PCB 3712 – General Physiology: Modules 06–09 Study Guide

Thermoregulation

The maintenance of a fairly stable temperature within the body or tissue, including the ability to alter temperature metabolically or behaviorally.

Endothermy

The physiological mechanism where body temperature changes or is maintained due to internal metabolism via the production of metabolic heat.

Ectothermy

The condition where an organism's body temperature changes and is determined primarily by the external environment.

Homeotherm

An organism that possesses the physiological ability to maintain a constant internal body temperature.

Poikilotherm

An organism whose internal body temperature fluctuates and directly follows the temperature of the external environment.

Behavioral thermoregulators

Ectotherms or poikilotherms that actively regulate their body temperature through behavioral adjustments rather than metabolic means.

Spatial heterothermy

Thermal variation characterized by differences in temperature across different anatomical locations within the same organism.

Temporal heterothermy

Thermal variation characterized by differences in body temperature occurring at different times within the same organism.

Phenology

The timing of recurring biological events in organisms, such as mating seasons, egg-laying seasons, and angiosperm flowering times.

Temperature

A quantitative measure of the average random motion of molecules within a substance.

Heat

The total energy of a substance resulting from molecular motion, which depends on both the temperature and the total amount of the substance.

Conduction

The transfer of heat through visibly motionless objects or substances, calculated as $H = k\frac{T_1 - T_2}{d}$, where $k$ is conductivity and $d$ is distance.

Thermal gradient

The temperature difference across a given distance separating two points, expressed mathematically as $\frac{T_1 - T_2}{d}$.

Convection

The rapid transfer of heat through moving fluid substances, described by the equation $H = h_c(T_S - T_A)$.

Convection coefficient proportionality ($h_c$)

The relationship governing convective heat exchange across a round surface of diameter $D$ with fluid velocity $V$, defined by $h_c \times \frac{\sqrt{V}}{\sqrt{D}}$.

Evaporation

Latent heat of vaporization

The substantial quantity of heat energy required to transform liquid water into gaseous vapor, approximately equal to $2400\,\text{J/g}$.

Thermal radiation

Heat emitted by all physical objects in the form of electromagnetic energy (infrared range), defined by $H = \varepsilon\sigma T_S^4$.

Temperature coefficient ($Q_{10}$)

A factor quantifying the sensitivity of metabolic reaction rates to temperature changes across a $10^\circ\text{C}$ interval, calculated as $Q_{10} = \frac{R_T}{R_{(T-10)}}$.

Pejus range

The worsening sub-optimal thermal range on a poikilotherm's performance curve where negative physiological responses occur, including reduced growth rates and increased food intake.

Supercooling

The physiological state in which liquid water drops below its freezing temperature without forming ice crystals, facilitated by removing ice nucleating agents.

What can happen if water crystallizes inside cells

Ice crystals can damage cells and cause them to burst.

What type of property is the freezing point of water?

colligative property, meaning it depends on the concentration of dissolved particles.

What are the two main strategies organisms use to survive freezing temperatures?

Freeze tolerance and freeze prevention (freeze avoidance).

Freeze tolerance

The physiological adaptation enabling certain animals to survive freezing by deliberately forming ice in extracellular fluids using ice nucleating agents and concentrated internal solutes.

What is supercooling?

When water drops below its normal freezing point without freezing.

How can organisms use extracellular ice formation to protect their cells?

Ice forms outside cells, increasing the solute concentration outside and drawing water out of the cells, reducing intracellular ice formation.

What is freeze tolerance?

The ability of an organism to survive freezing of some of its body water.

What are ice-nucleating agents?

How can ice crystals be used to reduce the risks of supercooling?

Introducing ice crystals can initiate controlled freezing in extracellular areas before dangerous uncontrolled ice formation occurs inside cells.

Why is metabolic thermoregulation energetically costly?

requires energy to produce heat in the cold or activate cooling mechanisms in the heat.

How does metabolic rate change outside the thermoneutral zone?

