Download Nerve meets muscle

Somatic nervous system
• Signals from CNS are sent to skeletal muscles.
Final result is a muscle contraction.
Alpha motor neurons branch into several
terminals (can be over 1000), each contacting
a separate muscle cell.
• Motor neuron starts in CNS and its axon ends
at a muscle cell.
Alpha
motor
neuron
Nerve meets muscle
Organization of cells
Axon of motor neuron
Action potential
of motor neuron
Terminal button
Voltage-gated
calcium channels
Action potential
propagation
in muscle fiber
Voltage-gated
Na+ channel
Acetycholinesterase
acetylcholine
Motor end plate
1
Sarcomere – the unit of contraction, made of
thin (actin) and thick (myosin) filaments
Myofibril
Sarcomere
sarcomere
Myofibril
Z band
sarcomere
Z band
Z band
Muscle cell
A band
Contraction of filaments
Before Contraction
Z
Z
Myosin
After Contraction
Z
Z
Length of sarcomere shortens with contraction
but filament length is unchanged
Myosin
Actin
2
• Tropomyosin normally covers the myosin
binding site on actin
• When calcium binds with troponin, it
pulls tropomyosin away from the binding
sites
Actin
troponin tropomyosin
myosin
actin
myosin
binding site
blocked
Calcium
myosin
actin
cross-sectional view
Figure 8.2 (3)
Page 259
• Signal from motor neuron causes action
potential in muscle cell
Sarcomere
Myosin
Muscle Contraction
Actin
• Calcium ions released (from sarcoplasmic
reticulum)
• Actin and myosin filaments slide relative to
each other
Actin
Myosin
3
BINDING Myosin cross
bridge binds to actin
Myosin cross bridge
Figure 8.13
Page 267
Myosin needs ATP to
change shape
Energized
POWER STROKE Cross
bridge bends, pulling thin
filament.
DETACHMENT Cross bridge
detaches and returns to
original shape- *ATP
required*
BINDING to next actin molecule;
repeat
Signal coming to muscle
motor
neuron
Sarcoplasmic reticulum
(Ca+2 storage)
T tubule
Resting
Binding
Detachment
Bending (power stroke)
Myosin has a binding site for ATPase
Rigor complex
From action potential to contraction
• Calcium is the link
– Acetylcholine released at the neuromuscular
junction - action potential on muscle fiber
– Action potential down “T tubule” to sarcoplasmic
reticulum at muscle fibers
– Calcium released from the SR to muscle fibers
4
Pathway review
Terminal button
T tubule
Action potential
Muscles contain groups of motor units
Units are recruited during motor activity
Muscle force depends on # muscle fibers
contracting
Acetylcholine-
Acetylcholine gated cation
channel (Na+
moves in)
Tropomyosin
Actin
Troponin
A calcium pump
in SR allows
muscle to relax
Cross-bridge binding
Motor unit = motor neuron +
muscle fibers it innervates
Myosin cross bridge
The number of muscle fibers varies
among different motor units.
– muscles can have many small units or a
few large units
Tension and frequency of stimulation
twitch - brief contraction
resulting from 1 action pot’l
tetanus - twitch summation
from sustained Ca+2
– Asynchronous recruitment of motor
units delays or prevents muscle fatigue.
5
Muscle length and force
Fast and slow twitch muscle cells
Differences in time when maximum tension is
reached
Slow twitch (Type I) - have myoglobin, many mitochondria, oxidative
Fast twitch (Type IIa) - myoglobin, mitochondria, oxidative & glycol.
“Very” Fast twitch (Type IIb) - use glycolysis, split ATP quickly
Fast and slow twitch muscle cells
Oxidative - resistant to fatigue, high rate
of O2 transfer from blood, recruited 1st
• People are born with certain ratio of slow
vs. fast twitch fibers
– usually an even mix in most skeletal muscles
Glycolytic - more prone to fatigue b/c
less ATP produced, harder to recruit
Endurance vs. Bursts of power
6
Sensation at muscle
Spindle muscle fibers (deep within muscle) sense
stretch, and Golgi tendon organs (in tendons)
sense tension.
Intrafusal (spindle) muscle fibers
Knee spinal reflex
Muscle
spindle
Extensor muscle
motor
neuron
Patellar tendon
Primary types of contraction
• Isometric contraction -
muscle tension is not enough
to move load. Muscle
doesn’t shorten.
• Isotonic contraction
– Concentric – muscle
shortens to lift a load.
– Eccentric - shortened
muscle has controlled
lengthening.
slowly lowering the weight
7
Exercising your muscles
Endurance training
type IIb
type IIa
Exercising your muscles
Strength training
hypertrophy of type
II fibers
more mitochondria, glycogen,
vascularization
Hypertrophy: how muscles get bigger
Muscle cells have satellite cells nearby that
respond to muscle injury and wear
Hypertrophy: how muscles get bigger
Satellite cells:
– activated at
microtears
– add nucleus to
muscle cell
– more myofibrils
made
– cell wider
Why are muscles sore after lifting?
8
Muscle hypertrophy vs. hyperplasia
How can muscle recruitment
change with exercise?
• The CNS can become trained to provide
more force (apparent in early training)
Hyperplasia
Hypertrophy
ATP sources at muscles
3a
1
– Better inhibition of antagonistic muscles
– Improved recruitment of different muscles
over a movement to gain power
What does creatine do?
2
3b
9
What are muscle cramps?
• When muscle fibers contract without our control it
is a muscle spasm or cramp
• Due to motor neurons being hyperexcited, often
b/c of a shift in body fluids or ion levels
(dehydration, low Ca, Mg, K) or vigorous activity
Smooth muscle
• Smooth muscle cells
are small and
unstriated
– No sarcomeres
– Smooth muscle cells
contract when Ca+2
enters
– Myosin cross bridges
are phosphorylated
and bind to actin
• Is there a point where being muscular negatively
affects our bodies?
• What is largest muscle in the body and why
• Why do our eyes twitch sometimes, seemingly
for no reason.
10
Smooth muscle
Smooth muscle
Striated muscle
• Multiunit - similar to
skeletal motor units
• Single unit - gap jxns b/w
muscle cells. Many cells
contract as a unit. (uterus,
intestine, bladder)
Cardiac muscle
Pacemaker muscle cells - action potential gradually
depolarizes, then repolarizes
Contraction spreads from pacemaker through gap jxns
Spontaneous action potential
Pacemaker cell
Action potential spread
to other cells
Gap junctions
11