Transcript Lesson 6: Diagram of a Motor Unit

Topic 4.1 Neuromuscular Function
• Neuron
Interneuron
• Dendrite
Motor neuron
• Cell body(soma)
Reflex arc
• Nucleus
Motor end plate
• Axon
Neuromuscular junction
• CNS/PNS
synapse
• Receptor
motor unit
• Sensory neuron
innervate
acetylcholine (Ach)
Cholinesterase
exocytosis
depolarization
T-tubule
Sliding Filament Theory
Tropomyosin/Troponin
Sarcomere structure
Basic vocabulary (p76 text)
• Nucleus: control center of cell
• Dendrites: carry signal to cell body (soma)
• Axon: Carry impulse away from soma to next neuron, muscle or gland.
Covered in myelin for insulation.
• Cell Body (Soma): in CNS axons and dendrites extend out.
Basic vocabulary (p76 test)
• Central Nervous System (CNS): Brain & Spinal Cord
• Peripheral Nervous System (PNS): extends from CNS into limbs
• Sensory Neuron (afferent neurons): pick up environmental stimuli
(senses, bp, blood O2..)
• Interneuron: link sensory & motor neurons in CNS.
• Motor neuron (efferent neuron): carry signal from CNS to muscles/glands
Diagram of a Neuron
dendrite
Myelin sheath
axon
Cell body
Nodes of
Ranvier
Terms to Know:
Dendrite
Cell body
Nucleus
Axon
Schwann Cell
Myelin Sheath
Node of Ranvier
Sensory
Relay (interneuron)
Motor
Reflex Arc
Reflex Arc
reflex arc video
Motor Unit
Motor Unit:
a single motor neuron
and the muscle fiber(s)
it innervates.
Innervate- supply an
organ /body part with
nerves
Innervation Ratio:
Number of muscle fibers stimulated
by one motor neuron.
Muscles that control fine movements
(fingers, eyes) have small motor
units
Large weight-bearing muscles
(thighs, hips) have large motor units
Motor Unit
Types of Muscle Units that are innervated.
Type I: Slow Twitch Motor Unitsslow contraction- small force produced – ex. Marathon runner
Type IIA: Fast Twitch Motor Unit
fast contraction- strong force produced – ex. sprinter
Type IIB: Fast Twitch Motor Unit (fastest)
fastest contraction- strongest force produced –ex. Punching
Control of Muscle Force.
How many muscle fibers are recruited to do an action?
Size Principle: When smaller motor units are recruited first and only
when larger forces are required will larger motor units be recruited.
Neuromuscular Function
1. What are the three types of muscle fibers? Briefly describe each
Slow Twitch (type I): slow contraction and high resistance
to fatigue
Fast Twitch A (type IIA): moderate resistance to fatigue
and are a transition between the other two types of fibers.
Fast Twitch B (type IIB): very sensitive to fatigue & used
for short anaerobic, high force production.
Neuromuscular Function
2. Discuss the size principle.
Type I
(Slow)
type IIA
type IIB
(intermediate)
(fast)
The order fibers are recruited to move the body
Neuromuscular Function
3. What can alter the size principle?
Eccentric muscle contractions (muscle lengthening) can
change the recruitment pattern.
Fast twitch can be recruited first, then the slow twitch if
the speed of the exercise is moderate to fast.
Neuromuscular Function
4. What is the only direct way to determine muscle fiber type?
Muscle biopsy
Neuromuscular Function
5. How can we indirectly determine fiber type
in a weight room?
Example:
establish a 1RM of any exercise.
lift 80% of 1RM as many times as possible.
7 or less reps most likely more than 50%FT fibers
12 or more reps most likely more than 50% ST fibers
Limitation: can only be used for muscle groups. Why?
Neuromuscular Function
6. Since we cannot change our muscle fiber composition,
how can we train our bodies to become better at certain
activities?
What we are born with is what we must live with.
You cannot increase the number of a specific fiber type but you
can hypertrophy a specific type of fiber through specific
training , thus increasing the volume of that specific fiber type.
Neuromuscular Function
7. What are the training recommendations for the following?
Increase maximum strength:
Increasing maximum strength
by stimulating muscle
hypertrophy:
Increasing muscle size with
moderate strength gains:
95% of 1RM, 1-3 rep range.
80% of 1RM, 5-8 rep range.
6-12 rep range.
Small
medium
large
High
High
High
high
high
high
low
low
low
Low
Low
high
medium
high
high
High
medium
low
Slow
Fast
Fast
Low
High
High
High
High
medium
Medium
low
Low
Low
High
High
Endurance
Sprint/ Walk
Power-pitch, hitting
All-or-Nothing Response
All the muscle fibers that are connected to a single motor
neuron either contract or relax at the same time.
Muscle Twitch
A muscle twitch is the response of a muscle to a single action
potential of its motor neuron. The fibers contract quickly and
then relax.
Neuromuscular Junction
Connection between nervous system and muscular system
Neuromuscular Junction
Motor End Plate: The
large and complex endformation by which the
axon of a motor neuron
establishes synaptic
contact with a striated
muscle fiber (cell).
Role of Neurotransmitters
What is a neurotransmitter?

