Chapter 12b
description
Transcript of Chapter 12b
![Page 1: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/1.jpg)
Chapter 12b
Neurophysiology
![Page 2: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/2.jpg)
• Processing of sensory information and communication– Messages are conveyed as action
potentials
– Communication depends on membrane potentials, graded potentials and action potentials
![Page 3: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/3.jpg)
• 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
![Page 4: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/4.jpg)
• 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
![Page 5: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/5.jpg)
• Synaptic activity: – releases neurotransmitters at presynaptic
membrane– produces graded potentials in postsynaptic
membrane
• Information processing: – response (integration of stimuli) of
postsynaptic cell
![Page 6: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/6.jpg)
Figure 12–7 (Navigator)
![Page 7: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/7.jpg)
How is resting potential created and maintained?
![Page 8: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/8.jpg)
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)
![Page 9: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/9.jpg)
Figure 12–8 (Navigator)
Product of both passive and active forces
![Page 10: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/10.jpg)
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
![Page 11: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/11.jpg)
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
![Page 12: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/12.jpg)
Electrochemical Gradients
Figure 12–9a, b
![Page 13: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/13.jpg)
Electrochemical Gradients
Figure 12–9c, d
![Page 14: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/14.jpg)
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)
![Page 15: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/15.jpg)
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
![Page 16: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/16.jpg)
Sodium and Potassium Channels
• Membrane permeability to Na+ and K+ determines transmembrane potential
• Sodium and potassium channels are either passive or active
![Page 17: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/17.jpg)
Passive Channels
• Also called leak channels
• Are always open
• Permeability changes with conditions
![Page 18: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/18.jpg)
Active Channels
• Also called gated channels
• Open and close in response to stimuli
• At resting potential, most gated channels are closed
![Page 19: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/19.jpg)
Gated Channels
Figure 12–10
![Page 20: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/20.jpg)
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
![Page 21: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/21.jpg)
2. Voltage-regulated channels:– respond to changes in transmembrane
potential– characteristic of excitable membrane– found in neural axons, skeletal muscle
sarcolemma, cardiac muscle
![Page 22: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/22.jpg)
3. Mechanically regulated channels:– respond to membrane distortion – found in sensory receptors (touch, pressure,
vibration)
![Page 23: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/23.jpg)
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
![Page 24: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/24.jpg)
• Opening sodium channel produces graded potential
Figure 12–11 (Navigator)
![Page 25: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/25.jpg)
Figure 12–11 (Step 1)
Graded Potentials: Step 1
![Page 26: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/26.jpg)
Figure 12–11 (Step 2)
Graded Potentials: Step 2
![Page 27: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/27.jpg)
• Repolarization– stimulus is removed, transmembrane potential
returns to normal
• Hyperpolarization– Increasing the negativity of the resting
potential– Result of opening a potassium channel
![Page 28: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/28.jpg)
Figure 12–12
![Page 29: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/29.jpg)
Effects of Graded Potentials
• At cell dendrites or cell bodies:– trigger specific cell functions
• At motor end plate:– releases ACh into synaptic cleft
![Page 30: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/30.jpg)
What events are involved in the generation
and propagation of an action potential?
![Page 31: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/31.jpg)
Action Potentials
• Propagated changes in transmembrane potential
• Affect an entire excitable membrane
• Link graded potentials at cell body with motor end plate actions
![Page 32: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/32.jpg)
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
![Page 33: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/33.jpg)
Generating the Action Potential
Figure 12–13 (Navigator)
![Page 34: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/34.jpg)
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
![Page 35: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/35.jpg)
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
![Page 36: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/36.jpg)
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
![Page 37: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/37.jpg)
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
![Page 38: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/38.jpg)
Comparison of graded and action potentials
![Page 39: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/39.jpg)
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
![Page 40: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/40.jpg)
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
![Page 41: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/41.jpg)
What factors affect the propagation speed
of action potentials?
![Page 42: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/42.jpg)
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
![Page 43: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/43.jpg)
3 Groups of Axons
• Classified by: – diameter– myelination– speed of action potentials
• Type A, Type B, and Type C fibers
![Page 44: Chapter 12b](https://reader033.fdocuments.in/reader033/viewer/2022061207/54864dedb4af9f6b6f8b4c76/html5/thumbnails/44.jpg)
• “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