BI100 Lecture 25.2

Nerve Structure

  • A nerve is a bundle of axons, also known as nerve fibers and dendrites. Nerves can vary in diameter and length, depending on their function and location in the body. Some nerves are microscopic, while others can be several feet long.

  • Axons communicate with dendrites to form nerves, transmitting electrical and chemical signals. The efficiency and speed of this communication are critical for proper nervous system function.

  • A ganglion is a cluster of cell bodies that make up nerves. Ganglia serve as relay points for nerve signals.

Nervous System Divisions

  • Central Nervous System (CNS): Contains interneurons, which process and transmit information within the brain and spinal cord. The CNS is responsible for higher-level functions such as cognition, emotion, and coordination.

  • Peripheral Nervous System (PNS): Contains sensory and motor neurons. These neurons collect and transmit signals to and from cellular systems. The PNS is divided into the somatic nervous system (voluntary control) and the autonomic nervous system (involuntary control).

Nerve Impulse

  • Detected as a wave of electrical activity. The speed and strength of the nerve impulse are crucial for rapid communication throughout the body.

  • Analogy: Neuron is like a copper wire with plastic insulation (Schwann cell).

    • The insulation prevents information from leaking out and ensures efficient signal transmission. Myelinated axons (those with Schwann cells) transmit signals much faster than unmyelinated axons.

    • Damaged insulation can lead to signal dissipation or potential fire hazards, such as in conditions like multiple sclerosis.

  • Impulses are received by the cell body, travel along the axon, and are transmitted at the synaptic area. The precise regulation of these impulses is essential for coordinated responses.

Signal Transduction and Action Potential

  • Cells have a positive charge on the outside and a negative charge on the inside in a resting state. This charge difference, known as the resting membrane potential, is typically around -70 mV.

  • Maintained by the sodium-potassium pump, which actively transports sodium (Na^+) ions out of the cell and potassium (K^+) ions into the cell, maintaining the electrochemical gradient.

  • Sodium (Na^+) and potassium (K^+) channels allow for ion movement across the cell membrane. These channels are voltage-gated, opening and closing in response to changes in membrane potential.

  • Action Potential:

    • If a nerve impulse reaches a certain threshold (typically around -55 mV), depolarization of the membrane occurs. This threshold must be reached for an action potential to be generated.

    • This allows the signal to move (e.g., left to right). The action potential is an all-or-nothing event; it either occurs fully or not at all.

    • Sodium (Na^+) rushes in, causing depolarization. The influx of sodium ions reverses the membrane potential, making the inside of the cell positive.

    • Potassium (K^+) moves out, repolarizing the membrane. The efflux of potassium ions restores the negative membrane potential.

    • The action potential moves along the axon due to these ion movements. The myelin sheath and the nodes of Ranvier facilitate saltatory conduction, greatly increasing the speed of action potential propagation.

  • Electrical signal conduction involves sodium (Na^+) influx (depolarization) and potassium (K^+) efflux (repolarization), changing the membrane charge. The precise timing and coordination of these ion movements are critical for proper nerve function.

  • Membrane potential can be detected using a physiograph, which measures the voltage difference across the cell membrane.

Ion Channel Dynamics

  • In a resting state, both sodium (Na^+) and potassium (K^+) channels are closed, maintaining the resting membrane potential.

  • A stimulus opens sodium (Na^+) channels, allowing sodium to flood in.

    • The inside becomes positive, and the outside becomes negative, leading to depolarization.

    • Reaching the action potential at a certain threshold initiates the cascade of events.

  • At the action potential:

    • Sodium (Na^+) channels close to prevent further influx of sodium ions.

    • Potassium (K^+) channels open, allowing potassium (K^+) to rush out, restoring the negative membrane potential.

    • The outside becomes more positive (resting state), completing the repolarization phase.

    • The inside becomes more negative, returning to the resting membrane potential.

  • Potassium (K^+) channels close relatively slowly, causing a slight undershoot in the action potential (hyperpolarization). This undershoot is due to the prolonged opening of potassium channels.

  • The change in membrane charge allows for energy derivation and information transmission along the axon. This electrochemical gradient is essential for nerve signal propagation.

Synapse

  • Synapse or synaptic cleft: the space between the axon terminus (axon terminal) and another cell. This gap is typically 20-40 nanometers wide.

  • Presynaptic Neuron: Includes dendrites, cell body, axon, and axon terminus. The presynaptic neuron is the neuron sending the signal.

  • Postsynaptic Neuron: Includes dendrites, cell body, and axon. The postsynaptic neuron is the neuron receiving the signal.

  • Synaptic Cleft: The space between the presynaptic neuron's axon and the postsynaptic neuron's dendrites. Neurotransmitters diffuse across this space to transmit signals.

  • Synaptic Knob/Bulb: Located at the axon termini; interacts with the dendrites or effector cells through direct contact or neurotransmitter release. Synaptic vesicles within the knob contain neurotransmitters.

  • Neurotransmitters: Chemicals released into the synapse that bind to receptors on the postsynaptic neuron, initiating a response. Examples include acetylcholine, dopamine, and serotonin.

Types of Synapses

  • Electrical Synapses

    • Occur when transmission is electrical between two cells (presynaptic and postsynaptic neurons). These synapses are faster but less versatile than chemical synapses.

    • Neurons are directly connected through gap junctions.

    • Good for steady, rhythmic contractions (e.g., heart, digestive system). These synapses allow for synchronized activity among cells.

    • Example: Tail flips in crayfish, lobsters, and some fish, enabling rapid escape responses.

Electrical Synapse Details

  • Synaptic Bulb:

    • Presynaptic neuron bulb connects to the dendrite of the postsynaptic neuron via gap junctions.

  • Signal Transmission: Electrical signals pass directly through a direct connection, allowing for rapid and synchronous communication.

  • Plasma Membrane: Axon end (synaptic bulb) and dendrite membranes are involved in forming the gap junction.

  • Connexons: Proteins connecting two cells.

    • Consist of six connexin proteins forming a channel-like structure that spans the intercellular space.

    • Embedded in the membrane, allowing for structural support and channel formation.

    • Allow ions (e.g., sodium, calcium) to pass directly between cells, facilitating electrical signal transmission.

  • Gap Junctions: Clusters of connexons in a particular area, allowing for efficient ion flow.

    • Found in heart cells, allowing electrical transmission between cells and coordinated contractions.

  • Chemical Synapse

    • A different type of synapse that uses neurotransmitters to transmit signals. These synapses are more versatile and can modulate the signal.