LOWER MOTOR NEURONS (16)

marathon runners

slow, small, red muscle fibers

muscle spindle (muscle passively stretched)

afferent discharge alot

golgi tendon organ (muscle passively stretched)

afferent discharge very little

muscle contraction by alpha motor neurons

alpha motor neurons release Ach at neuromuscular junction, cause EPP, if large enough, AP, muscle twitches (rapid sequence of contraction and relaxation)

Lateral motor neuron pool

distal extremities, skilled behavior, cervical enlargement: forearm/hand, governed by projections from cortex that is in lateral white matter

Golgi Tendon Organs

encapsulated afferent nerve endings located at the unction of muscle and tendon innervated by a single group of 1B sensory axon in series with extrafusal muscle fibers when muscle stretched, change occurs at muscle fibers because it is more elastic than tendon when muscle CONTRACTED (actively): force acts directly on tendon, leads to increase in tension Negative feedback system that regulates muscle tension: IB axons contract inhibitory local neurons in spinal cord that synapse on alpha motor neurons that innervate the same muscle -decreases activation of muscle when exceptionally large forces generate, protects muscle MAINTAINS STEADY LEVEL OF FORCE: counteracting effects that diminish muscle force (fatigue) feedback system that monitors and maintains muscle force

extrafusal muscle and spindle

in parallel, Ia afferent, sensitive to passive stretch, insensitive to muscle contraction (muscle contraction leads to Y motor neurons keeping gain control on spindle) sensitivity set by gamma motor neurons in spinal cord feedback system that regulates mucle length

extrafusal muscle and golgi tendon

in series, Ib afferent, sensitive to active muscle contraction, insensitive to passive stretch no known setting of sensitivity feedback system that monitors and maintains muscle force

motor neurons for proximal muscles

located medially

circuit

series of neurons synaptically linked that work together to perform some function usually have an input (afferent) end and an output (efferent) end

Lateral local circuit neurons

short axons, less than 5 segments ipsilateral finer/ differentiated control like finger manipulation

muscle spindle (muscle actively contracted by stimulation of their alpha motor neurons)

spindle unloaded, activity decreases

Ca2+ dependent K+ channel

If Ca2+ present, opens channel, allows K+to leave cell, HYPERPOLARIZE

gain control of stretch reflex

1. increase activity of Y motor neurons 2. increases tension in muscle fibers (increase gain) 3. increased SENSITIVITY of system contractile state of spindles adjust to changes in length of muscle, keeping sensory system working within its dynamic range

3 ways to increase muscle force

1. stimulate motor unit more frequently 2. recruit more motor units 3. recruit FAST motor units in addition to slow

Leech model system

A chain of 21 identical 'ganglia' form the segmental nerve cord Electrical recordings from ventral and dorsal longitudinal muscles in leech and corresponding motor neurons show a reciprocal pattern of excitation for the dorsal and ventral muscles of a given segment

when both alpha and y motor neurons activated

no decrease in 1a firing during muscle shortening. Thus, y motor neurons can regulate gain of muscle spindles so muscle spindle can operate efficiently at any length of parent muscle

most extrafusal skeletal muscles innervate by..

only a single alpha motor neurons. far more muscle fibers than motor neurons, individual motor axons branch within muscles to synapse on many extra fusel muscle fibers. reduced chance that damage to one or a few alpha motor neurons will significantly alter muscles action

Medial motor neuronal pool

postural contraol, maintenance of balance, input from upper motor neurons via LONG projections that run in medial and anterior white matter

golgi tendon organ (muscled actively contracted by stimulation of their alpha motor neurons)

rate of golgi tendon organ firing increases

when alpha motor neurons stimulated without Y motor neurons

response of Ia afferent fiber decreases as muscle contracts

Thomas Graham brown locomotion

rhythmic walking persists in animals without dorsal roots, i.e. in the absence of sensory information

