MOTOR SYSTEM – Muscle, LMC, Spinal cord mechanisms of control

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Nov 14, 2013 (3 years and 6 months ago)

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MOTOR SYSTEM



Muscle, LMC, Spinal cord mechanisms of control


-

Motion around a certain joint creates a mechanical effect such that each successive joint is
disturbed and must be stabilized

-

To localize lesion in motor system is presence of absence of

we
akness


-
U
pper motor neurons (UMNs
)

lower motor neurons (LMN
s
)
, NMJ, Muscle

fibers

present weakness

if damaged (motor cortex


muscles)


-

Motor cortex gets input form
basal

ganglia

(via thalamus),
cerebellum

(via thalamus),
and other
cortical areas
; Damag
e to one of these
does not result in weakness, but produces
disorders of the
quality of movement


2.


A muscle is innervated by several hundred LMNs (cell bodies in anterior horn of spinal
cord)

-

A motoneuron,
with its axon and all the muscle fibers that

it innervates is called a
motor unit

-

Each muscle fiber innervated by single
LMN

and has a single
NMJ

-

AP

along
LMN

reaches nerve terminal at
NMJ



calcium ions

enter nerve terminal due to
depolarization

and
Ach

is released from the nerve terminal

-

Ach

binds its
receptor

on surface of muscle fiber


depolarization



AP

along full length of
myofiber

(both directions)

-

As AP spreads along surface of myofiber, it travels
into T
-
tubules

(transverse tubules)


long
invaginations of plasma membrane into the
myofiber

-

T
-
tubules aligned along junction between A
-
band and I
-
band; T
-
tubules are closely apposed to
sarcoplasmic reticulum (SR). Two cistern of SR associated with single T
-
tubules


triad
.

-

At each triad, voltage
-
gated calcium channels in T
-
tubule mem
brane are coupled to a different
class of calcium channel in SR channel

-

AP in the T
-
tubule causes release of Ca
2+

from SR


rise in [Ca
2+
] causes activation of
sarcomeres and contraction of myofiber

-

One AP in LMN caused one AP in each of the muscle fib
ers it
innervates


1.
EMG



electrodes detect sum of electrical activity of active muscle fibers; APs of each
muscle fiber belonging to a single motor unit add up to produce a
compound AP

(EMG records
a sum of compound APs)


Myopathic disease

-

Muscle fibe
r dysfunction


no APs in muscle fibers


fewer surviving muscles in each motor
unit that can generate an AP


amplitude of EMG record is reduced


Neurogenic disease

-

Damage to
LMN

has abnormal finding
s


-

fibrillations



spontaneous contractions of indiv
idual
muscle fibers
(small spikes on EMG
record); small potential, low frequency; fibrillations arise from
increased expressions

of Na and
Ca
channels

in denervated muscles


nearby intact fibers activated previously and
residual
ionic currents
cause
depol
arization

in adjacent denervated fibers (with increased channels)


-
fasciculations



spontaneous activation of
motor units

(spontaneous activation of LMN
involving
Ach
); visible as
twitching
; can be abolished by Ach blocker and induced by AchAse
blocker;


-

EMG can detect
Giant units



abnormally large compound APs (show when voluntary
movements are attempted); occurs due to
sprouting
: motoneurons die and lose innervation of
muscle fibers


nearby intact motoneurons make
new collateral branches

and send tho
se
branches to innervate vacant slots
on

denervated muscle



surviving motor units

include
more muscle fibers



recorded compound AP increases in seize


Hyperactive reflexes


UMN lesion (motor cortex of CST)

Normal or reduced reflexes



LMN or muscle lesi
on (including NMJ); to distinguish between
LMN and muscle look at EMG (fibrillations, fasciculation, etc.


LMN, reduced amplitude


muscle if constant weakness or NMJ if fluctuating weakness)


3.
Automatic functions of spinal cord


automatic regulation o
f muscle force

Types of motor units

Motor Unit Type

Motoneurons
Properties

Muscle Fiber
Properties

Histological Type of
muscle fiber

Slow twitch
(S)

Small, slow
conducting axons

Low force, slow
contraction, fatigue
-
resistant, oxidative
metabolism

Type I (
smallest)

Fast
-
twitch, fatigue
resistant
(FR)

Medium size,
moderate
-
fast
conducting

Moderate
-
high forces,
fast contraction,
fatigue
-
resistant

Type IIa (medium
size)

Fast
-
twitch, fatigable
(FF)

Large, fast
-
conduction

High force, fast
contraction, fatigabl
e

Type IIb (largest)


-

Properties of motoneurons match properties of muscle fibers they innervate

-

Muscles contain a mixture of the 3 types of motor units


4.
How muscle force is increased



1
) increase

number of motor units activated, 2) increase
disch
arge rates of already firing motor units


Rule #1



Fixed order of recruitment

according to
size principle
; smallest motoneurons activated
first, and successively larger motoneurons activated as more force needed;
S