MR increases below and above the thermoneutral zone.

Tnz

The range of ambient temperatures where an animal maintains its body temperature without increasing metabolic heat production.

What is the lower-critical temperature in homeo

ambient temperature below which an animal must increase metabolic heat production to maintain body temperature.

What is the upper-critical temperature?

ambient temperature above which an animal must increase active cooling to maintain body temperature.

Colligative antifreeze

Soluble compounds such as glycerol that lower the freezing point of body fluids strictly by increasing total solute concentration.

Non-colligative antifreeze

Specialized proteins that bind directly to water molecules via hydrogen bonding to physically block ice crystallization.

Thermoneutral zone

The ambient temperature range bounded by lower-critical and upper-critical temperatures across which a homeotherm's metabolic rate ($M$) remains constant.

Why is behavioral thermoregulation important for poikilotherms?

Environmental temperatures fluctuate, so behavior helps them maintain a suitable body temperature.

How does metabolic rate (MR) change as temperature increases?

MR increases logarithmically with temperature

What is Q10?

The temperature coefficient that measures how much metabolic rate changes with a 10°C temperature increase.

One way of calculating is temperature coefficient, Q10, which compares at different temp

• Where RT is metabolic rate at a given temp and R(T – 10) is the metabolic rate ten degrees cooler

Exponential function (not linear)

𝑄

10 =RT/R(T-10)

Some poikilotherms can acclimate to a small extent over time (chronically)

This compensation buffers MR response to temp

What is acclimation?

A physiological adjustment that develops over time in response to environmental conditions.

Some poikilotherms can acclimate to a small extent over time (chronically)

This compensation buffers MR response to temp

What is a performance curve?

curve showing how an animal's performance changes with environmental temperature.

Important parts of the curve: low critical temp, peak, pejus range, high critical temp, death,

protein denaturation

In pejus range, many potential negative responses

Decreased growth rate, increased food intake, etc.

What is the low critical temperature?

The lower temperature limit near which normal physiological function begins to fail.

What is the high critical temperature?

upper temperature limit near which normal physiological function fails.

Dry heat transfer

All pathways of heat exchange excluding evaporative loss (conduction, convection, radiation), whose rate is directly proportional to $T_B - T_A$ .

Useful below and within thermoneutral zone

Can’t ignore evaporation above thermoneutral zone

Why is evaporation important above the thermoneutral zone?

Dry heat loss becomes less effective when the environment is hot, so evaporation helps remove excess body heat.

What is the linear heat-transfer equation?

MR = metabolic rate; C = thermal conductance; = body temperature; = ambient temperature.

Thermal conductance ($C$)

measure of how easily heat transfers between an animal and its environment.

substance-specific rate constant representing the ease of heat loss, used in the linear heat-transfer equation $M = C(T_B - T_A)$.

How does low thermal conductance affect an animal?

Heat escapes more slowly, allowing the animal to retain heat more effectively.

Insulation ($I$)

The resistance to heat loss, calculated as the inverse of thermal conductance ($I = \frac{1}{C}$), which modulates metabolic rate via $M = \frac{1}{I}(T_B - T_A)$.

What is the relationship between thermal resistance (insulation) and conductance?

They are inversely related

How does metabolic rate behave within the thermoneutral zone?

remains relatively constant at the minimum rate needed to maintain homeostasis.

What happens to insulation below the thermoneutral zone?

Insulation increases or is maximized to reduce heat loss while body temperature remains relatively constant.

insulation is maximized, TB is constant

What are three ways animals adjust insulation?

Fur/feather adjustments (to trap more insulative air)

Postural changes- Curling up reduces exposed surface area and decreases heat loss.

Blood flow alterations- Reducing blood flow to the skin and extremities conserves heat; increasing it promotes heat loss.

What are two ways animals produce additional heat below the thermoneutral zone?

Shivering

Non-shivering thermogenesis using brown adipose tissue

What is regional heterothermy?

Allowing different regions of the body to maintain different temperatures, often with extremities cooler than the core.

