lecture recording on 13 March 2025 at 01.41.58 AM

Objectives of the Lecture

  • Identify the different parts and functions of a neuron.

  • Understand the concept of resting membrane potential.

  • Explain graded potentials and action potentials.

  • Overview of chemical synapses.

Functions of the Nervous System

  • Sensory Input

    • Perception of the environment (e.g., lifting a glass of water).

    • Transmitted by sensory neurons to the central nervous system (CNS).

  • Integration

    • Sensory information interpreted by other neurons.

    • Determines responses and sends signals back.

  • Motor Output

    • Communicates responses away from CNS, typically through motor neurons to muscles or glands.

Overview of Neuron Structure

Key Components of a Neuron

  • Cell Body

    • Contains the nucleus and organelles.

  • Dendrites

    • Projections close to the cell body that receive signals.

  • Axon

    • Transmits signals away from the cell body towards axon terminals.

  • Axon Terminals

    • Release neurotransmitters into the synapse to communicate with other cells.

  • Myelin Sheath

    • Electrically insulates the axon for faster signal transduction.

Neuron Cell Body and Organelles

  • The cell body is essential for synthesizing neurotransmitters and contains most organelles.

  • Neurotransmitters are transported down the axon for storage in axon terminals, which contain vesicles for release.

Action of the Sodium-Potassium Pump

  • Resting Membrane Potential

    • Maintained by the sodium-potassium pump which moves 3 sodium ions out and 2 potassium ions in.

    • Creates an overall negative charge inside the neuron by unequal exchange.

  • Ion Concentration

    • High sodium (Na+) concentration outside and high potassium (K+) concentration inside at resting state.

Neural Signaling

Resting Membrane Potential

  • All cells have a resting membrane potential; neurons specialize in using it for communication.

  • Neuron resting potential typically around -70 mV (inside more negative than outside).

Graded Potentials (GP)

  • Initial response to stimuli; localized changes in membrane potential.

  • Effective when multiple stimuli occur close together, summating to potentially trigger an action potential.

Action Potentials (AP)

  • All-or-nothing response; generated when the threshold potential is reached.

  • Signals are long-distance and travel down the axon to the terminals.

Ion Movement in Action Potentials

Mechanisms of Action Potentials

  • Opening of voltage-gated sodium channels leads to depolarization as sodium enters the cell.

  • Followed by the inactivation of sodium channels and opening of potassium channels, leading to repolarization as potassium exits the cell.

  • The final phase includes an undershoot as additional potassium exits before resting potential is restored.

Key Vocabulary

  • Resting Membrane Potential: Voltage across the membrane when the neuron is at rest.

  • Graded Potential: Small, localized change in membrane potential in response to stimuli.

  • Action Potential: Rapid, large change in voltage that propagates along the axon.

  • Voltage-gated Channels: Specialized channels that open in response to changes in membrane voltage.

Objectives of the Lecture

  • Identify the different parts and functions of a neuron, including their roles in the transmission of signals within the nervous system.

  • Understand the concept of resting membrane potential, including the electrochemical gradients that contribute to it.

  • Explain graded potentials and action potentials, including the mechanisms that lead to their generation and propagation.

  • Provide an overview of chemical synapses, detailing how neurotransmitters facilitate communication between neurons.

Functions of the Nervous System

Sensory Input

  • The nervous system perceives environmental stimuli, such as lifting a glass of water, by processing sensory signals.

  • Sensory neurons are responsible for transmitting these signals from sensory receptors (e.g., eyes, ears) to the central nervous system (CNS) for analysis.

Integration

  • Integration involves the processing of sensory information interpreted by interneurons, which analyze incoming signals and determine appropriate responses.

  • The outputs from integrated information are sent back to the body through motor neurons, enabling responses to the stimuli.

Motor Output

  • The nervous system communicates responses away from the CNS, typically through motor neurons that innervate muscles or glands, leading to actions or hormone release.

