Basic Neuronal Synapse Flashcards
General Nerve Classification
Nerve fibers classified by the size of the fiber
Includes motor and sensory neurons
A-fibers
medium to large nerve fiber; myelinated
Aa, Ab, Agamma, Adelta–largest to smallest
C-fibers
small fiber, unmyelinated
Sensory Nerve Classification
Organized from largest to smallest
Ia : Muscle spindle primary ending (muscle stretch)
Ib: Golgi tendon organ: (muscle tension)
II: Muscle spindle secondary ending
III: Small, myelinated: (crude touch, sharp p!)
IV: Unmyelinated C-fibers: (dull, aching pain and temp)
Motor Neurons: (largest to smallest)
Alpha motor neuron
Gamma motor neuron
C-motor neuron
Alpha Motor Neuron
Extrafusal muscle fibers (skeletal muscle fibers)
Muscle fiber contraction
Gamma Motor Neuron
Intrafusal muscle fibers (muscle spindle)
Generate muscle response to sensory input from muscle spindles; control muscle spindle length
C-motor neuron
Autonomic nervous system fibers to smooth muscle
sympathetic and parasympathetic
Factors affecting NCV
Membrane myelination Neuron fiber diameter Temperature Cold – decreases NCV Heat – increases NCV Pharmacological agents: most decrease NCV Pathological processes: decrease NCV
Neuronal Synapses
Synapses are where neurons communicate with other neurons
axon terminal of one neuron that is adjacent to the dendrites of another neuron
consists of pre-synaptic neuron and post-synaptic neuron
Pre-synaptic neuron
releases neurotransmitter chemicals from the axon terminal
NTs bind to post-synaptic neuron and cause a change in MB permeability
Components of synapse
axon terminal
synaptic vesicles
synaptic cleft
post-synaptic receptors
Axon terminal
pre-synaptic neuron
Contains numerous synaptic vesicles and mitochondria
Synaptic vesicles
Contain neurotransmitters
Vesicles fuse with pre-synaptic membrane for exocytosis of contents
Synaptic cleft
Space between neurons that is maintained by reversible binding between membrane proteins of synaptic cells
Post-synaptic receptors
Protein receptors on membrane of post-synaptic neuron
Synaptic Sites: Axodendritic
Axon to dendrites and spines (most common type)
Synaptic Sites: Axosomatic
Axon to cell body synapse
Synaptic Sites: Axoaxonic
Axon to axon synapse
Most often occur at the initial segment or at the axon terminal of post-synaptic neuron
Synaptic Sites: Dendrodendritic
Dendrite to dendrite synapse
Occur infrequently and are typically reciprocal connections (synaptic communication in both directions)
Chemical Synapse
Release of neurotransmitter (NT) molecules into synaptic cleft
NT is released from the pre-synaptic terminal
NT binds to specialized receptors on the post-synaptic membrane
Each type of neurotransmitter has specific receptors on the post-synaptic membrane
Different NTs and their respective receptors have different effects on the post-synaptic cell
Electrical Synapse
Synapse that consists of gap junctions between adjacent cells
Gap junctions allow free passage of ions:
Any polarity change in one cell is easily passed to the adjacent cell
Allows synchronization of polarity changes among numerous adjacent cells
Abundant in connections between smooth muscle and cardiac muscle cells
Also contained within mammalian CNS
The easy transmission of changes in membrane potential between cells is important in contraction of:
Myocardium, smooth muscle of GI tract, and glandular epithelium
These synapses allow for coordinated contraction of smooth and cardiac muscle tissue
Electrical Synapses components:
Gap Jxn
fast transmission
common in smooth and cardiac muscle
Chemical Transmission of AP
- AP reaches the pre-synaptic terminal
- Change in membrane potential opens voltage-gated Ca 2+ channels to allow Ca 2+ to flow into the terminal
- Ca 2+ interacts with vesicles to cause vesicle fusion and exocytosis of neurotransmitter
- Neurotransmitter binds to receptor site on post-synaptic membrane
- Receptor is activated and creates channel opening in post-synaptic membrane
Synaptic Transmission: Post Synaptic processing of NT
Neurotransmitter degradation/removal must occur to clear the cleft for new signal transmission
NTs bind receptor, cause action and are released from receptor
Diffusion – molecule unbinds from receptor and simply “floats” away from cleft
Re-uptake - pre-synaptic terminal removes neurotransmitter and recycles it
Enzymes – chemically degrade neurotransmitters
Post-synaptic Response: potentials
Receptor-activated channels open to allow ion influx into post-synaptic cell
Amount of transmitter released (and bound) determines how many of these receptors/channels will be activated
