BIO 2301 Human Physiology Exam 1 2025

Homeostasis - -The body's ability to maintain internal stability, even in the face of external change Components of a homeostatic mechanism - - - Control center: usually found in the hypothalamus of the brain, contains the "set point" for the variable, receives info from the receptors and sends instructions to effectors - Receptors: detect or sense the variable and send that info to the control center - Effectors: receive instructions from the control center and carry them out to get the variable back to normal Examples of homeostatic mechanisms - - - The maintenance of body temperature - Blood glucose levels - Blood sodium levels - Blood pressure Negative feedback - -- When a change in the variable occurs, the body responds by causing a change in the opposite direction - The main way that homeostatic mechanisms work Positive feedback - -- When a change in the variable occurs, the body responds by causing more of a change in the same direction - Rarely used in the body - Examples: blood clotting, oxytocin release during child birth, and milk release Structure of the plasma membrane - -- Semi-permeable, phospholipid bilayer - Regulates passage of substances into and out of the cell - Principal components: lipids, proteins, carbohydrates Phospholipids in the plasma membrane - -- Hydrophilic head - Hydrophobic tail - Arranged in a double layer (bilayer) with the heads surrounded by fluid and the tails kept from the water Cholesterol in the cell membrane - -- Hydrophobic so found in the tail - Maintains the consistency of the cell membrane - Strengthens it by preventing smell molecules from crossing - Keeps the phospholipid tails from coming into contact and solidifying it to ensure the hydrophobic tail stays fluid and flexible BIO 2301 BIO 2301 Proteins in the cell membrane - -- Function as enzymes to speed up chemical reactions - Act as receptors for specific molecules or transport materials across the cell membrane - Integral proteins are located within the lipid bilater - Peripheral proteins are located outside the lipid bilayer Carbohydrates in the cell membrane - -- Sometime attached to proteins or lipids on the outside of the cell membrane - Form the glycocalyx which provides cushioning and protection for the membrane, important for cell recognition and determining if cells should be there or not, and acts as a glue to attach cells together Polar substances - -- Water soluble - Water, sodium, potassium Nonpolar substances - -- Fat soluble - Urea, carbon dioxide, oxygen, and alcohol Solubility - - - Like dissolves like - Polar liquids dissolve polar substances - Nonpolar liquids dissolve nonpolar substances What substances can easily pass through the phospholipid bilayer of cell membranes? - -- Nonpolar substances will pass through easily - Polar substances must go through a channel in the cell membrane Disorder - -- Disorder increases - Molecules want to go where there is more room for them to spread out Diffusion - -- The movement of small molecules from an area of high concentration to an area of low concentration - Does not require ATP energy - Consists of a solute (molecules being dissolved) and a solution (water) - Nonpolar, small lipid-soluble molecules will easily pass through the lipid bilayer Osmosis - -- The unassisted diffusion of water across a selective permeable membrane - Water will go through an aquaporin (a pore) and will move from where there is more water to where there is less water (high to low) OR movement of water from an area of low solute concentration to an area of high solute concentration - Happens when the membrane is not permeable to the solutes Osmotic pressure - -- The pressure needed to stop osmosis - Water follows salt concept - The more particles there are, the greater the osmotic pressure BIO 2301 BIO 2301 Osmolarity - - - The concentration of solute particles in a solution - NaCl and NH4Cl= x2 - Glucose and Urea = x2 - Must be constantly maintained or neurons will be damaged - osmoreceptors in the hypothalmus detect increased in osmolarity (due to dehydration) which triggers antidiuretic hormone release, thrist, and decresed excretion of water in urine Tonicity - - - The ability of a solution to affect a cell - cell volume will increase and decrease according to the concentration of the solute surrounding it - It involves the number of nonpenetrating solutes in a solution and take into account the permeability of the membrane to the solutes and water - 3 solution types: isotonic, hypertonic, and