Communication via Neurons
Neurotransmitter Anatomy and Components
Neurotransmitter: Chemicals that transmit signals across a synapse.
Neurotransmitter transporter: Proteins that transport neurotransmitters into synaptic vesicles.
Axon: The long, thin part of a neuron through which electrical impulses are transmitted.
Synaptic vesicle: Small sacs containing neurotransmitter molecules released into the synaptic cleft during synaptic transmission.
Voltage-gated Ca²+ channel: A channel that opens in response to changes in membrane potential, allowing Ca²+ ions to enter the cell, triggering neurotransmitter release.
Postsynaptic density: A specialized region of the postsynaptic membrane that contains receptors and proteins for signal transduction.
Cell body: The main part of the neuron, containing the nucleus.
Axon hillock: The conical region of the neuron where the axon originates; crucial for action potential initiation.
Dendrite: The branched part of a neuron that receives signals from other neurons.
Dendritic branches: Projections from dendrites that further increase the surface area available for synaptic connections.
Mitochondrion: Organelles that produce energy (ATP) through cellular respiration.
Synaptic cleft: The space between the presynaptic and postsynaptic neurons where neurotransmitter diffusion occurs.
Biological Structures in Auditory Systems
Auricle: The outer part of the ear, also known as the pinna.
Tympanic membrane (eardrum): The membrane that vibrates in response to sound waves.
Auditory canal: The passage leading from the outer ear to the eardrum.
Lobule: The lower part of the outer ear (earlobe).
Ossicles: Small bones in the middle ear that transmit sound vibrations; consist of:
Stapes: The stirrup-shaped bone at the base of the middle ear.
Incus: The anvil-shaped bone between the stapes and malleus.
Malleus: The hammer-shaped bone attached to the tympanic membrane.
Semicircular ducts: Structures within the inner ear that help maintain balance.
Oval window: The membrane that transfers sound vibrations from the stapes to the cochlea.
Vestibular nerve: A nerve associated with balance.
Cochlear nerve: A nerve that carries auditory sensory information from the cochlea to the brain.
Cochlea: A spiral-shaped organ responsible for converting sound vibrations into neural signals.
Round window: A membrane that allows for the movement of fluid within the cochlea, enabling sound perception.
Learning Objectives
The ionic basis of the resting membrane potential is established by the differential distribution of ions across the neuronal membrane, primarily involving K⁺ and Na⁺ ions. The sodium-potassium pump actively transports 3 Na⁺ ions out of the cell for every 2 K⁺ ions pumped in, resulting in a net negative charge inside the cell, typically around -70 mV. Action potentials occur when a neuron is stimulated, causing depolarization due to the rapid influx of Na⁺ ions through voltage-gated Na⁺ channels once a threshold is reached. Synaptic potentials, such as excitatory post-synaptic potentials (EPSPs) and inhibitory post-synaptic potentials (IPSPs), occur when neurotransmitters bind to chemically gated ion channels, leading to localized depolarization or hyperpolarization of the postsynaptic neuron.
Myelination plays a crucial role in increasing the speed of electrical signal conduction in neurons. Myelin sheaths insulate axons, allowing action potentials to propagate more rapidly through a process called saltatory conduction, where the impulse jumps between nodes of Ranvier instead of traveling continuously along the axon.
Electrical synapses involve direct electrical connections between neurons through gap junctions, allowing for the rapid transmission of signals. In contrast, chemical synapses involve the release of neurotransmitters from the presynaptic neuron into the synaptic cleft, where they bind to receptors on the postsynaptic neuron to transmit signals, leading to slower but more versatile communication due to different neurotransmitter types.
The information in an action potential is transmitted across a chemical synapse when the action potential reaches the axon terminal, causing depolarization and the opening of voltage-gated Ca²+ channels. The influx of Ca²+ triggers the fusion of synaptic vesicles with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft. These neurotransmitters then bind to ligand-gated ion channels in the postsynaptic neuron, causing depolarization (EPSP) or hyperpolarization (IPSP), and determining whether the postsynaptic neuron will fire its own action potential.
Excitatory neurotransmitters, such as glutamate and acetylcholine, lead to depolarization of the postsynaptic membrane, increasing the likelihood of action potential generation. Inhibitory neurotransmitters, such as GABA, cause hyperpolarization of the postsynaptic membrane, making it less likely for an action potential to occur. The balance between excitatory and inhibitory signals is critical for proper neuronal function and communication.
Key Terms
Acetylcholine: A neurotransmitter involved in muscle contraction and memory.
Action Potential: A rapid, temporary change in membrane potential that occurs when a neuron is activated.
Axon terminal: The endpoint of an axon where neurotransmitter release occurs.
Chemically gated ion channels: Channels that open in response to the binding of neurotransmitters.
Depolarization: The process of making the membrane potential more positive.
Dopamine: A neurotransmitter involved in reward and pleasure.
Endorphins: Neurotransmitters that act as natural painkillers.
Excitatory neurotransmitter: A neurotransmitter that causes depolarization of the postsynaptic membrane.
Excitatory post synaptic potential (EPSP): A postsynaptic potential that makes the neuron more likely to fire an action potential.
GABA: An inhibitory neurotransmitter that reduces neuronal excitability.
Gated ion channels: Channels that open or close in response to stimuli.
Glutamate: An important excitatory neurotransmitter in the brain.
Graded potential: Changes in membrane potential that vary in size and are dependent on the strength of the stimulus.
Habituation: A decrease in response to a stimulus after repeated exposure.
Hyperpolarization: The process of making the membrane potential more negative.
Inhibitory neurotransmitters: Neurotransmitters that cause hyperpolarization of the postsynaptic membrane.
Inhibitory post synaptic potential (IPSP): A postsynaptic potential that makes the neuron less likely to fire an action potential.
