4 LECTURES ON THE NERVOUS SYSTEM
3/25----INFORMATION FLOW AND THE NEURON OR HOW NEURONS WORK PART I
3/27----INFORMATION FLOW AND THE NEURON OR HOW NEURONS WORK PART II
4/1----INTEGRATION AND CONTROL: HOW NERVOUS SYSTEMS WORK
4/3----SENSORY RECEPTION AND PROCESSING
READING: CHAPTER 34-36 IN BIOLOGY THE UNITY AND DIVERSITY
NOTES: AVAILABLE ON THE WEB
DIAGRAMS AVAILABLE ON RESERVE AND IN THE CLASS HOLDER OUTSIDE OF THE BIOLOGY OFFICE
I THE ROLE OF A NERVOUS SYSTEM IN AN ORGANISM
II NERVOUS SYSTEMS ARE ORGANIZED
III THE CELLS OF THE NERVOUS SYSTEM
IV HOW NEURONS WORK
A nervous system allows on organism to sense information about its environment and respond rapidly.
An organism with complex behaviors will have a well developed nervous system.
The nervous system is responsible for complex behaviors.
Studying neurons and nervous systems will ultimately allow us to understand the biological basis for consciousness and the nature of the processes by which we perceive, act, learn and respond.
Sensory Neurons and their receptors detect a stimulus or the input into the nervous system. The nervous system detects changes in different forms of energy (light or pressure) or a change in the concentration of chemicals (taste or olfaction). This energy is detected by the receptors of specialized sensory neuron.
This information is then conducted by the sensory neuron to the interneurons that are involved in the processing of the signal. The information is transformed at this point either into a perception or to a command. Motor neurons transport the information to the effectors which are either muscles or gland cells.
INPUT (STIMULUS) OUTPUT (response ) | | RECEPTORS---->INTEGRATORS EFFECTORS of sensory interneurons--->motor----->muscle or neurons neurons gland cell
The nervous system is made up of neurons and glial cells.
Neurons are involved the transmission of information by both electrical and chemical mechanisms. Glial cells which out number neurons 10-50 to 1 are involved in supportive functions and form myelin which is needed for healthy and rapid nerve conduction.
See Diagram 34.2(d) in the book or figure1 in collection of diagrams
See Diagram 34.2(a-c) in the book or figure 2 in collection of diagrams.
Nerve cell bodies are surround by glial cells. They out number neurons 10-50 to 1. They take up half the volume in the brain. They are the cells that become malignant in brain tumors.
THERE ARE THREE TYPES OF GLIAL CELLS
GLIAL CELLS HAVE SEVERAL VITAL ROLES IN THE NERVOUS SYSTEM
1) Supporting elements
2) They produce myelin
see figure 4
All neurons can be described by a generalized model neuron that has 4 types of signals:
1) An input (dendrites and cell body)
2) An integration or trigger signal ( trigger zone)
3) A conducting signal ( action potential, axon)
4) An output signal (synapse)
see figure 5
Neurons propagate a signal as a wave of electrical activity moving along an axon.
Communication between neurons typically occurs by chemical neurotransmitters.
The unequal distribution of ions across the plasma membrane creates the resting potential.
THERE IS TEN TIMES AS MUCH Na OUTSIDE THE CELL AS INSIDE THE CELL
THERE IS 30 X MORE K INSIDE THE CELL AS OUTSIDE THE CELL
At rest this unequal distribution of ions is maintained by the sodium/potassium ATPase which is a pump that uses energy to maintain the ionic gradients and the fact that at rest the neuron's membrane has a greater permeability to potassium than sodium. At rest there is a "leakage " of mainly of K through open channels.
see figures 34.3 and 34.4 as well as figure 8.
Graded signals can vary in magnitude. Their size depends on the duration and intensity of the stimulus. The signals are local in that they do not spread fa from the point of stimulation
When a stimulus is intense or long lasting it will spread to the trigger zone. If the depolarization is beyond the threshold level an action potential will be generated.
The trigger zone has a high concentration of voltage sensitive sodium channels.
The ACTION POTENTIAL is the conducting signal of the neuron.
It is an All or none signal. Voltage -gated sodium channels in activate several milliseconds after they open Also voltage-gated potassium channels open in the middle of an action potential. It is their role to repolarize the membrane.
Note a stimulus below threshold will not produce an action potential . All stimuli above threshold produce the same signal. The amplitude and duration of an action potential are always the same,
(Figure 34.5 & 34.6 )
Action potential are self propagating. They only travel in one direction because voltage activated sodium channels are inactive immediately after opening.
Figure 34.7 and figure 11
The myelin sheath speeds up nerve conductance. It is responsible for saltatory conductance.