hypotonic Isotonic solutions - - - Causes no changed in the cell - Examples: 0.9% NaCl (normal saline), 5% dextrose (D5W), and lactated ringers solutions (LRS) - These are the solutions you want to give people intravenously Hypertonic solutions - -- Have less water than the cell and so water leaves the cell to enter the solution - Causes crenation (shriveling of the cell) - Examples: mannitol and 50% glucose Hypotonic solutions - -- Have more water than the cell so the water enters the cell from the solution - Causes the cell to swell and lyse (burst) - Examples: distilled water - never give this intravenously Rules governing permeability - -- Generally, nonpolar substances are permeable (can enter or leave a cell) - Polar substances may or may not be permeable depending on their size - small polar substances can fit through pores in the membrane where as large ones cannot Facilitated diffusion - -- Requires the use of carries (which are proteins) in the membrane to move the molecules from high to low concentration - Does not required ATP energy - It is both specific and limited - Carriers may always exist in the membrane or may be inserted when needed Active transport - -- Requires the use of carries to move the molecules from low to high concentration (against the concentration gradient) - Requires ATP energy BIO 2301 BIO 2301 - Example: Na/K pump Secondary active transport/coupled transport - - - Energy needed to move molecules across their concentration gradient is acquired by moving sodium back into the cell - Since sodium was originally pumped out of the cell using ATP, it is considered active transport - Cotransport: molecules move in with sodium (common with glucose) - Countertransport: molecules move the opposite direction of sodium Membrane behavior and vesicle formation - - - The cell membrane is not a static structure - It can be subtracted from (when vesicles form off of it) - It can be added to (when vesicles fuse with it) Endocytosis - -- Substances are taken into the cell - 3 types: phagocytosis, pinocytosis, and receptor mediated cytosis Phagocytosis - -- Where large particles (like bacteria) are engulfed by some cells - AKA "cell eating" - Only done by certain cells = some white blood cells and macrophages - Whatever is taken in by this method is placed in a vesicle Pinocytosis - - - Where extracellular fluid and dissolved solutes are taken into the cell - Routine activity of all cells - AKA "cell drinking" - Whatever is taken in by this method is placed in a vesicle Receptor mediated endocytosis - -- Allows the cell to take in a specific solute because that solute must bind to receptors on the cell membrane before it is engulfed Exocytosis - -The way some substances exit the cell Factors that influence rate of diffucion - -- Magnitude of concentration difference (larger difference = faster diffusion) - Mass of solute (heavier = slower, lighter = faster) - Solubility - Temperature of solution (colder = slower, warmer = faster) - Surface area of the membrane (less = slower, more = faster) Nervous system - -Involved in communication and control Neuron (nerve cell) - - - The main cell and the functional unit of the nervous system - Allows for the communication and control by nerve impulses - aka action potentials BIO 2301 BIO 2301 - Have a cell body (which contain organelles), many dendrites (the receptive regions of the neuron), and a single axon (the area of the neuron that generates and propagates the action potential) - At the end of the axon there are many telodendria (branches) and at the end of each telodendria there is a axonal terminal (aka bouton) - Neurotransmitters are chemicals found in the axonal terminals Somatic nervous system - -Controls skeletal muscle and is under voluntary control Autonomic nervous system - - - Controls smooth muscle, cardiac muscle, and glands - Involuntary control - Two divisions: sympathetic and parasympathetic Sympathetic nervous system - -Fight or flight Parasympathetic nervous system - -Rest and digest Action Potentials - - - Nerve impulses - Messages in the nervous system - the way neurons communicate - Electrical events so they involve the flow of ions (Na and K) into and out of the axon - Generated at the axon hillock (beginning of the axon) - Travels down the axon towards the axonal terminal where it will cause the release of a neurotransmitter - The neurotransmitter will carry the message from that neuron across the space to the next neuron Synapse - - - The functional connection between