Neurotransmitters are chemicals that are used for
communication between a neuron at the synaptic
cleft and another cell.
Acetylcholine (Ach):
increases the postsynaptic membrane’s
permeability to sodium
and potassium ions
spreading the impulse
over the entire muscle
fiber.
Neuromuscular junction
Role of Neurotransmitters
Neuromuscular Junction
Cholinesterase is an enzyme that breaks down Ach,
repolarizing the muscle fiber to await another nerve impulse.
Use with page in packet with the title, ‘The neuromuscular junction’.
1. Action potential arrives at
axon terminal
2. Depolarization opens Ca+
channels & Ca+ enters
axon terminal.
3. Ca+ stimulate synaptic
vesicles to fuse with
membrane.
4. Exocytosis of Ach into
synaptic cleft.
5. Ach binds to Ach-receptor
sites on post synaptic side.
6. This creates a
depolarization
7. Which allows the action
potential to continue to the
T-tubule which stimulates
the sarcoplasmic reticulum
to release Ca ions into the
muscle
8. Cholinesterase is released
and breaks down the Ach.
This will end the action
potential and the muscle
contraction.
What happens at the neuromuscular junction?
Parts of the Neuromuscular Junction
Excitation Contraction Coupling (Action Potential across the Neuromuscualr Junction)
A Quick Review of the Neuromuscular Junction
Terms to know:
Sliding Filament Theory
Tropomyosin: An actin-binding protein
which regulates muscle contraction.
Troponin: protein that is bound to tropomyosin
and blocks the binding of the myosin head.
The sarcoplasmic reticulum: specialized endoplasmic
That is a storage site for calcium ions used during muscle
contraction.
T-Tubules: are extensions of the sarcolemma. They allow
depolarization of the sarcolemma to quickly penetrate to the
interior of the cell.

Explains how muscle fibers shorten
during a contraction.
Sliding Filament Theory
Sliding Filament Theory
Starting position for muscle contraction:
Troponin is bound to
tropomyosin on the actin
filament
Myosin heads are waiting for
the binding sites to open
Steps of a muscle contraction:
1. Ca++ are released by the
sarcoplasmic reticulum.
2. Ca++ binds to troponin releasing
it opening up the binding sites
on the tropomyosin
3. The myosin heads can now bind
to tropomyosin
Sliding Filament Theory
Sliding Filament Theory
4. Using energy from break down of ATP*, the myosin
heads pulls on the actin causing a muscle contraction
(power stroke)and the ADP + P to be released.
*ATP
ADP + Pi + ENERGY
5. myosin head releases the actin when a new ATP** is
formed and binds to myosin head.
**ADP +Pi + Energy
ATP
Sliding Filament Theory
Sliding Filament Theory
6. Immediately after the myosin head tilts, it breaks
away from the active site, rotates back to its original
position, and attaches to a new active site farther
along the actin filament. Repeated attachments and
power strokes cause the filaments to slide past one
another, giving rise to the term sliding filament theory.
This process continues until the ends of the myosin
filaments reaches the Z-disks, or until the Calcium is
pumped back into the sarcoplasmic reticulum.
Terms to know:
Structure of a Sarcomere
Myofilaments : actin(thin) & myosin (thick).
Sarcomere: basic unit of muscle. (Z line to Z line)
H zone: thick filament (myosin) only. (gets smaller as muscle shortens)
I band: thin filament (actin) only. (Light color section)
A band: overlap of actin & myosin. (Dark color section)
Z line: (disc) connects I-bands. (Z to Z is a sarcomere)
M line: in the middle of H zone, connect thick filaments.
Structure of a Sarcomere
Sliding Filament Theory

During this sliding
(contraction), the thin
filaments move toward the
center of the sarcomere and
protrudes into the H-zone,
ultimately overlapping.
When this occurs, the H zone
is no longer visible.