Cell Communication and Neurons

Cell Communication

Basics

  • Cell communication involves basic steps, ligands, receptors, signal transduction pathways, and response.

Why Cells Communicate

  • Cells communicate in response to changes in their environment, whether internal or external.
  • This communication leads to temporary changes in the cell to conserve energy and materials.

Types of Signaling

  • Juxtacrine: Cell communication occurs through direct contact, such as cells touching or via plasmodesmata.
  • Paracrine: Ligands are released and bind only to local cells.
  • Local Signaling: Communication occurs between nearby cells.
  • Long Distance Signaling (Endocrine):
    • Communication occurs from one part of the body to another.
    • Signal molecules are typically made and stored in glands.
    • Hormones are released into the bloodstream and travel to target cells.

General Steps of Cell Communication

  1. Reception: A ligand binds to a receptor.
  2. Signal Transduction: The signal is passed along and amplified by molecules, where each molecule activates more than one of the next.
  3. Response: The cell temporarily changes its activity.

Signaling Molecules (Ligands)

  • Signaling molecules are called ligands.
  • Ligands are very specific to their receptors on target cells.
  • Examples of ligands include small molecules, ions, gases, and hormones, such as nitric oxide, calcium, iron, and growth factors.
  • Hormones are most common in endocrine signaling.

Types of Receptors

  • Receptors are usually proteins.
  • They are the first molecule to receive the message.
  • Surface receptors: Bind ligands and initiate a domino effect, passing the message into the cell.
  • Internal receptors: Bind with ligands to form a ligand-receptor complex, which often acts as a transcription factor.

Hormone Types

Protein Hormones

  • Made of amino acids.
  • Hydrophilic.
  • Cannot pass through the cell membrane.
  • Bind to surface protein receptors.
  • Initiate a signal transduction pathway.
  • Response: Activates or inhibits enzymes or protein synthesis.
  • Fast response.

Steroid Hormones

  • Made using cholesterol.
  • Hydrophobic.
  • Can pass through the cell membrane.
  • Bind to intracellular receptors.
  • The hormone-receptor complex acts as a transcription factor.
  • Response: Activates or inhibits protein synthesis.
  • Slow, sustained response.

Ligand-Receptor Specificity

  • The shape of the ligand must match the shape of the receptor.
  • R-groups in the receptor binding site interact with the ligand.
  • Ensures the correct cells receive the message.

G Protein Coupled Receptors

  • A ligand binds to a surface protein receptor, which modifies a G-protein.
  • The G-protein switches out GDP for GTP, becoming activated.
  • The activated G-protein activates an integral membrane protein (usually an enzyme that activates cyclic AMP).

Ligand-Gated Ion Channel

  • A channel protein that opens when the correct ligand binds to it.
  • Once open, it allows specific ions to pass through, changing the cell's activities.

Tyrosine Kinases

  • Ligands bind to receptors, causing them to move together and form a dimer.
  • Tyrosine amino acids become phosphorylated by ATP, thus activated.
  • The activated tyrosines can now start a signal transduction pathway.

Signal Transduction Pathway (STP): Phosphorylation Cascade

  • A series of kinase enzymes phosphorylate each other.
  • The final result either activates an enzyme or activates transcription factors to start protein synthesis.

STP: Secondary Messengers

  • Small non-protein molecules used to start a signal transduction pathway.
  • Usually activated by an integral membrane protein.
  • Examples: Cyclic AMP (cAMP), Cyclic GMP, Calcium.

Cell Responses

  1. Enzymes are activated or inhibited to start or stop a chemical reaction within the cell.
  2. Protein synthesis is started or stopped.

Examples of Cell Communication

Epinephrine (Fight or Flight Response)

  • Epinephrine binds to a receptor on the plasma membrane.
  • This activates a G protein, which in turn activates adenylyl cyclase.
  • Adenylyl cyclase converts ATP to cyclic AMP (cAMP).
  • cAMP activates protein kinase A, which initiates a phosphorylation cascade leading to cellular respiration, converting glycogen to glucose.

Insulin

  • Insulin binds to a receptor, activating IRS (Insulin Receptor Substrate) proteins.
  • This leads to the activation of PI 3-Kinase, then PDK, and finally Akt.
  • Akt promotes glycogen synthesis, lipid metabolism, and the insertion of GLUT 4 (glucose transporter) into the plasma membrane via transport vesicles, facilitating glucose uptake.