a neuron and the cell its signaling - The area between one neuron and the next - Consists of an axonal terminal of the presynaptic neuron, the synaptic cleft (space between), and the postsynaptic neuron (or neuronmuscular junction if the next cell is skeletal muscle) - Electrical or chemical Neurotransmitter - -The chemical that carries the nerve message across the synaptic cleft from the presynaptic neuron to the postsynaptic neuron From where is the neurotransmitter released? - -Axon terminal Nervous system's limited capacity for regeneration - -- Neurons do not do mitosis so they do not replace themselves if they are damaged or destroyed - If the cell body is still intact, they may be able to regenerate a portion of themselves - The cell body has the ability to produce substances needed for repair of a damaged axon or dendrite BIO 2301 BIO 2301 - Since most cell bodies are in the CNS, explains why CNS damage is usually permanent damage - regeneration does not occur in the CNS Neuronal cell body - -- The biosynthetic center (life source) of the neuron - Mostly found in the CNS Steps of axonal regeneration - - - Wallerian degeneration occurs and the cut end of the axon dies back toward the cell body, letting the cell body know there is a problem at the axon - The Nissl bodies (rER) in the cell body produce proteins needed for the repair of an axon - Those proteins are sent down to the damaged area where rebuilding occurs - The Schwann cell sheath remains in the area and serves as a guide or tunnel, telling the axon where to grow - Schwann cells also secrete growth factors to stimulate the growth of an axon - Once the axon has grown back to where it was initially, inhibitory factors are released to stop growth of the axon and function is regained Resting membrane potential - -- The charge across the axonal membrane when the axon is at rest - Equal to -70 mV - due to the relative permeability of the membrane (membrane is more permeable to K than to Na) and the Na/K pumps - When the charge changes, an action potential can occur Na/K pump - -- Keeps K inside the axon and Na outside of the axon - Pumps 2 sodium out and two potassium into the axon (sodium potassium ATPase) - Found in all body cells - Provides energy for couple transport of other molecules - Produces electrochemical impulse in neuron and other muscle cells - Maintains osmolarity Depolarization - -- The charge moves towards positive - Happens when Na enters the axon - During action potential, the voltage-gated Na channels open, sodium rushes in causing a positive charge around +30 mV Repolarization - -- Charge goes more negative - Happens when K leave the axon - Around +30mV, K channels open and potassium leaves the cell causing a negative charge in the cell Hyperpolarization - -- During repolarization, the charge actually dips lower than the RMP - The Na/K pump will pump the sodium out and the potassium in to get it back to its RMP BIO 2301 BIO 2301 What causes more stimulation of the postsynaptic neuron? - -A greater frequency of action potentials Voltage gated channels - -- Present on the axon cell membrane - Normally kept closed unless a change in voltage occurs - will open when it meets the threshold (-55 mV) - Voltage gated Na and K channels Steps of the action potential - - 1. Threshold: -55mV threshold value is met, resulting in the voltage gated Na channels opening 2. Sodium flow: Na flows into the axon through through the open voltage gated Na channels - depolarization occurs and the charge goes to +30 mV causing the voltage gated Na channels to close and the voltage gated K channels to open 3. Potassium flow: K flows out of the axon through the open voltage gated K channels - repolarization occurs and the charge goes to -70 mV 4. Sodium potassium ATPase: the Na/K pump will pump the Na back out and the K back in to the axon so the axon can be ready to have another action potential Action potential - -- All or none, meaning, once the threshold is reached, an action potential will occur - The size or strength of the stimulus will not affect the size or duration of the AP - Can only travel away from the site of initiation Continuous action potentials - -- Action potentials that occur on non-myelinated axons - AP are produced segment to segment down the entire length of the axon - Slow rate of conduction Saltatory action potentials - -- Action potentials that occur on mylinated axons - Nodes of Ranvier allow sodium and potassium to cross the membrane every 1-2mm so AP can jump from node to node