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Transcript
Biology 219 – Human Physiology
Clemens
Neurons and Neural Signals
Text: Ch 8
I. Neurons and the Nervous System
Functions of the nervous system:
sensation
communication
integration
control
neurons - functional cells of the nervous system;
- excitable cells - generate electrical signals (changes in membrane potential)
- communicate information in the form of electrical and chemical signals
DRAW NEURON HERE
Structure of Neurons
cell body contains the nucleus and most organelles
dendrites branch from the cell body, receive signals from other cells through synapses
axon extends from the cell body, conducts action potentials
axon hillock - region where axon joins the cell body
initial segment adjacent to the axon hillock is the trigger zone for AP
axon terminals - contain vesicles with neurotransmitter, form synapses with other cells
Nervous System Organization
1. Central Nervous System (CNS) - Brain and Spinal Cord.
2. Peripheral Nervous System (PNS) - nerves, ganglia and sensory receptors
a. Afferent Division - input sensory information to the CNS
b. Efferent Division - output motor signals from CNS to effector organs
Functional types of neurons
1. sensory (afferent) neurons - input to CNS from sensory receptors;
dendrites located at receptors, axons in nerves, cell bodies in ganglia outside the CNS
2. motor (efferent) neurons - output from CNS to effectors (muscles, glands)
cell bodies and dendrites in the CNS, axons in nerves
3. interneurons - located entirely within the CNS; most abundant and diverse types
II. Neural Signals
A.





Graded Potentials
small, localized changes in membrane potential
formed at the cell body and dendrites
can be depolarization (↑) or hyperpolarization (↓)
spread passively and weaken with distance
size depends on stimulus strength
+60
Vm 0
(mV)
GRADED POTENTIALS
-90
Time (ms)
Page 2
B. Action Potentials (= nerve impulses)
 large change in membrane potential
 actively conducted along the axon
 rapid depolarization followed by repolarization
 “all or none” - constant size, does not
depend on stimulus strength
+60
Vm 0
(mV)
ACTION POTENTIAL
Phases of the action potential
-90
Time (ms)
1. Depolarization (rising) phase
- triggered by an initial depolarizing stimulus, must be above threshold to form an AP
- voltage-gated Na+ channels open
activation gate opens in response to initial depolarization
→ rapid Na+ inflow → depolarization → more activation gates open (positive feedback)
2. Repolarization (falling) phase
- voltage-gated Na+ channels close
inactivation gate closes when depolarization reaches a peak (~ +30 mV)
- voltage-gated K+ channels open
→ rapid K+ outflow → repolarization
3. Hyperpolarization (undershoot) phase
- voltage-gated K+ channels remain open, high K+ permeability results in hyperpolarization
- resting states of channels and resting potential restored at the end of undershoot phase
Properties of action potentials
1. threshold - stimulus must be greater than a certain strength to evoke an AP
2. "all or none" - once threshold is reached, size of the AP is constant regardless of stimulus
3. regenerative - AP is regenerated and does not decrease in strength along the axon
4. refractory period - short delay following an AP before another AP can be formed
absolute refractory period (ARP) - another AP can not be formed
relative refractory period (RRP) – a stronger stimulus is required to get another AP
Importance of refractory period:
- Refractory period prevents AP from traveling backward along the axon
- During the RRP, a stronger stimulus can result in increased frequency of APs
Stimulus intensity is coded by the frequency of APs. _I___I____I_I_I_I____IIIIIIIIIIII_
Conduction of action potentials
a. unmyelinated axons
- AP depolarization spreads a short distance down the axon (local current flow)
- depolarization stimulates formation of AP farther down the axon
- axons are “leaky” to Na+ and K+; need to regenerate AP often along the axon
→ slow conduction speed
- increasing axon diameter increases conduction speed
b. myelinated axons
- myelin sheath formed by Schwann cells in the PNS, oligodendrocytes in CNS
- insulates the axon, reduces leakage of Na+ and K+
- nodes of Ranvier - gaps in myelin sheath are sites of AP regeneration
AP “jumps” from node to node (saltatory conduction)
→ very fast conduction speed (> 100 m/s)
Ion Channels and the Action Potential
0. Resting Potential
+
K Leak Channel
Voltage-gated
Na+ Channel
ECF
ICF
Channel:
activation gate
K+
OPEN
1. Depolarization Phase of A.P.
inactivation gate
CLOSED
Channel:
CLOSED
Na+
activation gate
ECF
ICF
Voltage-gated
K+ Channel
K+
OPEN
OPENS
CLOSED
2. Repolarization Phase of A.P.
Na+
ECF
ICF
Channel:
K+
OPEN
inactivation gate
K+
CLOSES
OPENS
3. Hyperpolarization Phase of A.P.
ECF
ICF
Channel:
K+
OPEN
gates reset
CLOSED
K+
OPEN, slowly closes