BIO192 - exam 4
Has more than one hominin species existed at the same time?
Not unusual for more than 1 hominin species to overlap in time
Some fish are capable of augmenting gill respiration with atmospheric respiration. What is the common environment for this adaptation?
Numerous groups of fish have evolved accessory breathing organs to cope with low dissolved oxygen in water Catfish absorb air through their digestive tract Electric eels absorb air in their mouth cavity Some fish gulp air and absorb O2 in specialized "labyrinth" organs Lungfishes have paired lungs that have the same origins as lungs in tetrapods
Describe the role of the stomach in humans. What is the significance of HCl?
The stomach is a collapsible muscular bag, mucus, HCl and pepsinogen are secreted - mucus (protects stomach lining), HCl (kills bacteria), and HCl causes (inactive) pepsinogen to transform into (active) pepsin, a protease. Some protein digested
What is unique about osmoregulation in sharks?
- urea levels are 100x the concentration found in mammals Enzymes and tissues have evolved to tolerate high urea concentrations
Describe how the human ear detects sound. Trace the path of sound waves from when they enter the ear until they send action potentials through the auditory nerves. What determine sound intensity? What determines sound frequency?
- vibration of the ear drum causes the bones of the inner ear to move, sound is amplified and transmitted to the oval window - Oscillations of the oval window cause fluid in the cochlea to move - hairs within the cochlea vibrate and bend, sending impulses to sensory neurons Loudness: depends on the number of hair cells stimulated Frequency: determined by location in cochlea where fibers in the basilar membrane resonate - The fibers vary in length and stiffness (short/stiff near base of the cochlea, long/flexible near the apex) - High frequency sounds resonate with short fibers, whereas low frequency sounds resonate with longer fibers
Define homeostasis. What challenges homeostasis in an animal's body?
A body's tendency toward internal stability Challenges - metabolic needs and byproducts, changes in the external environment
Do elephants sweat? How do they stay cool?
Elephant skin has many folds to hold water. They spray water over their bodies to promote evaporative heat loss
Describe the steps of a muscle contraction, starting with the formation of cross bridges between actin and myosin. What breaks the cross bridge between actin and myosin?
1) ADP and P1 attached to myosin head allow Myosin heads to bind to actin 2) Binding causes head to deform, pulling the actin filament toward center of sarcomere. ADP and P1 are released 3)Binding of new ATP molecule to myosin head detaches the link 4) Cleavage of ATP into ADP + P1 returns head to original shape REPEAT
Starting in the right ventricle, describe the path of blood flow in both the pulmonary and systemic circuits until you end up back in the right ventricle.
1) Right atrium receives venous blood 2) Blood flows to right ventricle and is pumped to lungs 3) Left atrium receives oxygenated blood from lungs 4) Blood flows to left ventricle and is pumped to organs in the body 2 cycle pump: atrial chambers pump simultaneously, followed by ventricular chambers
What are neuroglial cells? Are there different types? Explain.
1) Schwann cells produce myelin sheaths around axons in PNS 2) Oligodendrocytes do the same in CNS
Which tetrapods rely on two-way airflow, and which rely on one-way airflow? Is there an advantage of one over the other. Explain
2 way - amphibians, reptiles, and mammals 1 way - birds
Describe the sequence of events in an axon that produce an action potential. What is the role of the Na+/K+ pump? Why is an action potential described as an all-or-none event?
Action potential: a rapidly moving change in electrical membrane potential along the axon of a neuron - Action potentials are caused by voltage-gated ion channels (two types, Na+ and K+) - Gated channels open in response to chemical signals (neurotransmitters). Action potential begins if threshold (-55 mV) is reached - Voltage-gated Na+ channels now open, allowing Na+ to enter cell via diffusion - Na+ ions cancels out RMP (depolarization) - Na+ channels close in response - Depolarization causes voltage-gated K+ channels to open; K+ move outside cell (repolarization) - K+ channels close in response - Action potential passes; membrane returns to RMP
Why are gills ineffective in terrestrial environments? How do lungs overcome these challenges?
Air is less supportive than water (gills will collapse on land) and the large surface area of gills is a source of evaporative water loss on land (gills dry out on land) Lungs are air chambers with internal support and a moist environment to reduce water loss
In terms of hearing (and no other senses), how have some animals overcome the problem of determining the distance of a sound source? What animals have this feature (list two)?
