Chapter 6: Neural Communication and Signal Transduction

Electrical vs. Chemical Signals

Cells in the body communicate using both electrical and chemical signals. Understanding the differences between these types of signals is fundamental to physiology.

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

• Chemical signals: Involve the release of molecules (such as neurotransmitters or hormones) that bind to receptors on target cells.

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

Forms of Local Communication

Cells communicate locally through several mechanisms. Three main forms include:

• Paracrine signals: Chemical messengers that act on nearby cells.

• Autocrine signals: Chemical messengers that act on the same cell that secreted them.

• Synaptic (neurotransmitter) signals: Neurotransmitters released from neurons to act on adjacent target cells.

• Example: Nitric oxide acts as a paracrine signal in blood vessels.

Types of Chemical Messengers

• Hormones: Secreted by endocrine glands, travel through the bloodstream to distant targets.

• Neurocrine molecules: General term for chemicals secreted by neurons, including neurotransmitters and neurohormones.

• Neurotransmitters: Chemicals released by neurons at synapses to transmit signals to other neurons or effector cells.

• Neurohormones: Chemicals released by neurons into the blood, acting like hormones (e.g., vasopressin).

Signal Pathway Overview

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

1. Signal molecule (ligand) binds to a receptor protein.

2. Receptor activates intracellular signal molecules.

3. Intracellular signals alter target proteins to create a response.

Additional info: This is often called the "signal transduction pathway."

Lipophilic vs. Lipophobic Signal Molecules

• Lipophilic molecules: Can cross cell membranes easily; often bind to intracellular receptors (e.g., steroid hormones).

• Lipophobic molecules: Cannot cross cell membranes; bind to receptors on the cell surface (e.g., peptide hormones).

First Messenger and Second Messenger

• First messenger: The extracellular signal molecule (e.g., hormone or neurotransmitter).

• Second messenger: Intracellular molecules that relay signals from receptors to target molecules inside the cell (e.g., cAMP, Ca2+).

• Example: Epinephrine (first messenger) binds to a receptor, activating cAMP (second messenger).

Transducer

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

Types of Gated Channels

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

• Ligand-gated: Open or close when a specific chemical (ligand) binds.

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

Agonist vs. Antagonist

• Agonist: A molecule that binds to a receptor and activates it, mimicking the effect of the natural ligand.

• Antagonist: A molecule that binds to a receptor but does not activate it, blocking the action of the natural ligand.

Control Systems: Speed, Specificity, and Duration

• Neural control: Fast, specific, short duration.

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

• Neuroendocrine control: Combines features of both systems.

• Example: Reflexes are rapid (neural), while hormonal changes (e.g., growth) are slower and longer-lasting.

Stimulus Intensity Coding

• Stimulus intensity is coded by the frequency of action potentials and the number of neurons activated.

• Stronger stimuli generate higher frequency action potentials.

Chapter 8: The Nervous System and Neurons

Divisions of the Peripheral Nervous System (PNS)

• Somatic nervous system: Controls voluntary movements (skeletal muscles).

• Autonomic nervous system: Controls involuntary functions (smooth muscle, cardiac muscle, glands).

Structure and Function of a Neuron

• Dendrites: Receive incoming signals.

• Cell body (soma): Contains the nucleus and organelles.

• Axon: Conducts 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 from sensory receptors to the CNS.

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

Synapse: Presynaptic vs. Postsynaptic Cells

• Synapse: The junction between two neurons or a neuron and an effector cell.

• Presynaptic cell: The neuron sending the signal.

• Postsynaptic cell: The neuron or cell receiving the signal.

Types and Functions of Glial Cells

Myelination and Myelinating Cells

• Myelination: The process of wrapping axons with myelin sheaths to increase the speed of electrical conduction.

• Oligodendrocytes: Myelinate axons in the CNS.

• Schwann cells: Myelinate axons in the PNS.

Nodes of Ranvier

• Gaps between myelinated segments of axons where action potentials are regenerated.

• Enable saltatory conduction, increasing the speed of nerve impulse transmission.

Microglia

• Specialized immune cells in the CNS that remove debris and protect against pathogens.

Ion Concentrations Across the Cell Membrane

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

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

Graded Potentials vs. Action Potentials

Location of Graded and Action Potentials

• Graded potentials: Typically occur in the dendrites and cell body.

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

All-or-None Principle of Action Potentials

• Action potentials either occur fully or not at all, depending on whether the threshold is reached.

• Stimulus intensity is encoded by the frequency of action potentials, not their size.

Additional info: The threshold for action potential generation is typically around -55 mV. The formula for the Nernst equation, which predicts equilibrium potential for an ion, is: