Chapter 12: Neural Tissues Physiology Flashcards
General Neural Activity
Resting potential (stimulus produces) –> graded potential (may produce) –> action potential (triggers) synaptic activity –> information processing
Plasma Membrane
Polarized membrane; polarized at rest; -70 mV;
Outside = Net Positive
Inside = Net Negative
Functional Properties of Neurons
Polarized membranes
Intracellular: Low Na+; High K+
Extracellular: High Na+; Low K+
Resting Membrane Potential
The amount of energy it takes to keep the two charges separated.
Most neurons: -70mV
Membrane Channel Parts
- Passive (leak) channels = Always Open
- Active (gated) channels = channels that open and close in response to stimuli
Types of Gated Channels
- Voltage Gated Ion Channels
- On Axons - Chemically Gated Ion Channels
- On Dendrites & Cell Body - Mechanically Gated Ion Channels
Types of Change in Membrane Potential
- Depolarization
- Hyperpolarization
- Repolarization
Depolarization
Inside (Intracellular) becomes more positive
- Open Na+ channels
Hyperpolarization
Inside (Intracellular) becomes more negative
Open K+ channels
Repolarization
Return to resting membrane potential
Graded Potential
Could be either:
1. depolarization
2. hyperpolarization
- Function is to stimulate or inhibit an action potential
- Occur in dendrites or cell bodies
- Weaken as they travel
Action Potential
A self-propagated change in the membrane potential
Threshold = -60mV (What is required to change)
Action Potential Step 1
- Region of excitable membrane (axon) depolarizes to threshold
Action Potential Step 2
- Activation gates of voltage gated sodium channels open
Na+ rushes into cell
(Depolarization)
Action Potential Step 3
- Inactivation gates of voltage gated Na+ channels close
at peak potential +30mV
Action Potential Step 4
- Voltage gated K+ channels to open; K+ rushes out; Repolarization
reestablishment of
resting membrane potential
Characteristics of Action Potentials
- on axon; moves in one direction, away from cell body
Action potentials= nervous impulses - Always depolarization
- Involves voltage gated ion channels
- Self-propagates – stay the same strength as it travels down the axon (affects adjacent voltage gated ion channels)
- Action potentials follow the all-or-none principle
- All stimuli that exceed threshold will produce identical action potentials.
- Refractory period lasts from time action potential begins until normal resting potential returns
Generation & propagation of Action potential
- Action potential is generated when threshold is reached
- Change in voltage is strong enough to open voltage gated ion (Na+) channels
- Graded potential towards axon hillock towards open voltage gated channels
Continuous Propagation in Unmyelinated Fibers
- Spread of action potential across entire membrane in series of small steps
- Open voltage gated channels in adjacent membrane patches
Continuous Propagation in Myelinated Fibers
- Only exposed at Nodes of Ranvier
- Saltatory conduction / propagation (to leap)
- Action potential spreads from node to node, skipping down the membrane
Axon Classification
- Different axons conduct at different speeds (velocity)
- Myelinated (faster) and unmyelinated (slower) fibers
- Larger diameter axons conduct at faster speeds
Type A Fiber
Myelinated, fat & fast (muscle movement and sensory for position)
Synapse
Site of intercellular communication junction between two neurons;
- neuromuscular junction
- neuroglandular junction
Allows for continuation of impulse
Type B Fiber
Myelinated but smaller (general sensations)
Type C fibers
Unmyelinated, small (smooth muscle)
Presynaptic Neuron
- Before synapse
- Axon terminal
Postsynaptic Neuron
- After synapse
1. dendrite
2. cell body
3. axon
Types of Synapses
- Electrical
- Chemical
Electrical Synapse
- Rare
- Pre- and postsynaptic cells are connected by gap junctions
- Ions flow from one cell to another
Chemical Synapse
- Common
- Cells don’t touch (synaptic cleft)
- Electrical (nervous impulse) chemical electrical
- Results in a graded potential (Postsynaptic potential)
- May or may not generate an action potential
Synaptic Cleft
Space between two cell membranes of synapse
Neurotransmitter
- Chemicals released from the presynaptic neuron to stimulate postsynaptic neuron.
