Human Physiology: Neural Communication and the Nervous System (Chapters 6 & 8 Study Guide)

Chapter 6: Neural Communication and Signal Transduction

Electrical vs. Chemical Signals

Cells communicate using both electrical and chemical signals, each with distinct mechanisms and roles in physiology.

• Electrical signals: Involve changes in membrane potential, typically seen in neurons and muscle cells.

• Chemical signals: Involve the release and detection of molecules such as neurotransmitters, hormones, and neurocrine molecules.

• Example: Neurons use electrical signals to transmit information along their axons, and chemical signals to communicate across synapses.

Forms of Local Communication

Local communication refers to signaling between cells in close proximity. There are three main forms:

• Hormones: Chemical messengers released by endocrine glands, travel through the bloodstream to distant targets. Example: Insulin

• Neurocrine molecules: Chemicals released by neurons, including neurotransmitters and neurohormones.

• Neurotransmitters: Chemicals released at synapses to transmit signals between neurons or from neurons to muscles. Example: Acetylcholine

• Neurohormones: Neurocrine molecules released into the blood by neurons, acting like hormones. Example: Vasopressin

General Signal Pathway

Signal transduction involves a sequence of events from signal reception to cellular response.

• Outline: Signal molecule (ligand) binds to receptor → Activation of intracellular signaling pathways → Cellular response.

• Example: Epinephrine binding to a G protein-coupled receptor, activating cAMP as a second messenger.

Lipophilic vs. Lipophobic Signal Molecules

Signal molecules can be classified based on their solubility:

• Lipophilic (fat-soluble): Can cross cell membranes easily; receptors are often intracellular.

• Lipophobic (water-soluble): Cannot cross membranes; receptors are on the cell surface.

First Messenger and Second Messenger

In signal transduction, the first messenger is the extracellular signal molecule, while the second messenger is an intracellular molecule that relays the signal.

• First messenger: The original signaling molecule (e.g., hormone, neurotransmitter).

• Second messenger: Molecules such as cAMP, Ca2+, or IP3 that propagate the signal inside the cell.

• Equation: $\text{Signal} \rightarrow \text{Receptor} \rightarrow \text{Second Messenger} \rightarrow \text{Response}$

Transducer

A transducer is a device or molecule that converts one form of energy or signal into another. In cells, receptors act as transducers by converting extracellular signals into intracellular responses.

Types of Gated Channels

Gated channels regulate ion flow across membranes and are classified by their activation mechanism:

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

• Ligand-gated: Open when a specific chemical binds to the channel.

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

Agonist vs. Antagonist

Agonists and antagonists are molecules that interact with receptors:

• Agonist: Binds to a receptor and activates it, mimicking the natural ligand.

• Antagonist: Binds to a receptor but blocks its activation, preventing the natural ligand from binding.

Control Systems: Speed and Specificity

Physiological control systems differ in speed and specificity:

• Neural control: Fast, specific, short duration.

• Endocrine control: Slower, less specific, longer duration.

• Neuroendocrine: Combines features of both systems.

Stimulus Intensity Coding

Cells code stimulus intensity by varying the frequency of action potentials or the amount of neurotransmitter released.

• Example: Stronger stimuli produce higher frequency action potentials.

Chapter 8: The Nervous System and Neurons

Divisions of the Peripheral Nervous System (PNS)

The PNS is divided into two main branches:

• Somatic nervous system: Controls voluntary movements via skeletal muscles.

• Autonomic nervous system: Regulates involuntary functions (e.g., heart rate, digestion).

Structure and Function of a Neuron

Neurons are specialized cells for communication in the nervous system.

• Dendrites: Receive incoming signals.

• Cell body (soma): Integrates signals and contains the nucleus.

• Axon: Transmits electrical impulses away from the cell body.

• Axon terminals: Release neurotransmitters to communicate with other cells.

Types of Neurons

• Interneurons: Connect neurons within the CNS.

• Afferent (sensory) neurons: Carry information to the CNS.

• Efferent (motor) neurons: Carry commands from the CNS to effectors.

Synapse: Presynaptic vs. Postsynaptic Cells

A synapse is the junction between two neurons or a neuron and another cell.

• Presynaptic cell: The neuron sending the signal.

• Postsynaptic cell: The cell receiving the signal.

• Synaptic cleft: The gap between the presynaptic and postsynaptic cells.

Glial Cells: Types and Functions

Glial cells support and protect neurons. Main types include:

• Astrocytes: Maintain the blood-brain barrier, regulate nutrients.

• Oligodendrocytes: Myelinate axons in the CNS.

• Schwann cells: Myelinate axons in the PNS.

• Microglia: Act as immune cells in the CNS.

Myelination and Myelinating Cells

Myelination is the process of wrapping axons with myelin, a fatty substance that increases signal speed.

• Oligodendrocytes: Myelinate CNS axons.

• Schwann cells: Myelinate PNS axons.

Nodes of Ranvier

Nodes of Ranvier are gaps in the myelin sheath where action potentials are regenerated, allowing rapid signal transmission.

Microglia

Microglia are specialized immune cells in the CNS that protect against pathogens and remove debris.

Ion Concentrations Across the Cell Membrane

Different ions are distributed unevenly across the cell membrane:

• K+ (Potassium): Higher concentration inside the cell.

• Na+ (Sodium): Higher concentration outside the cell.

Graded Potentials vs. Action Potentials

Neurons use two types of electrical signals:

• Graded potentials: Small, variable changes in membrane potential, typically occur in dendrites and cell bodies.

• Action potentials: Large, all-or-none signals that travel along axons.

• Equation: $\text{Action Potential:} \quad V_m = V_{rest} + \Delta V$

Location of Graded and Action Potentials

• Graded potentials: Occur in dendrites and cell bodies.

• Action potentials: Originate at the axon hillock and propagate along the axon.

All-or-None Principle of Action Potentials

Action potentials follow the all-or-none principle: once threshold is reached, the action potential always occurs with the same amplitude.

• Perception of intensity: Intensity is coded by the frequency of action potentials, not their size.

Table: Comparison of Graded Potentials and Action Potentials

Feature	Graded Potential	Action Potential
Location	Dendrites, cell body	Axon
Amplitude	Variable	All-or-none
Propagation	Decreases with distance	Maintained along axon
Function	Local signaling	Long-distance signaling

Additional info: Academic context and definitions have been expanded for clarity and completeness.