Lecture Exam 3

Resting membrane potential

Occurs when the inside of the cell membrane is negative compared to the outside of the cell membrane

Voltage separation of oppositely charged particles

• Ranges from -50→ -100 mv depending on cell type

• Only at the cell membrane

Polarization

When the inside of the cell is more negative than the outside of the cell

How is the resting membrane potential produced?

• K+ is in higher concentration inside of the cell

• Na+ in higher concentration outside of the cell

Cell membrane is slightly permeable to K+ → K+ diffusion occurs through leakage channels

Most cell membranes are nearly impermeable to Na+, so not much Na+ can enter the cell

Slightly permeable

Allows for diffusion across the membrane (K+)

Nearly impermeable

Can’t (barely) diffuse across the membrane - Na+ can’t really enter the cell

Role of Na+

As a part of normal cell membrane processes, some Na+ enters the cell

With K+ leakage channels, diffusion of Na+ also contributes minimally

Role of protein anions (-)

Located inside of the cell in a high concentration, but unable to cross the membrane

• Further contribute to the negativeness of the inside of the cell

A or R: All cells have a resting membrane potential

Accept, although primarily focused with muscle and nervous cells

Where is the action potential generated?

On the sarcolemma

Synaptic cleft location

Space between the motor neuron and sarcolemma

Presynaptic membrane

In the motor neuron, before the the synaptic cleft

Postsynaptic membrane

The space after the synaptic cleft

What causes acetylcholine (ACh) to be released into the synaptic cleft?

The nerve impulse/action potential traveling to the end of the motor neuron

Exocytosis

Cells move materials from within the cell into extracellular fluid

In this case, ACh present in vesicles in the motor neuron is released into the synaptic cleft

What is exocytosis/vesicle fusion to presynaptic membrane caused by?

Influx of Ca++

Chemically gated (ligand) ion channels

The binding of the ACh receptor causes the channels to open and allow for the passive diffusion of Na+ across the membrane

A or R: The transportation of Na+ across the membrane is active

R, it is passive diffusion

What enzyme breaks down ACh in the synaptic cleft?

Acetylocholinesterase

Depolarization

The interior of the muscle cell becomes slightly less negative

What does depolarization cause?

An end-plate potential (increases positive charge inside of the sarcolemma)

Voltage-regulated Na+ channels

Stimulated to open by the end-plate potential reaching a threshold (if the stimulus is strong enough)

Action potential

The act of depolarization spreading down the sarcolemma, opening adjacent voltage channels

• Unstoppable

• Results in the contraction of a muscle cell

Excitation-Contraction Coupling

Events by which the transmission of an action potential along the sarcolemma leads to sliding of myofilaments

What is a key structure in nerve/muscle tissue?

Voltage-gated sodium channels

Step 1 of ECC

An action potential travels along the surface of a muscle fiber and down T tubules into internal parts of the cell

Step 2 of ECC

This impulse in the T tubules causes the sarcoplasmic reticulum to release Ca2+ into the sarcoplasm from terminal cisternae

Step 3 of ECC

Ca+ binds to the troponin on actin myofilaments

Step 4 of ECC

Troponin changes shape and pulls tropomyosin off active sites in the actin myofilament

• This action removes the blocking action of tropomyosin - the actin active sites are now exposed

Step 5 of ECC

Myosin binds to active sites on actin - “cross bridge formation”

Cross bridge formation

The attachment of myosin w/ actin within the muscle cell

Step 6 of ECC

Head of myosin changes shape - pulls on thin filament - sliding it toward the center of the sarcomere

• “Power stroke”

• Chemical energy → mechanical energy

• Myosin head = low energy configuration

• Detachment movement releases ADP/Pi

Low energy configuration

Immediately following the “power stroke” - energy has been expended

After EC Coupling

Cross-bridge detachment and “recocking” of the myosin head

Step 7 of ECC

ATP binds to myosin - myosin-actin bond is broken

• Release of myosin causes energy to be used to recock head into high energy configuration (in the presence of calcium)

High energy configuration

When the myosin head is “cocked” - has energy to expend for the “power stroke”

What requires the presence of Ca+?

