Neural Signalling Material

• Neurons: 


  The Electrical Messengers of the Nervous System

  Neurons are specialized cells within the nervous system that carry electrical impulses, acting as the body's communication network. 
• Structure of a Neuron

  1. Cell Body (Soma):
  2. Axon:
  3. Dendrites:
• Cell Body (Soma):

  • Contains the nucleus and cytoplasm
  • Houses organelles essential for cell function
• Axon:

  • Single, long fiber extending from the cell body
  • Conducts electrical impulses away from the cell body.

  The length of axons can vary greatly, from less than a millimeter to over a meter in some cases, allowing neurons to connect distant parts of the body.
• Dendrites:


  • Multiple shorter fibers branching from the cell body
  • Receive signals from other neurons or sensory receptors
• Electrical Impulse Conduction

  Neurons transmit information through electrical impulses, also known as action potentials. These impulses travel along the nerve fibers in the following manner:

  1. Dendrites receive signals
  2. Signals are integrated in the cell body
  3. If the threshold is reached, an action potential is generated
  4. The action potential travels along the axon
• Nerve Impulses:

  Action Potentials in Motion

  Nerve impulses, also known as action potentials, are the electrical signals that propagate along nerve fibers.
• What is an Action Potential?

  An action potential is a brief, rapid change in the electrical potential across a neuron's membrane. It's the fundamental unit of communication in the nervous system.
• The Electrical Nature of Nerve Impulses

  Nerve impulses are electrical because they involve the movement of charged particles, specifically positively charged ions, across the cell membrane
• Key Ions Involved

  1. Sodium ions (Na+)
  2. Potassium ions (K+)

  These positively charged ions move in and out of the neuron, creating changes in the electrical potential across the membrane.
• Stages of an Action Potential

  1. Resting State: The neuron is polarized (negative inside, positive outside)
  2. Depolarization: Na+ channels open, Na+ rushes in, making the inside more positive
  3. Repolarization: K+ channels open, K+ rushes out, restoring negative charge inside
  4. Hyperpolarization: Brief period where the neuron is more negative than resting state
  5. Return to Resting State: Ion pumps restore original ion concentrations
• Propagation Along Nerve Fibers

  Action potentials propagate along nerve fibers through a process of continuous regeneration:

  1. An action potential at one point triggers neighboring areas to depolarize
  2. This creates a wave of depolarization that travels along the axon
  3. The process is self-propagating and unidirectional
• The speed of action potential propagation can vary greatly, from as slow as 0.5 m/s in small unmyelinated fibers to as fast as 120 m/s in large myelinated fibers.
• The Resting Potential: A Delicate Ionic Balance

  The resting potential is a crucial concept in understanding how neurons function. It's the electrical state of a neuron when it's not actively transmitting a signal.
• Establishing Concentration Gradients

  The resting potential is generated and maintained by the active transport of ions across the neuron's membrane, creating concentration gradients:

  1. Sodium-Potassium Pump (Na+/K+ ATPase):

  • Pumps 3 Na+ ions out of the cell
  • Pumps 2 K+ ions into the cell
  • Uses energy from ATP hydrolysis
• Concentration Gradients at Rest

  • High Na+ concentration outside the cell
  • High K+ concentration inside the cell
• Membrane Polarization and Potential


  Membrane polarization refers to the separation of electrical charges across the cell membrane. The membrane potential is the electrical potential difference between the inside and outside of the cell.
• Why is the Resting Potential Negative?

  1. Unequal ion distribution: More K+ inside, more Na+ outside
  2. Selective permeability: Membrane is more permeable to K+ at rest
  3. Electrochemical gradient: K+ tends to diffuse out of the cell
  4. Negative proteins: Large, negatively charged proteins inside the cell

  The typical resting potential is around -70 mV (inside negative relative to outside).
• The Na+/K+ pump consumes a significant amount of cellular energy, accounting for about 20-40% of the brain's total energy use!
• Variation in Nerve Impulse Speed: 


  Size Matters!

