Neurons and Synapses Study Notes

Neurons Structure and Function

  • Neurons exhibit variability in structure and functions; however, they share fundamental mechanisms for signal transmission.

    • Signal Transmission:

    • Sensors: Detect external stimuli and internal conditions, transmitting information through sensory neurons.

    • Integration Site: Sensory information is processed in the brain or ganglia, where interneurons integrate this data.

    • Motor Output: Information exits the brain or ganglia via motor neurons, triggering muscle or gland activity.

  • Central Nervous System (CNS):

    • Integration occurs here.

  • Peripheral Nervous System (PNS):

    • Functions to relay information in and out of the CNS.

Neuron Structure

  • Neurons consist of organelles within the cell body (soma), with specialized components:

    • Dendrites: Receive signals from other neurons.

    • Axon: A longer structure transmitting signals to other cells at synapses, connecting with the cell body at the axon hillock.

Signal Reception

  • Dendrites and Soma:

    • Incoming signals cause changes in the membrane potential.

Signal Integration

  • Axon Hillock:

    • A strong stimulus converts the accumulated signal to an action potential (AP).

Signal Conduction

  • Axon:

    • Some are wrapped in a myelin sheath, allowing the AP to travel along the axon.

  • Synapse:

    • A junction between the axon of one neuron and another cell.

    • The synaptic terminal of a presynaptic cell transmits information across the synapse using neurotransmitters.

Electrical Signals in Neurons

Resting Membrane Potential

  • Neurons maintain a resting membrane potential like all cells.

    • Resting Condition: Membrane potential is negative.

    • Neurons are excitable and can rapidly change their membrane potential.

Changes in Membrane Potential

  • Depolarization:

    • Membrane potential becomes less negative.

  • Repolarization:

    • Membrane potential returns to resting value.

  • Hyperpolarization:

    • Membrane potential becomes more negative than resting value.

Formation of the Resting Potential

  • In mammalian neurons:

    • Sodium-Potassium Pump:

    • Maintains sodium (Na+) concentration higher outside the cell and potassium (K+) concentration higher inside the cell.

    • Energy Usage: ATP is used to maintain ion gradients across the plasma membrane.

    • These gradients represent chemical potential energy.

    • At rest, potassium (K+) channels are open, allowing K+ to diffuse out of the cell, while Na+ channels are less open.

Gated Ion Channels

  • Neurons alter their membrane potential through gated ion channels that respond to stimuli.

    • Opening or closing these channels selectively changes permeability and alters membrane potential.

  • Ion movement follows electrochemical gradients, affecting the degree of depolarization or hyperpolarization.

Signals in the Dendrites and Cell Body

  • Incoming signals (neurotransmitters) bind to membrane receptors, converting chemical signals to electrical ones by altering ion permeability of the membrane.

    • Changes in ion permeability result in graded potentials.

Graded Potentials

  • Vary in size based on stimulus strength:

    • More Neurotransmitter:

    • Leads to more open ion channels and larger graded potential magnitudes.

  • Processes:

    • Depolarization: Activates Na+ channels open.

    • Repolarization/Hyperpolarization: Opens K+ channels.

Graded Potentials Travel Short Distances

  • Conduction with Decrement:

    • Magnitude of graded potential decreases with distance from the origin.

    • Causes of Decrement:

    • Leakage of ions across the membrane.

    • Electrical resistance of cytoplasm.

  • Electronic Current Spread:

    • Positive charge disperses through the cytoplasm, causing depolarization of nearby membrane areas.

Action Potentials Travel Long Distances

Characteristics of Action Potential

  • Initiated by the net graded potential at the axon hillock.

  • All-or-Nothing Response:

    • Action potentials do not degrade over time or distance.

  • Threshold:

    • A minimum membrane potential must be reached to trigger an action potential.

    • Depolarizations below this threshold will not initiate an action potential.

Integration of Graded Signals

  • Spatial Summation:

    • Graded potentials from different locations contribute to net signal change.

  • Temporal Summation:

    • Graded potentials occurring at slightly different times also influence the net change.

Production of Action Potentials

  • Voltage-Gated Ion Channels:

    • Sodium (Na+) and potassium (K+) channels respond to changes in membrane potential.

    • Action Potential Threshold:

    • Generally about -50 mV.

    • An action potential is an all-or-nothing depolarization of the neuron's plasma membrane.

Action Potential Phases

  1. Resting State:

    • Most voltage-gated Na+ and K+ channels are closed; some non-voltage gated K+ channels are open.

  2. Depolarization Phase:

    • Na+ channels open, allowing Na+ to flow into the neuron.

  3. Threshold Crossed:

    • Excess Na+ entry causes rapid depolarization; voltage-gated K+ channels will open subsequently, and the membrane permeability for K+ increases.

  4. Repolarization Phase:

    • Voltage-gated Na+ channels become inactivated, and K+ channels are open, allowing K+ to exit the neuron, restoring the negative membrane potential.

  5. Hyperpolarization:

    • The membrane potential briefly becomes more negative than the resting potential due to lingering K+ channel openings before returning to the resting state.

Refractory Periods

  • Absolute Refractory Period:

    • The neuron cannot generate another action potential regardless of stimulus strength.

  • Relative Refractory Period:

    • A stronger-than-normal stimulus is required to elicit a response due to lingering K+ channel activity.

Key Ion Channel Dynamics

  • Voltage-Gated Na+ Channels:

    • Activation gate opens when the threshold is reached, while an inactivation gate closes after a brief period.

  • Action Potentials and Saltatory Conduction

    • Action potentials can only be generated at nodes of Ranvier (gaps in the myelin sheath) where voltage-gated channels are concentrated.

    • This allows the AP to jump between nodes, increasing signal speed (saltatory conduction).

Information Transfer by Action Potentials

  • The frequency of action potentials encodes information strength:

    • Stronger Stimuli: Result in higher frequencies of action potentials.

    • Mammalian nerves can conduct between 500-1000 action potentials per second.

  • Synapses: Location where neurons communicate with other cells.

    • Electrical Synapses: Direct electrical current flows through gap junctions.

    • Chemical Synapses: Use neurotransmitters to relay information across the synaptic cleft (most common).

Summary of Electrical and Chemical Synapses

  • Electrical Synapses:

    • Fast signal relay, less complex but less common.

  • Chemical Synapses:

    • Allow for greater integration and modulation of signals but are slower.