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Lesson 6 - Structure and Signaling
The Nervous System
Central Nervous System
Brain
Spinal Cord
Peripheral Nervous System
Somatic
Autonomic
Parasympathetic
Sensory
Motor
Sympathetic
Nervous System
• Afferent (sensory) Neurons – Carry impulses
toward the CNS
• Efferent (Motor) Neurons – Carry impulses from
the CNS
• Interneurons – conduct impulses within the
spinal cord (between afferent and efferent) (Syn.
Association, Internuncial)
• Ganglia are small masses of nervous tissue
located outside the brain and spinal cord.
Sensory Neurons
• INPUT From sensory organs to the
brain and spinal cord.
Sensory
Neuron
Brain
Spinal
Cord
Touch receptors in skin
Somatosensory neuron - spinal
Vision, hearing, taste and smell cranial
Motor Neurons
• OUTPUT From the brain and spinal cord to
muscles and glands
Brain
Spinal
Cord
Motor neurons in spinal cord
Motor
Neuron
Interneurons
• PROCESSING Relay information
between other neurons
Brain
Spinal
Cord
InterNeurons
Interneurons in brain
Divisions of the PNS
• Somatic Nervous System (SNS)
– Sensory neurons that convey information from
sensory receptors in the head, body wall and
limbs to the CNS
– Motor neurons from the CNS that conduct
impulses to the skeletal (voluntary) muscles
only.
Divisions of the PNS
• Autonomic Nervous System (ANS)
– Sensory neurons convey information from
receptors in the viscera (internal organs), to the
CNS.
– Motor neurons then convey information from
the CNS to smooth muscle, cardiac muscle,
glands, etc.
– Motor functions in the ANS are not normally
under conscious control; they are involuntary.
Neuron Resting Potential
• Uneven concentrations of Na+ (outside) and
K+ (inside) on either side of neuron
membrane results in the inside of the neuron
being 70 mV less positive than the outside
•
Text, pg. 419
A sodiumpotassium pump
maintains resting
membrane
potential after
ions “leak” down
their
concentration
gradient
- 3 Na+ ions are
actively pumped
out while 2 K+
ions are pumped
in.
ACTION POTENTIAL
• Nerve signals are transmitted by action
potentials that are abrupt, pulse-like
changes in the membrane potential that last
a few ten thousandths of a second.
• Action potentials can be divided into three
phases: the resting or polarized state,
depolarization, and repolarization
• The amplitude of an action potential is
nearly constant and is not related to the size
of the stimulus, so action potentials are allor-nothing events.
Action Potential
Saltatory Conduction
Terminology
• Synapse
– Region at which neurons come nearly together to
communicate. (neuron or effector organ)
• Synaptic Cleft
– Gap between neurons (at a synapse)
– Impulses can not propagate across a cleft
• Synaptic Vesicle
– Packets of neurotransmitter in presynaptic neuron
• Presynaptic Neuron
– Neuron sending a signal (before the synapse)
• Postsynaptic Neuron
– Neuron receiving a signal (after the synapse)
Neurotransmitters
5 general criteria:
1) synthesized and released by neurons
2) released at the nerve terminal in a 'chemically identifiable' form
3) the chemical should reproduce the activity of the presynaptic neuron
4) can be blocked by competitive antagonist based on concentration
5) active mechanisms to stop the function of the neurotransmitter
Classical transmitters are small molecules (often amino acid based)
Non-classical transmitters can be peptides or even gasses
5 Steps of Neurotransmission
1) synthesis of the neurotransmitter
precursors and enzymes should be in
the correct place
2) storage of neurotransmitter OR
precursor
often stored in presynaptic vesicles
5 Steps of Neurotransmission
3) release of the neurotransmitter
generally by vesicle fusion
4) binding to target receptor
ionotropic receptors open ion
channels
metabotropic receptors modulate
other signals
5 Steps of Neurotransmission
5) termination of the signal
active termination caused by
reuptake or chemical breakdown
*For e.g. acetylcholine is broken
down by . . .
passive termination uses diffusion
Types of Neurotransmitters
Acetylcholine
+ muscles, learning, memory
Serotonin (a derivative
of tryptophan)
Norepinephrine (aka
noradrenaline)
Dopamine
Endorphins
+ sleep, relaxation, self esteem,
too little = depression, perception
+ stress and fight/flight response,
sympathetic NS:+BP & heart rate
+ prolactin (milk production),
involved in pleasure, movement
(-) pain, involved in pleasure
GABA (gamma
aminobutyric acid)
Glutamate
(-) anxiety, too little in parts of
brain can lead to epilepsy
Most common NT, memory, toxic
“FLIGHT OR FIGHT RESPONSE”
Sympathetic component prepares body for
stress [neurotransmitter used=norepinephrine]
•Diverts blood from internal organs to skeletal
muscles, heart & brain
Parasympathetic brings things back to normal
[neurotransmitter used=acetylcholine]
•Work in conjunction/opposition to each other
–Ex. “on” / “off” switches
You come across a bear on your walk to
school…what happens?
•
•
•
•
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Sympathetic nervous system does what?
Increases heart rate
Increases breathing rate
Dilates bronchioles
Dilates pupils
Inhibits digestion