Countercurrent exchange

A vascular adaptation for regional heterothermy where heat transfers between closely positioned blood vessels flowing in opposite directions, cooling peripheral extremities while conserving core body heat.

What are three examples of evaporative cooling?

Sweating, panting, and

gular fluttering - Rapid movement or vibration of the throat region that promotes evaporative cooling, especially in birds.

What is integration?

Integration – combination of various processes that allow overall function

Cellular – specific to cell functioning

Whole-animal – integrative systems include sensory, nervous, endocrine

Nerve and endocrine cells are specialized for signaling

Nervous and endocrine systems have similarities and differences

Nervous is faster

Uses electrical signals through nerves

Secondary chemical neurotransmitters

Organs are brain, spinal cord

Central nervous system (CNS) and peripheral nervous system (PNS)

Endocrine is slower

Uses hormones through bloodstream

Organs are considered glands

Parts and outline of neurons

Neuron: Basic unit of the nervous system.

Dendrites: Receive neurotransmitter signals.

Excitatory signals: Increase likelihood of an action potential.

Inhibitory signals: Decrease likelihood of an action potential.

Action potential: Occurs when threshold is reached.

Soma (cell body): Contains nucleus and integrates signals.

Axon: Conducts action potentials away from the soma.

Presynaptic terminals: Send signals to other cells.

Postsynaptic cells: Receive signals; may be neurons, muscles, or endocrine cells.

Synapse: Junction between presynaptic and postsynaptic cells.

Synaptic cleft: Gap where neurotransmitters carry chemical signals across.

Afferent (sensory): Carries information to the CNS.

Interneuron: Located entirely within the CNS.

Efferent (motor): Carries signals away from the CNS to muscles or glands.

How are neurons adapted for signal transmission, and what are the functions of myelin and glial cells?

Neurons have specialized structures for transmitting signals. Myelin sheaths

Myelin sheaths are made by glial cells

Protect nerve cells

Increase speed of nerve impulse

Schwann cells form myelin in the PNS; oligodendrocytes form myelin in the CNS.

^ around axon

Astrocytes transfer nutrients from capillaries to neurons in CNS

microglial cells provide immune defense in the CNS.

characterize the ionic basis of how membrane potentials arise

Separation of charged ions creates an electrical gradient (voltage) that can do work.

Electrical current is the net movement of charges.

The Na⁺/K⁺-ATPase (NKA) uses ATP to pump 3 Na⁺ out and 2 K⁺ in, making the inside relatively negative

K⁺ leak channels allow K⁺ to move down its gradient, balancing the pump’s effects and helping maintain the

resting membrane potential- sustained grading

Afferent neurons

Sensory neurons specialized to relay incoming sensory information from peripheral tissues toward the central nervous system.

Interneurons

Neurons located entirely within the central nervous system that integrate, process, and relay neural signals between other neurons.

Efferent neurons

Motor neurons that convey regulatory electrical commands away from the central nervous system out to peripheral effector tissues like muscles or glands.

Schwann cells

Glial cells in the peripheral nervous system (PNS) that form protective myelin sheaths around neuronal axons to accelerate impulse conduction.

Oligodendrocytes

Glial cells in the central nervous system (CNS) that form myelin sheaths around axons to insulate and increase nerve impulse velocity.

Astrocytes

Central nervous system glial cells that facilitate nutrient and metabolic transfer between blood capillaries and neurons.

Microglial cells

Specialized glial cells in the central nervous system responsible for performing primary immune defense and scavenging functions.

$\text{Na}^+/\text{K}^+\text{-ATPase}$ Gradient set up by Nka

An active transport pump that consumes ATP to transport $3\,\text{Na}^+$ ions out of the cell for every $2\,\text{K}^+$ ions moved inward, establishing an electrically negative interior resting potential.

K+ leak channels allow K+ ions to flow down gradient

Keeps cell from becoming too negative

Balance between NKA and K+ leak channels

explain action potentials and how they can be measured

Resting potential: Maintained voltage gradient across the membrane; ions move both ways with no net movement at equilibrium.

