye socket of the skull. ▪ Light passes through the transparent cornea, enters the eye through an opening called the pupil, and then passes through a convex lens. o The eye is surrounded by the sclera—a tough, white outer layer. o Below the sclera, a thinner layer, called the choroid, carries blood vessels to nourish the eye. o Mucus secretions produced over the sclera keep the eye moist. o The portion of the sclera in the front of the eye is transparent; this is the cornea.

How are ion channels related to sensation?

• The influx of ions changes the membrane potential by altering the distribution of charged ions on either side of the membrane

How do sensory receptor cells detect stimuli such as light, heat, smell, taste, and pressure? What feature of a cell is necessary to detect these various stimuli?

• sensory receptors are organized into specialized sensory organs that convert particular physical and chemical stimuli into nerve impulses. o These signals are then transmitted to the central nervous system for processing and interpretation. • Sensory transduction typically beings with a receptor protein that opens or closes ion channels in response to a specific stimulus such as heat, light, chemicals, mechanical force, or electric fields.