Echolocation (distance) and ears on both sides of head (determine sound direction, not distance)
What is the difference between positive pressure breathing and negative pressure breathing. Provide an example of each.
Amphibians use positive pressure breathing air is taken into mouth nostrils close; floor of mouth raised air pushed into lungs; expelled by elastic forces Reptile lungs - negative pressure Rib cage expands to create negative pressure that draws air into lungs
What is the difference between the compound eyes of arthropods and the camera-type eyes of mollusks and vertebrates? Are the eyes of mollusks and invertebrates an example of convergent evolution or homology? Why?
Annelids, mollusks, arthropods, and vertebrates have evolved image-forming eyes independently - convergent evolution
What is the distinction between the venous (veins, venules) and arterial (arteries, arterioles) systems? Where does gas transfer, both to and from blood, occur? Which is higher pressure, the arterial or venous system?
Arteries: blood leaves heart via arteries and then to arterioles. Contain more elastic fibers than other blood vessels. - Vasoconstriction: Reduces heat loss, limits blood flow to injured vessels - Vasodilation: increases heat loss Veins: blood flows from venules and veins back to heart, aided by contractions of skeletal muscles - Low pressure - Most of body's blood is contained in veins - Pooling of blood causes varicose veins
Identify the four basic types of joints in a vertebrate skeleton and provide an example for each.
Ball and socket - rotated in 2 plane (ex. Hip; large range of motion but requires stabilization) Hinge - move in 1 plane (ex. Knee; limited range of motion but stronger and more stable Gliding - surfaces slide, lateral vertebral joints (ex. Spine; allows flexibility while maintaining alignment Combination - multiple joint types (ex. Mammal jaw; important for chewing)
How do ectothermic animals regulate body temperature?
Behavioral thermoregulation - Orientation to sun - Color change - Changes in shape and posture - Shuttling behavior - Basking and shivering
Did the evolution of bipedalism pre-date the evolution of large brain size in humans?
Bipedalism preceded larger brain size
Describe breathing in birds
Birds have one-way air flow through their lungs (parabronchi) which prevents the mixing of O2 and CO2 Inhale 1 - oxygen enters posterior air sacs Exhale 1 - oxygen moves into lungs and basses over parabronchi (air vessels) Inhale 2 - depleted oxygen leaves lungs and moves to air sacs Exhale 2 - depleted oxygen leaves body
1. Describe the nervous system of a vertebrate. Include information that allows you to explain how skeletal muscles and smooth muscles are stimulated. What is the difference between afferent and efferent neurons?
CNS - brain and spinal cord, composed of interneurons PNS - collects information and initiates responses - Sensory (afferent) neurons: impulses from sensory receptors to CNS - Motor (efferent) neurons: impulses from CNS to effectors: muscles and glands o Somatic nervous system: motor neurons that stimulate skeletal muscles o Autonomic nervous system: motor neurons that stimulate smooth muscles, cardiac muscles, and glands
Describe the importance of countercurrent exchange in thermoregulation.
Can limit heat loss. Cold blood is heated as it is returned to core
Why do carnivores typically have a more expandible stomach than herbivores? Relative to herbivores and controlling for body size, why do herbivores typically have a larger stomach than carnivores?
Carnivores - eat large items infrequently; need for expansion Herbivores - plants are difficult to digest and low in nutritional value. Most consume large quantities
Controlling for body size, is the digestive tract of carnivores longer or shorter than that of herbivores? Why is this?
Carnivores is shorter
Which invertebrate phyla have a closed circulatory system? Which ones have an open circulatory system? Are there phyla that have examples of both?
Closed - annelids and cephalopod mollusks Open - arthropods, mollusks (except cephalopods)
What two factors determine conduction velocity of an action potential? Explain their effects.
Conduction velocity of an AP is a function of axon diameter and the presence or absence of myelin - Action potential "jumps" between nodes (saltatory conduction)
What is the purpose/advantage of countercurrent exchange as it relates to respiration in fish?
Countercurrent exchange maximizes delta p
Identify structures in various animals that are used to store food prior to digestion.
Crop - storing food
What is cutaneous respiration and what kinds of animals use it?
Cutaneous respiration: amphibians and some aquatic reptiles exchange gases across their thin, membranous skin. Supplements lungs. Ex. Sea snakes and turtles
Compare and contrast endothermy and ectothermy. What are their strengths and limitations?
