Structures of the Nervous System
- Central Nervous System
(CNS)
- Brain & Spinal
Cord
- See interneurons
- Peripheral Nervous
System (PNS)
- Contains all nerves outside
of the brain & spinal cord.
- Somatic Nervous
System (Voluntary)
- Two types:
Sensory (or afferent) and motor (or efferent) nerves
- Autonomic
Nervous System (Involuntary)
- Sympathetic
- Activated by stress
- Increase heart rate,
breathing rate, pupil size, sweating
- Parasympathetic
- Maintains body
functions; in control when relaxed.
- Restores body to
pre-stress state
Structure of the Neuron (See Figure 1)
- Dendrites
- Filaments that receive
information from other neurons, receptors, etc.
- Soma (cell body)
- Contains nucleus
of the cell
- Manufactures enzymes and
molecules to maintain cell life.
- NOTE: In the PNS, a
group of cell bodies and dendrites are called ganglion and in the
CNS, it would be called a nucleus.
- Axon
- Filament that transmits
information to the dendrites or soma of other neurons.
- NOTE: In the PNS, a
group of axons is called a nerve and in the CNS it is called a tract.
- Myelin Sheath
- Insulation on axon to improve
speed of transmission.
- Also insulates axon
from activity of other neurons.
- Other manufacturing,
nurturing, and clean-up tasks
- Axon Terminal
- Where
neurotransmitters are stored.
How Neurons Fire (See
Figure 2)
- Electrochemical process
- Propagation of impulse
within neuron (electrical).
- Transmission of
impulse between neurons (chemical).
- Resting Potential
- Neuron at rest
- Not
firing.
- Stable, negative charge
inside neuron relative to outside (- 70 mV).
- Sodium (Na+) outside
the neuron.
- Action Potential
- Neuron receives
sufficient stimulation from receptor, other neurons, etc..
- Na+ ions rush in.
- Interior of axon
becomes positive relative to the outside (+ 40 mV).
- This release of energy
passes down the axon and is the firing of the neuron.
- Once neuron fires, resting
potential returns.
· All or None Law
- Once neuron fires, the
impulse moves down the axon at the same intensity.
· Impulse frequency
- While there are differences
in speed of transmission between neurons, increase in neuron
stimulation doesn’t cause stronger impulse. Speed and
electrical change remains the same regardless of intensity.
- Stronger stimulation
leads to more frequent firing of the neuron or more neurons
activated.
· Absolute Refractory Period
- Brief period of time after
firing when neuron won’t fire no matter how much
stimulation given.
- As brief as 1/1000 of
a second. HOWEVER, means a limit as to how many times neuron can fire per
second (the maximum is 500-600 times/second).
· Think of neuron like a toilet that works well
- Information flows in 1
direction (out to the sewage!)
- It is all or none - the
toilet doesn't have "half flush" days
- There is a period of time
that, no matter how much you jiggle the handle, no flushing will occur
How Neurons Send Messages
- Synapse
- Small gap or
cleft between the axon of one neuron and the dendrite of another.
- Axon terminal
- Contains synaptic
vesicles
- Neurotransmitters
- Chemical substances
released by synaptic vesicles.
- Taken up by the axon
of the receiving neuron
- Work in Lock
and Key fashion
- Inhibitory (tells
the neuron NOT to fire) or Excitatory (tells the neuron to fire)
- If gets more
excitatory information than inhibitory, neuron will fire, if more
inhibitory than excitatory, will not fire.
- A neuron AT
REST will occasionally spontaneously fire
- Excitatory
information - causes depolarization (an increase in neural firing)
- Inhibitory
information - causes hyperpolarization (a decrease in neural firing)
Neurochemicals
- Selected neurotransmitters
- Acetylcholine
- motor, learning
memory, contract muscles, REM sleep
- Alzheimers, black
widow spider bite
- Dopamine
- motor, attention,
emotions
- Parkinson’s,
schizophrenia
- Norepinephrine
- regulation of
arousal, eating and sleeping
- manic, depression
- Serotonin
- arousal, sleep,
learning, mood, aggression, appetite
- depression
- Endorphins
- Pain reduction,
pleasure
- possibly related to
"runner’s high"