1a afferents

sensory neurons whose nerve endings in MUSCLE SPINDLES cell bodies: dorsal root ganglia sensory processes (1a afferent terminals) wrap around intrafusal muscle spindles respond phasically to small stretches MYOTATIC REFLEX

muscle spindles

sensory receptors embedded in extrafusal muscles. they are intrafusal muscles.

motor unit

single motor neuron and the muscle fibers it innervates smallest unit of force that can be activated to produce movement sherringto was the first to recognized fundamental relationship between alpha motor neuron and fiber it innervates

motor unit plasticity

slow have low recency stimulate muscle at higher frequency takes on fast fatiguable properties (marathon)

fused tetanus

tension produced in individual motor unites no longer has peaks and troughs that correspond to individual twitches evoked by the motor neuron's action potential happens at highest irking rates, individual muscle fibers are in fused tetanus

SLOW motor units

threshold: easier to reach force: small soma size: small color: red energy: oxidative phosphorylation myboglobin content: rich mitochondria: high neuron: fiber: 1: 10 contraction speed: slow contract slowly, generate small force, but rich in myoglobin and lots of mitochondria and rich capillary beds: resistant to fatigue good for sustained muscular contraction, maintenance of upright posture

segmental muscle organization

The gastrocnemius and soleus are two synergistic calf muscles. Each extends the foot. These muscles overlap on the leg

FAST(fast fatiguable) motor neurons

threshold: requires more current force: greater soma size: big color: pale energy: glycolysis myoglobin content: poor mitochondria: poor neuron: fiber: 1: 200 contraction speed: fast innervate large, pale muscle fibers, generate more force, but have sparse mitochondria and are therefore easily fatigued

Reciprocal inhibition (rhythmic movement from simple circuit)

translate tonic excitation into alternating rhythmic excitatory interneuron, inhibited by inhibitory interneuron, inhibitory interneuron inhibited by autapse

proprioceptors

muscle spindles and their afferents provide information about position

Rhythmic activity of single interneuron

1. Glutamate comes on 2. binds to AMPA, Na enters and depolarizes cell 3. depolarization causes electrostatic repulsion of Mg2+ of NDMA 4. NDMA opened by glutamate and displacement of Mg 5. Na and Ca enter through NMDA channel 6. Ca that entered through NMDA opens Ca dep K+ channel 7. K leaves cell, hyper polarize 8. Hyperpolarization causes Mg to bind to NMDA (inactive) and AMPA desentize 9. Cycle repeated when AMPA resensitized

Homonymous muscle

muscle that contains or is associated directly with sense organ producing the reflex. 'The muscle in question'

How do gamma motor neurons keep muscle spindles sensitive

Small Y motor neurons adjust tension (gain) of spindle so that partially contracted muscle does not lose important 1a afferent feedback

alpha motor neurons

VENTRAL HORN OF SPINAL CORD innervate extrafusal muscle fibers, which are the striated muscle fibers that actually generate forces needed for posture and movement Exrafusal fibers are innervated by neurons whose cell bodies lie in the ventral horn of the spinal cord

antagonist muscle

muscle that produces opposite motor action ex: flexors versus extensors, such as biceps vs triceps when flexor contract, limb flexes when extensor contracts, limb extends

Gain

amount of Y motor activity, tension in the muscle spindle adjusted by upper motor neurons or local circuit neurons it is the amount of muscle force generated in response to given stretch it is continually adjusted to meet different functional needs high gain, then small stretch produces high muscle tension low gain, big stretch produces low muscle tension high gain (baseline level of y motor neuron activity) when movement is difficult or precise gain can also be controlled by local circuits that can inhibit alpha motor neurons

sensory feed back (rhythmic)

avoid barrier: modulated by descending input Ascending information from the spinal cord is sent to higher brain centers during locomotion; in essence, a 'copy' of what is going on is sent up.

