Types of nervous systems

Differ not in basic function of cells, but in organization

• all function via electrical and chemical signals
• electrical signals based on movement of ions across neuron cell membrane

• provide information on internal and external conditions
• process information
• coordinate appropriate responses

• no real centralization, some clustering of nerve cells
• coordination of movement
• Hydra, jelly fish, echinoderms

• more clustering of nerve cell bodies to form ganglia
• ganglia of increasing size and complexity to form brain
• increasing cephalization

Direction of signal flow:

dendrites ------> axon -------> terminal branches (cell body somewhere in between)
 

Electrical signal

1. Generation of an action potential

• resting neuron in polarized, negative internal state
• upon stimulation, changes to depolarized, positive internal state
• due to entry of Na+ into cell
• returns to polarized state due to exit of K+ from cell

Action potentials are all-or-none phenomena

• if a stimulus is below threshold, no action potential will occur
• if a stimulus is at threshold or above, an action potential will occur
• the magnitude of the action potential does not change with the magnitude of an above-threshold stimulus

2. Propagation of impulse along axon of neuron

• current flow generated along membrane in vicinity of depolarized membrane
• influx of Na+ leads to new action potential in adjacent area
• propagation = series of action potentials generated along length of axon

Synaptic transmission from one cell to another - DO NOT TOUCH each other!

• electrical signal arrives at terminal branches
• causes release of neurotransmitter (NT) from pre-synaptic cell
• NT binds briefly to receptors on post-synaptic cell
• causes ion movement across membrane of post-synaptic cell
• may stimulate action potential, may be inhibitory
• NT removed by re-uptake into pre-synaptic cell, or by enzymatic destruction

Question: Why is it important to remove the NT from the synapse right away?
 
 
 
 
 

INTEGRATION

Involves spinal cord (simple responses) and brain (complex integration such as discrimination, learning, problem solving, emotion, planning, language...)

Reflex arcs

Simplest form of integration is reflex arc via spinal cord - stimulus elicits a response by very direct pathway

Question: Draw a diagram that shows the pathway of spinal reflex, correctly labeling all the components.
 
 
 
 
 
 
 
 
 

Sensory discrimination - intensity of stimulation

How can we distinguish between a weak stimulus and a strong stimulus if action potentials are all or none?

• greater stimulation causes more neurons to fire
• greater stimulation causes individual neurons to fire more frequently
• brain interprets number and frequency of arriving signals as indicators of stimulus intensity

Sensory discrimination - location and type of stimulus

How can we distinguish different types or sources of sensation (e.g. sound vs. temperature; toe vs. finger) if all signals arriving at the brain are fundamentally, qualitatively the same - electrical impulses?

• "mapping" of body parts and sense organs to specific locations in the brain
• different locations for receiving and processing visual, auditory, etc. signals
• different locations for receiving and processing signals from face, hand, elbow, etc.
• perception = function of location in brain that input is received

Experiments and observations related to processing of stimuli

1. Blocking transmissions of nerve impulses from an organ to brain results in no perceived sensation when a stimulus is applied to the organ.

Question: What does this tell us about perception?
 
 
 

2. Direct electrical stimulation of various parts of the brain results in perception of specific sensations, even though the actual body part is not being stimulated.

Question: What does this tell us about perception?
 
 
 
 

3. A person who has had an limb amputated will continue to feel pain or other sensation in the amputated limb for some time.

Question: What does this tell us about perception?
 
 
 
 

4. Question: What experience do all of us have on a regular basis that demonstrates perception is an integrative process occurring in the brain, not in sense organs?
 
 
 

Processing multiple inputs

Most neurons receive inputs from more than one other neurons - two to hundreds or even thousands

• inputs may be any combination of stimulatory and inhibitory inputs
• response determined by balance of inputs
• example: to cough or not to cough?
• example: Parkinson's disease

 

 
 
 
 
 
 
 
 
 

• limbic system consists of several parts of brain that are involved in emotion and personality
• Phinneas Gage - personality change after brain injury
• direct electrical stimulation of specific location in brain lead to specific emotional responses
• delusions and hallucinations of schizophrenia associated with excess dopamine in limbic regions

• some forms of depression associated with not enough serotonin at synapses

WHAT IS PERCEPTION? REALITY? CONSCIOUSNESS?

What does a bat perceive when it "sees" the world via sound?

What color is ultraviolet? (Can be detected by many insects).

What does the wavelength 550 nm (green to us) look like to another animal?

What does the electromagnetic field of Earth feel like? (Can be detected by many animals).