Epidermal Growth Factor (EGF)

  • EGF binds to its receptor, activating GRB2, which activates SOS, which activates RAS (bound to GTP).
  • RAS activates RAF (a kinase), which phosphorylates MEK.
  • MEK phosphorylates ERK.
  • ERK enters the nucleus and phosphorylates ELK-1.
  • Phosphorylated ELK-1 activates the expression of genes for cell division.

Testosterone

  • Testosterone enters the cell and binds to a receptor.
  • The complex enters the nucleus and acts as a transcription factor.
  • RNA polymerase transcribes mRNA from DNA.

Neurons and Synaptic Communication

  • Covers neuron parts, types of neurons, synapses, stimulus, resting potential, and action potential.

Generic Neuron Parts

  • Dendrites: Receive a signal (stimulus) and convert it to an electric signal.
  • Axon terminals: Convert the electric signal to a chemical signal called neurotransmitters.
  • Cell Body: Houses the organelles.
  • Myelin Sheath: Insulates the axon and speeds up the electric signal.
  • Axon: Carries the electric signal toward the next neuron.
  • Synapse: The space between two neurons.

3 Main Types of Neurons

  • Sensory neurons: Gather information and send it to the brain.
  • Interneurons: Interpret information (the brain is primarily made of interneurons; also found in the spinal cord).
  • Motor neurons: Carry information away from the brain to the body (usually to muscles or glands).

Synapse (Synaptic Cleft)

  • The electric signal is converted to a chemical signal to cross the synapse.
  • Vesicles in the axon terminal of one neuron release neurotransmitters, which float across the gap and bind to receptors on the dendrites of the next neuron, starting a new electric signal.

Stimulus

  • Anything that activates a sensory neuron and causes it to send a signal.
  • Examples:
    • Sensory neurons in the eyes: light
    • Sensory neurons in the ears: sound waves
    • Sensory neurons in the body: pressure, temperature
    • Sensory neurons in the nose: molecules
    • Sensory neurons in the mouth: molecules

Stimulus Examples

  • The fire alarm goes off, causing you to jump. The stimulus: Soundwaves from the alarm.
  • You get a whiff of your mom's chocolate chip cookies, and your mouth begins to water. The stimulus: Molecules from the cookies.
  • You touch an unexpectedly hot pan, causing you to let go and drop the pan. The stimulus: Heat from the pan.

Membrane Potential

  • The charge difference inside and outside a cell.
  • A specific balance of ions inside and outside the cell creates the membrane potential.
  • For neurons, it is maintained by using Na^+ and K^+ and the sodium/potassium pump.

Resting Potential

  • Resting Membrane Potential is +outside/-inside.
  • The sodium-potassium pump continuously pumps sodium ions out and potassium ions in.
  • Resting membrane potential charge is -70 mV.
  • Resting Potential: When a neuron is not sending an electric signal.

Action Potential

  • Depolarization: Shift in membrane potential from +outside/-inside to -outside/+inside.
  • In a neuron, this is accomplished by sodium channels opening to allow sodium to rush in.
  • Action Potential: When a neuron is sending an electric signal.

Action Potential Phases

  1. Resting State: The activation gates on the Na^+ and K^+ channels are closed, and the membrane's resting potential is maintained.
  2. Depolarization: A stimulus opens the activation gates on some Na^+ channels. Na^+ influx through these channels depolarizes the membrane.
  3. Rising Phase: Depolarization opens the activation gates on most Na^+ channels, while the K^+ channels' activation gates remain closed. Na^+ influx makes the inside of the membrane positive with respect to the outside. Action potential is reached at +40 mV.
  4. Falling Phase: The inactivation gates on most Na^+ channels close, blocking Na^+ influx. The activation gates on most K^+ channels open, permitting K^+ efflux, which again makes the inside of the cell negative.
  5. Undershoot: Both gates of the Na^+ channels are closed, but the activation gates on some K^+ channels are still open. As these gates close on most K^+ channels, and the inactivation gates open on Na^+ channels, the membrane returns to its resting state.

Action Potential at the Synapse

  1. Action potential reaches the axon terminal and depolarizes the membrane.
  2. Voltage-gated Ca^{2+} channels open, and Ca^{2+} flows in.
  3. Ca^{2+} influx triggers synaptic vesicles to release neurotransmitter.
  4. Neurotransmitter binds to receptors on the target cell, causing positive ions to flow in, leading to depolarization and making an action potential more likely.