General Biology/Tissues and Systems/Sensory Systems

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Sensory systems

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Categorized by

  1. nature of stimulus, such as mechanical, chemicalor light stimulus, and
  2. where stimulus received, such as outside (exteroceptors,such as the eye and skin temp receptors) or insidebody (interoceptors, such as blood body temperature receptors).

Transduction of sensory input into signal. Means to “carry across”, signal transduced, or carried, from environment into nervous signal.

Three sensory processes we cover

  1. taste and smell (chemoreception)
  2. gravity and movement 
  3. light

Taste and smell (chemoreception)

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Found in mammal nose and mouth, fly feet, fish bodies, moth antennae.

Papilla: bumps on tongue, contain taste buds down between. Sweet, sour, salty and bitter. Some act directly by ion channels, others act indirectly. Other “taste” sensations really smell.

Smell: received in nasopharynx. Airborne molecules go into solution on moist epithelial surface of nasal passage. Approximately 1000 genescode for sensory neuron receptors. “Fried onions” odor not one receptor but a mixture of many odors registered in our mind as one. Very sensitive, habituates rapidly (don’t notice a smell after a bit). Odor sensation has relatively unfiltered root to higher brain centers.

Snakes more chemosensory focused than us.

Response to gravity and movement

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Registered in inner ear. Three semicircular canals loop in three planes at right angles to each other, responsible for transduction of movement messages. Method: hair cells deformed by gelatinous membrane. Vestibular apparatus, gives us perception of gravity and movement. Due to physical response, not chemical binding.

Cochlea: bony, coil shaped part of inner ear, where hearing occurs.

Sound enters through auditory canal, vibrates tympanic membrane,moving three bones of middle ear (malleus, incus, and stapes)against oval window opening in front of cochlea. Cochlea has three fluid filled ducts, one of these the organ of Corti. Sound waves in air go to vibration in organ of Corti; fluid tickles hair cells, which register the movement along basilar membrane in cochlea. Different sound frequencies move different portions of basilar membrane. Hearing loss due to loss of hair cells.Humans normally smell more than 300 odors in a day(Facts and Truth).

Transduction of sound accomplished through physical deformation,not chemical binding.


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Light enters pupil, focused by lens onto retina.

Sclera: hardened part behind retina.

Optic nerves and neurons attached to retina. Blind spot where optic nerve attaches, has no receptors. 

Two types of photoreceptors

  1. rods - black and white low-light vision, 100 million in each retina in humans.
  2. cones - color vision, work best under better illumination. 3 million in each retina.

Fovia: region of most acute vision, has most of the cones, few rods.

Transduction process of light to signal a molecular change, to light absorbing molecule called photopigment. Located in outer parts of rods and cones in pigment discs. The rod photopigment is called rhodopsin,cone has three photopigments, called photopsins. This molecular change initiates pathways to result in action potential in downstream neuron leading to vision center in brain.Parul Godika

Each of the three photopsins has a different peak of sensitivity: blue,green or red, and changes isometric form (from cisto trans) based on light from a particular wavelength range. Color blindness:inherited lack of one or more types of these cones. Gene carried on X chromosome, therefore more common in men than women.

General Biology | Getting Started | Cells | Genetics | Classification | Evolution | Tissues & Systems | Additional Material