Psychology 1000 Study Guide - Final Guide: Peripheral Nervous System, Parasympathetic Nervous System, Somatic Nervous System

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Chapter 3 Biological Foundations of Behaviour
Neurons
Basic building blocks of the nervous system
100 billion at birth, lose 10,000 every day
Three main parts:
o Cell body or Soma
Contains biochemical structures needed to keep the neuron alive
Nucleus carries genetic information that determines how the cell develops and
functions
o Dendrites
Branch-like fibres that emerge from the cell body
Collect messages from neighbouring neurons and send them to cell body
o Axon
Extends from cell body, conducting electrical impulses to other neurons, muscles, or
glands
Branches out to form axon terminals
Connect with dendritic branches from numerous neurons
Glial cells surround neurons and hold them in place
o Also manufacture nutrients, form myelin sheath, absorb toxins and waste
o Guide newly divided neurons to place in brain during development
Blood-brain barrier prevents substances from entering brain
Nerve Conduction
Neurons surrounded by salty liquid environment
o High concentration of sodium (Na+)
o Inside of neuron is more negative, causing it to be more electrically negative
o Resting potential across the membrane: -70 mV
Action potential (nerve impulse) is a sudden reversal in neuron’s membrane voltage
o Depolarization - changes from 70 mV to +40 mV
o Graded potentials changes proportional to the amount of incoming stimulation
If potentials aren’t very strong, the neuron will be partially depolarized, but not
enough for action potential
If strong enough, graded potential reaches action potential threshold about 55
mV (obeys all-or-none law)
o Graded potentials changes membrane potential by acting on tiny protein structures in the
cell membrane called ion channels
Open channels allow rushing in of Na+, making neuron less negative
Creates state of partial depolarization that may reach action potential
o When membrane reaches action potential threshold, Na+ rushes in due to attraction to
negative force in cell
Ion channels close quickly, K+ channels open and K+ leaves cell
Restores neuron to resting potential
Na+ and K+ flow back to respective positions to restore distribution
o Refractory period time period during which the membrane is not excitable and cannot
discharge another action potential
Occurs immediately after impulse passes
Limits rate at which action potentials can be triggered (300 impulses per second in
human)
o Rate of firing or number of neurons fired help differentiate between strength of stimuli
Myelin sheath is a fatty, whitish insulation layer derived from glial cells that covers axons
o Thins out at regular intervals, by nodes of Ranvier
o Allow for high conduction speeds along axon (still slower than speed of electricity in
electrical wire)
Synaptic Transmission
Otto Loewi discovered that neurons release chemicals to pass over to next neuron
Researchers found synaptic cleft between axon terminals of one neuron and dendrite of the next
Neurons produce neurotransmitters to carry messages across synapse to excite or inhibit other
neurons
o Process involves five steps:
Synthesis chemical molecules formed inside neuron
Storage molecules stored in synaptic vesicles
Release Action potential causes vesicle to move to surface of terminal, molecules
are released into fluid-filled space
Binding molecules cross the space and bind to receptor sites (large protein
molecules embedded in the membrane)
Deactivation
o Binding of neurotransmitters to receptor site causes two possible effects:
Excitation depolarizes the postsynaptic cell membrane by stimulating flow of Na+
(excitatory transmitters)
Inhibition hyperpolarizes the postsynaptic cell membrane by stimulating ion
channels that allow K+ to flow out of the neuron, or negatively charged ions to flow
in (changes potential from 70 mV to 72 mV)
Makes it more difficult for excitatory transmitters at other receptor site to
depolarize the neuron to the threshold
o Neurotransmitters continue to function until deactivation:
Some deactivated by other chemicals in synaptic space that break them down
Reuptake transmitters reabsorbed into presynaptic axon terminal
o Examples of neurotransmitters:
Acetylcholine (Ach) functions in excitatory and inhibitory systems (related to
memory, motor, behavioural inhibition)
Norepinephrine (NE) functions in excitatory and inhibitory systems (related to
arousal, eating)
Dopamine (DA) functions in inhibitory, sometimes excitatory, systems (related to
arousal, voluntary movement)
Seratonin (5-HT) functions in inhibitory and excitatory systems (related to sleep,
thermoregulation)
Gamma Aminobutynic Acid (GABA) functions in inhibitory systems (related to
motor behaviour)
o Drugs function by affecting neurotransmitters
Increase or decreases amount of transmitter, stimulates or blocks receptor sites,
terminates transmitter function
Examples:
Cocaine stimulates release of dopamine, prevents reuptake
Curare blocks receptor sites for ACh, causes complete paralysis
Black widow venom stimulates release of ACh
Botulism toxin Blocks release of ACh
Nicotine stimulates receptor molecules, “duplicating” effects of ACh
Caffeine blocks adenosine receptor sites
o Disinhibition inhibition of inhibitory neurons to bring system back to normal state
The Nervous System
Three major types of neurons the carry out functions:
o Sensory neurons carry input messages from the sense organ to the spinal cord and brain
o Motor neurons transmit output impulses from the brain and spinal cord to the muscles and
organs