Overview of Neuron Structure

Key Components of a Neuron

  • Cell Body

    • Contains the nucleus and organelles crucial for maintaining cellular health and synthesizing neurotransmitters.

  • Dendrites

    • These branched projections from the cell body receive signals from other neurons and convey information towards the cell body.

  • Axon

    • This long, tail-like structure transmits electrical impulses away from the cell body and towards axon terminals where neurotransmitter release occurs.

  • Axon Terminals

    • Contain specialized vesicles filled with neurotransmitters that are released into the synapse to communicate signals to adjacent neurons.

  • Myelin Sheath

    • A protective layer of fatty tissue that insulates the axon, enhancing the speed and efficiency of signal transmission through saltatory conduction, where action potentials jump between nodes of Ranvier.

Neuron Cell Body and Organelles

  • The cell body, or soma, is vital for synthesizing and transporting neurotransmitters and houses the majority of the neuron’s organelles, including mitochondria and ribosomes.

  • Neurotransmitters synthesized in the cell body are transported down the axon to the axon terminals for release during neuronal signaling.

Action of the Sodium-Potassium Pump

Resting Membrane Potential

  • The resting membrane potential is maintained by the sodium-potassium pump, which actively transports 3 sodium ions (Na+) out of the neuron and 2 potassium ions (K+) into it against their concentration gradients.

  • This activity creates an overall negative charge inside the neuron, resulting in a resting membrane potential typically around -70 mV, where the inside is more negative compared to the outside.

Ion Concentration

  • At the resting state, sodium ions are more concentrated outside the neuron, while potassium ions are more concentrated inside, establishing the electrochemical gradients necessary for neuronal signaling.

Neural Signaling

Resting Membrane Potential

  • All cells possess a resting membrane potential; however, neurons have developed specialized mechanisms to utilize these potentials for communication and signal propagation.

Graded Potentials (GP)

  • Graded potentials are localized changes in membrane potential resulting from stimulus-induced openings of ion channels. They can vary in magnitude depending on the strength of the stimulus.

  • These potentials can summate if multiple stimuli occur simultaneously or in rapid succession, potentially leading to the generation of an action potential if the threshold is crossed.

Action Potentials (AP)

  • Action potentials are described as an all-or-nothing response generated when the depolarization threshold is reached, causing a rapid rise in membrane potential.

  • They propagate along the axon to the axon terminals, facilitating long-distance communication within the nervous system.

Ion Movement in Action Potentials

Mechanisms of Action Potentials

  • The initiation of an action potential begins with the opening of voltage-gated sodium channels, allowing sodium to enter the cell, leading to rapid depolarization.

  • Following depolarization, sodium channels become inactivated, and voltage-gated potassium channels open, allowing potassium to exit the cell, facilitating repolarization of the membrane.

  • The final phase of an action potential includes a temporary undershoot due to the continued exit of potassium ions before the membrane potential is restored to resting state.

Key Vocabulary

  • Resting Membrane Potential: The voltage across the neuronal membrane when at rest, essential for the generation of action potentials.

  • Graded Potential: A small, localized change in membrane potential triggered by stimuli that can either depolarize or hyperpolarize the neuron.

  • Action Potential: A rapid and substantial change in voltage that propagates down the axon, crucial for neuronal communication.

  • Voltage-gated Channels: Specialized ion channels that open in response to changes in membrane voltage, playing a critical role in the generation of action potentials.

Ligand-gated channels generally have two gates associated with their functioning:

  1. Activation Gate (or Open Gate): This gate opens when a specific ligand, such as a neurotransmitter, binds to the receptor site on the channel. When this gate is open, it allows ions to flow through the channel, leading to changes in the membrane potential of the cell.

  2. Inactivation Gate (or Close Gate): After the channel has opened and ions have flowed through, the inactivation gate closes shortly after. This ensures that the channel does not remain open indefinitely and helps to reset the channel for future signaling.

Together, these two gates allow ligand-gated channels to control ion flow in response to specific signals, playing a crucial role in neural communication.