Chapter 13 - Spinal Control of Movement

Organization of Lower Motor Neurons in Ventral Horn

-Motor neurons controlling flexors lie dorsal to those controlling extensors -Motor neurons controlling axial muscles lie medial to those controlling distal muscles

Inputs to Alpha Motor Neurons

1)Upper motor neurons 2)Proprioceptive input from muscle spindles 3)Spinal interneurons (excitatory and inhibitory)

Two Types of Lower Motor Neurons

1.) Alpha Motor Neurons - Directly trigger muscle to contract •Motor unit -All the muscle fibers and the one alpha motor neuron that innervates them •Motor neuron pool •A single muscle and all the alpha motor neurons that innervate it (made up of multiple motor units) 2.) Gamma Motor Neurons •proprioception

Spinal Motor Programs for Walking steps

1.Glutamate binds NMDAR, membrane depolarizes 2.Na+ and Ca2+ flow into the cell through NMDAR •Assumption: Mg2+ block removed! 3.Ca2+ opens potassium channel 4.K+ flows out of cell 5.Hyperpolarization...causes... 6.NMDAR closes (Mg2+ block) •Important: Ca2+ entry stops! 7.K+ channels close with low Ca2+ 8.Membrane depolarizes again due to continued presence of glutamate...cycle repeats rhythmically

Flexor Withdrawal Reflex circuit steps

1.Step on tack 2.Activate Aδ (pain) axon 3.Pain axon activates excitatory interneurons in adjacent segments of spinal cord 4.Interneurons activate alpha motor neurons innervating flexor muscles (not shown - extensor muscles on that side would need to relax)

Knee-jerk reflex

1.Tap on tendon of quadriceps causes small stretch in quadriceps muscle 2.Muscle spindle is also stretched, the (Aα) Ia sensory axon discharges 3.Causes discharge of alpha motor neuron (monosynaptic) 4.Quadriceps muscle contracts (reflexively)

Excitation-Contraction Coupling (Part 1 of Muscle Contraction) 1.Action potential generated in ________________ 2.__________ is released onto the ________________, bind ____________. 3. ___________ _________ channels in _____________ (membrane) open, _______________________. 4. ____________________ channels in sarcolemma open 5.Resulting _______________ sweeps down _____________ and into _________________. 6.Depolarization of ____________ causes __________ release from _____________ into the __________________.

Alpha motor neuron ACh, neuromuscular junction, nAchRs nAchR, Na+, sarcolemma, sarcolemma depolarizes (EPSP) Voltage-gated Na+ action potential, sarcolemma, T tubules T tubules, Ca2+, sarcoplasmic reticulum, muscle fiber

Muscle contraction requires liberation of intracellular __________

Ca2+

Sliding Filament Model of Muscle Contraction (Part 2 of Muscle Contraction) 7. __________ (released from ___________ into muscle fiber) binds to a _________ on ___________ that causes a second __________ to move and _______________________. 8.Allows _________________________. 9._______________, causing ______________ filaments to slide (walk) along ____________. 10. ___________ binds each ____________ and they ____________________. 11.The cycle continues as long as _____________________.(muscle relaxation requires _______________.)

Ca2+, sarcoplasmic reticulum, protein (troponin), actin filament, protein (tropomyosin), myosin binding sites on actin are exposed myosin heads to bind actin Myosin heads pivot, thick (myosin), thin (actin) filaments ATP, myosin head, disengage from actin Ca2+ and ATP are present, Ca2+ reuptake into the sarcoplasmic reticulum

Maintenance of the Myotatic (Stretch) Reflex

Gamma motor neuron (2nd type of lower motor neuron)

Role of Motor System

Generation of coordinated movements

Motor system

Muscles and neurons that control muscles

Reciprocal Inhibition of Flexors and Extensors of the Same Joint

Reciprocal inhibition: •Contraction of one muscle accompanied by relaxation of antagonist muscle •Example: Myotatic reflex in arm •Contraction of biceps muscle (monosynaptic), relaxation of triceps muscle (polysynaptic) Presence of an inhibitory interneuron is key

Types of Muscles of Somatic Motor System

Smooth •digestive tract, arteries, related structures •Striated Cardiac (heart) •Skeletal (bulk of body muscle mass) •Work in pairs to flex or extend joint (cannot push, only contract or relax)

Sliding Filament Model of Muscle Contraction

Within each myofibril are sarcomeres that have alternating thin (actin protein) and thick (myosin protein) filaments - muscles contract as a result of them moving past each other.