Depolarization: Cell becomes more positive.

Repolarization: Cell returns toward a more negative voltage.

Hyperpolarization: Cell briefly becomes more negative than resting potential.

Return to rest: Membrane returns to resting potential.

Action potential: Begins when threshold is reached; follows the all-or-none rule, has a consistent magnitude for a given cell type, and travels the entire neuron.

Voltage-gated ion channels: Trigger the action potential.

How does an action potential occur and travel in one direction?

Cell starts at resting potential

Electrical signal causes voltage-gated Na+ channels to open

Allows ions to flow down gradient (result is depolarization)

Na+ channels close, voltage-gated K+ (not leak) channels open

Allows NKA to begin to set up gradient again

K+ channels stay open a little too long

Hyperpolarizes cells

K+ leak channels allow cell to go back to resting potential

Action potentials only go in one direction

When Na+ ions enter, they cause a locally positive environment inside cell

Attracts negative and repels positive charges

Na+ ions move downstream and open next Na+ channel

Refractory period (slow reopening) makes certain signal goes in one direction

How do scientists determine the function of individual ion channels?

Patch clamping: Technique used to isolate and measure the activity of individual ion channels.

Toxins: Used to identify channel functions because they often target specific ion channels.

$\text{K}^+$ leak channels

Transmembrane channels that permit $\text{K}^+$ ions to diffuse down their electrochemical gradient out of the cell, preventing excessive negative hyperpolarization.

Depolarization

A shift in membrane potential making the inside less polorized

Presynaptic/ post

prior to synapse / after

What are the characteristics of electrical synapses?

Rare compared to chemical synapses.

Faster because there is no chemical conversion delay.

Can be bidirectional.

Signals weaken (attenuate) with distance.

Less modifiable than chemical synapses.

Use gap junctions formed by connexons made of connexin.

Have a shorter distance between cells.

What are the characteristics of chemical synapses?

More common than electrical synapses.

Slower due to conversion between electrical and chemical signals.

Unidirectional (one-way).

Signals do not attenuate along the postsynaptic membrane; responses can follow an all-or-none action potential.

More modifiable and can amplify signals.

What are the structures and steps involved in chemical synaptic transmission?

Synaptic vesicles: Store neurotransmitters.

Active zone: Where vesicles fuse with the presynaptic membrane.

Postsynaptic density: Contains receptors.

Steps:

Action potential reaches presynaptic terminal.

Voltage-gated Ca²⁺ channels open; Ca²⁺ enters.

Vesicles fuse with the membrane and release neurotransmitters into synaptic cleft

Neurotransmitters cross the synaptic cleft and bind to postsynaptic receptors on ligand-gated channels

Two main types of responses

Ionotropic

Metabotropic

Two main types of responses

Ionotropic – opening of channels, ions flow across membrane

Metabotropic – no channel opening, GPCR elicits action potential in postsynaptic cell

How are chemical synapses amplifiable and modifiable.. important?

Chemical synapses

Amplifiable – regulate amount of neurotransmitter dumped into synaptic cleft

Active zone can change in size

Presynaptic terminal size can change

Both allow more or less neurotransmitter to be released

Can be excitatory or inhibitory

Modifiable – use makes them stronger, disuse weaker

Important in development and learning

How do excitatory and inhibitory synaptic potentials affect action potentials?

EPSPs (excitatory postsynaptic potentials): Make an action potential more likely.

IPSPs (inhibitory postsynaptic potentials): Make an action potential less likely.

Integration: Neurons receive input from many neurons; EPSPs and IPSPs combine to move the cell toward or away from threshold.

Temporal summation: Repeated signals from one presynaptic neuron add together over a short time.

Spatial summation: Signals from multiple presynaptic neurons add together.

How does a fast chemical synapse work at the neuromuscular junction?

Sometimes called motor end-plate

NMJ: Connection between a motor neuron and a skeletal muscle fiber.

