Human Physiology: Signal Transduction, Endocrinology, Nervous System, Sensory and Muscle Physiology

Receptors and Signal Transduction

Cellular Response to Signals

Cells respond to external messages through specialized proteins called receptors. The specific response depends on the type of receptor present and the intracellular signaling pathways activated.

• Receptor: A protein that binds a signaling molecule (ligand) and initiates a cellular response.

• Signal specificity: Only cells with the appropriate receptor can respond to a given signal.

Signal Transduction

Signal transduction is the process by which a cell converts an extracellular signal into a functional response. This often involves a series of molecular events inside the cell.

• Second messengers: Small intracellular molecules that relay signals from receptors to target molecules. Examples include cyclic AMP (cAMP), inositol triphosphate (IP3), and calcium ions (Ca2+).

• Cascades and amplification: Signal transduction pathways often involve a cascade of events, where one molecule activates another, leading to amplification of the signal.

Types of Receptors

• Tyrosine kinase receptors: Enzyme-linked receptors that phosphorylate tyrosine residues on themselves and other proteins, initiating signaling cascades.

• G protein-coupled receptors (GPCRs): Receptors that activate G proteins, which then modulate other enzymes or ion channels.

• Comparison: Tyrosine kinase receptors directly phosphorylate targets, while GPCRs use intermediary G proteins and second messengers.

Endocrinology

Hormones and the Endocrine System

Hormones are chemical messengers secreted by endocrine glands into the bloodstream, affecting distant target organs.

• Endocrine vs. Nervous System: The endocrine system uses hormones for slower, longer-lasting effects; the nervous system uses electrical impulses for rapid, short-term responses.

Hormone Classes and Mechanisms

Class	Synthesis & Storage	Transport	Half-life	Mechanism of Action
Peptides	Made in advance, stored in vesicles	Dissolved in plasma	Short	Bind membrane receptors, activate second messengers
Steroids	Made on demand, not stored	Bound to carrier proteins	Long	Bind intracellular receptors, alter gene expression
Amines (Catecholamines)	Made in advance, stored	Dissolved in plasma	Short	Bind membrane receptors
Amines (Thyroid hormones)	Made in advance, stored	Bound to carrier proteins	Long	Bind intracellular receptors

Hormone Release and Regulation

• Trophic hormones: Hormones that regulate the secretion of other hormones.

• Simple endocrine reflex: Direct regulation of hormone release by physiological variables (e.g., blood glucose).

• Synergism: Combined effect of hormones is greater than the sum of individual effects.

• Permissiveness: One hormone enables another to act.

• Antagonism: One hormone opposes the action of another.

Membrane Potential

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the plasma membrane of a cell at rest, typically around -70 mV in neurons.

• Caused by unequal distribution of ions (mainly Na+, K+, Cl-) and selective membrane permeability.

• Na+/K+ pump: Maintains ion gradients by pumping 3 Na+ out and 2 K+ in, using ATP.

Equilibrium Potentials and Driving Force

• Equilibrium potential: The membrane potential at which there is no net movement of a particular ion across the membrane. Calculated using the Nernst equation:

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

• Driving force: Determined by the difference between the membrane potential and the equilibrium potential for an ion.

Introduction to the Nervous System

Divisions and Cell Types

• Central Nervous System (CNS): Brain and spinal cord.

• Peripheral Nervous System (PNS): All neural tissue outside the CNS.

• Neurons: Excitable cells that transmit electrical signals.

• Supporting cells (glia): Non-excitable cells that support, protect, and nourish neurons.

Neuron Classification

• Structural: Multipolar, bipolar, unipolar neurons.

• Functional: Sensory (afferent), motor (efferent), interneurons.

Information Coding and Action Potentials

• Depolarization: Membrane potential becomes less negative.

• Hyperpolarization: Membrane potential becomes more negative.

• Action potential: Rapid, all-or-none electrical signal along the axon.

• Refractory period: Time during which a neuron cannot fire another action potential.

• Stimulus intensity: Coded by frequency of action potentials, not amplitude.

Synaptic Transmission

• Synapse: Junction between two neurons where neurotransmitters are released.

• Ligand-operated channels: Open in response to neurotransmitter binding.