Endotherms - rely on metabolic heat (birds and mammals) - Use much of their energy to maintain body temperature Ectotherms - rely on external sources of heat (fish, amphibians, reptiles, arthropods) - Can tolerate wider ranges of temperature than endotherms - Activity is constrained by environmental temperature; endotherm activity is less affected
Compare and contrast removal of nitrogenous waste in fish, mammals, and reptiles/birds/insects. Which method is the most energetically expensive? Which method is a major source of water loss?
Fish - direct elimination: immediate removal via gills or dilute urine Mammals, amphibians, and sharks - urea: less toxic than ammonia, can be diluted. Source of water loss (urine) Reptiles, birds, and insects - uric acid: crystal; can be excreted without water, energetically expensive
Identify the osmoregulatory organ found in (a) flatworms, (b) annelids and mollusks, (c) insects, and (d) vertebrates.
Flatworms - protonephridium with flame cells Annelids and mollusks - nephridium Insects - Malpighian tubules Vertebrates - kidneys
Describe the various ways that herbivores break down cellulose in plant material. What kind(s) of animals engage in coprophagy?
Foregut fermentation: ruminants - the rumen is a large chamber housing bacteria. Fermented food is regurgitated (cud), rechewed, and then swallowed again Hindgut fermentation: cecum - many non-ruminant herbivores (horses, rodents, rabbits) have a large cecum that contains bacteria needed to break down cellulose Hindgut fermentation: colon - elephants
What is the significance of this difference in terms of function and efficiency?
Four chambered heart with complete separation of oxygenated an deoxygenated blood
Explain the different challenges faced by freshwater versus marine (saltwater) fish in terms of osmoregulation.
Freshwater - water tends to enter body from environment, solutes tends to leave the body and enter environment, excrete large volumes of dilute urine Saltwater - water tends to leave the body and enter environment, solutes tends to enter the body from environment, drink large amounts of water and produce little urine
What is a gill? What variables in Fick's Law are essential to the design and function of gills? List the various types of animals that use gills, broadly defined, for respiration.
Gills: specialized vascular filamentous structures that project into water. Their design increases surface area (A) and minimizes membrane width (d) for efficient gas exchange Papulae and tube feet in echinoderms serves as gills "naked gills" in nudibranchs (a type of gastropod) Many marine annelids breathe through their skin and parapodia Some aquatic insect larvae have external gills that connect to their tracheal system. These gills are lost in adults
Identify structures in various animals that help in the grinding of food.
Gizzard (birds) and gastric mill (crustaceans) - assist in grinding food
What anatomical features help to define primates?
Grasping finger and toes, most with opposable first digit, most with nails Binocular vision
What two features define mammals and exclude all other vertebrates?
Hair (follicles in dermis, shaft made of keratin) females produce milk for offspring
What is an herbivore? What is an omnivore? What challenges do herbivores face when digesting food?
Herbivore - only eats plants. Gut microbes are required to digest cellulose, adaptations for crushing plant material (surface area), large amounts needed to meet energy demands Omnivores - unspecialized feeders, adapted for consuming a range of food types, including plant and animal
How are antlers different from horns? What is unique about rhinoceros' horns?
Horns have a core of bone surrounded by a sheath of keratin, not shed Antlers are made of bone and shed annually Rhino's horns are made from keratin but are not attached to the skull
Provide a general description of digestion in a vertebrate once food is swallowed.
Human saliva contains the enzyme amylase, which initiates the breakdown of starch The esophagus transfers food to digestive region (peristalsis - rhythmic waves of contraction to move food to stomach) Birds and many invertebrates have a crop for storing food The gizzard of birds and gastric mill of crustaceans assist in grinding food The stomach is a collapsible muscular bag, mucus, HCl and pepsinogen are secreted - mucus (protects stomach lining), HCl (kills bacteria), and HCl causes (inactive) pepsinogen to transform into (active) pepsin, a protease. Some protein digested Food enters small intestine (duodenum, jejunum, and ileum) from stomach Duodenum receives pancreatic juice - neutralize gastric acids, enzymes critical for digestion Bile is produced by liver, stored in the gallbladder, and transported to the duodenum - bile pigments: worn out blood cells. Bile salts: help emulsify fat Internal surface area of small intestine is increased by coiling and by villi and microvilli The large intestine (colon) begins at the point where the appendix and cecum meet small intestine - colon is shorter than SI and lacks villi. No digestion occurs in colon. Colon is for consolidation of waste via water absorption and absorption of products of bacterial metabolism Colon empties into rectum; feces are excreted
Identify and describe (in writing) the four types of "skeletons" covered in class, and then illustrate them with drawings and provide an example(s). Identify any strengths and/or weaknesses of each design.
