Intercellular Communication Types

Intercellular Communication

• It is essential for physiological processes, varying by signal source and distance to target.
• Endocrine signaling is a type where hormones travel through the bloodstream to reach distant target cells.

Types of Intercellular Signaling

• Endocrine: Hormones released into extracellular fluids travel through the bloodstream to reach distant target cells; it regulates homeostasis, responses to stimuli, and growth/development. Examples include responses to stress and sexual maturity.
• Paracrine: Signals act on nearby cells of a different type.
• Autocrine: Signals act on the same cell that secreted them for self-regulation.
• Quorum sensing: A form of autocrine signaling where cells sense the density of similar cells in a region.
• Synaptic: Neurons release neurotransmitters to target cells at synapses.
• Neuroendocrine: Neurosecretory cells release hormones into the bloodstream; for example, antidiuretic hormone regulates kidney function.
• Pheromone: Chemicals are released into the external environment to communicate between animals of the same species.
• Functions include territorial marking, mating behavior, and predator warning.

Chemical Classes of Intercellular Signaling Factors

• Gaseous Signaling Factors include gases like nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H2S) which are involved in local signaling.
• Nitric oxide (NO) helps with vasodilation, playing a role in sexual function and blood pressure regulation.
• Protein Hormones: An example is Insulin, which is produced in an inactive form and activated when needed.
• Amine Hormones: Derived from amino acids, examples include catecholamines and thyroid hormones.
• Lipid Signaling Factors: Derived from fatty acids and steroids, steroid hormones, like glucocorticoids and sex steroids, play key roles in metabolism, water balance, and reproductive biology.
• Prostaglandins: Modified fatty acids involved in immune responses, pain, and blood clotting.

Cellular Response Pathways

• Hydrophilic Hormones: Water-soluble hormones bind to receptors on the cell surface, triggering intracellular signaling events
• Hydrophobic Hormones: Lipid-soluble hormones pass through the cell membrane and bind to intracellular receptors, often influencing gene expression.

Example Pathways

• Water-Soluble
• Epinephrine triggers a cascade via a G protein-coupled receptor to break down glycogen in the liver, releasing glucose into the bloodstream during stress.
• Lipid-Soluble
• Estrogens bind to intracellular receptors, activating transcription of vitellogenin genes in liver cells, essential for egg yolk production.

Feedback Regulation

• Hormone secretion is controlled by feedback loops
  • Negative Feedback: Response reduces the initial stimulus
    • Pancreas releases bicarbonate to neutralize acid in the intestine, stopping the signal.
  • Positive Feedback: Response increases the stimulus
    • Oxytocin during childbirth causes uterine contractions, which cause even more oxytocin to be released until birth.

Elements of Endocrine Signaling Pathways

Syntesis

• Hormones are made in endocrine glands.
• Some chemical factors are proteins/polypeptides encoded by genes and are typically stored in vesicles, released on demand.
• Others are synthesized by enzymes, and their storage depends on lipid solubility. Secretion
• Lipid-soluble hormones are released upon synthesis, diffusing through cellular membranes.
• Most hydrophilic hormones are secreted via the ER-Golgi system using exocytosis. Transport
• Hydrophilic hormones do not need carriers as they dissolve in the blood.
• Hydrophobic hormones bind to proteins in the blood, which carry them to the target tissue. Reception
• A tissue can control its responsiveness to a hormone by deciding whether or not to make a receptor.
• A hormone binds with a receptor, causing it to undergo a conformational change that alters its function.
• Ligand specificity means only specific ligands can bind to the receptor.

Minetics

• Agonist: Same shape as a ligand and triggers a response.
• Antagonist: Same shape as a hormone but does not trigger a response.

Receptor Types

• Ligand-gated ion channel
• Receptor-enzyme
• G protein coupled receptor
• Intracellular receptors

Transduction

• Cascades and 2nd messengers

Response

• Ultimate consequences of the signaling cascade
• Antagonistic hormones antagonize each other to regulate blood glucose levels

Neuron Structure

• Cell Body: Contains the nucleus and other organelles.
• Dendrites: Branched extensions that receive signals from other neurons.
• Axon: A single long extension that transmits signals
  • Axons can be very long, like those from the spinal cord to the feet of a giraffe.
• Axon Hillock: Cone-shaped base from where signals are generated before traveling down the axon.
• Synaptic Terminal: The end of the axon that forms synapses with other cells.
• Synapse: The gap between the axon terminal and the receiving cell; signals are transmitted across via neurotransmitters.
• Glial Cells (Glia): Support neurons by nourishing them, insulating axons, and regulating extracellular fluid.
  • Glia outnumber neurons 10-50 times in the mammalian brain.

Neuron Functions & Shapes

• Highly Branched Neurons: Integrate information from many synapses
• Long Axons: Transmit signals to many target cells.

Information Processing Stages

• Sensory Input: Sensory neurons detect external stimuli or internal conditions.
• Integration: The brain or ganglia process sensory input, interpreting the information in context.
  • Interneurons form circuits for integration.
• Output: Motor neurons trigger an appropriate response based on processed information.

