Biol 2110 Chapter 12-13: Neurons and Synapses

What are the similarities between the nervous system and endocrine system?

Both are specialized for control and coordination to ensure smooth and coordinated function of organisms

Nervous system functions

Initiates responses rapidly

Short-duration responses

Acts via action potentials and neurotransmitters

Acts at specific locations determined by axon pathways

Nuerotranmitters act over short-distances

Endocrine system function

Initiates responses slowly

Long-duration responses

Acts via hormones released into blood

Acts at diffuse locations (targets can be anywhere blood reaches)

Hormones act over long-distances

Neuronal signalling and hormonal signalling

What are the 4 basic parts of a neuron?

Dendrites, cell body, axon and presynaptic terminal

What are the four functional properties of neurons?

Synaptic input (dendrites)

Integration of synaptic potentials (cell body)

Conduction of action potentials (axon)

Synaptic output (presynaptic terminal)

Structural classifications of neurons

Unipolar, bipolar and multipolar

Functional classifications of neurons

Sensory (afferent) neurons

Interneurons (association neurons)

Motor (efferent) neurons

Sensory neurons

Relay sensory signals from PNS to CNS for integration

Interneurons

Exclusively in CNS, between sensory and motor neurons.

Shuttle signals through CNS pathways for integration

Motor neurons

convey motor from CNS to PNS to appropiate effectors (muscles and glands)

Glial cells

Supportive cells of neurons

Membrane potential

Voltage across plasma membrane

Resting membrane potential

Electrical potential difference across the plasma membrane in a non-excited

Ion channels

Cause changes in membrane permeability, as ions can flow in/out and RMP can change

Ligand-gated channel

ion channel that opens/closes in response to specific ligand stimulus

mechanically gated channel

Ion channels that open/close in response to mechanical stimulation (touch, pressure, tissue stretching, vibration)

voltage-gated channel

Ion channel that opens in response to change in membrane potential

Determinants of resting membrane potential

Unequal distribution of ions in ECF and cytosol

Differences in membrane permeability

Action of Na+/K+ ATPase (maintains high intracellular K+ and extracellular Na+)

In a K+ only cell, how is membrane potential maintained?

K+ flows out, K+ electrical gradient increases

K+ flows back into cell, the electrical
gradient and concentration gradient
would be equal (equilibrium potential)

equilibrium potential close to typical RMP

In a Na+ only cell, how is membrane potential maintained?

Na+ flows in, Na+ electrical gradient increases

Na+ flows back out of cell, the electrical
gradient and concentration gradient

would be equal (equilibrium potential)

equilibrium potential not close to typical RMP

Graded potential

Small deviation from RMP, graded due to amplitude depending on stimulus strength

Can result in hyperpolarizing or depolarizing

What is the basic mechanism of a graded potential?

Neurotransmitter binds to ligand-gated Na+ channel

Na+ enters cell through channel, and current spreads through. cell

Strength of signal decreases with distance

Why does the current spread in graded potentials?

Positive ions attracted to negative ions, causing spread from initial site

Why do graded potentials decrease with distance?

Resistance in cytoplasm, leak channels and membrane properties

In addition to ligand binding, what else triggers graded potentials?

Mechanical deformation in mechanically-gated ion channels

Light energy activating GPCR in photoreceptor cells

What is the main difference between graded and action potential?

Graded potentials decrease with distance while action potentials maintain their amplitude over long distances.

What are the three phases of action potentials?

Depolarizing, Repolarizing, After-hyperpolarizing

Shape, magnitude or duration of each AP reflects…?

Type and distribution of ion channels in membrane

Specialized function of cell

Spatial summtion

Graded potentials occur from different sites, then summate at axon hillock

Can result in action potential if both stimuli are polarizing and reaching threshold, but also could cancel each other out

Temporal summation

Graded potentials can occur
from the same site but with increased frequency of stimuli and then summate at the axon hillock

Occurence of action potential depends on sum of graded potentials reaching threshold at the axon hillock

Resting state

Na and K channels closed, RMP

Depolarizing/Rising phase

Membrane potential reaches threshold, Na channel opens. As Na moves in, membrane becomes depolarized

Repolarization/Falling stage

Na inactivation gate close, K channels open. As K leaves, the membrane starts to repolarize towards RMP.

Na activation gates close and inactivation gate opens, K gates close, and RMP is restored.

Conduction velocity of AP is influenced by..?

Axon diameter

Myelination of axon

Temperature

Myelin sheath function

Increases speed of conduction, insulates axon

How does myelin speed up conduction?

Prevents ions from leaking out as charge spreads through axon

Charge spreads unimpeded until reaching node of Ranvier

Signals continue to jump down the axon

How does axon diameter affect conduction velocity?

Large diameter axons conduct action potentials more rapidly, due to longer length constants, thus a more distant spread of local currents

How does temperature affect conduction velocity?

Higher temperatures increase conduction velocity, as the gating kinetics of voltage-gated ion channels are temperature-dependent

Synapse

Point at which one neuron communicates with another neuron, or target tissue (skeletal muscle)

What are the two types of synapses?

Electrical synapse

Chemical synapse

Summarize events of chemical synaptic transmission

Action potential reaches the axon terminal, Ca2+ ions flow into presynaptic terminal, triggering the release of neurotransmitters from synaptic vesicles into the synaptic cleft.

Neurotransmitters bind to receptors on the postsynaptic membrane, leading to depolarization or hyperpolarization of the postsynaptic neuron.

Excitatory postsynaptic potentials

Increased likelihood of an action potential in the postsynaptic neuron due to the binding of neurotransmitters, causing depolarization.

Inhibitory postsynaptic potentials

Decreased likelihood of an action potential in the postsynaptic neuron due to the binding of neurotransmitters, causing hyperpolarization.

Signal transmission at the
neuromuscular junction

End plate potential is a..?

Graded potential

Microglia function

Mediate immune responses in neural tissue, consume pathogens and cell debris

Astrocytes

Take up neurotransmitters from extracellular space, help supply metabolic substrates to neurons

Compare and contrast Electric synapse and Chemical Synapse

Chemical synapses are either excitatory or inhibitory, while electrical synapses are nearly always excitatory

Chemical synapses are one-way, while electrical synapses are two-way

Chemical synapses are more modifiable (use makes them stronger, disuse makes them weaker) in their properties than electric synapses