Hydrostatic - muscular force developed by contracting against fluids Muscular hydrostat - movement achieve via muscle action (no bones, cartilage, or fluid for support); complex muscle arrangements required Exoskeletons - surrounds the body as a rigid, hard case, interior surfaces are sites of muscle attachment; animal must mold periodically to grow, area/mass ratio doesn't support large body size Endoskeletons - internal skeletons to which muscles attach, made of cartilage and/or bone, both of which are living tissues; efficient framework for muscle attachment and continuous growth is possible
Compare and contrast the design and function of the circulatory systems of insects and crustaceans. What is the oxygen-binding pigment contained in crustaceans and chelicerates? Why is this absent in insects?
Insects - hemolymph transports nutrients and metabolic waste, but not O2 and CO2 Crustaceans and Chelicerates - hemolymphs also transports O2 and CO2. Blood has oxygen binding pigments (hemocyanin, not hemoglobin)
How do endotherms maintain elevated body temperatures when it's cold?
Large quantities of food are required and behavior can reduce energy demands Heat loss can be influenced by size, insulation, control of blood flow to the surface, and adjustment of hair/fur
Describe how the lateral line of fishes works. What does it detect?
Lateral line allows fish to sense objects that reflect pressure waves and vibrations
Describe breathing in humans. What takes place for inhalation to occur? How about exhalation.
Lika amphibians and reptiles, mammals have a two-way air flow through their lungs, which allows some mixing of fresh and stale air Alveoli increase surface area for improved gas exchange Air is drawn into lungs by negative pressure - contraction of intercostal muscles raises ribs and expands thoracic cavity; contraction of diaphragm lowers base of thoracic cavity - relaxation of these muscle allows elastic forces to expel air
Why are the terms "warm blooded" and "cold blooded" not favored by biologists? Same question for homeotherm and poikilotherm.
Many cold-blooded animals achieve warm body temperatures (opposites is also true) Some poikilotherms have stable body temperatures and some homeotherms modulate their temperatures
In osmoregulators, what does "metabolic water" refer to in terms of a source of water?
Metabolic water = byproduct of cellular metabolism
What are the three lineages of modern mammals? Identify the features that define each lineage, and provide examples of animals within each lineage. Are mammals considered part of the amniote clade?
Monotremes - lay shelled eggs; young drink but don't suckle. Represented by echindnas and the platypus Marsupials - pouched mammals, brief gestation; no implantation of embryo in uterus, offspring live and feed in pouch (kangaroos, koalas, opossums) placental mammals - long gestation period during which embryo remains attached to mother's placenta, young suckle following birth
Describe what determines the magnitude of a muscular response. Can muscle fibers contract "extra-hard" to exert greater forces? Why or why not?
Motor unit - a motor neuron plus all the muscle fibers contacted by the axonal branches of the motor neuron The number of motor units recruited - precise contractions require smaller motor units than large, forceful contractions. Most muscles contain motor units in a variety sizes which are selectively activated by the nervous system
What is the only thing a muscle can do? Given your answer, how is it that animals can move skeletal structures in multiple directions? i.e., what kind of muscle arrangement is required?
Muscles can only contract, muscle arrangement is typically antagonistic (opposing)
Turning to osmoregulation, what is an osmoconformer? Provide examples.
Osmoregulation: the regulation of internal fluid and its compositions Osmoconformers: fluids inside the animal are isotonic with sea water - therefore, no osmotic gradient exists
Can fish hear, and if so, how?
Otoliths in the head of fishes vibrate against hair cells - otoliths are much denser than water, and thus slow sound waves relative to the rest of the fish's body
Where does perception of a sense occur? Why do we perceive sensory signals differently (e.g., why is smell different than touch or sight)?
Perception is a product of the brain and where that signal goes
The goal of eating is to obtain nutrients from food. What are the three key steps in this process?
Physical fragmentation - increase exposure to enzymes Chemical digestion - break down carbohydrates, lipids, and proteins into smaller units Absorption and waste excretion
What kind of animals use pit organs for thermoreception of prey?
