Biol216 Exam 2

4.1-7.3: Thermoregulation, Neurons, Ion Channels and Action Potentials, Neuronal Functioning, Brain and Nervous System, Sensory Receptors, Hearing, Vision, Taste and Smell

Ectotherm

Obtains heat from the environment. Typically live in a favorable environment, have lower metabolic rate that falls at low environmental temp, and have small surface-to-volume ratio.

Endotherm

Maintains body temp and balances. internal heat production with heat loss from body surface. Metabolic rates rise at low environmental temps to generate body heat.

Vasoconstriction

Blood vessels constrict to retain heat.

Vasodilation

Blood vessels expand to increase blood flow and release heat.

Countercurrent exchange

Blood flow currents move in opposite directions to ensure a heat gradient that facilitates for maximum retention.

Temperature homeostasis

Hypothalamus triggers mechanisms to instate vasodilation or vasoconstriction when body temperature deviates from a set point.

Ways animals manage heat conductance

1. Feathers/fur/blubber insulation to retain heat

2. Reflective surfaces to minimize heat uptake

3. Blood flow redirection to retain or release heat

Anatomical hierarchy

cells, tissues, organs, organ systems, animal

Basic tissue types

Epithelial, connective, muscle, nervous

Epithelial tissue

Lines body structures and cavities to form a protective, secretory, and absorptive covering.

Types of epithelial tissue

Squamous, cuboidal, columnar.

Connective tissue

Provides bodily support and binds parts.

Types of connective tissue

Loose, fibrous, cartilage, bone, blood, adipose

Muscle tissue

Contracts or shortens to facilitate movement.

Types of muscle tissue

Skeletal, cardiac, smooth.

Nervous tissue

Transmits information by carrying electrical messages.

Torpor

A period of inactivity aligned with variations in environmental temperature.

Hibernation

Winter torpor.

Estivation

Summer torpor.

Central Nervous System

Brain and spinal cord.

Peripheral Nervous System

Afferent and efferent neurons.

Interneurons

Neurons in the CNS that integrate information and formulate a nervous response.

Afferent neurons

Sensory neurons that pick up stimulus and transfer it to interneurons/the CNS.

Efferent neurons

Sensory neurons that carry neuronal response signals to the muscles and glands.

Motor neurons

Efferent neurons that carry signals specifically to the skeletal muscle.

Neuron

A highly specialized cell built to receive and pass on electrical signals.

Dendrite

Branched extensions of a neuron that receive incoming signals.

Axon

A threadlike extension of a neuron that carries nerve impulses away from the cell body.

Nerve

A collection of axons that provide a common pathway for electrochemical impulses to travel upon.

Synapse

The junction between the axon tip of the sending neuron and the dendrite tip of the receiving neuron.

Membrane Potential

The voltage difference across a cell's plasma membrane.

Nervous system organization

Stimulus, PNS (afferent neurons), CNS (interneurons), PNS (efferent neurons), effectors, action.

White matter

Myelinated axons and glial cells.

Gray matter

Neuronal cell bodies.

Glial cells (neuroglia)

Non-neuronal cells in the PNS and CNS that support, protect, and nourish neuronal cells.

Types of glial cells

Ependymal, microglial, astrocytes, satellite, Schwann, oligodendrocytes

Ependymal cells

CNS. Produce cerebrospinal fluid.

Microglial cells

CNS. Defensive cells that ingest and break down pathogens and waste products.

Astrocytes

CNS. Provide structural support to blood vessels and maintain ion concentration in surrounding fluid.

Satellite cells

PNS. Protect neuron cell bodies (astrocytes but in PNS).

Schwann cells

PNS. Form myeline sheath around PNS nerve fibers.

Oligodendrocytes

CNS. Form myelin sheath around CNS nerve fibers.

Synapse types

Electrical and chemical

Electrical synapses

Have a direct flow of current between cells where the action potential of one cells causes the formation and travel of an action potential in the next. Located in smooth and cardiac muscle, and tooth pulp. Important for uniform, synchronized, contractile activity across a group of cells.

Chemical synapses

Electrical impulse arrives at the terminal on the presynaptic cell before a neurotransmitter is released and diffuses across the synaptic cleft and binds to a receptor on the postsynaptic cell. With enough neurotransmitters bound, a new electrical impulse is generated on the postsynaptic cell.

Electrical vs chemical synapses

Electrical:

near-instantaneous, poor modulation, synchrony.

Chemical:

slower, delayed by diffusion and binding, better modulation, no synchrony

Action potential

The change in electrical potential associated with the passage of an impulse along the membrane of a muscle cell or nerve cell

Resting neuron

Not conducting an action potential. Closed Na+ and K+ voltage gated channels, while K+ leak channels remain open. Na+/K+ ATPase pump continues pumping.

Resting membrane potential

Equilibrium condition of a cell. No net flow of ions across the membrane; describes a cell not currently transmitting an impulse.

Equilibrium potential

Membrane potential when the voltage gradient of a particular ion exactly balances that ion’s concentration gradient

Resting potential

-70mV

[Na+]>[K+] outside, [Na+]

Influences where ions cross membrane

1. Diffusion due to a concentration gradient

2. Electric fields due to charge differences

Resting potential causal mechanisms

1. Na+/K+ pump

2. Selective permeability (resting membrane more permeable to K+ than Na+ (leak channels), so net negative charge within the cell)

Ungated/leak channel

Channels that are always open. Often found in resting neurons.