Ion flux results in local potential change called:
Post-Synaptic Potential or graded potentials
Post-synaptic potentials are short-lived (milliseconds) changes in membrane potential
Excitatory Post-Synaptic Potential (EPSP)
Influx of Na+ and Ca 2+ results in local depolarization–moves closer to threshold
Inhibitory Post-Synaptic Potential (IPSP)
Influx of Cl- results in local hyper-polarization
further away from threshold
Neuronal Intergration
The post-synaptic membrane contains thousands of transmitter-gated channels that create EPSPs or IPSPs
Post-synaptic neurons compute (integrate) numerous EPSPs and IPSPs = (summation)
Integration/summation occurs in order to determine whether the overall set of stimuli creates enough depolarization to reach threshold for the post-synaptic neuron
Excitation of post-synaptic neuron requires numerous EPSPs to provide enough polarity change to reach action potential threshold
Spatial summation
Summation of all the EPSPs that are occurring simultaneously on neuron
Temporal summation
Summation of EPSPs at the same synapse when they occur in rapid succession
Within 3-15 milliseconds of each other
Divergence
Process by which a signal from a single neuron is transmitted to a large number of post-synaptic neurons
Allows amplification of a signal from one neuron to many neurons
Convergence
Process by which multiple signals from different neurons excite a single post-synaptic neuron
Allows summation of input of several neurons
Presynaptic Fxn
Functions of the pre-synaptic terminal can be affected by axoaxonal synapses it receives
Axoaxonal synapses alter NT release from axon terminal
Presynaptic inhibition
Axoaxonal synapse causes anion (Cl-) channel opening
Reduces the positive charge of the presynaptic cell, and reduces the amount of NT released
Presynaptic Facilitation
Axoaxonal synapse slows K+ channel opening of pre-synaptic neuron
Prolongs depolarization and increases NT release
Characteristics of Neurotransmitters
Chemicals contained in vesicles in the pre-synaptic terminal
Released into synaptic cleft as mode of transmission of signals between neurons
Neurotransmitters are either excitatory or inhibitory
Most cells produce only one neurotransmitter
Most receptors bind only one particular NT
Some NTs can bind to more than one receptor
As a rule, no 2 NTs bind the same receptor but the same NT may bind to many different receptors- each different receptor that a NT binds to is called a receptor subtype receptors often named by their agonists
Acetylcholine: Neurotransmitter
Excitatory NT
Major NT of the ANS and the neuromuscular junction
Utilizes nicotinic receptors at the neuromuscular junction
Utilizes muscarinic receptors at several areas of CNS and in the heart PNS (parasympathetics system)
Receptor Blockade
Curare: blocks nicotinic receptors (creates paralysis)
Atropine: blocks muscarinic receptors (increase heart rate)
BoTox: blocks nicotinic receptors (creates paralysis)
Glutamate: NT
Major excitatory neurotransmitter of CNS
Utilizes three different receptors with variable locations within the CNS
AMPA, NMDA, Kainate
Released in large quantities in cerebral injury and/or pathology
Excess excitation can have deleterious effects on neurons and CNS function:
Epilepsy
Cerebrovascular accident (CVA)
Seizures
GABA: NT
Major inhibitory neurotransmitter of CNS
Utilizes GABAa and GABAb receptors
A receptors: open Cl- channels
B receptors: use intracellular messenger to affect gene transcription and ultimately neuron function
Important in long-term memory/learning
Barbituates and benzodiazepenes (anti-anxiety, anti-depressants)
Bind to GABAa receptor and lengthen duration of channel opening when GABA is bound
Catecholamines: NT
Dopamine, serotonin, norepinephrine, epinephrine
Dopamine
Inhibitory NT with receptors throughout CNS
Involved with motor function, memory, mood and emotions
Important function of maintaining smooth, controlled movements
Lack of dopamine leads to Parkinson’s disease
Excess dopamine associated with schizophrenia
Serotonin (5-HT)
(5-hydroxytryptamine)
Inhibitory NT associated with mood (euphoria), emotion, sleep
LSD
Agonist of serotonin (binds serotonin receptor
Prozac
Inhibits reuptake of serotonin; allows it to be longer acting at synapses
Tryptophan
Molecular precursor to serotonin
Norepinephrine
noradrenaline
Epinephrine (adrenaline)
Major NTs of the autonomic nervous system
Both have excitatory and inhibitory functions
Function of these NTs is dependent on which receptor type they bind
Use adrenergic receptors:
Alpha 1 and alpha 2 receptors
Beta 1 and beta 2 receptors