F删





-

Smaller motoneuron

has fewe
r parallel ion channels and has a
higher total cell input
resistance

-

Larger motoneuron

has more parallel ion channels and has a
lower total cell input resistance

-

according to
V=IR

(I is constant as the CST UMN);
R

is
high

in
small

motoneurons and
low

i
n
large

motoneurons

-

V

of the
small

LMN is
higher
, and
V

of the
large

LMN is
lower
;
small motoneurons

will
reach
threshold first
, and the
large motoneurons

will reach
threshold later


Rule #2



fixed sequence for
firing rate of motoneurons
;

when fi
r
st act
ivated, a motoneuron may
fire at 10 Hz; when it receives more excitation is will increase its firing up to 25 Hz

-

Increasing the
discharge rate

of an already active unit increases its
force output

until it
reaches
maximum force

(tetany)


-

Both rules lead

to
increases

in
force

that are
smooth


5
.
-

A network of interneurons responsible for generating an automatic rhythmical movement is
called
central pattern generator (CPG)
; CPGs are in the
spinal cord
and
brainstem

for
respiration, chewing, swallowing, an
d locomotion

-

once a CPG is activated it can product the complex muscle instruction by its own subroutine;
does not need descending cortical input or sensory feedback

-

A CPG
reduces

the amount of
planning

that the nervous system at other levels need; A C
PG
established the
timing signals

and
basic motor pattern

at a lower level;
cortical areas

and
sensory feedback

modulate

these elementary instructions


Locomotor central pattern generator



network of
interneurons

residing in
spinal cord
; most
unconscious
and automatic part of the motor system (like walking)

-

capability for
rhythmical stepping

after cord transection and deafferentation; CPG is
dependent on
descending influences
; the locomotor CPG provides
timing instructions

for
alternating activity

in
fle
xor

and
extensor

muscles at each joint of the leg: hip, knee, ankle

-

locomotor
CPG

requires
sensory sources

and
cortical, higher motor center sources
:
cutaneous sense, position sense, and force feedback from limbs


6.
Reflexes during walking



the network

of spinal cord circuitry for reflexes must interact with
the circuitry for the locomotor CPG

-

If the dorsum of the foot is pinched during the
swing phase

of locomotion


withdrawal
response

in which
flexor muscles

of the stimulated leg are activated


fo
ot is drawn up

and
away

for stimulus

-

If the dorsum of the foot is pinched during the
stance phase
of locomotion (leg is accepting
body’s weight)


extensor muscles

are activated to produce an
extension response

in the
standing leg

to move away from the s
timulus


7. Brainstem



CPGs

for breathing, chewing, and swallowing
; receive

sensory

and
cortical

input

to adapt the basic instructions for the functional and environmental context


Respiration



nuclei in
pons/medulla;

two medullary centers


a
dorsal

and

ventral

respiratory group in
reticular formation
,
pons,
and
medulla



timing instructions

to
inspiratory and expiratory muscles;

-

output of two respiratory centers activates

trigeminal, facial, glossopharyngeal, vagus, and
hypoglossal nerve (and spinal
input to
diaphragm

via
phrenic nerve

and intercostals,
abdominal muscles)

-

neurons in respiratory gro
u
p
s include
pacemaker neurons



spontaneous rhythmical
oscillations


APs to quiet to APs; CPG provides
alternating EPSPs and IPSPs

to
respiratory
motoneu
rons

-

breathing requires
sensory

and
cortical

input

to respiratory
CPG

-

chemoreceptors, pulmonary stretch receptors

and muscle spindles, provide
sensory
feedback

about blood gases and muscle work to
regulate breathing muscle pattern

-

cortical input

nece
ssary for adjusting breathing pattern to hold breath, whistle,
talk
.


Chewing



repetitive jaw opening and closing;
initial prep phase



food from anterior to
posterior of mouth;
reduction phase



food is torn and crushed to form bolus

-

the
CPG

for chewin
g isolated to gigantocellular reticular nucleus in
reticular formation

in
medial pons/medulla to motoneurons (
trigeminal

and
facial)

-
sensory
and
cortical inputs

sculpt the rhythmical jaw movements into everyday patterns of
chewing;
muscle spindle

informat
ion

(trigeminal mesencephalic nucleus) for jaw closure
muscles
modifies timing

of
switch

from jaw
opening

to jaw
closing
; feedback from
pressure
sensors

in teeth
modify force
and
timing

of chewing (jaw closure);
cortical input

involved in
modulating chewin
g pattern


Swallowing



three phase: oral, pharyngeal, and esophageal;

Pharyngeal phage
:

1. Hyoid bone elevated and moved anteriorly

2. Larynx is adducted

3. Soft palate elevated

4. Pharynx is constricted

5. Esophageal sphincter is relaxed

-

Muscles recei
ve alternating
excitatory

and
inhibitory
input t p produce sequenced EMG
pattern producing these steps

-

CPG in
nucleus of Solitary tract

and

dorsal motor nucleus

(X)


LMN (nucleus ambiguus)

-

CN X (Superior laryngeal nerve)


on
-
switch

for pharyngeal

CP
G

(activated by cortical or
sensory input)

-

sensory/cortical info
;
chemoreceptors

at dorsum of tongue, epiglottis, pharynx walls carries
with V3, IX, and X


modify duration of EMG

and
force

of muscle contractions of
swallowing pattern;
cortical areas



v
oluntary swallowing