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Neurophysiology: Neural Signals

Neurophysiology: Neural Signals (Page 1)

This section introduces the fundamental concepts of neurophysiology, focusing on how neurons generate and transmit electrical signals.

Clinical Case Study: Sheila (UK) (Page 2)

Sheila, a patient from the UK, presented with a clinical scenario highlighting the severe consequences of neurological dysfunction. Initially, she sustained a leg cut in her English garden, which received standard emergency care (cleaning and stitching). However, three days later, she returned to the ER exhibiting concerning symptoms:

  • Face ache and difficulty opening her mouth (trismus).

  • Generalized unwell appearance and diffuse pain.
    Her condition rapidly deteriorated within 24 hours, leading to:

  • Increased jaw stiffness.

  • Severe back and limb spasms.
    She was subsequently transferred to the Intensive Care Unit (ICU).

This case strongly suggests a diagnosis of tetanus, an acute, often fatal, disease caused by an exotoxin produced by Clostridium tetani, which typically enters the body through a contaminated wound. The toxin interferes with neurotransmission, leading to uncontrolled muscle contractions and spasms, illustrating the critical importance of proper neural signaling.

The Big Picture: Components of the Nervous System (Page 3)

The study of neural signals involves examining structures at various magnifications, from macroscopic views to electron microscopic details.

  • Microscopy: Both electron microscopes and light microscopes are employed to visualize the intricate structures of nerve cells.

  • Nerve Cells (Neurons): The fundamental units of the nervous system responsible for transmitting electrical and chemical signals.

  • Parts of Neurons: Key components include dendrites, soma (cell body), axon, and axon terminals.

  • Synapse: The specialized junction between two neurons where information is transmitted. It consists of a presynaptic neuron, a synaptic cleft, and a postsynaptic neuron.

  • Synaptic Cleft: A tiny gap (approximately 5 ext{ nm} wide) separating the presynaptic and postsynaptic membranes, across which neurotransmitters diffuse.

  • Neuronal Membrane: The outer boundary of the neuron, crucial for maintaining ion gradients and regulating signal transmission.

  • Ion Channel: Protein pores within the neuronal membrane that allow specific ions to pass through, essential for electrical signaling.

Simple Ion Forces Permit Electrical Signaling (Page 4)

Electrical signaling in neurons is governed by two primary forces acting on charged ions across a semipermeable membrane (analogous to a screen door):

  1. Diffusion: This force drives ions from an area of high concentration to an area of low concentration, moving them along their concentration gradient.

  2. Electrostatic Pressure: This force drives ions towards areas of opposite charge, moving them along their electrical gradient. For instance, positively charged ions (cation) are attracted to negatively charged areas, and negatively charged ions (anion) are attracted to positively charged areas.

The Resting Potential (Page 5)

Neurons behave like biological batteries, storing an electrical charge that is utilized for signal transmission. The resting potential is the electrical potential difference across the neuronal membrane when the neuron is not actively signaling.

  • Measurement: The resting potential is measured using a microelectrode inserted into the axon and a reference electrode placed outside. An amplifier is used to record the voltage difference.

  • Voltage: When the microelectrode enters the cell, the potential inside the axon typically drops from 0 ext{ mV} (outside) to a negative value, usually between -60 ext{ mV} and -90 ext{ mV}. This negative charge inside the cell relative to the outside is the resting potential.

Cell Membrane and Ion Channels (Page 6)

  • Cell Membrane Composition: The neuronal cell membrane is a lipid bilayer. Its hydrophobic (water-repelling) nature means ions, which are surrounded by water molecules (hydrated ions), cannot directly pass through.

  • Ion Channels: These are specialized proteins that span the lipid bilayer, creating aqueous pores through which ions can pass in and out of the cell.

  • Gated Channels: Many ion channels are