Pit vipers use thermoreception for detection of prey. These organs are wired to the visual processing center of brain
Identify the functional changes in the components of the amniotic egg in placental mammals.
Placenta - organ that serves as provisional lungs, intestines, and kidneys of fetus without mixing maternal and fetal blood - amnion remains unchanged - yolk sac greatly reduced - allantois contributes to umbilical cord - chorion forms the placenta
What are predators? How do they differ from filter feeders such as whales that consume large numbers of small fish in a single gulp?
Predators - animals that eat other animals (one on one interactions)
What is the difference between the pulmonary and systemic circuit of blood flow? Name the classes of vertebrates that exhibit both circuits.
Pulmonary circuit - blood is pumped from the heart to the lungs and back Systemic circuit - blood is pumped from the heart to the tissues and back
Write down Fick's Law of Diffusion. Which variables increase R? Explain in words why each relationship makes sense. In terms of Fick's equation, what are the villi and microvilli of the small intestine doing?
R=(DxAxdeltap)/d D= diffusion constant A= surface area Deltap= difference in concentration on either side of membrane D=width of membrane R can be increased by increasing A, deltap, or lowering d
Describe the differences between radiation, conduction, convection, and evaporation as it relates to heat transfer.
Radiation - transfer of heat by electromagnetic radiation Conduction - direct transfer of ehat between objects Convection - transfer of heat via movement of gas/liquid Evaporation - energy needed to transfer from liquid to gas phase (vaporization)
describe the role of rods and cones in vertebrate vision. What are their abilities and limitations? What is the fovea?
Rods - detect low levels of light. Hundreads may converge on a single nerve cell Cones - detect color when light there is enough light
Animals have evolved various ways to perform gas exchange (O2 and CO2). What are they, and what features do they have in common? How is the function of an arthropods respiratory system different from vertebrates?
Some invertebrates acquire O2 and liberate CO2 via diffusion across their moist skin Some invertebrates lack respiratory organs but circulate water over bodies to maximize deltap Larger organisms have evolved lungs, gills, or other means for gas exchange. These structures increase A and/or minimize d, in some cases, deltap is also increased
How do sponges digest food? What about cnidarians and flatworms?
Sponges digest their food intracellularly Cnidarians and flatworms - the alimentary canal of most eumetazoans allows for extracellular digestion - cnidarians and flatworms have a cavity with a single opening
Trace the path of sensory information from the moment of stimulation until it is relayed to the brain. Are action potentials associated with one type of sense (e.g., smell) different from another (e.g., sight)? Explain
Stimulation: a stimulus impinges on a sensory receptor Transduction: energy from a stimulus transformed into a graded potential in the associated sensory (afferent) neuron Transmission: action potential develops in axon of sensory neuron and is conducted to CNS Interpretation: all nerve impulses are qualitatively alike, perception of a stimulus is a product of the brain
What is a graded potential, and how is it related to an action potential? Why does Na+ flood into cells when sodium ion channels open? Why don't these ions flood out of the cell?
Stimulus causes ion channels to open, which allows a flood of Na+ to enter cell via diffusion Causes depolarization If sufficient depolarization across the cell membrane is reached (-55 mV in mammals), a nerve impulse, or action potential is produced
Compare and contrast the tree types of particulate feeders - suspension feeders, deposit feeders, and filter feeders. Provide example(s) of each).
Suspension feeders - sessile animals that filter suspended particulates Deposit feeders - consume substrate and extract whatever nutrients they can Filter feeder - actively acquire and filter smaller organisms (from water)
Describe the respiratory system of insects from air intake to exhalation. Is it associated in any way with the circulatory system?
Terrestrial arthropods breathe via a network of air-filled tubes (tracheae and air sacs) that branch into finer tubes (tracheoles) where gas exchange occurs. Air enters and exits via spiracles. Circulatory system is not involved with gas exchange - Air enters through abdominal spiracles and exits through posterior spiracles - Abdominal pumping improves circulation
Define resting membrane potential (RMP) in your own words, and describe how it is achieved.
The electrical gradient between a resting neuron and surrounding environment - Fixed anions with cells (proteins and nucleic acids) - Membrane more permeable to K+ than to Na+ (leakage channels) o Concentration gradient: favors K+ out of cells o Electrical gradient: K+ attracted to anions in cell - Equilibrium point between the concentration and electrical gradient is the resting membrane potential (-70 mV)
Draw the labyrinth of the inner ear along with the cochlea. Describe in words how the inner ear is involved with (a) detection of linear motion and (b) detection of angular motion. Be certain to identify the key structures involved.
