1. Biology 201 Dr. Edwin DeMont St. Francis Xavier University Communication I: Nervous and Sensory Systems

2. St. Francis Xavier University Communication systems Two forms of communication integrate body function that helps to maintain homeostasis: (1) Neurons which transmit electrical signals to initiate a rapid response in a tissue. (2) Hormones which are a slower chemical signals that initiate a widespread and often prolonged response and often in a variety of tissues.

3. St. Francis Xavier University Nervous system All animals depend on information on their surroundings, whether for finding food and mates or escaping from predators. They must be able to rapidly assess important quantities of the environment. Most information is obtained through specialized sense organs, i.e. for vision, hearing, taste, smell and touch.

4. St. Francis Xavier University Nervous system The variety of stimuli and the receptors (sense organs) is large, but the general sequence of events initiated by the external stimulus is similar.

5. St. Francis Xavier University Nervous system The nerve impulses carried in the different sensory nerves are all of the same fundamental nature.

6. St. Francis Xavier University Neurons Filmed live The function unit of the nervous system is the neuron. Neurons have two important properties: (1) excitability, the ability to respond to stimuli (2) conductivity, the ability to conduct a signal

7. St. Francis Xavier University Ion Channels Plasma membrane of neuron contains channels in which certain ions can pass through. Some of the channels are gated and only open under certain circumstances.

8. St. Francis Xavier University Resting Membrane Potential The resting neuron (non-conducting) has a resting membrane potential of approximately -70 mV. There is a higher concentration of sodium ions (+) outside the plasma membrane and potassium (+) and various proteins (-) and ions (chlorine ions) are found inside the membrane. Why?

9. St. Francis Xavier University Resting Membrane Potential Plasma membrane contains open channels only for potassium. This ion moves out of the cell down its concentration gradient.

10. St. Francis Xavier University Resting Membrane Potential As potassium moves out the inside of the cell becomes more negatively charged and this tends to attract the positively charged potassium back into the cell. Diffusional forceElectrical force Animation

11. St. Francis Xavier University Action potential Changes in the resting electrical potential across the plasma membrane is the most important factor in the creation and subsequent conduction of a nerve impulse.

12. St. Francis Xavier University Action potential When a stimulus is applied to a region on the resting plasma membrane the permeability of sodium (Na + ) increases at that point (some sodium channels are opened). Membrane potential becomes more positive.

13. St. Francis Xavier University Action Potential At a certain level of membrane potential special voltage sensitive sodium channels open and the membrane potential rapidly rises to around 30 mV.

14. St. Francis Xavier University Action Potential Shortly after the voltage sensitive Na + channels open special voltage sensitive potassium channels open and potassium pours out and the voltage drops (Na + channels close.) Animation

15. St. Francis Xavier University Action Potential The sodium inside the cell can be removed by the sodium-potassium pump.

16. St. Francis Xavier University Action Potential This series of changes in membrane potential trigger a similar cycle in adjacent regions of the membrane and the wave of depolarization moves down the axon.

17. St. Francis Xavier University Action Potential The action potential is caused by the concentration of ions on the two sides of the membrane. Therefore the stimulus causes a full strength action potential or none at all. This is called an all-or-none law. So if all the action potentials are the same, how is the strength of the stimuli conveyed?

18. St. Francis Xavier University Between Neurons Information that travels in axons is transmitted to other neurons at the synapse. The transmission at the synapse is of two distinct kinds: electrical or chemical. Electrical transmission is not as widespread as chemical transmission, but it has characteristics that in certain situations confer considerable biological advantage.

19. St. Francis Xavier University Chemical Chemical transmission across a synapse. The neurotransmitter (eg. acetylcholine) binds to the receptor site and causes a depolarization in the postsynaptic membrane.

20. St. Francis Xavier University Chemical Post-synapse action potentials can be generated many ways: Thus information is ‘integrated’ at the synapse. (2) Input from many action potentials on the same axon. (1) Input from many different axons. (3) Any combination

21. St. Francis Xavier University Sensory Reception The human ear Biology 403 – Physiology of Sensations (Dr. Beye) Inner ear includes the semicircular canal, which is involved in equilibrium and the cochlea, which is involved in hearing.

22. St. Francis Xavier University Sensory Reception The sense of equilibrium Cilia of receptor hairs bend and initiates a generator potential

23. St. Francis Xavier University Sensory Reception The fish inner ear 1. Sound waves enter the mouth 2. Transmitted to gas in the swim bladder 3. Vibrates at corresponding amplitudes and frequencies as the incoming sound 4. Weberian ossicles vibrate 5. Vibrations move forward to inner ear 6. Response initiated – example fast start

24. St. Francis Xavier University Sensory Reception Demonstration of a fast start in a fish My computer apps on ‘caesar’ (S:) Biomechanics Programs FastStart Select Image Series Accept Settings Step >

25. St. Francis Xavier University Neural Network Fast start in fish: Mauthner Cells Image source