Psy 111 Basic concepts in Biopsychology

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Psy 111 Basic concepts in Biopsychology Lecture
8 Spinal Control of Muscle
Website http//mentor.lscf.ucsb.edu/course/summer
/psyc111/
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Objectives

• Describe the anatomy, structure, and activation
  of skeletal muscle.
• Describe the neuromuscular junction and its
  control of skeletal muscle.
• Define motor units, pools, and muscle groups.
• Describe the basic reflexes myotatic, reverse
  myotactic, reciprocal inhibition, and the flexsor
  reflexes.
• Explain the relation between basic reflexes and
  walking at the spinal level.

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Motor Systems

• Structure and activation of muscle
• Neuronal innervation of muscle
• Spinal reflexes
• Descending motor pathways (next lecture)
• Brain control of motor system (next lecture)

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Appearance and Innervation of Muscle
Muscle is made up of fibers that appear striated
and innervated by axons from the spinal cord in a
non-overlapping fashion. Motor neuron excitation
produces muscle contraction basis of all
movement.
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Anatomy of a Muscle fiber

• Myofibril bundles comprise a muscle fiber.
• Fiber is encased in sarcoplasmic reticulum.
• T-Tubules connect the sacroplasmic reticulum to
  sarcolemma.
• Sarcolemma is an excitable membrane (nAChRs)
  surrounding muscle fibers.

Activation Sarcolemma -gt t-tubule -gt
sarcoplasmic reticulum -gt myofibrils.
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Structure of myofibril

• Myofibril comprises of two types of filaments
  thick and thin.
• Thin filaments are attached to Z-line.
• Thick filaments held in place between the thin
  filaments.
• The basic unit of a myofibril is the sacromere
  which is comprised of the thin and thick
  filaments between a Z-line.

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Contraction of Myofibril
Sacromere shortens and generates force through
the movement of thick and thin filaments relative
to each other. This decreases the distance
between the Z-lines. Note filament length does
not change but distance between serial thin
filaments decreases.
Process activated by Ca
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Functional Components of a Sacromere
Thin Filaments contain actin, tropomyosin, and
troponin. Thick Filament contain myosin ATP
binding enzyme.
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Thick Filaments

• Thick filament is comprised of multiple myosin
  proteins which can
• bind ATP
• (2) crosslink to thin filaments.
• (3) move/bend (generate force)

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Thin Filaments

• Thin filaments contain 3 proteins
• G actin which binds myosin
• Tropomyosin which can block myosin-actin binding
• Troponin which binds Ca to remove tropomyosin
  block of myosin-actin binding.

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Filament Movement

• At rest
• Tropomyosin blocks Myosin-binding site on Actin.
• Contraction Triggered by Ca
• Ca binds troponin producing shape change in
  tropomyosin exposing myosin binding site on
  actin.
• Myosin binds to actin.
• Myosin bends, pulling the actin filament.
• System resets.

Movement requires Ca signal and uses ATP energy.
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Filament Movement(detailed)
Binding controlled by Ca via troponin
influencing tropomyosin.
Contraction During contraction, the thin actin
filaments slide over the thick myosin
filament. When Calcium is present the blocked
active site of the actin clears. 1. Myosin head
attaches to actin. (High energy ADP P
configuration) 2. Power stroke  myosin head
pivots pulling the actin filament toward the
center. 3. The cross bridge detaches when a new
ATP binds with the myosin. 4. Cocking of the
myosin head occurs when ATP binds to myosin as
ADP P. Another cross bridge can form.
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Sarcoplasmic reticulum Ca
Sarcoplasmic reticulum stores Ca and releases
it in response to muscle cell depolarization. Note
similar to axon terminal vg-Ca channels or
smooth endoplasmic reticulum in 2nd messenger
system
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Control of Contraction
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Excitation-Contraction Coupling
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Motor Systems

• Structure and activation of muscle
• Neuronal innervation and recruitment of muscle
• Spinal reflexes
• Descending motor pathways
• Brain control of motor system

