Spinal Control of
Movement
Midterm 2 Results
15
15
12
9
Count
Frequency
12
9
6
6
3
3
Mean = 27.90
Std. Dev. = 5.05985
N = 50
0
15.00
0
20.00
25.00
30.00
mid2score
35.00
40.00
A
B
C
mid2grade
D
F
Types of Muscles


Smooth – digestive tract, arteries
Striated:



Cardiac – accelerates or slows heart rate
Skeletal – moves bones around joints, moves
eyes, facial expression, respiration, speech
Skeletal muscles are the somatic motor
system and are under voluntary control.
Types of Movement



Extension – takes limb away from body (opens
penknife)
Flexion – brings limb toward body (closes penknife)
Muscles cannot push so any movement requires
coordination



Synergists – muscles that work together
Antagonists – muscles that pull in opposite directions
Muscles are also named by location:

Axial (trunk), proximal (shoulder, elbow, knee), distal
(toes)
Motor Units


Each muscle fiber is innervated by an alpha motor
neuron.
Bundles of fibers form large and small motor units.



Small motor units act first and control fine motor
movement.
Finer control is possible under lighter loads.
Fast contracting, fast fatiguing white fibers form
“fast” motor units (slow ones are red).

Alpha neuron firing rate makes a fiber/motor unit fast or
slow.
Input to Alpha Motor Neurons

Dorsal root ganglion input to muscle spindle.


Upper motor neurons in the motor cortex and
brain stem.


Provides feedback about muscle length.
Voluntary control of muscles.
Interneurons
Muscle Fiber Structure



Muscle fibers are enclosed by an excitable
cell membrane called sarcolemma.
Within the muscle fiber are cylindrical
structures called myofibrils which contract in
response to the sarcolemma’s action potential.
The action potential releases Ca++ from the
sarcoplasmic reticulum which starts the
chemical reaction resulting in contraction.
How Muscles Contract



Myofibrils are divided into segments by disks
called z lines. The myofibril in between two
disks is called a sarcomere.
Each sarcomere includes both thick and thin
filaments.
During contraction, thin filaments slide along
the thick filaments bringing the z lines closer
together. This is reversed during relaxation.
Chemical Reaction


Calcium binds with troponin (in the thin
filament). This exposes myosin binding sites
on the thin filament’s actin.
Myosin heads in the thick filament bind with
actin in the thin filament, causing the myosin
heads to rotate. This results in the sliding.


This continues as long as calcium is present.
An ATP-driven pump returns calcium to the
sarcoplasmic reticulum and muscles relax.
Reflexes


Reciprocal inhibition – cannot flex and extend
the same muscle
Myotatic (knee-jerk)



Opposes gravity
Uses spindle sensory feedback
Reverse myotatic (knife-clasp)


Relaxes overloaded muscle
Responds to golgi tendon organ feedback
More Reflexes


Flexor reflex – response to pain
Crossed-extensor reflex – compensates for
flexor reflex



One side extends as the other flexes
The circuit for coordinated control of walking
resides in the spinal cord.
Circuits called “central pattern generators” give
rise to rhythmic motor activity.