Neuroscience: Nerve Impulse Mechanisms

Animal Physiology (3rd Level Biology English)

• Course Instructor: Dr. Hiba Allah Abdul Rahman Ahmad
• Course Level: 3rd Level (Biology English)

General Functions of Neuroglia

• Support: Provide supportive scaffolding for neurons.
• Neuron Health Growth: Promote neuron health growth.
• Chemical Guidance: Some neuroglia produce chemicals to guide young neurons to proper connections.
• Speed Up Action Potential Conduction: Others wrap around and insulate neuronal processes to speed up action potential conduction.

Differences Between Neuroglia & Neuron

• Size: Neuroglia are smaller than neurons.
• Quantity: There are 5 to 50 times more neuroglia than neurons.
• Action Potentials: Glial cells do not generate or propagate action potentials.
• Replication: Glial cells can multiply and divide in the mature nervous system.

The Neuron

• Signal Reception: Dendrites receive signals.
• Signal Integration: The cell body integrates signals.
• Signal Transmission: The axon transmits action potentials.
• Speed Increase: The myelin sheath increases the speed of signal transmission.
• Signal Transmission: Synaptic terminals transmit signals.

Nerve Impulse

• Progressive Wave: A progressive wave of electrical and chemical activity along a nerve fiber.
• Action of Nerve Cell: Stimulates or inhibits a muscle, gland, or other nerve cell.
• Information Transmission: Information moves from sensory neurons to interneurons to motor neurons.

Factors Determining Nerve Impulse Speed

• Temperature: Higher temperature leads to faster speed.
• Axon Diameter: Larger diameter results in faster speed.
• Myelin Sheath: Vertebrates have a myelin sheath surrounding their neurons.
• Nodes of Ranvier: Voltage-gated ion channels are found only at these nodes.
• Saltatory Propagation: The myelin sheath isolates the axon, and the impulse jumps between nodes to significantly increase speed, while impulses in unmyelinated neurons only travel at approximately 1 m/s, those in myelinated neurons can reach up to 100 m/s.

Ion Channels

• Plasma Membranes: Contain various ion channels.
• Passive Channels (Leakage Channels): Always open.
• Active Channels (Gated Channels): These channels open and close in response to specific stimuli and include ligand-gated, voltage-gated, and mechanically-gated channels.

Membrane Potential

• Measurement: Measured in millivolts.
• Resting State: When a nerve is inactive (not transmitting a signal), the membrane potential is around -70 mV.

Membrane Potentials

• Separation of Ions: Voltage exists across the plasma membrane due to the separation of oppositely charged ions.
• Resting Membrane Potential: The potential difference in a resting membrane.
• Polarization: The membrane is polarized in a resting neuron.

Changes in Membrane Potential

• Stimuli: Responses to stimuli such as ion concentrations (e.g., temperature, light, or pressure) and chemical stimuli (e.g., neurotransmitters such as dopamine, serotonin, and amino acids)

Resting Membrane Potential

• Charge Differences: Inside and outside of the cell have a difference in charge.
• Ion Concentrations: Sodium is in greater concentration outside the cell, potassium inside, and anions inside.
• Large Molecules: Maintain a negative charge inside the cell..
• Membrane Permeability: Greater for potassium than sodium.
• Active Transport: The Na+/K+ pump moves sodium outside and potassium inside the cell.

Membrane Potentials

• Signals: Neurons use changes in membrane potentials as signals.
• Signal types: Two types of signals: graded potentials and action potentials.

Graded Potentials

• Triggers: Triggered by change in neuron's environment.
• Opens Ion Channels: Opens gated ion channels to change membrane state.
• Depolarization: A small area of the neuron's plasma membrane becomes depolarized (less negative).
• Current Flow: Current flows on both sides of the membrane (+ moves toward − and vice-versa).

Graded Potentials

• Short-lived: Short-lived, local changes in membrane potential (either depolarizations or hyperpolarizations).
• Decreasing Magnitude: Cause current flows that decrease in magnitude with distance.
• Stimulus Strength: Magnitude depends on stimulus strength; stronger stimuli lead to larger voltage changes.
• Current Flow: Larger voltage changes lead to farther current flow..

Graded Potentials

• Short-Distances: Signals over very short distances.
• Action Potential Initiation: Important in initiating action potentials.

Action Potentials

• Phases: Describes four phases of an action potential: resting states, depolarization, repolarization, and undershoot.
• Neuron Response: A nerve signal begins as a change in membrane potential.
• Membrane Permeability Changes: Stimuli alter the membrane permeability, allowing ions to pass through, changing the membrane voltage.
• Action Potential: The nerve signal, termed an action potential, is a change in the membrane voltage that goes through the resting potential to a maximum and back.

