Integrative Physiology: The Nervous System and Neuronal Excitability

Integrative Physiology: The Nervous System and Neuronal Excitability

Overview of Content

• Focus on cellular and network properties of the nervous system.
• Exploration of neuronal excitability and the role of electrical signals in communication.
• Detailed examination of neuron functions and types, including synapses and glial cells.

Cellular and Network Properties

Key Concepts

• Cells of the Nervous System: Neurons and glial cells.
• Electrical Signals in Neurons: Action potentials as the primary electrical signal.
• Cell-to-Cell Communication: Mechanisms of synaptic transmission.
• Integration of Neural Information Transfer: Combining signals within the nervous system.

Synonyms in Neuroscience

Organization of the Nervous System

Hierarchical Layout

• Central Nervous System (CNS): Comprises the brain and spinal cord.
  • Components:
    • Sensory receptors receive signals.
    • Afferent pathways transmit sensory information.
    • Integrating center processes information.
    • Efferent pathways send impulses to effectors.
    • Effectors include various muscles and glands responsible for a response.
• Peripheral Nervous System (PNS): All nerves connecting to the CNS.
  • Divisions:
    • Afferent Division: Carries sensory information to the CNS.
      • Divided into somatic senses and special senses.
    • Efferent Division: Carries motor commands from the CNS.
      • Includes somatic nervous system and autonomic nervous system (further divided into sympathetic and parasympathetic systems, as well as enteric nervous system).

Model Neuron

Anatomy of a Neuron

• Dendrites: Receive incoming signals.
• Cell Body (Soma): Contains the nucleus and integrates synaptic input.
• Axon: Transmits outgoing information; includes myelin sheath and axon terminal.
• Synapse: The junction between neurons, involving a synaptic cleft and postsynaptic neuron.

Neuron Classification

• By Function:
  1. Sensory Neurons: Carry signals from sensory receptors to the CNS.
  2. Interneurons: Process and relay information within the CNS.
  3. Efferent (Motor Neurons): Transmit signals from the CNS to effectors (muscles and glands).
• By Structure: Classified as multipolar, bipolar, or unipolar, but multipolar are most common in the CNS.

Glial Cells and Their Functions

Overview of Glial Cells

• Functions:
  • Physical and biochemical support for neurons.
  • Supply nutrients and maintain extracellular environment.
  • Form the blood-brain barrier and create myelin sheaths around neurons.
  • Act as scavengers to remove debris.
  • Have roles in repair and regeneration of neural tissues.
• Types of Glial Cells in CNS:
  • Astrocytes: Support neurons, maintain blood-brain barrier.
  • Oligodendrocytes: Produce myelin sheaths in CNS.
  • Microglia: Immune response in the CNS, act as phagocytes.
  • Ependymal cells: Line brain ventricles and form the choroid plexus, producing cerebrospinal fluid (CSF).

Myelin Sheath Functionality

Overview of Myelin

• Components:
  • Schwann Cells: Form myelin in PNS.
  • Oligodendrocytes: Form myelin in CNS.
• Functions:
  • Provide electrical insulation.
  • Increase speed of conduction of action potentials.
  • Present in both CNS and PNS.
  • Include nodes of Ranvier which are critical for rapid signal transmission.

Resting Membrane Potential

Definition and Importance

• Resting Membrane Potential: The electrical potential difference across the plasma membrane of a neuron when not actively transmitting an impulse.
• Typical Value: Approximately -70 mV, results from differences in ion concentrations, particularly K+ and Na+.
• Ion Concentration:
  • K+ is high intracellularly, Na+ high extracellularly.
  • Maintained by the Na+/K+ ATPase pump which typically moves 3 Na+ out and 2 K+ in.

Ion Channels and their Types

Leak Channels

• Function: Randomly alternate between open and closed states allowing ions (like K+) to move across the membrane.

Ligand-gated Channels

• Response Mechanism: Open or close in response to a specific chemical stimulus.
  • Example: Acetylcholine binds to receptors, opening channels and allowing Na+ influx.

Mechanically-gated Channels

• Triggers: Open in response to mechanical forces like touch or pressure.

Voltage-gated Channels

• Function: Open when the membrane potential reaches a critical threshold, enabling action potentials.
  • Example: Voltage-gated Na+ channels activate causing depolarization.

Graded Potentials Instead of Action Potentials

Definition and Features

• Graded Potentials: Small localized changes in membrane potential that can be depolarizing or hyperpolarizing.
• They decrease in strength as they spread from the origin.
• Can summate if close together in both space and time, leading to potential action potential generation.

Action Potentials

• Definition: A rapid rise and subsequent fall in voltage or membrane potential across a cellular membrane.
  • Represents a neural signal that propagates along the axon.
• The all-or-nothing principle states that once the threshold is reached, an action potential will occur.
• Sequential opening and closing of voltage-gated channels generate the action potential’s characteristic spike.

Factors Affecting Action Potential Speed

• Diameter of Axon: Larger diameters allow faster conduction.
• Myelination: Myelin sheath allows faster conduction via saltatory conduction.

Chemical Signaling at Synapses

Neurotransmission Steps

1. Action potential arrives at the axon terminal causing voltage-gated calcium channels to open.
2. Ca2+ influx triggers synaptic vesicles to fuse with the membrane and release neurotransmitters into the synaptic cleft via exocytosis.
3. Neurotransmitters bind to receptors on the postsynaptic neuron, initiating a response (either EPSP or IPSP).

Types of Neurotransmitters

• Classes of Neurocrines: Include acetylcholine, amines, amino acids, purines, gases, peptides, and lipids.
• Examples:
  • Amino Acids: Glutamate (excitatory), GABA (inhibitory).
  • Biogenic Amines: Norepinephrine, dopamine, serotonin.
  • Gases: Nitric oxide (NO), carbon monoxide (CO).

Inhibition in Neural Communication

• EPSP (Excitatory Postsynaptic Potential): Electrical change that depolarizes the postsynaptic cell.
• IPSP (Inhibitory Postsynaptic Potential): Makes the postsynaptic cell more negative, less likely to fire.

Integration in Neural Networks

Spatial Summation

• Graded potentials from multiple presynaptic inputs combine to reach threshold at the axon trigger zone, resulting in action potential generation.

Temporal Summation

• Summation of subthreshold graded potentials arriving in quick succession can also lead to action potentials.

Presynaptic and Postsynaptic Inhibition

• Presynaptic Inhibition: Reduces the amount of neurotransmitter released from a presynaptic neuron, decreasing the postsynaptic response.
• Postsynaptic Inhibition: Results in postsynaptic hyperpolarization, thus no action potential is generated.

Divergent and Convergent Pathways

• Divergence: One neuron influences multiple neurons.
• Convergence: Multiple neurons synapse onto a single neuron, allowing integration of diverse signals.

Conclusion

• Understanding the anatomy of neurons, their classification, electrical signals propagation mechanisms, and synaptic transmission processes is crucial to mastering integrative physiology and the functions of the nervous system.