The labyrinth of inner ear is fluid-filled membranous chamber with semicircular canals Linear motion - acceleration distorts the matrix, causing hair cells to bend and stimulating sensory neurons Angular motion - each semicircular canal loops in different planes. At the end of the semicircular canals are swollen chambers called ampullae, withing these ampullae are hair cells
List the six classes of receptors and describe their function.
Thermoreceptors - temperature changes Nociceptors - impulses perceived as pain Electromagnetic receptors - sense electric and magnetic fields Mechanoreceptors Chemoreceptors - taste and smell Photoreceptors - vision
What is the purpose of villi and microvilli in the small intestine? Are these structures also found in the large intestine?
They are not found in the large intestine and they increase surface area
Fact: the circulatory system in humans is closed. However, some water and solutes from blood plasma filter through capillaries and enter the lymphatic system. Why then is the circulatory system of humans a closed system? Why is the lymphatic system an open system?
This fluid is called interstitial fluid. Amount of fluid that flows out of capillaries is greater than that that flows in. the remainder enters the lymphatic system (an open system). Excess interstitial fluid drains into lymph capillaries, which eventually passes to lymph nodes ( in neck) and drains back into veins
What is the difference between torpor and hibernation?
Torpor: a temporary reduction in body temperature during periods of inactivity. Smaller endotherms only - large animals have too much mass to allow rapid cooling Hibernation: extended torpor that lasts several weeks to months. Body temperature may drop 20 C below set point
What is the general purpose of a circulatory system? What is the difference between and open and closed circulatory system?
Transportation - all substances essential to cellular metabolism are transported by the circulatory system a) Red blood cells (erythrocytes) transport oxygen to tissues b) Blood carries absorbed products of digestion to cells in the body c) Metabolic wastes and excess water in blood are filtered through kidneys Regulation - transports hormones, participates in temperature regulation Protection - circulatory system protects against injury and foreign substances a) Blood clotting - platelets adhere to damaged areas b) Immune defense - white blood cells (leukocytes) Open: no distinction between blood and extracellular (interstitial) fluids of body tissues Closed - blood is enclosed within vessels that transport the blood away from and back to the heart
What is a tympanum? What does it detect? Is the presence of a tympanum in (some) insects and (most) vertebrates an example of homology or analogy. Would your answer change if I was to point out that tympana on insects occur on a variety of body parts (e.g., the front legs of katydids and the abdomen of grasshoppers)?
Tympanic membrane = ear drum vibrates in response to sound waves
Some animals can detect bioelectric fields. What is the commonality of these animals in terms of their habitat? Name a sense organ involved with detection of bioelectric fields, and what type of organism has it?
Water is a good conductor of electricity, and thus many fish have evolved the ability to detect electric fields - sharks use ampullae of Lorenzini
Consider the following: A motor neuron delivers an electrochemical impulse to a muscle. Describe how this impulse results in the formation of cross bridges between actin and myosin. Be sure to specify the role of the two regulatory proteins associated with actin.
When a muscle is relaxed the myosin heads are not bound to actin - Tropomyosin blocks the myosin heads from binding to actin For a muscle to contract, tropomyosin must be moved out of the way. This is controlled by troponin, a regulatory protein complex. When nerves signal muscles to contract Ca++ is released from the sarcoplasmic reticulum. Ca++ binds to troponin, which alters the configuration of the troponin-tropomyosin complex and allows the myosin heads to bind with actin
What is a synapse? Describe the chain of events that propagates an action potential in a neuron to a muscle contraction
When an action potential passes along a neuron it must eventually cross a synapse Axon terminals contain vesicles of neurotransmitters that relay messages across synapse 1) Action potential opens calcium channels and triggers the inward diffusion of Ca+, which causes synaptic vesicles to bind with plasma membrane and then release neurotransmitters into synapse 2) Neurotransmitter binds to receptor proteins on postsynaptic cell, opening their gated ion channels 3) If the resulting voltage change across the cell membrane reaches the threshold, an action potential in the postsynaptic cell (e.g., neuron, muscle) is initiated 4) Neurotransmitters are rapidly removed from synaptic cleft by enzymatic digestion and reuptake by the neuron 5) Neuron reabsorbs and resynthesizes components into neurotransmitter