Nervous Tissue Physiology: Structure, Function, and Signal Transmission

Nervous Tissue Physiology

Organization of the Nervous System

The nervous system is organized into two main divisions: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS consists of the brain and spinal cord, while the PNS includes cranial and spinal nerves. The PNS is further divided into sensory (afferent) and motor (efferent) divisions. The motor division is subdivided into the somatic (voluntary) and autonomic (involuntary) systems, with the autonomic system split into sympathetic and parasympathetic branches.

Structure of Neurons

Neurons are the fundamental units of the nervous system, specialized for transmitting electrical and chemical signals. Key structural components include the cell body (soma), dendrites, axon, myelin sheath, nodes of Ranvier, and synaptic terminals. Myelin, produced by oligodendrocytes in the CNS, increases the speed of signal transmission.

Neuronal Function: Signal Transmission

Neurons communicate by changing the voltage across their cell membranes, resulting in the release of neurotransmitters at synaptic terminals. The process involves presynaptic and postsynaptic membranes, with the synaptic cleft serving as the site of neurotransmitter release.

Membrane Potentials

The resting membrane potential is the voltage difference across the neuronal membrane when the cell is not actively sending a signal. This potential is established by concentration and charge gradients of ions, primarily sodium (Na+), potassium (K+), and chloride (Cl-).

• Equilibrium potential: The membrane potential at which there is no net movement of a particular ion.

• Typical values: Na+ = +62 mV, K+ = -90 mV, Cl- = -70 mV.

• Resting membrane potential depends on ion concentration differences and permeability.

Ion Channels and Membrane Permeability

Ions move across the membrane through various channels:

• Ligand-gated channels: Open in response to binding of a specific molecule (e.g., neurotransmitter).

• Mechanically-gated channels: Open in response to physical deformation of the membrane.

• Voltage-gated channels: Open in response to changes in membrane potential.

• Leak channels: Allow ions to move passively, contributing to resting membrane potential.

Overview of Signal Transduction in Neurons

Neuronal signaling involves a sequence of events:

• Resting potential: Baseline state of the neuron.

• Graded potential: Local changes in membrane potential, often occurring on dendrites.

• Action potential: Rapid, all-or-nothing change in membrane potential, propagated along the axon.

• Synaptic activity: Release of neurotransmitter and activation of postsynaptic cell.

Graded Potentials

Graded potentials are initiated by the opening of ion channels on dendrites. Their size is proportional to the intensity of the stimulus and they decay over space and time.

• Ligand-gated and mechanically-gated channels are involved.

• Graded potentials can summate to trigger an action potential if threshold is reached.

Action Potentials

Action potentials are initiated when the membrane depolarizes to a threshold voltage (typically -55 mV). They are all-or-nothing events, self-propagating, and utilize voltage-gated Na+ and K+ channels.

• Absolute refractory period: Time during which a new action potential cannot be initiated.

• Saltatory conduction: Action potentials jump between nodes of Ranvier in myelinated axons, increasing speed.

Refractory Periods

Refractory periods ensure one-way flow of action potentials and prevent overstimulation.

• Absolute refractory period: Due to inactivation of Na+ channels.

• Relative refractory period: Due to continued outward diffusion of K+.

Synaptic Transmission

Synaptic transmission converts electrical signals into chemical signals via neurotransmitter release. Neurotransmitters bind to receptors on the postsynaptic membrane, opening ion channels and changing the voltage.

• EPSP (Excitatory postsynaptic potential): Opening of Na+ channels causes depolarization.

• IPSP (Inhibitory postsynaptic potential): Opening of K+ or Cl- channels causes hyperpolarization.

Neurotransmitter Receptors

Neurotransmitters act via two main types of receptors:

• Ligand-gated ion channels: Directly open ion channels (e.g., nicotinic acetylcholine receptor).

• G-protein linked receptor proteins: Indirectly affect ion channels via second messengers (e.g., norepinephrine receptors).

Summation of Postsynaptic Potentials

Summation is the process by which multiple graded potentials combine to influence the postsynaptic cell:

• Temporal summation: One presynaptic cell fires repeatedly, increasing neurotransmitter concentration.

• Spatial summation: Multiple presynaptic cells fire simultaneously, combining their effects.

Termination of Synaptic Signals

Neurotransmitter action is terminated by:

• Degradation by enzymes

• Diffusion away from the synaptic cleft

• Uptake by astrocytes or presynaptic membrane

Pharmacological Modulation of the Nervous System

Drugs can affect the nervous system by interfering with neurotransmitter production, secretion, binding, reuptake, or signaling. They may act as agonists (activators) or antagonists (inhibitors).

Key Terms and Concepts

• Neuron: A cell specialized for transmitting electrical and chemical signals.

• Action potential: A rapid change in membrane potential that propagates along the axon.

• Synapse: The junction between two neurons where neurotransmitters are released.

• Neurotransmitter: A chemical messenger released by neurons to communicate with other cells.

• EPSP/IPSP: Excitatory/Inhibitory postsynaptic potentials, which depolarize or hyperpolarize the postsynaptic cell.

Equations

Nernst Equation (for equilibrium potential):

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

Resting Membrane Potential (Goldman-Hodgkin-Katz equation):

$V_{m} = \frac{RT}{F} \ln \left( \frac{P_{K}[K^+]_{out} + P_{Na}[Na^+]_{out} + P_{Cl}[Cl^-]_{in}}{P_{K}[K^+]_{in} + P_{Na}[Na^+]_{in} + P_{Cl}[Cl^-]_{out}} \right)$

Example: The opening of ligand-gated Na+ channels by acetylcholine at the neuromuscular junction causes an EPSP, leading to muscle contraction.

Additional info: Academic context was added to clarify the mechanisms of ion channel function, synaptic transmission, and pharmacological modulation.