Transmit signals between neurons by influencing the excitability of the postsynaptic neuron.

Delay, diverge, converge, repeat or sustain transmission

Continuously modify excitability of all CNS cells

Electrical:

• Flows thru gap junction channels of adjacent cells
• Has cytoplasmic continuity
• uses small molecules and has bidirectional flow
• virtually no synaptic delay
• 3.5nm b/t pre and postsynaptic cells

Chemical: 

• most predominant
• not structurally connected
• uses transmitter and and storage vesicles
• 30nm dist b/t pre and postsynaptic cells
• synaptic delay of at least 0.3 ms; usually 1-5ms

Axosomatic: synaptic current generated at this site has stronger influence and greater influence on outcome at trigger zone vs axodendritic contacts.

Axodendritic: proximity to trigger zone is important is determining its effectiveness.

Axo-axonic: most have no direct effect on trigger zone of postsynaptic cell. They affect activity by controlling transmitter release.

One action potential in the presynaptic cell results in a single action potential in the postsynaptic cell.

Ex: neuromuscular junction

aka Divergence 

A single cell can have widespread influence by sending its signal to many target cells.

Ex: Renshaw cells of spinal cord...one action potential in the motorneuron causes Renshaw cells to fire off many action potentials

Aka convergence

A target cell can integrate diverse info from many sources. Most common input-output relation.

Ex: motor neurons of the nervous system receive convergence of information

1. generation of action potential in presynaptic cell
2. depolarization of plasma membrane of presynaptic axon terminal
3. entry of calcium ions into presynaptic terminal
4. release of transmitter from presynaptic terminal
5. binding of transmitters to specific receptors on postsynaptic plasma membrane
6. transient change in conductance of the postsynaptic membrane to certain ions
7.  Transient change in membrane potential of postsynaptic cell

Describe the effect of neurotranmitter gated channels on voltage-gated channels during synpatic input

What mechanisms can result in Excitatory Postsynaptic Potentials

(EPSP)

opening of Na or cation channels

closure of Cl or K channels ( since Cl and K normally flow out of the cell)

Changes in cell metabolism to inc excitability, # of excitatory receptors, or dec # of inhibitory receptors

closing Na or cation channels

opening Cl or K channels or both

activation of receptor enzymes that inhibit metabolic function, dec # excitatory receptors, or inc # inhibitory receptors.

Resting potential

Type: mostly K+ and Cl- channels; some Na

Mechanism: Channels usually non-gated (sometime voltage-gated K)

Signal Prop: usually steady; ranging in different cell from -35 to -90mV

Type: separate Na and K+ channels

Mechanism: Voltage

Signal Props: all-or-none; usually 100mV in amplitude; duration = 1-10ms

Receptor Potential

Type: Single class of channels for both sodium and potassium

Mechanism: Sensory stimulus

Signal props: graded; fast; several ms in duration; several mV in amplitude

Type: gap junctions (permeable to many small ions and organic molecules)

Mechanism: change in voltage, pH, Ca2+

Signal Props: passive spread of presynaptic potential charge

Type: EPSP depends on class of channels for Na and K+; IPSP depends on channels for Cl- or K+

Mechanism: chemical transmitter

Signal props: graded; fast; several ms; several mV in amplitude

Type: Closure of channels for K+, Na+, or Cl-

Mechanism: chemical transmitter

Signal props: graded; slow; seconds to minutes in duration; one to several mV in amplitude; contributes to action potential's amplitude and duration.

substance is synthesized in the neuron

released from presynaptic terminal in amt sufficient to exert its action of the postsynaptic neuron

when applied exogenously, it mimics the action of the endogenously released transmitter

a specific mechanism exists for removing it from its site of action (synaptic cleft)

small-molecule, rapidly acting transmitter: cause most of acute responses of nervous system

- ex: transmission of sensory signals to brain and motor signals back to muscles

Large-molecule, slow acting transmitter: cause more prolonged action

- ex: long-term changes in number of receptors, and long term opening/closing of certain ion channels

• can bind to receptor of postsynaptic terminal
• can diffuse into the blood
• can catalyze/influence other enzymatic reactions of the body
• can be taken back up by the presynaptic cell

• Acetylcholine
• epinephrine (catecholamine)
• norepinephrine (catecholamine)
• Dopamine
• seritonin
• glutamate
• glycine

What are the characteristics of Acetylcholine (ACh)?

• found in large pyramidal cells of motor cortex, neurons in basal ganglia, and motor neurons innervating skeletal muscle
• has inhibitory effect on cardiac muscle (vagus nerve)
• neurotranmitter of all preganglionic and postganglionic neurons of the parasympathetic nervous system
• neurotransmitter of all preganglionic neurons of sympathetic nervous system
• binds to nicotinic and muscarinic receptors of postsynaptic cells
• can also activate acetylcholinesterase enzyme that breaks it down to makae choline.
• choline is taken up by the presynaptic neuron, where it is combined with acetyl CoA (catalyzed by choline acetyltransferase) to reform acetylcholine

• EPI is secreted by chromaffin cells of the adrenal medulla
• plays a role in regulating BP in CNS
• EPI is main circulating excitatory transmitter in the periphery - released during fight or flight response
• NE originate in locus coreleus of the CNS - inhibitory effect
• NE involved in alertness and wakefulness
• primary transmitter for postganglionic sympathetic nervous system
• NE is released by internal organs ie the gut, spleen heart
• Dysfunctions include bipolar disorder, schizophrenia, regulation of mood, responsible for some symptoms of depression

• also a catecholamine
• NE is synthesized from dopamine
• dopaminergic neurons prominent in certain midbrain regions
• Dysfunctions: degeneration of dopaminergic neurons in basal ganglia of substantia nigra occurs in parkinson's disease

• synthesized from tryptophan 
• 5 hydroxytryptamine
• found throughout the brain, mainly synthesized in brain stem
• mostly inhibitory effects on postsynaptic membrane
• involved in control of eating, sleeping, and arousal
• Dysfunction: implicated in bipolar disorder and depression

• it is a principle excitatory transmitter in the brain
• plays a role in learning and memory
• glutamate is synthesized from glutamine from the glial cells and alpha ketoglutarate from the TCA cycle
• glial cells reuptake glutamate and convert to glutamine which is transferred to presynaptic neuron

• Gamma amino butyric acid
• widely distributed throughout brain and spinal cord
• has inhibitory effect in order to stabilize electical activity (produces IPSPs)
• GABAa receptor is linked to ligand gated Cl channels - inc Cl influx to produces inhibitory effect (Benzodiazepines such as librium, are anticonvulsants and sedatives that facilitate GABAa
• GABAb receptor: activation of inc'd K conductance - creates IPSP (hyperpolarization) via activation of G protein
• GABA plays a role in learning and memory
• is made from glutamate

• important inhibitory transmitter in spinal cord and brain stem
• blockage of glycine receptors causes continous contractions (lock jaw)

• they are large molecule, slow acting transmitters
• most serve as neuromodulators, but some act as neurotransmitters
• synthesized in neuronal cell body (not terminal), packaged and sent to terminal by axonal streaming of axon cytoplasm
• smaller amts are release compared to classical neurotransmitters, but they are more potent
• Ex of action: prolonged changes in metabolism in cells, prolonged activation/deactivation of genes, prolonged changes in number of receptors
• Actions are terminated by proteases or diffusion, there is NO reuptake into presynaptic cell
• can be co-released with classical transmitters
• can trigger complex coordinated behavior
• actions don't require point to point synaptic connectons
• opiates: regulate pain info (ex. beta-endorphin)