Parts of the Nervous System

• Central Nervous System (CNS): Neurons that handle integration.
• Peripheral Nervous System (PNS): Neurons carrying signals to and from the CNS.

Depolarization and Hyperpolarization

• Resting Membrane Potential: Neurons maintain a typical resting membrane potential, primarily influenced by ion movement.
• Depolarization: Occurs when positive ions enter, making the membrane potential less negative.
• Hyperpolarization: Happens when positive ions exit, making the membrane potential more negative.

Graded Potentials and Action Potentials

• Graded Potentials: Small, temporary changes which can either depolarize/hyperpolarize the membrane, depending on the ion channels involved.
  • The magnitude of a graded potential depends on the strength of the stimulus.
• Action Potentials: Triggered when a graded potential reaches a threshold, they are all-or-none events
  • Once the threshold is reached, the neuron will fire an action potential regardless of the stimulus's strength

Generation of Action Potentials

• Voltage-Gated Channels: Play a critical role in action potential generation.
  • When a stimulus depolarizes the membrane to the threshold, voltage-gated sodium channels open, leading to a rapid influx of sodium ions and depolarization.
• Repolarization: As the action potential reaches its peak, sodium channels inactivate, and potassium channels open, causing potassium ions to flow out, repolarizing.
• Hyperpolarization: The membrane potential may briefly become more negative than the resting potential, before returning to its resting state.
• Refractory Period: After an action potential, there is a period during which the neuron cannot fire another action potential.

Conduction of Action Potentials

• Saltatory Conduction: In myelinated axons, action potentials "jump," allowing for faster transmission.
• Unmyelinated Axons: Action potentials move along continuously, which is slower.

Evolutionary Adaptations of Axon Structure

• Axon Diameter: A larger diameter reduces resistance to current flow, allowing faster conduction.
• Myelination: Insulates axons, speeding up signal transmission by allowing saltatory conduction.

Postsynaptic Potentials (PSPs)

• Excitatory PSP (EPSP): Depolarizes the postsynaptic membrane, making the neuron more likely to fire
  • Involves neurotransmitters that bind to ion channels permeable to sodium
• Inhibitory PSP (IPSP): Hyperpolarizes the postsynaptic membrane, making it less likely to fire.
  • Involves neurotransmitters that open chloride or potassium channels, moving the membrane potential away from the threshold.

Summation of PSPs

• Temporal Summation: When two EPSPs arrive at a synapse in rapid succession, their effects combine, leading to stronger depolarization.
• Spatial Summation: When multiple EPSPs from different synapses occur nearly simultaneously, their effects can add to reach the threshold for firing an action potential.
• Integration at the Axon Hillock: The axon hillock integrates all the EPSPs and IPSPs and determines whether the postsynaptic neuron will fire an action potential.
• Termination of Neurotransmitter Signaling: Neurotransmitters need to be cleared after triggering a response, that is done by enzymatic breakdown, reuptake, or removal by glial cells.

Modulated Signaling at Synapses

• The neurotransmitter binds to metabotropic receptors, activating second messenger pathways to influence ion channels indirectly, these pathways last longer, allowing regulation.

Neurotransmitter Examples

• Acetylcholine: Plays a role in muscle contraction, memory, and learning. Receptors can be ionotropic or metabotropic.
• Amino Acids: Glutamate (excitatory) and GABA (inhibitory) are amino acids that are important in the brain.
• Biogenic Amines: Dopamine, serotonin, and norepinephrine regulating mood, attention, and CNS functions.
• Neuropeptides: Substance P (pain perception) and endorphins (natural pain relief).
• Gases: such as Nitric oxide (NO) acts as a neurotransmitter involved in blood vessel dilation, and carbon monoxide (CO) has a role in the nervous system.

Glia and Their Role in the Nervous System

• Glial (Glia): Support cells in the nervous system. Schwann Cells
  • Produce myelin sheaths in the PNS. Oligodendrocytes
  • Myelinate axons in the CNS. Astrocytes
  • Involved in forming the blood-brain barrier (BBB) and regulating the CNS's extracellular environment. Radial Glia
  • Guide neuron migration during development.
• Blood-Brain Barrier (BBB): Formed by astrocytes, restricts entry of substances into the CNS but allows some molecules to cross
  • Incomplete in certain regions for the brain to detect blood-borne metabolites.
• Stem Cells in Glia: Radial glia & astrocytes can act as stem cells
  • These stem cells can potentially replace damaged brain tissue in the future.

Nervous System Development

• Central Nervous System (CNS): Develops from the hollow dorsal neural tube, forms ventricles and the central canal filled with cerebrospinal fluid (CSF)
  • CSF supplies nutrients, removes waste, and circulates through the CNS.
• Grey Matter: Contains neuron cell bodies, dendrites, unmyelinated axons, and glia.
• White Matter: Consists of myelinated axons.
• Spinal Cord: Carries information and generates reflexes.
• Reflexes: Involuntary and rapid responses to stimuli, bypassing the brain (e.g., knee-jerk reflex).

