Biology 201 Dr. Edwin DeMont

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

2. 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. St. Francis Xavier University

3. 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. St. Francis Xavier University

4. 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. St. Francis Xavier University

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

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

7. 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. St. Francis Xavier University

8. 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? St. Francis Xavier University

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

10. 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 force Electrical force Animation St. Francis Xavier University

11. 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. St. Francis Xavier University

12. 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. St. Francis Xavier University

13. 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. St. Francis Xavier University

14. 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 St. Francis Xavier University

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

16. 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. St. Francis Xavier University

17. 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? St. Francis Xavier University

18. 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. St. Francis Xavier University

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

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

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

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

23. Sensory Reception 1. Sound waves enter the mouth The fish inner ear 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 St. Francis Xavier University

24. Sensory Reception Demonstration of a fast start in a fish My computerapps on ‘caesar’ (S:)BiomechanicsPrograms FastStartSelect Image Series Accept Settings Step > St. Francis Xavier University

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