Graded Control of Muscle Contraction

a)Single action potential leads to muscle twitch b)Summation of twitches leads to sustained contraction •Occurs when the frequency of incoming action potentials increase •Greatest frequency = smoothest contraction

Golgi Tendon Organ •Provides ______________________. •located at __________________. •acts like ___________________. •monitors muscle tension ____________________ operate with _________________ •When muscle fibers __________, tension ___________, ______________________(if the muscle isn't stretched, the __________________) •__________________ discharges

additional proprioceptive input to alpha motor neuron junction of muscle and tendon a very sensitive strain gauge Muscle spindles (muscle length), GTOs (muscle tension) contract, increases, activating mechanoreceptors of Golgi Tendon Organ, Ia axon "stays quiet" Ib (Aα) sensory axon

Spinal Motor Programs for Walking Require

central pattern generator in spinal cord

All three types can ________________, but each motor unit has _______________.

coexist in a single muscle, fibers of only one type

Golgi Tendon Organs Circuit •Maintains ________________ via _______________. •Activation of _________________ by ____________ excites an ___________________, which in turn ______________________________ •Allows for _____________________.

muscle tension within optimal range, feedback loop Ib (Aα) sensory axon, Golgi tendon organ, inhibitory interneuron, inhibits alpha motor neuron (reducing force of contraction) fine motor control (e.g., handling a fragile object)

Rhythmic Activity in a Spinal Interneuron produced by

presence of NMDA Receptors

Reflex arc allows for

withdrawal of limb from aversive stimulus

Hypothetical Circuit for Rhythmic Alternating Activity - walking without "thinking"

•Alternating activation of flexor and extensor muscles •Again, requires continuous excitatory input from motor cortex •Again, requires both excitatory and inhibitory interneurons •and that rhythmic central pattern generator activity

•Proprioceptive axons also in joint tissues, respond to movement in joint

•Angle •Direction •Velocity

•Information from joint receptors

•Combined with other proprioceptive information to determine position in three dimensional space (what is my body doing?) •muscle spindle •Golgi tendon organs •skin receptors

White muscle fibers

•Contain few mitochondria •Contract rapidly •Two types: •Fast Fatigable (FF) •Fatigue-Resistant (FR) •Found in fast motor units •Flight or Fight responses; arm muscles

Red muscle fibers

•Contain many mitochondria •Contract slowly •Fatigue slowly •Sustained contraction possible •Found in slow motor units •Antigravity muscles of legs

You don't fall over because

•Crossed-extensor reflex •Activation of appropriate extensor muscles and inhibition of flexors on opposite side •Involves both inhibitory and excitatory interneurons - one of which crosses

•Primary Afferent Axons

•Enter via the vertebral column via the dorsal root •Four types: Aa, Ab, Ad, C

•Gamma motor neuron (2nd type of lower motor neuron)

•Innervates intrafusal fibers within the muscle spindle •Changes set point of the myotatic/stretch feedback loop -Keeps tension on intrafusal fibers (muscle spindles) "wakes it back up" -Allows for continued proprioceptive feedback even when muscle is contracted

Lower Motor Neurons ("final common pathway for motor control")

•Located in ventral horn of spinal cord •Send axons through the ventral root •Different from upper motor neurons in primary motor cortex (the focus of the next chapter)

Organization of Lower Motor Neurons in Spinal Cord: •Cervical enlargement

•Motor neurons innervate arm muscles

Organization of Lower Motor Neurons in Spinal Cord •Lumbar enlargement

•Motor neurons innervate legs

Myotatic (Stretch) Reflex

•Muscle pulled -> spindle stretched -> muscle contracts (pulls back) •Monosynaptic feedback loop makes possible •Discharge rate of Aα Ia sensory axon depends on length of stretched muscle •Motor neuron responds and contracts muscle briefly (antigravity feedback loop)

What controls the motor neuron (in the spinal cord)?

•Sensory (proprioceptive) feedback from muscle spindles •stretch receptors in the muscle fiber •Sensory signal carried by Aα sensory axon (Group Ia sensory axons) •Large and myelinated

Two connected motor systems?

•Spinal cord - motor programs control coordinated muscle contraction (largely reflexive) - this chapter! •Brain - controls motor programs in spinal cord - next chapter!

Reflex requires

•excitatory interneurons to complete circuit