Standard Operational Procedure 12b (SOP 12b) Filing CYPF ...
Chapter 12b
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Transcript of Chapter 12b
Chapter 12b
Neurophysiology
• Processing of sensory information and communication– Messages are conveyed as action
potentials
– Communication depends on membrane potentials, graded potentials and action potentials
• Resting potential: – transmembrane potential (TMP) of resting cell– Results from uneven distribution of ions
across membrane – Usually -70mV for average neuron
5 Neural Membrane Processes
• Graded potential: – temporary, localized change in TMP– caused by stimulus– Generated in soma or dendrite
• Action potential: – electrical impulse– produced by graded potential– moves along surface of axon to synapse
• Synaptic activity: – releases neurotransmitters at presynaptic
membrane– produces graded potentials in postsynaptic
membrane
• Information processing: – response (integration of stimuli) of
postsynaptic cell
Figure 12–7 (Navigator)
How is resting potential created and maintained?
1. Concentration gradient of ions (Na+, K+)• ECF has high concentration of Na+ & Cl-• Cytosol has high concentration of K+
2. Selectively permeable through channels
3. Maintains charge difference across membrane (-70 mV)
Figure 12–8 (Navigator)
Product of both passive and active forces
Passive Forces Across the Membrane
• Chemical gradients:– concentration gradients of ions (Na+, K+)
• Electrical gradients:– potential difference across membrane– Slightly negative on inner surface – Slightly positive charge on outer surface
• Electrochemical gradient:– Sum of chemical and electrical forces
Electrical Currents and Resistance
• Electrical current:– movement of charges to eliminate potential
difference
• Resistance:– the amount of current a membrane resists – May be altered by opening/closing channels
creating a current
Electrochemical Gradients
Figure 12–9a, b
Electrochemical Gradients
Figure 12–9c, d
Active Forces Across the Membrane
• Sodium–potassium ATPase (exchange pump): – are powered by ATP– carries 3 Na+ out and 2 K+ in– balances passive forces of diffusion– maintains resting potential (—70 mV)
Changes in Transmembrane Potential
• Transmembrane potential rises or falls:– in response to temporary changes in
membrane permeability– resulting from opening or closing specific
membrane channels
Sodium and Potassium Channels
• Membrane permeability to Na+ and K+ determines transmembrane potential
• Sodium and potassium channels are either passive or active
Passive Channels
• Also called leak channels
• Are always open
• Permeability changes with conditions
Active Channels
• Also called gated channels
• Open and close in response to stimuli
• At resting potential, most gated channels are closed
Gated Channels
Figure 12–10
3 Classes of Gated Channels
1. Chemically regulated channels:– open in presence of specific chemicals at a
binding site– found on neuron cell body and dendrites
2. Voltage-regulated channels:– respond to changes in transmembrane
potential– characteristic of excitable membrane– found in neural axons, skeletal muscle
sarcolemma, cardiac muscle
3. Mechanically regulated channels:– respond to membrane distortion – found in sensory receptors (touch, pressure,
vibration)
Graded Potentials
• Any stimulus that opens a gated channel: – produces a graded potential – Also called local potentials
• Changes in transmembrane potential:– can’t spread far from site of stimulation
• Opening sodium channel produces graded potential
Figure 12–11 (Navigator)
Figure 12–11 (Step 1)
Graded Potentials: Step 1
Figure 12–11 (Step 2)
Graded Potentials: Step 2
• Repolarization– stimulus is removed, transmembrane potential
returns to normal
• Hyperpolarization– Increasing the negativity of the resting
potential– Result of opening a potassium channel
Figure 12–12
Effects of Graded Potentials
• At cell dendrites or cell bodies:– trigger specific cell functions
• At motor end plate:– releases ACh into synaptic cleft
What events are involved in the generation
and propagation of an action potential?
Action Potentials
• Propagated changes in transmembrane potential
• Affect an entire excitable membrane
• Link graded potentials at cell body with motor end plate actions
Initiating Action Potential
• Initial stimulus: – a graded depolarization to change resting
potential to threshold level (—60 to —55 mV)
• All or none principle – stimulus exceeds threshold amount and
action potential is triggered or it wont
Generating the Action Potential
Figure 12–13 (Navigator)
Steps of A P Generation
1. Depolarization to threshold
2. Activation of Na+ channels and rapid depolarization
3. Inactivation of Na+ channels, activation of K+ channels
4. Return to normal permeability
The Refractory Period
• time period:– from beginning of action potential – to return to resting state– during which membrane will not respond
normally to additional stimuli– Absolute vs. Relative
Propagation of Action Potentials
• moves along entire length of axon– series of repeated actions, not passive flow
1. Continuous propagation:– unmyelinated axons
2. Saltatory propagation:– myelinated axons
Saltatory Propagation
• Faster and uses less energy than continuous propagation
• Myelin insulates axon, prevents continuous propagation
• Local current “jumps” from node to node
• Depolarization occurs only at nodes
Comparison of graded and action potentials
Graded Potential
• Depolarizes or hyperpolarizes
• No threshold value
• Dependent of intensity of stimuli
• Effect decreases with distance
• No refractory period
• Occurs in most cell types
Action Potential
• Depolarizes only
• Distinct threshold value
• All or none phenomenon
• No decrease in strength along axon
• Refractory period occurs
• Occurs only in excitable cells
What factors affect the propagation speed
of action potentials?
Axon Diameter and Propagation Speed
• Ion movement is related to cytoplasm concentration
• Axon diameter affects action potential speed
• The larger diameter, the lower the resistance
3 Groups of Axons
• Classified by: – diameter– myelination– speed of action potentials
• Type A, Type B, and Type C fibers
• “Information” travels within the nervous system as propagated electrical signals (action potentials)
• The most important information (vision, balance, motor commands) is carried by large-diameter myelinated axons