Innervation: Each skeletal muscle fiber usually receives input from one motor neuron.

Neurotransmitter: Acetylcholine (ACh).

Mechanism: ACh binds ligand-gated channels, allowing Na⁺ and K⁺ to flow; the resulting depolarization triggers a muscle action potential.

Acetylcholinesterase: Breaks down excess ACh to stop prolonged signaling.

Choline recycling: Choline is taken back up and reused to make ACh.

Fast chemical synapses: Especially well studied in the PNS because they are simpler to study than CNS synapses.

Small molecule neurotransmitters

Biogenic amines (ACh, dopamine, serotonin, norepinephrine)

ACh most well-known, but <10%

Norepinephrine 2nd most well-known, but <1%

Amino acids (glycine, GABA, glutamate)

Peptides

Common in CNS

Often co-released with small molecule neurotransmitters

How do neurotransmitters and their receptors differ in function

Receptor diversity: The same neurotransmitter can bind to multiple receptors and cause different effects.

Ionotropic: Directly opens ion channels; usually fast.

Metabotropic: Activates intracellular signaling; usually slower.

Glutamate: Usually causes EPSPs in the CNS.

GABA & glycine: Usually cause IPSPs in the CNS.

Biogenic amines: Used by relatively few neurons and often act slowly through metabotropic receptors.

Peptides: Common in the CNS.

ACh: Can produce different effects depending on the receptor and target cell.

What are sensory receptors, and how do animals detect sensations?

Main senses: Sight, smell, touch, taste, and sound.

Sensitivity: Ability to distinguish stimulus intensity.

Specificity: Ability to distinguish types of sensory input.

Stimulus: External energy that causes a sensory response.

Sensory receptor cell: Converts a stimulus into an electrical signal (sensory transduction).

Receptor potential: Change in membrane voltage caused by a stimulus.

Receptors: Can be ionotropic or metabotropic and detect different stimuli through specialized receptor molecules.

Labeled lines principle: Sensation depends on which receptor pathway is activated; signals are processed in specific brain regions.

Pathway: Sensory signals travel directly to the CNS or through the PNS.

How do mechanoreceptors detect stimuli and encode intensity?

Mechanoreceptors: Detect physical changes, such as touch, pressure, vibration, or stretch.

Insect sensory bristle: Bending the bristle changes the sensory neuron's dendrite and can trigger action potentials.

Threshold: Stimulus must reach a certain level to trigger an action potential.

Stimulus intensity: Stronger stimuli generally produce faster action-potential firing; weaker stimuli produce slower firing.

Train of action potentials: Repeated action potentials transmit sensory information.

TRP channels: Detect diverse stimuli; examples include capsaicin (chili heat), menthol (cooling), and compounds in wasabi.

Stretch receptors: Respond to mechanical changes in cell or tissue shape.

Maybe due to ankyrin repeats in channel

What do the main mammalian skin sensory receptors detect?

Meissner corpuscles: Light touch.

Ruffini endings: Skin stretch and sustained pressure.

Merkel discs: Touch and pressure.

Free nerve endings: Pain, itch, and temperature.

Pacinian corpuscles: Vibration and deep pressure.

Dorsal root ganglia (DRG): Contain sensory neuron cell bodies.

Receptor diversity: Different sensory neurons detect different kinds of stimuli.

How do chemical synapses in the CNS differ from the neuromuscular junction?

Neurotransmitter: CNS commonly uses glutamate; NMJ uses ACh.

Signal strength: CNS postsynaptic potentials are usually smaller and often require multiple impulses to reach threshold.

NMJ response: Usually reliably triggers a muscle action potential.

Inhibition: IPSPs are common in the CNS but generally absent at the skeletal muscle NMJ.

Anatomy: CNS and NMJ synapses differ in synaptic cleft distance and synaptic vesicle shape.

Adaptation – continuous response decreases frequency of action potentials over timE

Tonic – slow-adapting responses

Phasic – rapidly-adapting respon