Human Physiology module 1

Diffusion

Passive movement of molecules down a concentration gradient (High to Low).

Name 4 factors that increase Diffusion Rate.

1. Higher Concentration Gradient. 2. Larger Surface Area. 3. Thinner Membrane. 4. Higher Lipid Solubility.

Define Osmosis.

Movement of water across a membrane toward higher solute concentration.

Osmotic Pressure

The pressure needed to stop osmosis. (Higher solute = Higher pressure/pull).

Molarity:

Concentration of the compound

Osmolarity

Concentration of individual particles

Colligative Property

A property depending only on the number of particles, not their identity.

Isosmotic

Same particle count.

Hyperosmotic

More particles

Hyposmotic

Fewer particles

Tonicity

Effect of a solution on cell volume (based on non-penetrating solutes)

Hypertonic

Water leaves cell; cell shrinks

Hypotonic

Water enters cell; cell swells/bursts

Uniport

One molecule, one direction

Symport

Two molecules, same direction.

Antiport

Two molecules, opposite directions

PrimaryActive Transport.

Uses ATP directly (Pumps).

Secondary Active Transport

Uses a concentration gradient (Potential Energy) created by a primary pump.

Autocrine

Signal acts on the same cell that fired it

Paracrine:

Signal acts on neighboring cells

Endocrine signaling.

Hormones travel through the blood to distant targets

The 4 steps of Signal Transduction.

1. Reception (Ligand binds). 2. Transduction (Relay). 3. Amplification (Second Messengers). 4. Response (Cell action).

cAMP

second messenger, uses to carry message from hormone that cannot enter the cell

Gap Junctions

Direct physical "tunnels" connecting the cytoplasm of two cells for ion passage

Contact-Dependent Signaling.

Surface molecules on one cell bind to surface receptors on another (requires physical touch)

Neural Signaling

Neurotransmitters cross a synapse to a specific target

What is the function of a Signal Cascade?

To amplify a small external signal into a massive internal response.

Why do different tissues respond differently to the same hormone?

Because the hormone binds to different receptor subtypes, which activate different intracellular signal cascades

Receptor-Channels Membrane Receptors

Open ion gates.

GPCRs Membrane Receptors

Use G-proteins + Second messengers.

Receptor-Enzymes Membrane Receptors.

Activate internal enzymes.

Integrins Membrane Receptors.

Alter the cytoskeleton.

Agonist

Mimics the ligand and activates the receptor.

Antagonist

Blocks the ligand and prevents activation

Synergistic effects

Combined effect is greater than the sum (1+1=5)

Additive effects.

Effect is the sum of individual parts (1+1=2)

How does a Negative Feedback Loop work?

The output of a pathway shuts off the original stimulus

Anterior pitutatory

True endocrine gland; produces/secretes its own hormones

Posterior Pituitary

Neural tissue; stores/releases hormones made in the Hypothalamus

relationship: Hypothalamus and Anterior Pituitary

Hypothalamus secretes hormones into blood vessels to tell the Anterior Pituitary to release trophic hormones

Afferent

Delivers input (Sensory).

Efferent

Delivers output (Motor/Response).

Dendrites

Receive incoming signals.

Cell Body (Soma)

It receives signals from dendrites, performs protein synthesis.

Axon

Carries the electrical signal (Action Potential) to the target.

Equilibrium Potential

The membrane potential at which the electrical gradient exactly opposes the chemical gradient (no net movement of that specific ion).

Depolarization

Na channels open; Na rushes in.

Peak:

Na channels close, K channels start to open

Repolarization

K rushes out

Hyperpolarization

K channels stay open too long, goes neg

Why is Depolarization a Positive Feedback Loop?

Na entering the cell makes the inside more positive which opens more voltage-gated Na channels which lets in more Na. This continues until the inactivation gate closes.

What stimulus opens Voltage-Gated K Channels?

Depolarization (The same stimulus that opens Na channels, but K channels are just slower to open).

Absolute Refractory Periods

Impossible to fire another AP. (Cause: Na channel inactivation gates are closed).

Relative Refractory Periods.

Possible to fire, but needs a stronger stimulus. (Cause: Some K channels are still open).

Why do Action Potentials move in only one direction?

Because the section of the axon behind the signal is in the Absolute Refractory Period, preventing the signal from moving backward.

Relationship between Resistance and Ion Current.

Inversely proportional. Higher resistance = Lower current flow.

What is the function of Myelin?

It acts as an insulator to increase membrane resistance and decrease "leakage," allowing for Saltatory Conduction (the signal "jumps" between nodes, moving much faster).

How are Graded Potentials conducted, and where do they go?

travel by diffusion of ions across the cell body toward the Axon Hillock. they lose strength as they move.

graded potentials

Why can’t the Cell Body fire an Action Potential?

It lacks a high density of Voltage-Gated Na Channels.

Subthreshold stimuli.

Too weak; doesn't reach threshold at the axon hillock (No AP).

• Suprathreshold stimuli

Strong enough to hit threshold (Starts an AP)

Spatial summation

Multiple different neurons fire at once on one targett

temporal

One neuron fires repeatedly in a short time

Does Summation occur in Graded Potentials, Action Potentials, or both?

Only Graded Potentials. Action Potentials cannot summate because of the Refractory Period.

Why is an Action Potential "All-or-Nothing"?

: Once threshold is hit, the Positive Feedback Loop (Na channels opening) is unstoppable. It either fires at full strength or not at all

How is Synaptic Transmission terminated?

1. Enzymatic Breakdown (e.g., AChE). 2. Reuptake into the axon. 3. Diffusion away from the cleft.

Ionotropic

Receptor is an ion channel (Fast)

Metabotropic receptors

• Receptor uses a G-protein/2nd Messenger (Slower).

Sympathetic

Fight or Flight

Parasympathetic

Rest and Digest

Dual Innervation

Most organs receive signals from both systems to create opposite effects (Antagonistic Control)

: How do Sympathetic/Parasympathetic produce opposite effects on one cell?

they use different neurotransmitters and receptor subtypes.

• Symp: Releases Norepinephrine onto Adrenergic receptors.

• Para: Releases Acetylcholine onto Muscarinic receptors