Voltage-gated channel

A channel triggered by a change in membrane voltage. Found in the axon membrane.

Ligand-gated channel

A channel triggered by the binding of a ligand. Found in synapses.

Mechanically-gated channel

A channel triggered by physical deformities. Found in sensory receptors.

Universal functions of members of voltage-gated channel family

Ion conductance, pore gating, and regulation.

Voltage-gated Na+ channel

Voltage-gated K+ channel

Action potential firing sequence

1. Stimulus causes positive charges to flow into the neuron

2. Cell membrane becomes less negative (depolarization)

3. Depolarization occurs slowly until the membrane potential reaches the threshold point

4. Firing (increase in membrane potential) due to increase in positive ions

5. Membrane potential falls below resting potential (hyperpolarization)

6. Membrane returns to resting potential

All-or-nothing principle

Depolarization and firing will occur once the threshold is reached regardless of the strength of the stimulus

Key features of action potentials

1. All-or-nothing principle

2. Maintain size as AP travels along axon

3. Propagate

Action potential propagation mechanism

1. APs generated locally. Resulting currents trigger APs on neighboring membrane stretches.

2. Ions flow between currently-firing and unfired axon segments. Each firing causes the next one.

Refractory period

A period after an action potential fires in which the membrane temporarily cannot respond to new stimulus.

Absolute refractory period

Period where an excitable membrane (absolutely) cannot generate an action potential.

Absolute period mechanistic basis

Occurs because voltage-gated Na+ channels are already open or are in an inactive state. This state may only be removed by repolarization and closing of the membrane pore.

Relative refractory period

Period when an excitable membrane will produce an action potential ONLY in response to a stimulus of greater strength than usual.

Relative period mechanistic basis

Occurs because some voltage-gated Na+ channels have returned to resting (closed) gate status, with some still inactivated and some voltage-gated K+ channels are still open.

Relative period re-firing requirements

1. High magnitude stimulus

2. Stimulus outlasts relative refractory period

Reasoning behind difference in appearance of refractory-period fired action potentials

1. Smaller-than-normal size occurs because of some K+ channels still being open

2. Some Na+ channels still being inactivated

Stimulus intensity encoding

Because an action potential’s peak size never changes regardless of stimulus strength, intensity cannot be encoded by AP amplitude, therefore, intensity is encoded by the frequency of APs. Increased frequency = increased intensity.

Axon diameter differences

Increased diameter yields lower internal resistance, resulting in increased speed of neuronal firing

A-alpha axons

Carry proprioception-related information

A-beta axons

Carry touch-related info

A-delta axons

Carry pain and temperature-related info

C axons

Carry pain, temperature, and itch-related info. Unmyelated, slow.

Quantitative model for relationship between axon diameter and conduction velocity

Cable Theory

Capacitances

Stored electric energy caused by electrostatic forces acting through the phospholipid bilayer.

Resistance (cause)

Due to cytosol’s resistance to movement of electric charge

Measure of the length constant

37% decrease for each length. Approximately 1/3.

Effect of length constant on conductance velocity

Increased measure of lambda (length constant) yields an increase in distance voltage travels down the axon. Voltage reaches a further threshold the greater the value of lambda.

Membrane resistance

Increases with myelin

Benefits of the chemical synapse

Allows neurons to receive inputs from numerous axon terminals simultaneously and facilitates modulation

2 Types of chemical synapses

Direct and indirect

Direct chemical synapses process

1. Neurotransmitter binds directly to ligand-gated ion channel

2. Gate opens/closes, affecting the flow of ions in the postsynaptic cell

Indirect chemical synapses process

1. Neurotransmitter binds to G-protein coupled receptors on postsynaptic membrane

2. Secondary pathway is activated

3. Ion channels are opened/closed and signal is propogated

Direct vs indirect chemical synapses

Direct: Fast. Ionotropic receptors form an ion channel pore.

Indirect: Slower, effects last longer. Metatropic receptors indirectly link to ion channels via signal transduction, often G-proteins.

Neurotransmitter types

Acetylcholine, GABA, glycine, glutamate, norepinephrine and epinephrine, dopamine, serotonin, endorphins, enkephalins, substance P, dissolved carbon monoxide (CO), dissolved nitrogen monoxide (NO)

Acetylcholine

Neurotransmitter between nerves and muscle; in brain and in heart. Related to Alzheimer’s Disease (AD)

GABA (Gamma aminobutyric acid)

Cl- channel opening neurotransmission inhibitor.

Glycine

Neurotransmission inhibitor

Glutamate

Excitatory neurotransmitter involved in learning and memory

Norepinephrine and epinephrine

Dual-role hormones and neurotransmitters that can be excitatory and inhibitory depending on the receptor they bind to. Involved in attention, mental focus, memory, motor control, and the pleasure/reward pathway.

Dopamine

Neurotransmitter/neurohormone involved in behavior, cognition, voluntary movement, motivation and reward, prolactin production inhibition, sleep, mood, attention, and learning. Degeneration of neurons that release this neurohormone is related to Parkinson’s.

Serotonin

Regulates intestinal movements, involved in mood, appetite, and sleep

Endorphins

Works on PNS, released during pleasurable experiences

Enkephalins

A subset of endorphins that works in the CNS and modulates pain response

Substance P

A neurotransmitter released by the spinal cord that increases pain perception

Dissolved CO (carbon monoxide)

A neurotransmitter that regulated the release of hormones from the hypothalamus