Action Potentials

• Phases of Action potential: Different phases of repolarization (e.g. channels close and open) are detailed including the refractory period (absolute refractory period and relative refractory period)

Action Potential Propagation

• Electric Current: Action potential causes an electric current that stimulates adjacent membrane portions.
• Sequential Activation: A series of action potentials occurs sequentially along the axon as a nerve impulse.

Action Potential Propagation

• Self-Propagation: The action potential propagates itself along the neuron.
• One-Way Chain Reaction: Action potentials are self-propagated in a one-way chain reaction along a neuron.
• All-or-None Events: Action potentials are all-or-none events..

Action Potentials

• All-or-Nothing: Action potentials are all-or-nothing phenomena; either they happen completely or not at all.
• Stimulus Strength Independence: Independent of stimulus strength once generated.
• Strong Stimuli: Strong stimuli generate more impulses of the same strength per unit time.
• Intensity Determination: Determined by the number of impulses per unit time

Action Potentials

• Frequency Changes: The frequency of action potentials changes with the strength of the stimulus.
• Strength Stability: The strength of the action potential itself is stable (all-or-none law)

Action Potentials- Refractory Period

• Second Stimulus Response: Neurons cannot respond to a second stimulus while Na⁺ channels are still open from the previous stimulus.
• Absolute Refractory Period: This period is called the absolute refractory period.
• Relative Refractory Period: This is followed by the relative refractory period where repolarization is occurring, and the threshold for impulse generation is elevated.
• Strong Stimuli: Only strong stimuli can generate impulses during this period.

Synapses

• Neuron Connection: Neurons usually do not connect directly, but instead, a junction between dendrites of one neuron and the axon of another, called a synapse, controls signal transmission.
• Neurotransmitters: Neurotransmitters cross the synapse and stimulate the next neuron.

Synapse

• Types: Synapses can be chemical (most common) or electrical.
• Electrical Synapses: Ions flow directly between neurons.
• Chemical Synapses: Neurotransmitters transmit the signal, crossing the synaptic cleft and binding to a receptor, leading to excitation or inhibition.

Synaptic Integration

• Excitatory Postsynaptic Potential (EPSP): Neurotransmitter binding causes depolarization, Na⁺ and K⁺ flow through the membrane, and net depolarization occurs.
• Inhibitory Postsynaptic Potential (IPSP): Neurotransmitter binding reduces a postsynaptic neuron's ability to generate an action potential by increasing permeability to K⁺ and Cl⁻.

Synaptic Integration

• Temporal Summation: Repeated stimulation from a single synapse within a short timeframe leads to summation..
• Spatial Summation: Simultaneous stimulation from separate synapses leads to summation.

Basic Concepts of Neural Integration

• Circuit Patterns: Different patterns of synaptic connections in neuronal pools (e.g., diverging, converging, reverberating, etc.) are shown.
• The diagrams show how signals travel within each circuit type

Synapse

• Function: Mediating information transfer from one neuron to another or to an effector cell..
• Types: Axodendritic, axosomatic, axoaxonal synapses.

Synapse

• Electrical Signals: Pass between cells at electrical synapses, as well as at chemical synapses.
• Chemical Signals: The sending neuron secretes a neurotransmitter, which crosses the synaptic cleft and binds to a receptor on the receiving cell, leading to excitation or inhibition.

Synapse Types

• Electrical synapses: Less common than chemical synapses; correspond to gap junctions; cytoplasm of adjacent neurons are connected through protein channels, allowing ions to flow directly between neurons, and transmission is very rapid.

Neurotransmitters

• Chemicals: Chemicals released from the presynaptic neuron.
• Functions: Assist, stimulate, or inhibit postsynaptic neurons.
• Synthesis Locations: Synthesized in the cytoplasm of synaptic knobs/vesicles.

Neurotransmitters

• Types: Various types including small molecules (e.g., acetylcholine, biogenic amines, amino acids) and neuropeptides (e.g., endorphins).
• Classification: Classified by chemical and anatomical/physiological characteristics.

Criteria for Neurotransmitters

• Synthesis: Synthesized in the presynaptic neuron
• Storage: Stored in vesicles in the presynaptic terminal
• Release: Released upon appropriate stimulus (e.g., action potential)
• Binding: Bind to receptors on the postsynaptic membrane
• Termination: Mechanisms for clearing the neurotransmitter from the synapse.