physiology notes: neurophysiology
- a. Cell body: contains the nucleus; control center; carries out metabolism
b. Dendrite: thin branched processes that project from cell body; receives
impulses from other neurons
c. Axon hillock: narrow region where axon begins; nerve impulse originates
d. Axon and axonal terminals: axon is a long process that conducts impulses away
from the cell body; axonal terminals are the enlarged ends of the axon.
- Myelin sheath in peripheral system: from Schwann cells; leaves exposed patches
(Nodes of Ranvier)
Myelin in central nervous system: from oligodendrocytes (neuroglia)
- Nodes of Ranvier: gaps in the myelin sheath; rapid ion exchange area.
- Membrane potential: when cells have an electrical charge across their membrane due to negative proteins and K+ inside the cell and Na+, Ca² and Cl- outside the cell; it is about -70mv.
- a. Depolarization: when the membrane potential becomes less negative (more positive); can be caused by Na+ moving into the cell±
b. Repolarization: when membrane potential moves back toward -70mv; can be
caused by K+ moving out of the cell or Cl- moving in.
c. Hyperpolarization: when membrane potential becomes more negative than
-70mv; can be caused by K+ moving out of the cell or Cl- moving into cell
6. Gated ion Channels: found in axons; open due to a certain stimulus; will stay open a
fraction of a second before being blocked; there are ion channels for Na+, K+,
- Voltage gated: channels that open at a certain membrane potential; axon terminals have Ca+² voltage gated channels
Ligand gated ion channels: open when a certain molecule binds to a receptor
the membrane surface
- Action potential: rapid changes in membrane potential across a small section of
the axon membrane caused by ion movement cross the membrane
- Na+ movement: depolarization below -50mv opens voltage gated Na+ channels to
Open; Na+±the cell; membrane potential rises to +30mv and then blocks Na+
channels and Na+ movement stops.
K+ movement: after Na+ channels open, gated K+ channels open letting K+ out of the cell; membrane potential drops to -90mv;
- Na+/K+ pump: moves Na+ back out of the cell and K+ into the cell, resetting the
proper ion gradient
- Myelinated axons: the myelin sheath will not allow ions to cross the membrane;
at the nodes of Ranvier, the axon is bare and here ions can cross at these areas; this is called saltatory conduction.
Non-myelinated axons: have no sheath of myelin; action move down the entire
length of the axon; every stretch of the axon will experience depolarization and
repolarization; the action potential moves like a wave.
- All or None Law: voltage gated channels will open all the way once the membrane potential reaches the threshold; therefore, every action potential has
- Refractory period: time period during an action potential in which the axon
cannot respond to a new change in the membrane potential.
- Absolute refractory period: when Na+ ion channel is opened it can not respond to
another depolarization until it moves from the active state to the closed state.
Relative refractory period: if a 2nd depolarization occurs while the K+ channels are open , it takes a more intense depolarization to overcome the effect of the
K+ leaving through the open K+ channels.
- Synapse: junction point between two neurons or between a neuron and an effector
cell (muscle or gland).
- Neurotransmitters: chemicals stored in vesicles in the axon terminal of the
Function: when an action potential moves down the axon to the terminal, the
membrane depolarization opens voltage gated Ca+² channels;
-Ca+² enters the cell causing the vesicle to fuse with the presynaptic membrane
releases neurotransmitters in the synaptic cleft
- neurotransmitters cross the synaptic cleft and bind to receptors on the
- Excitatory postsynaptic potential: depolarization; more neurotransmitters= more
Depolarization; no refractory period; several EPSP’s can be summed to create
a greater depolarization
Inhibitory postsynaptic potential: hyperpolarization; lessens the chance for
depolarization due to increased outward diffusion of K+ ions and > increase
of positive charge outside the membrane.
- Acetylcholine: excitatory neurotransmitter in the CNS and neuromuscular
junction; can be excitatory or inhibitory
- Ligand-gated channels: nicotinic acetylcholine receptors: acetylcholine binds to receptors and opens channels that let Na+ in and K+ out; more
Na+ comes in so they produce EPSPs in neuromuscular junction
- G-protein mediated channels: muscarinic acetylcholine receptors: binding
of acetylcholine to these receptors activate a G-protein; one type of G protein opens K+ channels letting K+ out causing an IPSP; another type
closes K+ channels causing an EPSP; therefore, two different cells can have opposite response to muscarinic stimulation.
d. Acetylcholinesterase: enzyme in the synaptic cleft that degrades
e. Monoamines: neurotransmitters derived from amine groups; similar in
action to acetylcholine
f. Second messenger mediation: rather than activating ion channels directly,
monoamines work through a 2nd messenger, Cyclic AMP
which is the second messenger; cyclic AMP±ion channels to open
g. Monoamine inactivation: monoamines are reabsorbed by the pre-synaptic
cell and degraded by the enzyme monamine oxidase
h. Serotonin: involved in mood, behavior, appetite, cerebral circulation.
i. Dopamine: two separate systems: one involved in control of movements
and one involved in behavior and reward
j. Norepinephrine: involved in general arousal
k. Amino acids as neurotransmitters: some create EPSP’s and others, IPSP’s by opening voltage gated Cl- channels (glycine, glutamate, GABA)
l. Polypeptides: analgesics, opioids; (endorphins)
m. Nitric oxide: diffuses out of pre-synaptic cell and into postsynaptic cell and causes muscle relaxation in target organ; can produce engorgement of spongy tissue with blood in males
- Signal integration: all the EPSP’s and IPSP’s that occur on the dendrites of the
post-synaptic cell will add together to alter membrane potential of axon hillock
- Spatial summation: occurs when IPSP’s and EPSP’s from several synapses
combine to alter the membrane potential
- Temporal summation: occurs when several EPSP’s or IPSP’s are generated from
the same synapse in a very short period of time and the collected effect is added
together to alter the membrane potential