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

The Central Nervous System

Brain Organization

• Cerebrum: Divided into lobes (frontal, parietal, temporal, occipital), each with specialized functions.

• Cerebral lateralization: Functional differences between left and right hemispheres.

• Language, memory, emotion, motivation: Involve multiple brain regions, including the limbic system and association areas.

• Diencephalon: Includes thalamus (sensory relay), hypothalamus (homeostasis, endocrine control), and pituitary gland (hormone secretion).

• Midbrain and hindbrain: Involved in basic functions such as movement, arousal, and autonomic control.

• Diffuse modulatory system: Networks of neurons that modulate large areas of the brain, affecting mood, arousal, and attention.

The Autonomic Nervous System

Somatic vs. Autonomic Neurons

• Somatic motor neurons: Control voluntary skeletal muscle.

• Autonomic neurons: Control involuntary functions (smooth muscle, cardiac muscle, glands).

Divisions of the Autonomic Nervous System

Division	Origin	Ganglia	Main Functions
Sympathetic	Thoracolumbar spinal cord	Close to spinal cord	"Fight or flight" responses
Parasympathetic	Craniosacral (brainstem, sacral spinal cord)	Near or in target organs	"Rest and digest" responses

Neurotransmitters and Receptors

• Cholinergic stimulation: Involves acetylcholine (ACh).

• Nicotinic receptors: Ion channels, fast response, found at neuromuscular junctions and autonomic ganglia.

• Muscarinic receptors: G protein-coupled, slower response, found on target organs of parasympathetic system.

• Adrenergic stimulation: Involves norepinephrine and epinephrine.

• Alpha receptors: Generally cause constriction or stimulation.

• Beta receptors: Generally cause relaxation or increased heart rate.

• Adrenal sympathetic pathway: Adrenal medulla releases epinephrine into the bloodstream as a hormone.

• Effects: Sympathetic and parasympathetic divisions can have antagonistic, cooperative, or complementary effects on target organs.

Sensory Physiology

Sensory Receptors and Coding

• Types: Mechanoreceptors, thermoreceptors, nociceptors, chemoreceptors, photoreceptors.

• Phasic receptors: Adapt rapidly, signal changes in stimulus.

• Tonic receptors: Adapt slowly, signal duration of stimulus.

• Lateral inhibition: Enhances sensory acuity by inhibiting neighboring neurons, improving contrast.

Cutaneous Sensations

• Receptor fields: Area monitored by a single sensory neuron.

• Sensory acuity: Ability to distinguish between two points of stimulation.

Taste and Olfaction

• Taste buds: Contain taste receptor cells; detect sweet, salty, sour, bitter, umami.

• Olfaction: Olfactory receptors send signals to the olfactory bulb, then to the brain.

Equilibrium and Hearing

• Vestibular apparatus: Detects head position and movement for balance.

• Ear anatomy: Outer, middle, and inner ear structures involved in hearing.

• Transduction: Hair cells convert mechanical vibrations into electrical signals.

• Hearing impairments: Can result from damage to any part of the auditory pathway.

Vision

• Rods: Sensitive to low light, no color vision.

• Cones: Responsible for color vision and visual acuity.

• Transduction: Photoreceptors convert light energy into electrical signals.

• Accommodation: Adjustment of lens shape for focusing.

• Visual defects: Myopia (nearsightedness), hyperopia (farsightedness), astigmatism (irregular curvature).

• Retina: Contains photoreceptors and neural circuits for initial processing.

Muscle Physiology

Skeletal Muscle Structure and Contraction

• Microanatomy: Muscle fibers contain myofibrils, which are composed of sarcomeres (the contractile units).

• Types of contractions: Isotonic (muscle changes length), isometric (muscle length unchanged), twitch (single contraction), summation (increased force), tetanus (sustained contraction).

• Sliding filament theory: Actin and myosin filaments slide past each other to shorten the muscle.

• Regulation: Ca2+ binds to troponin, allowing myosin to bind actin.

• ATP use: Required for cross-bridge cycling, detachment, and Ca2+ reuptake.

• Excitation-contraction coupling: Sequence from action potential to muscle contraction.