local circuit neurons

cell bodies and axons are all located in spinal cord

projection neuron

cell body and nerve terminal are in separate brain areas

interneuron

cell body and terminal are in same general brain area

CENTRAL PATTERN GENERATORS

circuits that give rise to rhythmic motor activity also known as oscillators central components of these networks are capable of producing rhythmic patterns of activity, in the ABSENCE of sensory input (however, sensory info may be essential for appropriate modifications)

Lamprey model system

continuous spinal cord that innervates muscle segments. discovered rhythmic activity in spinal interneuron

NMDA channel

gluatmate receptor open with glutamate and Mg2+ displacement (from AMPA depolarization) lets in Na AND Ca2+, activates Ca2+ dep K+ channels inactive when K+ leaves and hyper polarized and Mg+ goes back into channel

AMPA Channel

glutamate receptor immediately lets Na+ in (ONLY) depolarize, causes Mg2+ displacement inactive with desensitization

2 types of motor neurons

in ventral horn of lower motor neuron pool: 1. gamma: innvervate muscle spindles 2. alpha: innervate extrafusal muscle fibers

gamma motor neurons

innervate sensory receptors called muscle spindles function: regulate sensory input by setting the intrafusal muscle fires to an appropriate length

sprinters

larger proportion of fast fatigue (pale) muscles

motor neurons for distal muscles (hands)

located laterally

medial local circuit neurons

lower motor neurons in medial ventral horn axons in MANY spinal cord segments cervical/lumbar enlargements mediate posture entire length of cord cross over commisure so that both sides act in concert to maintain posture

Flexor Reflex

mediates withdrawal of limb from painful stimulus WITHDRAWAL: stimulation of nociceptive sensory fibers excites IPSILATERAL flexor and inhibits extensor SUPPORT: CONTRALATERAL: stimulus inhibits flexor and stimulates extensor (provides postural support during withdrawal)

myotatic stretch reflex

monosynaptic, no interneuron passive weight or patellar reflex 1. Passive weight: extrafusal muscle lengthen 2. intrafusal spindles lengthen 3. activate mechanosensitive ion channels on 1a afferent 4a: let in + ions, 5a: AP fires at higher frequency 6a: synapse onto Alpha motor neuron (in dorsal horny of gray matter) 7a: muscle contracts 4b:synapse with inhibitory inerneuron axon colateral. GABA onto interneuron 5b: inhibits antagonist

signal about muscle fiber arises from?

motor neuron motor neuron determines muscle type (forcing slow motor neuron to innervate fast muscle causes fast muscle to act like slow muscles

gastronemius

muscle comprised of both small and large motor units, 1 neuron: 1000 muscle fibers can generate force needed for sudden changes in body position

soleus

muscle important for posture, comprised mostly of small motor units 1 neuron: 200 muscle fibers

contralateral muscle

muscle of opposite limb

synergist muscle

muscle produces a similar motor actions (soles and gastocnemius)

Patellar Reflex

type of myotatic reflex 1. tap on patellar tendon stretches quad 2. stretch activates muscle spindle 3. burst of AP in 1a spindle afferents 4. SYNAPSE ACTIVATE EXTENSOR: 1. 1A afferent release glutamate on alpha motor neuron of externsor 2. alpha motor neuron of extensor AP 3. release Ach onto muscle 4. EXTENSOR CONTRACT DEACTIVATE FLEXOR 1. 1A afferent synapse onto interneuron 2. interneuron release GABA/glycine onto alpha motor neuron of FLEXOR 3. flexor hyperpolarize 4. FLEXOR RELAX

axial muscles (trunk)

used for posture, most medial

Sherrington (locomotion)

vertebrate locomotion by chaining together a series of reflexes (require sensory stimuli) WRONG

size principle

weak synaptic stimulation: smallest motor units activates (slow) as synaptic activity increases, low threshold slow motor units recruited, then Fast fatiguable recruited. stand: slow run: fast fatiguable