ch. 13 - spinal control of movement Flashcards
motor system
all muscles and the neurons that control them
behavior requires the coordination of how many muscles?
almost 700
smooth muscle
lines the GI tract, arteries, and related structures; innervated by autonomic nervous system
what is the role of smooth muscles?
peristalsis and blood flow
cardiac muscle
heart muscle that functions without innervation but can be modified by ANS
skeletal muscle
the bulk of muscles mass; muscle within a sheath and composed of hundreds of fibers. innervated by CNS
functions of skeletal muscle
- move bones around joints
- move eyes within the head
- control respiration
- control facial expression
- produce speech
muscles that work in opposite directions
synergists and antagonists
T or F: muscles ONLY pull (shorten), not push
true
axial muscles
move trunk, posture
proximal muscles
move shoulder, elbow, knee, and pelvis - locomotion
distal muscles
move fingers, feet, and hands - manipulation of objects
which neurons command muscle contraction?
ONLY lower motor neurons
lower motor neurons (LMN)
distributed in proportion to skeletal muscle density
LMN controlling flexors are located…
dorsal to extensors
LMN controlling axial muscles are located…
medial to those controlling distal muscles
which type of LMN triggers muscles?
alpha motor neurons
which typer of LMN innervates intrafusal fibers?
gamma motor neurons
motor unit
one alpha motor neuron and all muscle fibers it innervates
motor neuron pool
all alpha motor neurons which innervate a single muscle
what is NT used by alpha motor neurons?
acetylcholine (ACh)
how does the CNS grade muscle contraction?
by controlling firing rate of motor units and by recruiting more motor units
size principle
small motor units have small motor neurons, large have large
are small or large neurons more easily excited?
small
twitch
rapid sequence of contraction and relaxation
“dark meat”
- red muscle fibers
- slow to contract
- slow to fatigue
- present in antigravity muscles
- many mitochondria
- use oxidative metabolism
“white meat”
- fewer mitochondria
- anaerobic metabolism
- rapid
- powerful
- fatigue rapidly
- present in “escape muscle”
fast motor units contain
white fibers
- 30-60 AP/sec bursts
slow motor units contain
red fibers
- 10-20 AP/sec steady
crossed-innervation experiment
muscle switched phenotype to match motor neuron type
sarcomere
a segment compromised of two Z lines and the myofibril in between
myosin
major thick filament protein
actin
major thin filament protein
why is Ca2+ required for muscle contraction?
- it initiates contraction by binding to troponin
- contraction continues as long as Ca2+ and ATP are available
steps of excitation
- AP occurs in alpha motor neuron
- ACh is released by axon terminal at NMJ
- nicotinic receptor channels in sarcolemma open, posysynaptic sarcolemma depolarize (EPSP)
- voltage-gated Na+ channels open, AP is generated in muscle fiber, which sweeps down the sarcolemma and into T tubules
- depolarization of T tubes causes Ca2+ release from sarcoplasmic reticulum
steps of contraction
- Ca2+ binds to troponin
- myosin binding sites on actin are exposed
- myosin heads bind actin
- myosin heads pivot
- myosin heads disengage at the expense of ATP
- cycle continues as long asCa2+ and ATP are present
steps of relaxation
- as EPSPs end, the sarcolemma and T tubules return to their rest potentials
- Ca2+ is sequestered by sarcoplasmic reticulum by an ATP-driven pump
- myosin binding sites on actin are covered by troponin
excitation-contraction coupling
this AP, excitation, triggers release of Ca2+ from organelle inside muscle fiber, which leads to the contraction of the fiber
sliding filament theory
process of sacromere shortening when thin filaments slide toward one another on the thick filament
muscle spindles
- stretch receptor
- specialized muscle fibers in a fibrous capsule
Ia sensory axons
- largest and fastest
- innervate spindle
- enter spine at dorsal roots
- synapse on interneurons and alpha motor neurons
proprioception
body sense
myotatic reflex
- aka stretch reflex
- involves sensory feedback from muscle, shown by cutting the dorsal roots
what is an example of myotatic reflex?
knee jerk
intrafusal fibers
modified skeletal fibers within the muscle spindle
gamma motor neurons
- innervate intrafusal fibers
- activation keeps the muscle “on air” by contracting the poles of the spindle
alpha activation alone..
decreases Ia activity
gamma activation alone…
increases Ia activity
circuit of myotatic feedback loop
gamma motor neuron to intrafusal muscle fiber to Ia afferent axon to alpha motor neuron to extrafusal fiber
golgi tendon organ
- monitors muscle tension or force of contraction
- located in series
what innervates golgi tendon organ?
Ib sensory neurons
reverse myotatic reflex
- keeps muscle from being overloaded
- muscle tension in optimal range
- important for manipulation of fragile objects
joint receptors
- combined with muscle spindle, golgi tendon organ, and skin receptors
- rapidly adapting
synaptic input from spinal interneurons
- primary sensory axons
- descending axons from brain
- collateral of LMN axons
- other interneurons
reciprocal inhibition
the contraction of one set of muscles accompanied by the relaxation of the antagonists muscle
flexor reflex
complex reflex arc used to withdraw limb from adverse stimulus
crossed-extensor reflex
the activation of extensor muscles and the inhibition of flexor muscles on the OPPOSITE side
central patter generators
produce rhythmic motor activity
rhythms are produced by…
pacemaker cells and synaptic connections
rhythmic activity steps
- membrane depolarizes
- Na+ and Ca2+ flow into cell through NMDA receptors
- Ca2+ activates potassium channels
- K+ flows out of cell
- membrane hyperpolarizes
- Ca2+ stops flowing into cell
- potassium channels close
- membrane depolarizes, cycle repeats
