Nerve Cells Study Notes

NERVE CELLS Study Notes

A. Introduction to the Nervous System

  • Contextual Knowledge Required: Understanding of basic concepts from biochemistry, cell biology, ion channels, and electrochemical gradients.

  • Learning Objectives:

    • Mechanism of action potential movement.

    • Transmission of nerve signals from one neuron to another.

    • Integration of information from various inputs by neurons.

B. Functions of the Nervous System

  1. Rapid Communication:

    • Electrical communication is significantly faster than chemical communication.

    • Sensory inputs from receptors are transmitted rapidly to the central nervous system (CNS).

    • Motor outputs rapidly travel to effector cells (muscles, glands).

  2. Integration of Information:

    • Neurons process multiple input signals (additive and subtractive), leading to a singular output decision.

C. Cells of the Nervous System

  1. Neurons: Principal signaling cells of the nervous system.

    • Types of Neurons:

      • Sensory Neurons: Transmit information from sensory receptors to the CNS.

      • Motor Neurons: Convey signals from the CNS to muscles or glands.

      • Interneurons: Facilitate communication between sensory and motor neurons.

    • Structure of Neurons:

      • Cell Body: Contains the nucleus and organelles.

      • Dendrites: Branch-like structures that receive messages and transmit them to the cell body.

      • Axon: Long structure that carries messages away from the cell body.

  2. Glial Cells: Supportive cells in the nervous system that do not conduct nerve impulses.

    • Types of Glial Cells:

      • Schwann Cells: Form the myelin sheath around axons in the peripheral nervous system (PNS).

      • Oligodendrocytes: Form the myelin sheath around axons in the central nervous system (CNS).

      • Astrocytes: Form the blood-brain barrier, regulating blood flow to neurons, and provide structural support.

      • Microglia: Act as macrophages, cleaning up dead cells and pathogens in the nervous system.

D. Transmission of Signals in Neurons

  1. Membrane Potential:

    • Defined as the voltage difference across the neuronal membrane, created by the separation of charges (positive and negative).

    • Electrochemical Gradient: Influences the movement of ions across the neuronal membrane.

  2. Ion Channels:

    • Allow passive movement of ions according to their electrochemical gradient.

    • Specific for each type of ion, may be gated (open or close under certain conditions).

    • Function rapidly to facilitate signal transmission.

  3. Resting Potential:

    • The resting membrane potential is maintained at approximately -70 mV.

    • Indicates the interior of the neuron is negatively charged compared to the exterior.

  4. Action Potential:

    • Triggers when depolarization of the membrane exceeds a specific threshold.

    • Associated with a rapid rise and fall in membrane potential due to the opening of voltage-gated channels.

  5. Voltage-Gated Channels:

    • Two types of voltage-gated channels are necessary for action potentials:

      • Sodium Channels: Allow Na+ ions to enter, leading to depolarization.

      • Potassium Channels: Allow K+ ions to exit, aiding in repolarization.

    • Graded Potentials:

      • Graded hyperpolarization (making the cell more negative) and graded depolarization (making the cell less negative) precede the action potential.

  6. Unidirectionality of Transmission:

    • Action potentials propagate in one direction due to the delayed reopening of the Na+ channel, ensuring that the signal only moves forward along the axon.

  7. Conduction Velocity:

    • Factors influencing speed include:

      • Larger axon diameter (less resistance to electrical flow).

      • Presence of myelinated axons and Nodes of Ranvier (facilitating saltatory conduction).

E. Transmission of Signals Between Neurons

  1. Synapse:

    • Junctions between neurons for signal transmission.

  2. Chemical Synapse:

    • Involves diffusion of neurotransmitters from the presynaptic neuron to receptors on the postsynaptic neuron.

    • Mechanism:

      • Action potentials open Ca2+ channels at the synaptic terminal, leading to neurotransmitter release into the synaptic cleft.

      • Detection of neurotransmitters opens ion channels in the postsynaptic membrane, affecting its potential:

      • Na+ channel -> leads to depolarization of the postsynaptic cell.

      • Cl– channel -> leads to hyperpolarization of the postsynaptic cell.

  3. Neurotransmitter Inactivation:

    • Neurotransmitters are inactivated rapidly to reset the postsynaptic cell for future signals.

    • Various types of neurotransmitters exist, each exerting different effects based on the ion channels they influence.

  4. Signal Integration:

    • Summation: Process by which one postsynaptic cell integrates signals from many presynaptic cells.

    • Postsynaptic potentials:

      • Excitatory Postsynaptic Potential (EPSP): Makes the postsynaptic cell less negative (more likely to trigger action potential).

      • Inhibitory Postsynaptic Potential (IPSP): Makes the postsynaptic cell more negative (less likely to trigger action potential).

    • Neurons decide to fire an action potential based on the summation of these potentials, particularly at the axon hillock.

  5. Electrical Synapses:

    • Comprise direct cytoplasmic connections (gap junctions) between cells.

    • Advantages: Extremely fast signal transmission.

    • Disadvantages: Lack of integration capability for incoming signals.

F. Important Terms and Concepts

  • Neurons:

    • Types: Sensory neurons, motor neurons, interneurons.

    • Structure: Cell body, dendrites, axon.

  • Glial Cells: Schwann cells, oligodendrocytes, astrocytes, microglia.

  • Membrane Potential & Ion Channels:

    • Resting potential, action potential, gated ion channels.

  • Synapses:

    • Chemical synapse, presynaptic cell, postsynaptic cell, synaptic cleft, neurotransmitter.

  • Signal Integration: Summation, EPSP, IPSP, axon hillock, electrical synapses.

G. Study Questions & Answers

  1. What is the function of the myelin sheath surrounding the axons of neurons?

    • The sheath creates insulated zones along the axon, preventing membrane depolarization in these regions and forcing depolarization to occur at the Nodes of Ranvier, which accelerates signal transmission.

  2. What is the direction of the chemical and electrochemical gradients for an anion with high intracellular concentration?

    • Chemical gradient is from inside to outside; electrochemical gradient is also from inside to outside.

  3. Movement of Na+ and K+ during depolarization and repolarization phases.

    • Na+: Moves with both electrical and chemical gradients upon channel opening.

    • K+: Moves with both electrical and chemical gradients during repolarization.

  4. How does Na+ leakage affect resting potential?

    • Increased Na+ would depolarize the membrane, making resting potential more positive.

  5. What ensures one-way transmission of action potentials?

    • The slow opening of the Na+ channel inactivation gate prevents immediate reopening after activation.

  6. Factors affecting conduction speed along an axon?

    • Formation of myelin sheaths and larger axon diameters.

  7. Gating differences in ion channels for action potentials vs. postsynaptic responses?

    • Voltage-gated for action potentials; ligand-gated (chemically gated) for postsynaptic responses.

  8. Why is synaptic transmission unidirectional?

    • Neurotransmitter release occurs exclusively from presynaptic vesicles, while receptors are only on the postsynaptic membrane.

  9. How do neurons integrate conflicting signals?

    • By summing excitatory and inhibitory impulses at the axon hillock, an action potential is triggered if net depolarization reaches the threshold.

  10. Why must neurotransmitters be quickly degraded?

    • To avoid continuous stimulation of the postsynaptic cell, allowing it to be responsive to subsequent signals.