Outline
?
Review of Muscle Contraction
–F
rom AP generation to contraction of fibers
–
Muscle proprioceptors (spindles and Golgi tendons)
–
Afferent and efferent axons
Muscle Strength Loss in Spaceflight
?
S
tart with Earth-normal muscle magnitudes and directions:
for mid-stance
[Cheal
et a
l
.,1992]
:
Mag
(
BW
)
X
(med-
lat)
Y (post-ant)
Z
(dist-prox)
Gluteus
medi
u
s
0.80
-0.67
0
.18
0
.72
Gluteus
minim
u
s
0
.30
-
0.78
0.21
0.59
Iliopso
a
s
1
.30
-
0.10
0.73
0.68
?
R
educ
e muscle strength with duration of weightlessness:
–
40% lowe
r a
t
6 mo
nth
s
, 6
0
%
lower
at 1
2
mont
hs, ba
sed on
lit.
?
21% lo
w
e
r pe
a
k
activated force 17 da
y flig
ht
[W
idrick
et
al.,
1999]
?
120 da
ys of H
D
T
bed rest
[Ko
r
y
a
k, 19
99
]
:
–
44% / 33% (M/F
)
decline i
n
is
ometric ma
x. voluntar
y co
ntra
ction (MVC)
–
36% / 11% (M/F
)
decline i
n
is
ometric t
w
itc
h
contraction (Pt)
–
34% / 24% (M/F
)
decline i
n
tetanic
c
ontractio
n force (Po)
?
M
aximal e
x
plo
s
ive po
w
e
r (MEP) reduced to
67% after 31 d
a
y
s, an
d to
45% after 180
da
ys of space flight
[Anto
n
u
t
to
e
t
al.,
19
99
]
Muscles: Effectors of the Motor System
?
The major output of the elaborate information processing that takes place in our brain is the generation of a contractile force in our skeletal muscles.
?
Muscle fasciculus
–
Muscle fiber
?
Myofibril
–
Sarcomere
?
Each muscle fiber is innervated by only one motor neuron, although each motor neuron innervates a number of muscle fibers
?
The motor neuron and all the fibers it innervates is called a motor unit (the smallest functional unit controlled by the motor system)
Muscles: Effectors of the Motor System
?
The number of muscle fibers innervated by one motor neuron is called the innervation ratio. The innervation ratio can vary between 10 and 2000
?A
low innervation ratio indicates a greater
capacity for finely grading the muscle total force
Muscles: Effectors of the Motor System
A simplified sequence from AP generation to muscular contraction ?
Motor neuron fires an action potential
?
It propagates down the motor axon until it reaches the
neuro
-muscular junction
?
It triggers an AP in the muscle fiber
?
This AP is propagated rapidly over the surface of the fiber and conducted into the myofibril by mean of the T-tubule system
?
This in turn releases Ca
++
from the Sarcoplasmic Reticulum (SR)-the SR serves as
a store of Ca
++
?
This in turn triggers the cyclic motion of Myosin heads, attaching and detaching on the
Actin filaments, thus forming cross-bridges and generating the pulling force
?
C
a
++
are pumped back to the SR
Muscles: Effectors of the Motor System
?
The force of contraction depends on the length of the muscle (length-tension relationship)
?
The force of contraction also depends on the relative rates of movement of the Actin
and Myosin filaments (tension-velocity
relationship, Hill
’s curve)
?
Motor units are recruited in a fixed order from the weakest to the strongest (Henneman size principle): The weakest inputs recruit the slow units which generate the smallest force and are most resistant to fatigue. The fast fatigue-resistant are recruited next, followed by the fast fatigable units which generate the strongest force.
Muscles: Effectors of the Motor System
?
Muscle Proprioceptors (spindles and Golgi tendons) There are different types of receptors which respond to light, sound, odor, heat, touch, pain, etc. The receptors which lead to conscious sensations are called
exteroceptors
, those which are not responsible for conscious
sensation are called-primary in motor functions- are called
proprioceptors
–
Spindle organs
Those are stretch receptors scattered deep within all muscles. They are usually attached in parallel with a muscle fiber, and therefore experience the same relative length change.
Spindles give information about its length and rate of change of its
length
–
Golgi tendon
They are found very close to the junction between tendon and muscle fibers.
They are placed in series with the muscle fibers and respond to the tendon stretch which accompanies a muscle tension. Thus they are
force
transducers
for the muscle.
Muscles: Effectors of the Motor System
?
The nerve axons which run out of the spinal cord are called efferent
, the ones that carry information to the cord are
afferent
?
Group I afferent fibers have large diameters therefore relatively high conduction velocities. They bring information from the spindle (
I
a
)
and the golgi (Ib) to the
cord
?
The efferent which innervate the main muscle mass are the α
, and those that serve the intrafusal fibers within the
spindles are called
γ
?
The stretch reflex, co-activation of
α
-
mn
and
γ
-mn
Muscles: Effectors of the Motor System
Stretch reflex stiffness ?
Until recently, it was supposed that the tendon organ served as a sensor which turned off muscle activity (inhibited
α
-mn) when muscle force rose beyond safe
levels
?
Afferent activity from both spindles and Golgi tendons balance in such a way that neither muscle force nor muscle length should be considered as controlled quantity, rather their ratio (the stiffness or change in force per change in length) appears to be fixed by the stretch reflex
Muscles: Effectors of the Motor System
?
The sensorimotor cortex
is at the top of the chain of command in the
sensorimotor area of the cerebral cortex. There is a specialized area in the cerebral cortex devoted to movement of the limbs (1691, the case of a knight with a fractured skull and paralysis of the left side of the body)
–
The fraction of the cerebral cortex controlling each part of the body is by no means proportional to the size of that part
–
If the cerebral cortex is removed, the animal continues to display all the locomotion reflexes, but cannot learn new skills
?
Basal ganglia
are a set of specialized nerve cells in the brain stem.
?
Cerebellum
is a major focus of incoming sensory information. The
information reaching the cerebellum has to do with length, force, velocity of muscles and position of joints.