Chemical Messengers and Receptor Properties in Physiology

Chemical Messengers in Physiology

Neurohormones

Neurohormones are a specialized class of hormones produced by neurons and released into the bloodstream, allowing them to act on distant target cells. This mechanism is essential for integrating nervous and endocrine system functions.

• Definition: Neurohormones are chemical messengers synthesized by neurosecretory cells and secreted directly into the blood.

• Examples:

  • Antidiuretic hormone (ADH) – regulates water balance in the body.

  • Oxytocin – involved in childbirth and lactation.

• Mechanism: Neurohormones travel through the blood to reach target cells, similar to classic hormones but originate from neurons.

Messenger & Receptor Properties

Specificity of Messenger and Receptor

Cell signaling relies on the specificity between chemical messengers and their receptors. Only target cells with the appropriate receptor can respond to a given messenger.

• Receptor Specificity: Each receptor binds only to its specific messenger (ligand).

• Target Cell Response: A cell's response depends on the presence of the correct receptor for the messenger.

• Example: Messenger 1 binds to Receptor A, while Messenger 2 binds to Receptor B; only cells with the correct receptor will respond.

Classification of Chemical Messengers

Classification by Chemical Class

Chemical messengers are classified based on their chemical properties, which determine their location of receptors and functional class. The main distinction is between lipophobic (water-soluble) and lipophilic (fat-soluble) messengers.

• Lipophobic Messengers:

  • Cannot cross the plasma membrane easily; receptors are located on the cell surface.

  • Include neurotransmitters and some paracrines.

• Lipophilic Messengers:

  • Can cross the plasma membrane; receptors are located in the cytosol or nucleus.

  • Include steroid hormones and some paracrines.

Chemical Property	Location of Receptors on Target Cell	Functional Class
Lipophobic	Plasma membrane	Neurotransmitters
Lipophobic	Plasma membrane	Paracrines, neurohormones
Lipophilic	Cytosol	Hormones
Lipophilic	Cytosol	Paracrines
Lipophobic	Plasma membrane	Paracrines, neurotransmitters

Additional info: Some amines, although lipophilic, have receptors in the nucleus of target cells.

Signal Transduction Mechanisms

Types of Signal Transduction

Signal transduction refers to the process by which a cell converts a chemical messenger's signal into a functional response. There are two major types based on the chemical nature of the messenger.

• Membrane-bound (Lipophobic) Receptor-Mediated Responses:

  • Receptors are located on the plasma membrane.

  • Responses include opening ion channels or activating enzymes.

  • Types of membrane receptors:

    • Channel-linked receptors

    • Enzyme-linked receptors

    • G protein–linked receptors

• Intracellular (Lipophilic) Receptor-Mediated Responses:

  • Receptors are located in the cytosol or nucleus.

  • Responses typically involve gene activation and changes in protein synthesis.

Organization of the Nervous System

Functional Classes of Neurons

The nervous system is organized into different functional classes of neurons, each with distinct roles in processing and transmitting information.

• Afferent Neurons: Carry sensory information from the body to the central nervous system (CNS).

• Efferent Neurons: Transmit motor commands from the CNS to muscles and glands.

• Interneurons: Connect neurons within the CNS and integrate sensory and motor information.

Organization of Neurons

Neurons are organized into groups based on their location and function.

• Cell Bodies (Nuclei): Groups of neuron cell bodies within the CNS.

• Pathways, Tracts, Commissures: Bundles of axons within the CNS.

• Cell Bodies (Ganglia): Groups of neuron cell bodies outside the CNS.

• Nerves: Bundles of axons outside the CNS.

Additional info: This organization is essential for efficient communication and processing within the nervous system.