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Neuromuscular Junction
Motor neuron releases Ach on muscle to activate
nAchR (Na/K channel) resulting depolarizaiton
leading to contraction. -large/massive synapse
-postsynaptic cell has 1 input -strong/reliable
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Lower Motor Neurons (a.k.a alpha motor neurons)
Lower motor neurons cell bodies are in the
ventral horn of spinal cord and send axons via
the ventral root to innervate muscle fibers
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Motor Unit
Muscle fibers of motor units do not have
overlapping innervation i.e. only 1 neuron to
each fiber/no convergence.
Motor Unit Single motor neuron and the muscle
fibers that it innervates. Note alpha motor
neuron lower motor neuron.
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Motor Pools
Rem each motor neuron and innervated fibers
motor unit, thus, motor pool is the group of
motor units to a muscle
Motor neuron pool all the motor neurons that
innervate a single muscle
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Organization of Muscles and Bones
Muscles attach to joints via tendons. Muscle
contraction causes either -flexion (shortening
of limb) or -extension (lengthening of
limb). Muscles are termed either flexors or
extensors. Muscles with common functions are
synergistic. Muscles with opposite functions
are antagonistic.
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Spinal Organization of lower motor neurons
The arrangement of motor neurons is matched to
arrangement of muscle fibers (i.e. topographical
arrangement of neurons) -medial to lateral
portions of the ventral horn innervates axial to
distal muscles. -dorsal to ventral portions of
ventral horn innervates flexors to extensors.
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Spinal Organization of lower motor neurons
Size of ventral horn is proportional to number of
lower motor neurons. Number of lower motor
neurons is determined by number of muscles.
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Action Potentials Muscle Contraction
Increased motor neuron activity produces
increased muscle fiber contraction through motor
unit recruitment
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Fast Slow Motor Units
Slow fibers -slow activation -prolonged
contraction -a lot of mitochondria
Fast fibers -fast activation -fatigue -few
mitochondria
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Fast and Slow muscle fibers
Activation properties of slow and fast units are
due to innervation. Note fatigue properties due
to innervation but take time to change.
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Muscle Recruitment
(fast)
(fast)
(slow)

• Fast units recruited first but provide small
  amount of force.
• Slow units recruited last but provide large
  amount of force.

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Motor Systems

• Structure and activation of muscle
• Neuronal innervation of muscle
• Spinal reflexes
• Descending motor pathways
• Brain control of motor system

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Inputs to lower motor neurons
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Muscle spindles
Muscle spindles are proprioceptors that provide
information about muscle length
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Spindle input of alpha motor neuron
Spindle sensory neuron (Ia afferent) firing is
proportional to muscle length. Spindle afferent
activates alpha motor neuron, contracting muscle,
producing negative feedback to muscle.
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Myotatic Relex

• Knee-jerk or patellar reflex is a monosynaptic
  reflex
• Muscle is stretched, activating muscle spindle
  and Ia afferent, activating motor neuron,
  contracting muscle.

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Control of Spindle length
Gamma motor neuron stimulates contraction of
fibers within the spindle i.e. intrafusal fibers
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Control of Spindle Length
Gamma motor neuron contracts the muscle spindle
(decreases length) in response to loss of tension
on intra-fusal fibers.
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Golgi tendon organ
Golgi tendon organ detects muscle tension
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Activation of the Golgi tendon organ
Contraction of extrafusal fibers increases
tension of muscle and tendon which activates the
Golgi tendon organ
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Reverse myotatic reflex
Activation of the Golgi tendon organ results in
increased firing of Ib afferents which produces
inhibition of alpha motor neuron (e.g. muscle
failure). Prevents muscle or tendon tearing.
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Proprioception (Muscle Info) Summary

• Muscle Spindle Ia fiber -gt muscle length
• (Ia to gamma motor neuron -gt intrafusal fibers)
• Golgi tendon organ Ib fiber -gt muscle tension

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Reciprocal Inhibition
Ia afferents also synapses on inhibitory
interneurons that connect to opposing alpha motor
neurons. Activation of muscle spindle leads to
inhibition of antagonistic muscles Reciprocal
inhibition Prevents opposing muscles working
against each other.
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(Polysynaptic) Flexsor Reflex
Pain afferents stimulate excitatory interneurons
producing synergistic activation of flexsor
muscles.
Synergistic muscles are activated together and at
the same time have inhibition of extensor muscles
(reciprocal inhibition).
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Spinal control of walking reflexes
Rhythmic spinal interneurons use reflex arcs to
produce coordination of synergistic and
antagonistic muscles of the legs during walking.
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Rhythmicity produced by dual-channel coupling