Download Chapter 12 Notes Part 3 File

Action Potential
• Action potential
– The membrane potential of a neuron that is conducting
an impulse
– Also known as a nerve impulse
• Mechanism that produces the action potential
– An adequate stimulus triggers stimulus-gated Na+ channels to open,
allowing Na+
to diffuse rapidly into the cell, producing a local depolarization
– As threshold potential is reached, voltage-gated Na+ channels open and
more Na+ enters the cell, causing further depolarization
– The action potential is an all-or-none response
– Voltage-gated Na+ channels stay open for only about 1 millisecond before
they automatically close
– After action potential peaks, membrane begins to move back toward the
resting membrane potential when K+ channels open, allowing outward
diffusion of K+; process is known as repolarization
– Brief period of hyperpolarization occurs and then the resting membrane
potential
is restored by the sodium-potassium pumps
Action Potential
• Refractory period
– Absolute refractory period
• Brief period (lasting approximately half a millisecond) during
which
a local area of a neuron’s membrane resists restimulation
and will not respond to a stimulus, no matter how strong
– Relative refractory period
• Time during which the membrane is repolarized and is
restoring the resting membrane potential
• The few milliseconds after the absolute refractory period
• Membrane will respond only to a very strong stimulus
Action Potential
• Conduction of the action potential
– At the peak of the action potential, the plasma membrane’s polarity
is now the reverse of the RMP
– The reversal in polarity causes electrical current to flow between
the site of the action potential and the adjacent regions of
membrane and triggers voltage-gated Na+ channels in the next
segment to open; this next segment exhibits an action potential
– This cycle continues to repeat
– The action potential never moves backward, as a consequence of
the refractory period
– In myelinated fibers, action potentials in the membrane only occur
at the nodes of Ranvier; this type of impulse conduction is called
saltatory conduction
– Speed of nerve conduction depends on diameter and on the
presence or absence of a myelin sheath
Synaptic Transmission
• Two types of synapses
– Electrical synapses - occur where cells joined by gap junctions
allow an action potential to simply continue along postsynaptic
membrane
– Chemical synapses - occur where presynaptic cells release
chemical transmitters (neurotransmitters) across a tiny gap to the
postsynaptic cell, possibly inducing an action potential there
Synaptic Transmission
• Structure of the chemical synapse (Figure 12-21)
– Synaptic knob
• Tiny bulge at the end of a terminal branch of a presynaptic
neuron’s axon that contains vesicles housing
neurotransmitters
– Synaptic cleft
• Space between a synaptic knob and the plasma membrane of
a postsynaptic neuron
– Arrangements of synapses
• Axodendritic—axon signals postsynaptic dendrite; common
• Axosomatic—axon signals postsynaptic soma; common
• Axoaxonic—axon signals postsynaptic axon; may regulate
action potential of postynaptic axon
– Plasma membrane of a postsynaptic neuron has
protein molecules that serve as receptors for the
neurotransmitters
Synaptic Transmission
• Mechanism of synaptic transmission
– Action potential reaches a synaptic knob, causing calcium ions
to diffuse into the knob rapidly
– Increased calcium concentration triggers the release of
neurotransmitter via exocytosis
– Neurotransmitter molecules diffuse across the synaptic cleft and
bind to receptor molecules, causing ion channels to open
– Opening of ion channels produces a postsynaptic potential,
either an excitatory postsynaptic potential (EPSP) or an
inhibitory postsynaptic potential (IPSP)
– The neurotransmitter’s action is quickly terminated by either
neurotransmitter molecules being transported back into the
synaptic knob (reuptake) and/or metabolized into inactive
compounds by enzymes and/or diffused and taken up by nearby
glia
Synaptic Transmission
• Summation (Figure 12-28)
– Spatial summation
• Adding together the effects of several knobs being activated
simultaneously and stimulating different locations on the
postsynaptic membrane, producing an action potential
– Temporal summation
• When synaptic knobs stimulate a postsynaptic neuron in
rapid succession, their effects can summate over a brief
period of time to produce an action potential
Neurotransmitters
• Neurotransmitters
– Means by which neurons communicate with one another
– More than 30 compounds known to be neurotransmitters
• Classification of neurotransmitters
– Function
•
•
•
•
Determined by the postsynaptic receptor
Excitatory neurotransmitters
Inhibitory neurotransmitters
May be classified by whether the receptor directly opens a channel or
instead uses a second messenger mechanism involving G proteins
and intracellular signals
– Chemical structure
• Mechanism by which neurotransmitters cause a change
• Four main classes; since the functions of specific neurotransmitters
vary by location, they are usually classified according to chemical
structure
Neurotransmitters
• Small-molecule neurotransmitters
– Acetylcholine
• Unique chemical structure; acetate (acetyl coenzyme-A) with
choline
• Acetylcholine is deactivated by acetylcholinesterase, with the
choline molecules being released and transported back to
presynaptic neuron to combine with acetate
• Present at various locations; sometimes in an excitatory role
and at other times, an inhibitory one
Neurotransmitters
• Small-molecule neurotransmitters (cont.)
– Amines
• Synthesized from amino acid molecules
• Two categories: monoamines and catecholamines
• Found in various regions of the brain; affecting learning,
emotions, motor control, etc.
– Amino acids
• Believed to be among the most common neurotransmitters of
the CNS
• In the PNS, amino acids are stored in synaptic vesicles and
used as neurotransmitters
Neurotransmitters
• Small-molecule neurotransmitters (cont.)
– Other small transmitters
• Nitric oxide (NO) derived from an amino acid
• NO from a postsynaptic cell signals the presynaptic neuron,
providing feedback in a neural pathway
Neurotransmitters
• Large-molecule neurotransmitters—neuropeptides
– Peptides made up of 2 or more amino acids
– May be secreted by themselves or in conjunction with a
second or third neurotransmitter; in this case,
neuropeptides act as a neuromodulator, a
“cotransmitter” that regulates the effects of the
neurotransmitter released along with it
– Neurotrophins (neurotrophic [nerve growth] factors)
stimulate neuron development but also can act as
neurotransmitters or neuromodulators
Cycle of Life: Nervous System Cells
• Nerve tissue development
– Begins in ectoderm
– Occurs most rapidly in womb and in first 2 years
• Nervous cells organize into body network
• Synapses
– Form and reform until nervous system is intact
– Formation of new synapses and strengthening or elimination of
old synapses stimulate learning and memory
• Aging causes degeneration of the nervous system,
which may lead to senility
The Big Picture: Nervous System and the Whole Body
• Neurons act as the “wiring” that connects
structures needed to maintain homeostasis
• Sensory neurons—act as receptors to detect
changes in the internal and external
environment; relay information to integrator
mechanisms in the CNS
• Information is processed and a response is
relayed to the appropriate effectors through the
motor neurons
The Big Picture: Nervous System and the Whole Body
• At the effector, neurotransmitter triggers a
response to restore homeostasis
• Neurotransmitters released into the bloodstream
are called hormones
• Neurons are responsible for more than just
responding to stimuli; circuits are capable of
remembering or learning new responses,
generation of thought, etc.