Lecture 9: Sensory Systems

What is the benefit of sensory systems?

All animals depend on info from their surroundings, most info is obtained through sensory organs, which have specialized cells that respond to particular stimuli.

What type of stimuli can organisms recieve?

Chemical (Gustation, olfaction, humidity) (oldest) if you percieve you must have that molecule attatched to a sensory receptor and change its voltage (dendrite has a pore that accepts the chemical), electromagnetic and thermal energy (light, heat, magnetic fields, infaredradiation), Force (gravity, inertia, sound, sonar, touch, pressure)

General principles of sensory perception

Sensory cells are transducers (recieve external info, encode it as impulses). Our sensory cells transduce signals into an action potential. Sensory nerves are the transmission lines carrying the info to the CNS. in the CNS signals are decoded, then the info is integrated and used. Think about where the signal goes when it reaches the end.

Sequence of events in sensory perception

1. Stimulus produces an alteration of a receptor protein in the plasma mebrane of receptor cell

2. THis change in membrane permeability shifts the membrane voltage (receptor potential)

3. As intensity of stimulus increases, more channels respond, producing increased receptor potential

   1. If the sensory information reaches threshold, it is converted to an action potential (spike initiating zone)

Transmission of information

Action potentials are always of the same magnitude irrespective of stimuli. How is it possible to convey info about variations in strength of stimulus? By temporal summation-the frequency of action potentials.

Spontaneous activity

In some receptor cells, constantly producing action potentials even without stimulation (always firing). Still hits threshold for an action potential, rests, etc., but does it without stimulation. If the nerve is already firing you dont have to worry about hitting threshold. An animal using spontaneous activity dectects low levels, or low changes, in stimuli. This increases sensitivity of the receptor. Changes in either direction can also be encoded

Sensory adaptation

Prolonged exposure to a stimulus produces a change in ampligication of the receptor events, changing the way it encodes the stimulus intensity. Prolonged exposure will cause a cell to stop firing. this filters out unneeded information

Principle summary

1. Sensory organs serve as transducers where external signals generate
receptor potentials.
2. Information is conveyed in the sensory nerves as action potentials
3. Action potentials in all sensory nerves are of the same nature and magnitude.
4. The magnitude of the action potential is unaffected by stimulus
intensity.
5. Stimulus intensity is coded by frequency modulation of the actions potentials

Statolith

Bone floating over hair cells, add definition

Mammalian ear

Two functions; detecting the force of equilibrium and sound pressure.

Equilibrium: three semicircular canals filled with fluid allow vertebrates to detect head movement by collecting information on Z Y and Z planes. At the base of the canals are calcaneous stones over a bed of hair. Disorientation comes from two conflicting streams of information. Alcohol makes the fluid in the canals more viscous (thicker).

Sound: External ear acts as a funnel to collect sound and concentrate air pressure onto the eardrum (tympanic membrane). Energy is transferred from air to aquatic medium of the inner ear where hair cells transduce the sound input. Middle ear (containing hinges and ossicles) function is to amplify the sound. Sound hits the membrane, the membrane vibrates and pushes against the bones. Hinged bones help sound amplify better. The stapes pushes against the oval window which is always smaller than the tympanic membrane, increasing force per square millimeter. The inner ear (cochlea) is a fluid filled coil containing hair cells all along it. the oval window vibrates and creates a standing wave of water moves along the entire cochlea. Shorter waves affect more hair cells, so where the hair cells are located determines what frequency you are hearing. If a sound is not hitting a hair cell, it does not hear that frequency. This is because all sound waves (respective to frequency) are identical. Hair cells get damaged from constant exposure. If a sound is loud, the waveform does not change, but the force does. If a hair cell gets stuck bent down. If the sound is too loud it will get cut off entirely.

A lot of aquatic vertebrates do not have equilirbium. WHY? Because it does not matter to them since gravity does not affect them dramatically underwater.

Insect vibration and sound detection

The easy way, tympanal organs are located i a variety of locations in insects are associated with respiratory passages (trachea). Some insects

Echolocation (high frequency)

What is it? Emmiting a sound and listening for its echo. Echo tells distance and shape of objects using very high frequencies, the frequency distorts. The intensity of the sound bouncing back is also damped. The melon is made entirely of fat, but of various types with different densities. The sound is made through the blowhole with two muscular valves that are clicked together to produce a noise. This noise is focus and directed out through the melon. Dr bartols pHD partner discovered dolphin jaws contain distinct fats that run along the jaw and extend to the tympaic membrane to funnel sound to the ear. Fats are used because they are a different density than water.

Infrasound

Low frequency sound detection. Homing pigeons can detect aroun d0.05 Hz, which is on cycle per 20 s. Elephants use 14-24 Hz to communicate over long distances through the earth and pick them up with the pads of their feet.

Photoreceptors

Transduce photons of light into electrical signals that can be interpreted by the nervous system. Widespread from unicellular to multicellular organisms. Compound eyes contain many optic units called ommatidia aimed at different parts of optic field. Each photoreceptor contains its own lens. Each ommatidium samples a 2 degrww portion of the visual field, making a coarse image. This type of eye is very good at tracking movement

Single lens eye (vertebrates)

Light hits the cornea and bends because the lens is a different density. The purpose of the lens is refraction/to bend light by changing the thickness of the lens. The retina contains the photoreceptors. The light has to bend at different angle's for various distances. As people age, the lens starts to stiffen. This is why near-vision goes first. When underwater, the light does not bend when it hits the lens because its the same density. Many bony fish focus images on retiuna by moving spherical lens forwards and backwards. In higher vertebrates, the curvature and thickness of the lens is changed to focus images, this is called accommodation.

Light and vision

Humans see between 400-700 nanometers called visible light. Photoreceptors are located in the retinal lining: rods and cones. Rods (detect intensities) function best in dim light and have high sensitivity with low resolution. The edges of the eyes are mostly rods. Rods can be overexposed and change shape, then stop working. Rods contain photopigments. Rhodopsin (497 nm) is found in the rods.

Cones (detect colors) function best in bright light, low sensitivity, and high resolution. Photopigments found in cones are blue (420), green (524), and red (563).

Light hits the back of the eye and reflects back onto the photoreceptor cells. In cats the back of the eye is reflective, which is why they can see at night. Horizontal streaks are seen sometimes instead of fovea. This is advantageous for animals that need to scan a horizon

Infrared radiation

Pit organs on snakes are specialized receptors for infrared radiation.

Sorting and processing of information

Convergence is many different receptor cells connect to a single neuron (retina). Based on the architectrure of this, it amplifies the signal. Divergence is one receptor cell connecting to many neurons (auditory cell). Serial processing is when two information across their respective transmission lines, and is processed sequentially (seen in language, lip reading vs hearing the words). Parallel processing is pathways processing different functional properties of sensory signals simultaneously (hearing)

information procesing in the visual system

THE EDGE EFFECT!!! contrast enhacement is a mechanism allowing for more recognition of stimulus change. Lateral inhibition is photoreceptors coupled to one another so when one cell is active, its neighbors are inhibited. The benefit is amplification of edge effect.

IN VERTEBRATES: If you are cells in entirely in bright light, they have action potentials at a higher rate than in dim light. Discharge occurs at a lower rate than if the cell were by itself because it is slightly inhibited.