Cellular Communication and Signaling

Cellular Communication

• Cellular communication uses neurotransmitters, paracrines, and endocrine hormones as communicators.
• Signals are messengers, also known as ligands.
• Hormones, paracrines, and neurotransmitters facilitate communication.
• Receptors are proteins that bind to ligands based on shape and charge.
• Binding to a receptor results in a shape change.
• The shape resulting from the bond must be complementary and opposite if identical.
• Activation occurs when a ligand binds to a receptor.
• Specificity occurs when the ligand and receptor have complementary shapes and charges.
• Affinity measures how well ligands bind, and is only considered if specificity exists.
• Saturation is the percentage of receptors bound.
• 50% saturation is shown in the figure provided.
• Competitors must have specificity.
• An agonist is a competitor that elicits the same shape change and response when it binds.
• An antagonist elicits no response; medicines act as antagonists.
• Up regulation- more receptors
• Down regulation- less receptors removed through endocytosis

Signal Transduction

• Signal transduction converts an incoming signal into a response by the target cell.
• Responses exhibit a difference in the membrane, acting as a discriminating membrane.
• Responses include changes in metabolism
• Preparing for anaerobic respiration, turning the Krebs cycle off & Fat breakdown instead of glucose
• Modifications in contractile activity- Secrete product
• Increase of collagen/elastin, which fibroblasts produce.
• Changes in proliferation rate (mitosis).
• Changes in differentiation determine what a cell will become.

Receiving Signals

• Receptor location depends on the polarity of the ligand.
• Transmitters, endocrines, and paracrines can be polar or non-polar.
• Non-polar ligands can bind to receptors in the cytosol or nucleus
• Polar ligands bind to integral membrane-bound receptors.
• Polar ligands cannot access intracellular receptors.
• Neurotransmitters are typically polar.
• Intracellular receptors bind non-polar ligands.
• Paracrines and neurotransmitters travel through polar interstitial fluid, but do not mix well due to opposite polarity, being a short distance allows this to proceed.
• Endocrines spend more time, so plasma binding proteins combat polarity, as they are amphipathic
• Plasma binding proteins interact with endocrines (non-polar) and water (polar).

Intracellular Receptors

• A complex forms or enters the nucleus, increasing or decreasing transcription (DNA reading).

Membrane Bound Receptors

• Polar ligands travel in polar water without issue, but cannot enter the cell because of polarity.
• To combat not being able to enter the cell a "hand off" message is required with a ligand.
• The ligand is the first messenger- results in a change to charge distribution (more + on inside/outside)
• Activation of enzymes results in amplification.
• Kinase is the enzyme that gets "turned on" a majority of the time.
• Amplification occurs. It does its job faster.

Impacts to Signal Transduction and Receiving

• There there can be more or less communication.
• A decrease in activation creates less communication
• Ligands can be destroyed before reaching the signal through catabolization
• Receptors- catabolized but it is a costly action
• See the above action in intracellular receptors
• There can be a change in charge or shape, lowering affinity
• During endocytosis we can remove part of the plasma membrane with the receptor (ex, insulin).
• Impact a component later in a relay.
• Increase activation through anabolization. Simply make more receptors
• The above action happens with intracellular receptors or put used receptor back through exocytosis

Nervous System Organization

• The nervous system comprises the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS includes the brain and spinal cord.
• The PNS includes efferent (action) and afferent (sensory) components.

Functions of the Nervous System

• The nervous system facilitates communication through electrical impulses over long distances.
• Neurotransmitters (NTs) pass messages to the next cell (short distances).
• Limited by number of connections
• Communication is fast and quick.
• It is responsible for processing and controlling information.
• The nervous system acts as an integrator in a reflex template.
• It is key for homeostasis.

Anatomy of the Nervous System (NS)

• Neuron is the basis.
• Cell body contains the nucleus and facilitates protein production.
• Processes extend from the main component
• Processes allow connections to and from other cells.
• Dendrites gather process
• Dendrites number 5/6 in figures.
• A neuron can have up to 400,000 dendrites to receive information.
• Dendrites branching out to increase surface area, create more surface area for connections
• Axons (nerve fibers) extend longer.
• Single, elongated >1m
• Axons get weakened and break as body moves in people with ALS
• Axons process from the cell body, and sends information.
• Axon terminal is the end of the axon.
• Branches offer more communication and the release of NTs.
• Myelinated axons are made of additional supporting cells (not part of the neuron).
• Extensions of the plasma membrane wrap around the axon and insulate it from the ECF.
• In the PNS, myelin is created by Schwann cells. The brain and spine rely on glial cells to supply myelin
• One Schwann cell is needed to create one section.
• In the CNS, oligodendrocytes work with multiple axons
• A single oligodendrocyte can work with multiple axons and make many sections.
• Loosing an oligodendrocyte is more costly than loosing a single Schwann cell
• Breaks in myelin are called nodes of Ranvier.
• Axons needs to be protected for electrical signals
• Breaks are critical to access ECF

Main Neuron Types

• Afferent neurons conduct afferent signals.
• Afferent neurons send signals to the CNS from receptors.
• They lack dendrites.
• They have axons to send/receive sensory signals and 2 terminals.
• All in PNS, besides one axon terminal.
• Terminology: P. term gathers and C. term receives.
• Ratio- 1 afferent: 10 efferent: 200,000 interneurons.
• Efferent neurons originate from the CNS and go to the effector.
• Efferent neurons are entirely within the CNS, except the innervating axon.
• Efferent neurons innervate effectors (talk to the axon).
• Interneurons connect afferent and efferent neurons.
• Interneurons are all in the CNS (by definition).
• The CNS is all space that interneurons occupy.

Synapse Types

• Neurotransmitters (NTs) a synaptic connection
• Electrical synapses are quicker.
• There is bidirectional flow in electrical synapses by use of gap junctions
• Chemical synapses- NTs release sites which bind to ligand-g channels and induce electrical activity of NTS .
• Electrical activity signals "exocytosis," move through space.
• Chemical synapses, in contrast to electricals which are quick, have a trade off to be slower.
• Slower chemical directionality is unidirectional.

Synapse Terms

• A synapse involves two cells (pre/post).
• White space is interstitial fluid (ECF).
• Almost every neuron can be pre and post, depending on which synapse is referenced.
• Convergence vs divergence:
  • Convergence is coming together (# pre > # post).
  • Divergence is apart (# pre< # post).

Electrical Potentials and Gradients

• Potential is stored energy and a membrane potential.
• Potential makes others possible.
• With electricity there is charges across the membrane due to distribution of charged components.
• Most cells are slightly negative inside.
• All cells exhibit membrane potential.
• Impacts electrochemical gradient.
• Net gradient = concentration gradient + electrical gradient.
• Directions align same ex (Na+)
• Opposite- one dominates
• Need to know sizes
• There's an electrical gradient which can be in the same or different direction than concentration.