- Found in synaptic vesicles in synaptic terminals
Types of Neurotransmitters
- Excitatory
- Inhibitory
Acetylcholine is both:
- Stimulates skeletal muscle , some synapses in CNS
Inhibits cardiac muscle
- Depends on what channels the receptors are sitting on (Na+ or K+)
Inhibitory Neurotransmitter
- Hyperpolarizes the postsynaptic membrane
- Harder to generate an action potential
Excitatory Neurotransmitter
- Depolarizes the postsynaptic membrane
- More likely to generate and action potential
Synapses involving Acetylcholine
- Cholinergic synapses
- All Cholinergic synapses release acetylcholine
- Most widespread neurotransmitter
- Neuromuscular junctions
- Some synapses in CNS
- Neuron to neuron synapses in PNS
Cholinergic Synapse Event 1
- Action potential arrives at axon terminals depolarizing synaptic knob
Cholinergic Synapse Event 2
- Depolarization opens voltage gated Ca++ channels
- Ca++ rushes in
- Ca++ causes the exocytosis of ACh (synaptic vesicle fuse with membrane)
Cholinergic Synapse Event 3
- ACh diffuses across the synaptic cleft
- binds to receptors on postsynaptic membrane
- chemically gated Na+ channels open
- Na+ rushes in and causes depolarization (graded potential)
Cholinergic Synapse Event 4
- ACh is removed
- Breakdown by acetylcholinesterase (an enzyme)
- Reabsorb breakdown products to resynthesize ACh
- Return to resting potential
Synaptic Fatigue
Use up Ach faster than it can be produced weakens synapse
Synaptic Delay
- Events at a synapse take time
- More control in complicated neural pathways involving thousands of synapses
- Reflexes – simple neural pathways
- Fastest=one synapse
- Very quick responses
Norepinephrine/Noradrenaline
- Adrenergic synapses release (NE)
- CNS & ANS (typically excitatory)
Dopamine
- Basal nuclei (part of brain) - CNS
- Inhibitory neurotransmitter important in muscle control
Serotonin
- Happy neurotransmitter in CNS
- Low levels = depression
GABA
Gamma Aminobutyric Acid
- CNS – reduces anxiety
- Other neurotransmitters – difficult to study
- Amino acids, peptides, polypeptides, ATP and gases (NO, CO)
Function of Neurotransmitters
Function through 3 mechanisms
1. Direct effect on membrane potential
2. Indirect effect by G proteins
3. Indirect effect by intracellular enzymes (Lipid soluble substance)
Direct Action
- Change in permeability of membrane
- Move ions change membrane potential
- ACh
Indirect Effect by Intracellular Enzymes Lipid soluble gases
- Pass through membrane
- Bind to enzymes
- Alter permeability or cellular activity
Indirect Effect by G Proteins
- Neurotransmitter is the 1st messenger
- Bind to receptor on cell membrane
- Activates enzyme(G protein)
- Results in a second messenger (cAMP) Second messenger causes change in membrane permeability or cellular activity
Information Processing
- Neural pathways can involve thousands of synapses
- A typical interneuron synapses with 1000 10,000 other neurons
- Simplest level of information processing occurs at the cellular level
- Postsynaptic neurons are either stimulated or inhibited by postsynaptic potentials
- Axon hillock integrates excitatory and inhibitory stimuli affecting cell body and dendrites.
Post Synaptic Potentials
- Graded potentials that develop in the postsynaptic membrane in response to a neurotransmitter.
- Can be either excitatory or inhibitory
EPSP
- EPSP (excitatory postsynaptic potential)
- Depolarization
- More likely to generate an action potential
IPSP
- IPSP (inhibitory postsynaptic potential)
- Hyperpolarization
- Harder to generate an action potential
Summation
Combining EPSPs and IPSPs to generate or inhibit an action potential
Temporal Summation
1 presynaptic neuron
- Fires in rapid succession
- Reach threshold
Spatial Summation
- Several presynaptic neurons
- All fire at the same time
- Reach threshold
Facilitation
- A membrane whose membrane potential shifts closer to threshold.
- Larger facilitation, less stimuli needed for AP
- Some drugs; ex. Nicotine