Peripheral Nervous System (PNS)

• PNS Components: Afferent Neurons
  • Carry sensory information to the CNS. Efferent Neurons
  • Carry instructions from the CNS to muscles and glands.
• Motor System & Autonomic Nervous System (ANS): Motor System
  • Controls voluntary skeletal muscles. Autonomic Nervous System
  • Controls involuntary functions, with Sympathetic, Parasympathetic, and Enteric Divisions.
  • Sympathetic increases alertness, Parasympathetic promotes energy conservation, and Enteric controls digestive organs.
  • Sympathetic and parasympathetic divisions often have opposite effects.
  • These roles are Antagonistic

Sleep and Arousal

• Arousal: The state of being aware of external stimuli.
• Sleep: A state where external stimuli are not consciously perceived.
• Regulation of Arousal and Sleep:
• Reticular Formation: Controls arousal by filtering sensory input. Brainstem and Cerebrum
  • Work together in managing sleep and wake cycles.
• Sleep Studies: EEG
  • Measures brain activity during sleep. Dolphins
  • Sleep with one hemisphere at a time.
• Circadian Rhythms: Daily biological cycles regulated by an internal clock (SCN).

Emotions and the Limbic System

• Limbic System: Composed of structures like the amygdala, hippocampus, and thalamus, involved in emotions, motivation, memory, and behavior.
  • The amygdala plays a major role in emotional memory storage.
• Emotional Memory Studies: Brain damage to the amygdala can impair emotional memory.

Functional Imaging of the Brain

• (fMRI): Measures brain activity by detecting changes in blood Functional MRI Emotional Responses and fMRI
  • Happy music activates the nucleus accumbens.
  • Sad music activates the amygdala. Positron-Emission Tomography (PET)
  • Used to monitor metabolic activity in the brain.

Biological Clock Regulation

• Circadian Rhythms: The body's internal clock regulates daily cycles, is synchronized by the SCN These rhythms persist even without external light cues, maintaining roughly a 24-hour cycle.

Scientific Skills Exercise

• Hamsters
• Experiment: SCN transplanted in mutant hamsters to understand its role
  • Findings show restores rhythmic activity.
• Implications: SCN plays a crucial role in determining the period.

Brain Structure

• Cerebral Cortex is responsible for cognitive functions i.e. awareness, memory, language, and consciousness which is then divided into 4 lobes
• Sensory Input and Processing: Sensory information enters the cortex through sensory organs and somatic sensory receptors. primary sensory areas
  • association areas
  • the motor cortex

Homunculus

• Motor Commands and : The motor cortex controls body movements and shows that the size of the cortical region dedicated to a body part that is not proportional to it's size but rather to its level of neural control
  • face has a large cortical area
• Lateralization of Cortical Function: There is a division of labor between the two cerebral hemispheres Language and Logical Tasks: Left hemisphere Spatial Relations
  • face recognition
  • nonverbal thinking
  • Right Hemisphere

damage to language & comprehension

• Broca's and Wernicke's Areas: Damage to this area affects the ability to produce speech & impairs language comprehension.
• Frontal Lobe and Executive Functions: The prefrontal cortex plays a crucial role in decision-making and personality.
• Evolution of Cognition: other animals exhibit advanced cognitive behaviors

Neural Plasticity

• ability of the nervous system to reshape itself, especially in response to activity, occurring in the synapses
• Neural changes may occur after birth.
  • Visual changes

and pain

• Phantom limb syndrome: Can be reorganized using mirrors or different ways to to reduce it
• Short- and long-term memory are the main types.
• memory consolidation happens during sleep.
• learning skills
• Motor skills are learned through repetition
• LTP (Long-Term Potentation): Help with making long-term connections

Nervous System Disorders

• A mental disorder characterized by distorted reality, hallucinations, and delusions Schizophrenia
• disruptions to dopamine
• glutamate signaling -genetic factors and environmental influences
• treat by blocking dopamine

Disorders and Addictions

• Characterized by low mood, loss of interest, and fatigue, depression can be treated by prozac and drugs
• addiction effects the brains reward system especially dopamine pathways

Diseases and Solutions

• Alzheimer's Disease and Parkinson's Disease create damage to brain tissue
• Research shows these can be helped, new brain research with zebrafish models are being worked on. sensory function & signalling
• Sensory reception and signalling happens throughout the body in specialised non-neural and neural receptor cells for detection.

Potential Difference

• stimuli with potential difference, transmitted into CNS, sensory stimuli are indicated by frequency

Stimuli Types

• perception happen via CNS
• amplified to become transduced correctly.
• responsiveness to prolonged stimuli, with 5 general receptors in the body:
• mechanoreceptors and pressure sensors
• Detect chemical signals -Detect light
• electricity and magnetic fields
• Detect heat and cold, and the various pain receptors with examples

Intercellular communication is essential for physiological processes. Types include endocrine, paracrine, and autocrine signaling. Synaptic and neuroendocrine signaling are also important for communication between cells.