1. Action potential on nerve impulse

2. Terminal cisternae of sarcoplasmic recticulum

3. Troponin on actin filaments

A or R: If nerve impulses cease, Ca+ is actively transported back into sarcoplasmic recticulum without the use of ATP

R, use of ATP is necessary

Active transportation of Ca+ requires…

• Requires a carrier molecule

• Requires ATP

Step 9 of ECC

Tropomyosin blocks active sites again

Step 10 of ECC

Cross-bridge cycling ends

• Relaxation occurs - actin sliding back to resting position

Step 11 of ECC

Cross-bridge cycling occurs until Z lines are up against the edges of the A band

• Not all myosin heads are attached at the same time

A or R: The myosin head attaches/detaches once during a contraction

R, the myosin head is constantly attaching and detaching many times during a contraction

A or R: Sliding of thin filaments occur as long as calcium ion levels are high enough and ATP is present

Accept

Refractory Period

The interval following stimulation when a muscle fiber will not respond to a second stimulus

• About 1 millisecond (1/1000 of a sec)

When does the refractory period occur?

During depolarization/repolarization of the action potential

A or R: The refractory period is completed before the muscle is fully relaxed

A

A or R: Contractions can build on one another

A, this increases muscle tension

Unfused (incomplete) tetanus

Partial relaxation of the muscle

Tetanus

The point where the pressure can’t build anymore - max force is obtained

Wave summation

The building of contractions on top of one another - partial relaxation - peaks become higher and higher

• The process of going from incomplete to complete tetanus

What happens to muscles in rigor mortis?

Muscles shorten, contract, and stay contacted (tense)

Muscle tension

The force exerted on an object by a contracting muscle

Isotonic contraction

Two types: Concentric/eccentric contractions

• Muscle length changes

• Moves the object

• Thin filaments are sliding/moving

Concentric contraction

Muscle shortens - does work

Eccentric contraction

Muscle generates force when it lengthens

• Contributes more to the strength building of the muscle

Isometric contraction

Tension increases until reaching peak tension, but muscle doesn’t shorten or lengthen

• Cross-bridges are generating force

• “Clenching” a muscle

• Thin filaments are not moving

Muscle twitch

Muscle response to a single stimulus above the threshold

• Duration varies with % of fiber type in muscle

• All humans are a mixture of fiber types

Three phases of a muscle twitch

1. Latent period: First few milliseconds following the stimulation of the excitation-contraction coupling is occurring

2. Period of contraction: Lasts 10-100 milliseconds

3. Period of relaxation: 10-1000 milliseconds, initiated by the re-entry of the Ca+ into the sarcoplasmic recticulum

Types of skeletal muscle fibers

1. Slow twitch oxidative fibers - resist fatigue

2. Fast twitch oxidative fibers

3. Fast twitch glycolytic fibers

Muscles contain a mixture of all three types - vary among different muscles

Oxidative

Use oxygen to generate ATP

Glycolytic

Don’t use oxygen - just go through glycolysis

Graded responses

Influence force of skeletal muscle contraction

• Whole muscles vary in producing tension

• Graded response occurs in two ways

How does graded response occur?

1. Changing frequency of stimulus: Can achieve greater muscular force by increasing the firing rate of motor neurons - wave summation, incomplete tetanus, complete tetanus

2. Changing the strength of the stimulus: Occurs due to motor unit recruitment - by varying the number/size of the motor units that are being recruited - the nervous system can control the degree of contraction

How many of the muscle’s motor units are recruited for maximum stimulation of the muscle?