Leakage channels: Channels that are always open and allow ions to move across the membrane.
Ligand gated channels: Channels that open in response to the binding of a chemical messenger (ligand).
Membrane potential: The voltage difference across a membrane.
Nerve impulse: The electrical signal that travels along the axon of a neuron.
Neurotransmitters: Chemical messengers that transmit signals across synapses.
Nicotine: A stimulant that primarily affects acetylcholine pathways.
Postsynaptic cell: The neuron receiving the signal.
Presynaptic cell: The neuron sending the signal.
Propagation: The process of signal transmission along the axon.
Refractory period: The period during which a neuron cannot fire another action potential.
Resting potential: The baseline membrane potential of a neuron at rest (approximately -70 mV).
Saltatory conduction: The rapid transmission of action potentials along myelinated axons.
Serotonin: A neurotransmitter that influences mood, emotion, and sleep.
Sodium-potassium pump: A membrane protein that pumps Na+ out of the cell and K+ into the cell.
Spatial summation: The process by which multiple synaptic inputs combine to influence the postsynaptic neuron's activity.
Summation: The process of combining multiple inputs to generate a stronger output.
Synapse: The junction between two neurons where neurotransmitter release and reception occurs.
Synaptic cleft: The narrow gap between the presynaptic and postsynaptic neurons.
Synaptic integration: The process by which multiple synaptic potentials combine in the postsynaptic neuron.
Synaptic terminal: The end of an axon where neurotransmitter release occurs.
Synaptic vesicle: Membrane-bound vesicles that store and release neurotransmitters.
Temporal summation: The process of combining multiple stimuli over a short time to influence neuron activity.
Threshold potential: The minimum depolarization needed for an action potential to occur.
Voltage-gated channels: Channels that open in response to changes in membrane voltage.
Action Potentials
Establishment of the resting membrane potential:
Concentration of ions: 2 K⁺ ions are pumped into the cell for every 3 Na⁺ ions pumped out by sodium-potassium pumps.
Negatively charged molecules inside the cell also contribute to the resting potential.
Disruptions to the resting potential:
Graded potentials: Changes in membrane potential that can be either depolarizing or hyperpolarizing.
Depolarization: Moves the membrane potential towards a more positive value, often caused by excitatory stimuli.
Hyperpolarization: Moves the membrane potential towards a more negative value, often caused by inhibitory stimuli.
Gated channels: Ion channels that open in response to specific signals, which can lead to either depolarization or hyperpolarization.
Action potentials (AP):
Triggered when depolarization reaches a threshold (approximately -55 mV in mammal axons).
Steps involved in the action potential generation:
A stimulus causes the neuron to depolarize toward the threshold potential.
If threshold is reached, all Na⁺ channels open, leading to membrane depolarization.
At the peak of the action potential, K⁺ channels open, allowing K⁺ to exit while Na⁺ channels close.
Excess K⁺ outflow leads to hyperpolarization; during this refractory period, the neuron cannot fire again.
The Na⁺/K⁺ transporter returns the membrane to its resting potential after K⁺ channels close.
Characteristics of Action Potentials:
They are all-or-nothing phenomena; once the threshold is achieved, an action potential will fire.
The action potential is characterized by rapid depolarization followed by repolarization and hyperpolarization phases.
Action Potential Initiation
Location: Action potentials initiate at the axon hillock, where the cell body connects with the axon.
Phases of Action Potential:
Resting Phase: K⁺ ions diffuse out of the cell at an equilibrium state between diffusion and electrical pull.
Rising Phase: A sufficient stimulus depolarizes the cell, leading Na+ entry, which sharply raises the membrane potential (up to +55mV).
Peak of Curve: This represents the maximum voltage reached; Na⁺ channel activation occurs here.
Falling Phase: K⁺ ions exit the cell due to the opening of voltage-gated potassium channels, causing repolarization, and then undershooting occurs as excess potassium diffuses out.
Restoration of Equilibrium: The Na⁺/K⁺ pump resets the membrane potential back to the resting state.
Saltatory Conduction
Definition: In myelinated axons, action potentials are generated only at the nodes of Ranvier.
Impulses jump from node to node, allowing for faster conduction velocities compared to unmyelinated axons.
Factors Affecting Conduction Velocities
Effects of Diameter and Myelination on Conduction Velocities:
Squid giant axon (500 μm): Conduction velocity is 25 m/s (unmyelinated).
Large motor axon (20 μm): Conducts at 120 m/s (myelinated).
Human skin axon (10 μm): Conducts at 50 m/s (myelinated).
Temperature receptor (5 μm): Conducts at 20 m/s (myelinated).
Internal organ motor axon (1 μm): Conducts at 2 m/s (unmyelinated).
Neurotransmission at Chemical Synapses
An action potential arrives at the axon terminal.
Presynaptic membrane depolarizes, opening voltage-gated Ca²+ channels, allowing Ca²+ influx.
Calcium entry triggers synaptic vesicles to fuse with the membrane, releasing neurotransmitters into the synaptic cleft.
Neurotransmitters diffuse across the synaptic cleft to bind to ligand-gated ion channels on the postsynaptic membrane.
This binding results in either localized depolarization (EPSP) or hyperpolarization (IPSP) of the postsynaptic neuron, depending on the type of neurotransmitter released.
Neurotransmitters are then either degraded or reabsorbed by the presynaptic neuron.
Types of Neurotransmitters
Excitatory Post Synaptic Potentials (EPSPs): Depolarization effects that promote action potential generation.
Inhibitory Post Synaptic Potentials (IPSPs): Hyperpolarization effects that decrease the likelihood of action potentials.
Each neuron receives synaptic inputs from multiple axons, integrating excitatory and inhibitory signals to determine overall neuronal activity.