Cell Communication and Signaling Physiology Exam 3 and 4

Fluid & Electrolyte Balance Overview

• Low blood volume or blood pressure activates volume receptors in the atria, carotid, and aortic baroreceptors.
• These receptors trigger homeostatic reflexes through the cardiovascular system, behavior adjustments, and kidney actions.
• High blood volume or blood pressure stimulates volume receptors in the atria and endocrine cells in the atria, along with carotid and aortic baroreceptors, which trigger similar homeostatic reflexes via the cardiovascular system and kidneys.

Countercurrent Multiplier

• The loop of Henle uses a countercurrent multiplier system.
• Solutes, specifically Na+ and Cl-, are actively transported from the ascending limb into the medulla.
• This process results in elevated ECF osmolarity, crucial for urine concentration.

Aldosterone Action

• Aldosterone, a steroid hormone from the adrenal cortex, responds to decreased blood volume/pressure and increased ECF K+.
• It facilitates Na+ reabsorption and K+ secretion in the collecting ducts and distal tubules.
• Aldosterone targets P cells (principal cells) in the late distal convoluted tubule (DCT) and collecting duct.
  • Aldosterone combines with a cytoplasmic receptor.
  • The hormone-receptor complex then initiates gene transcription in the nucleus.
  • Translation and protein synthesis leads to the production of new protein channels and pumps.
  • Aldosterone-induced proteins modulate existing channels and pumps.
  • The ultimate result is increased Na+ reabsorption and K+ secretion.

Renin-Angiotensin-Aldosterone System (RAAS)

• Angiotensinogen from the liver is converted to Angiotensin I by renin from the kidneys.
• Angiotensin I is converted to Angiotensin II by ACE (Angiotensin Converting Enzyme) released from the lungs.
• Angiotensin II causes vasoconstriction in arterioles, increasing blood pressure.
• It also leads to aldosterone creation, which increases BP, Na+ reabsorption, and K+ secretion.

Acid-Base Disturbances

• Metabolic Acidosis: Low pH and low HCO3-, caused by diabetes mellitus, obesity, or aspirin. Compensation involves hyperventilation to release CO2 and raise pH.
• Metabolic Alkalosis: High pH and high HCO3-, due to vomiting or bulimia. Compensation includes hypoventilation to retain CO2 and lower pH.
• Respiratory Acidosis: Low pH and high PCO2, caused by asthma, emphysema, or pulmonary fibrosis. Compensation involves secreting H+ and reabsorbing HCO3- to raise pH.
• Respiratory Alkalosis: High pH and low PCO2, caused by panic attacks, high altitude, or behavioral disorders. Compensation includes reabsorbing H+ and secreting HCO3- to lower pH.

Nervous System Organization

• The Central Nervous System (CNS) consists of the brain and spinal cord.
• The Peripheral Nervous System (PNS) includes sensory (afferent) neurons and efferent neurons.
• Efferent division includes:
  • Somatic nervous system controls voluntary motor movements.
  • Autonomic nervous system regulates involuntary functions, divided into sympathetic (fight or flight) and parasympathetic (rest and digest) branches.

Neuron Structure and Function

• Dendrites receive incoming signals.
• The cell body acts as the control center.
• The axon transmits electrical impulses to send a message.
• The axon terminal releases neurotransmitters at synapses.

Synapses

• The synapse, or junction, is the region where the axonal terminal meets the target cell.
  • Presynaptic neuron sends the signal.
  • The synaptic cleft is the narrow gap where neurotransmitters diffuse.
  • The postsynaptic neuron receives the signal via neurotransmitter receptors.

Glial Cells

• CNS:
  • Oligodendrocytes wrap around axons and form myelin sheaths.
  • Astrocytes have multiple roles, including BBB creation and nerve nourishment.
  • Microglia are specialized immune cells.
  • Ependymal cells produce cerebral spinal fluid and line brain ventricles/central canal of the spinal cord, and aid in the removal of toxins.
• PNS:
  • Schwann cells wrap around axons and produce myelin sheaths.
  • Satellite cells are nonmyelinating Schwann cells providing nourishment to nerves.

Graded vs Action Potentials

• Graded Potentials: Travel short distances and vary in strength (amplitude), occurring on dendrites or the cell body, are influenced by ion permeability.
• Action Potentials: Travel long distances via brief, large depolarizations that is initiated at the axon hillock.
• If the signal does not reach threshold, there is no response.

Refractory Periods

• A refractory period is necessary to prevent additional action potentials on the same neurons.
  • Absolute Refractory Period: Due to voltage-gated Na+ channels resetting; no additional action potential can be triggered.
  • Relative Refractory Period: Follows the absolute refractory period; an action potential can only be triggered by a strong stimulus.

Myelin's Role in Action Potential Conduction

• Conduction is faster in myelinated neurons.
• Myelin provides resistance of the axon membrane to ion leakage out of the cell.
• Saltatory conduction occurs between nodes of Ranvier.
• Demyelinating diseases cause loss of myelin, such as multiple sclerosis and Guillain-Barre Syndrome.

Summation

• Temporal Summation: Graded potentials overlap over time, having an additive effect.
• Spatial Summation: Two or more neurons simultaneously fire, having an additive effect.

Presynaptic vs Postsynaptic Inhibition

• Presynaptic Potential: Multiple presynaptic neurons provide input on dendrites and the cell body of the postsynaptic neuron.
• Postsynaptic Potential:
  • Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizing.
  • Excitatory Postsynaptic Potential (EPSP): Depolarizing.

CNS Components

• Gray Matter (absence of myelin):

  • Nerve cells
  • Clusters of cell bodies in the CNS (nuclei)
  • Dendrites
  • Axon terminals
  • Unmyelinated axons

• White Matter (presence of myelin):

  • Myelinated axons
  • Axon bundles connecting CNS regions (tracts)

Brain Enclosure

• The membranes enclosing the brain consists of meninges including (deep to superficial):
  • Pia Mater
  • Arachnoid Membrane
  • Dura Mater
• Ventricles suspend the cerebrospinal fluid (CSF)

Cerebrospinal Fluid (CSF)

• Formed by choroid plexuses (capillaries) in ventricles.
• Function is physical protection, lightening brain weight, nutrient supply, and waste exchange between the CSF and interstitial fluid of the CNS.

Blood-Brain Barrier (BBB)

• Protects the brain from toxic water-soluble compounds and pathogens.
• Formed by endothelial cells with tight junctions in brain capillaries.
• Astrocytes help maintain the barrier by secreting chemical compounds like glucose, amino acids, O2, and CO2.
• Pericytes provides nutrition and removal of waste

Spinal Cord Organization

• White Matter is divided into columns and tracts (containing myelinated axons).
  • Ascending Tract (afferent): Takes sensory information to the brain.
  • Descending Tract (efferent): Carries motor signals from the brain.
  • Propriospinal Tracts: Tracts of white matter that remain within the cord.
• Gray Matter:
  • Dorsal Root Ganglion: Collection of sensory bodies on the dorsal root.
  • Dorsal Horns: Contain visceral (involuntary) organs and somatic sensory nuclei (skin, muscle, joints, etc.).
  • Dorsal Root: Branch that carries sensory information.
  • Lateral Horns: Contain visceral (autonomic) motor nuclei to smooth muscle, cardiac muscle, and glands.
  • Ventral Horns: Contain somatic (skeletal muscle) motor efferent signals to muscles and glands.
  • Ventral Root: Carries information from the CNS to muscles and glands.