Sensory systems
  1. Senses
    1. information about the external and internal environment
    2. not just the 5
  2. Sensors
    1. a cell transducing physical or chemical stimulus into action potentials
    2. most but not all are modified neurons
    3. each responds to one specific stimulous type
      1. specificity varies from general to highly specific
    4. each type of sensory cell sends its signal to a specific portion of the CNS
    5. perception and distinction are confounded in text
      1. depends upon diversity of sensors and processing in CNS
    6. frequency of action potentials proportional to intensity
      1. response of receptor amplifies the signal
  3. Types of sensors
    1. chemosensors
      1. taste, smell, some aspects of homeostasis
    2. mechanosensors (pressure sensors in text)
      1. touch, pressure, sound, including some aspects of homeostasis
    3. photoreceptors and heat receptors
      1. electromagnetic radiation in long- and shortwave frequencies
    4. other sensors exist in other organisms
      1. only through experimentation (not observation) can sensory abilities of animals be examined
  4. We will focus on the visual system as it is the best understood from sensors to brain
    1. All known photosensitive cells use rhodopsin
      1. a two-protein molecule
        1. opsin: "housing"
        2. cis-retinal: photosensitive tertiary structure
        3. the change in conformation initiates a cascade of chemical reactions
    2. Vertebrate eye rod cell is best studied
      1. Rhodopsin packed on disks in outer portion of rod (fig. 43.9)
      2. activiation of rhodopsin causes hyperpolarization
      3. signal magnification: one photon can cause over 1 million channels to close
      4. depolarization or hyperpolarization changes rate of neurotransmitter release
    3. Color vision
      1. variation in rhodopsin yields variation in light sensitivity
        1. when several types of rhodopsin are present, animal likely has color vision
        2. different animals perceive light in different wave-lengths
    4. Vertebrate and cephalopod eyes: convergent evolution
      1. components: cornea, iris, lens, lens muscles, retina
      2. cephalopods have direct eyes, vertebrates have indirect eyes
      3. retina is the location of photosensory cells and neurons
        1. layer of light-sensitive cells + layer of nervous tissue
        2. cells are packed heterogeniously
        3. fovea is species-specific region of dense packing and increased visual acuity
      4. cellular composition of retina varies
        1. rod cells are more light sensitive
        2. cones are color specific
        3. surface behind retina also variable: light absorbing or light reflecting
      5. bipolar cells (labeled "neurons" in fig. 43.8) transmit signal to sensory neuron cell(s)
      6. each sensory neuron (ganglia) receives information vertically and horizontally
        1. this defines receptive field of each sensory neuronganglion cell (sensory neurons) release action potential
        2. receptive fields are overlapping
        1. interconnectedness reduces quantity of information
      7. rate of neurotransmitter release from photosensor is proportional to light intensity in sensitive range
      8. rate of firing of neurosensory cell proportional to concentration of neurotransmitter & number of presynaptic cells firing
  5. Muscles
    1. three types: skeletal, cardiac, smooth
    2. Skeletal and cardiac best studied
      1. each muscle cell = several fused cells
      2. excitable
    3. Sliding-filament theory
      1. histological structure (fig. 43.15, pg 842)
        1. sarcomeres with light and dark bands
      2. actin and myosin
        1. actin anchored at A in figure
        2. myosin "crawls" along actin
      3. Binding and hydrolysis of ATP (fig. 43.17 pg 844)
      4. Muscles will continue to contract so long as ATP is available
        1. relaxation due to binding of alternative substrate (troponin) to actin binding sites
      5. excitation of muscle cells
        1. Initial local depolarization due to acetylcholine-activated sodium channels
        2. if threshold passed, action potentials initiated
        3. action potentials propogate through transverse tubules
          1. transverse tubules are connected with sarcoplasmic reticula of cells
          2. calcium sequestered in sarcoplasmic reticulum
        4. cascade initiated by action potential results in contraction by conformation change in troponin


Study questions

1.  What are the mechanoreceptors? What "senses" utilize these organs?

2.  What is one homeostasis function that uses mechanoreceptars?  Chemosensors?

3.  Contrast the eyes of octopi and diurnal vertebrates. Clearly identify those characteristics that are evidence of convergent evolution.

4.  What gated channels open to cause depolarization in the hair sencillae of the ear?

5.  The postsynaptic region of the muscle cell always depolarizes in response to acetylcholine.  How is this different from the response of a postsynaptic nerve cell?  Why might this response be adaptive?

6.  Rigor mortis is temporary, post-death contraction of the muscles that starts with the elimination of ATP from the muscle and lasts until protein degredation starts.    Can you explain this phenomenon using the sliding-tubule model of muscle action?

See also:  Content review #1 - 5;  Concept review #1, 2, 5;  Applying ideas # 1, 2, 3, 4
 

Answers