All of the muscle’s motor units

Electromyograph

Records the changes in electrical potential

• Relaxation/contraction

Motor unit

1 motor neuron and all the muscle fibers it innervates/ supplies

Each motor neuron has ____ axon(s)

1 axon

Nerve

All of the axons wrapped together (larger)

Neuron

1 axon from one motor neuron

The more motor unit attachments to fibers = __________

= the more force produced

A or R: Each axon divides and extends to one muscle fiber

R, each axon divides and extends to many different muscle fibers - forms a neuromuscular junction w/each

A or R: When a motor neuron fires, all muscle fibers that it innervates contract

Accept

A or R: Muscles that perform fine movements have many fibers

R: Few (4-10) fibers per motor unit

ex: ocular muscles

Muscles which perform large movements have: _____

Several hundred fibers per motor unit

ex: weight-bearing muscles like the gluteals, back muscles

A or R: Muscle fibers in a motor unit are concentrated in one place in the muscle

R, muscle fibers are spread throughout the muscle

A or R: Weak stimulation causes a weak contraction of the entire muscle (not just a small section)

Accept

Tonic contraction

Motor units frequently contract asynchronously (not at the same time)

• Produces continual, partial contraction of a muscle

• Ex: muscle tension that maintains posture

• Helps prolong a strong contraction by delaying fatigue

Asynchronous contraction

Alternating motor units

ATP’s role in muscle contraction

1. Disconnect the myosin cross bridge from binding side of actin at the conclusion of the power stroke

2. Re-energize the myosin head in preparation from the next power stroke

3. Actively transport Ca+ back into the sarcoplasmic recticulum

A or R: ATP is used in disconnection and connection of the myosin head

R, ATP is not used in connection

ATP’s role in calcium

Allows calcium to not be available anymore → results in relaxation

Central Nervous System (CNS)

Consists of the brain and spinal cord

• Processes and interprets sensory input - decides what should be done - sends an output/signal

• Integration/control

A or R: The CNS includes cranial nerves

R, the CNS does not include cranial nerves

Peripheral Nervous System (PNS)

Consists of the neural structures outside of CNS

Primarily nerves (bundles of axons)

• Cranial nerves - 12 pairs

• Spinal nerves - 31 pairs

A or R: There are more spinal nerves in the body than cranial nerves

Accept

What is the peripheral nervous system involved in?

• Sensory input - monitors changes inside/outside the body

• Motor output - causing a response by activating effector organs

Afferent division of PNS

Conveys impulses to CNS

Sensory division → PNS → CNS

Efferent division of PNS

Conveys impulses from CNS to effector organs through:

1. Somatic nervous system

2. Autonomic nervous system

Somatic nervous system

• Voluntary nervous system

• Conscious control of skeletal muscles

Autonomic nervous system (ANS)

• Involuntary nervous system

• Cardiac muscle, smooth muscle, and glands

Gland

Anything that secretes

Ex: Adrenal gland, thyroid gland

Divisions of the ANS

1. Sympathetic division: E-division - dominates in exercise, excitement, emergency, embarrassment - speeds up organs associated

2. Parasympathetic division: D-division - dominates in digestion, defacation, diuresis (urination) - speeds up organs that aid in these functions

Unique Properties of Nervous tissue

1. Excitability - ability to respond to changes in stimuli (internal/external) and produce electrical signals

2. Conductivity - can transfer impulse along the neuron (action potential) - voltage-gated Na channels

Electrical signal

Also called an action potential

• Only occurs in muscle and nervous tissue

A or R: Nervous tissue is a very dense tissue

Accept, cells are densely packed/tightly intertwined

Less than ___% of nervous tissue is extracellular space

20%

Neuroglia/glial cells

• Supporting/helper cells

• Make up the majority of cells in the brain/spinal cord - account for 50% of brain mass

• Do not conduct nerve impulses

A or R: Glial cells do not conduct nerve impulses, therefore they do have voltage-gated sodium channels

R, because glial cells don’t conduct nerve impulses, they do not have voltage-gated sodium channels

Astrocytes

Found in the brain - primary type of cells

• Attach neurons to capillaries

• Control chemical environment (ions, neurotransmitters) around neuron

• Provide physical support - prevent bending

• Make lactic acid to provide to the brain

Microglial cells

Found in the brain

• “Nurse cells”

• Monitor health of neurons and transform into macrophages to engulf microorganisms/debris to protect neurons