  The speed of nerve impulses can vary significantly depending on several factors.
• Comparing Nerve Fiber Types

  1. Giant Axons of Squid
  2. Smaller Non-myelinated Nerve Fibers
  3. Myelinated Nerve Fibers
• Giant Axons of Squid:


  • Very large diameter (up to 1 mm)
  • Extremely fast conduction speed (up to 25 m/s)
• Smaller Non-myelinated Nerve Fibers:

  • Small diameter (0.2-1.5 μm)
  • Slow conduction speed (0.5-2 m/s)
• Myelinated Nerve Fibers:


  • Medium to large diameter (2-20 μm)
  • Fast conduction speed (3-120 m/s)
• Correlations in Nerve Impulse Speed


  1. Axon Diameter and Conduction Speed
  2. Animal Size and Conduction Speed
• Animal Size and Conduction Speed:

  • Negative correlation
  • Larger animals = slower conduction (relative to body size)
  • Correlation coefficient (r) typically between -0.6 and -0.8
  • If r = -0.7, then R² = 0.49
  • This means 49% of the variation in conduction speed can be explained by animal size
• Axon Diameter and Conduction Speed:

  • Positive correlation
  • Larger diameter = faster conduction
  • Correlation coefficient (r) typically between 0.7 and 0.9
  • If r = 0.8, then R² = 0.64
  • This means 64% of the variation in conduction speed can be explained by axon diameter
• Mathematical Tools for Correlation

  1. Correlation Coefficient (r)
  2. Coefficient of Determination (R²)
• Coefficient of Determination (R²):

  • Square of the correlation coefficient
  • Ranges from 0 to 1
  • Indicates the proportion of variance in the dependent variable explained by the independent variable
• Correlation Coefficient (r):

  • Ranges from -1 to +1
  • -1: perfect negative correlation
  • 0: no correlation
  • +1: perfect positive correlation
• The giant axon of the squid, which can be up to 1 mm in diameter, was crucial in early neuroscience research due to its large size, allowing for easier study of action potentials.
• Synapses:

  The Communication Hubs of the Nervous System

  Synapses are specialized junctions where information is transferred from one neuron to another, or from a neuron to an effector cell (such as a muscle or gland)
• Structure of a Chemical Synapse

  1. Presynaptic terminal: Contains synaptic vesicles filled with neurotransmitters
  2. Synaptic cleft: Narrow gap between pre- and postsynaptic membranes
  3. Postsynaptic membrane: Contains receptors for neurotransmitters
• Unidirectional Signal Transmission


  A key characteristic of chemical synapses is that signals can only pass in one direction:

  1. From the presynaptic neuron to the postsynaptic cell
  2. Never from the postsynaptic cell back to the presynaptic neuron

  This unidirectional flow ensures organized and controlled signal transmission in neural circuits.
• Types of Synapses Based on Target Cells

  1. Neuron-to-Neuron synapses:

  • Most common in the central nervous system
  • Allow for complex information processing

  1. Neuron-to-Effector Cell synapses:

  • Found at neuromuscular junctions (neurons to muscle cells)
  • Present in autonomic nervous system (neurons to gland cells)
• The synaptic cleft is incredibly narrow, typically only about 20-40 nanometers wide. This small distance allows for rapid diffusion of neurotransmitters and precise signal transmission.
• Neurotransmitter Release:

  The Calcium-Triggered Cascade

  The release of neurotransmitters from the presynaptic membrane is a crucial step in synaptic transmission. This process is intricately linked to calcium (Ca²⁺) influx and its role as an intracellular signaling molecule.
• The Calcium-Dependent Release Process

  1. Action Potential Arrival: Depolarization reaches presynaptic terminal
  2. Calcium Channel Activation: Voltage-gated Ca²⁺ channels open. Ca²⁺ rapidly enters the presynaptic terminal
  3. Calcium as a Signaling Molecule: Intracellular Ca²⁺ concentration rises. Ca²⁺ binds to specific proteins (e.g., synaptotagmin)
  4. Vesicle Fusion: Ca²⁺-sensitive proteins trigger SNARE complex activation. Synaptic vesicles fuse with presynaptic membrane
  5. Neurotransmitter Release: Vesicle contents expelled into synaptic cleft
• Calcium's Role as an Intracellular Signal

  Calcium acts as a second messenger within the neuron:

  1. Resting State: Low intracellular Ca²⁺ concentration (~100 nM)
  2. Activated State: High intracellular Ca²⁺ concentration (~1-10 μM)
  3. Signaling Cascade:

  • Ca²⁺ binds to calmodulin
  • Ca²⁺/calmodulin complex activates various enzymes

  1. Cellular Responses:

  • Neurotransmitter release
  • Gene expression changes
  • Synaptic plasticity
• The speed of neurotransmitter release is remarkably fast, occurring within less than a millisecond after calcium influx. This rapid response is crucial for precise neural signaling.