1.6 motor system, LMN, and spinal motor function

what are the complex steps involved in voluntary motion?

1. decision making in anterior frontal love

2. motor planning and control

3. upper motor neurons deliver signal

4. LMN trigger skeletal muscle contraction

where does motor planning and control happen in voluntary motion?

motor planning: frontal lobe

control circuits: cerebellum, basal ganglia

where do the UMN deliver signals in voluntary motion?

•Brainstem

•Interneurons

•Lower motor neurons (LMNs)

what are characteristics of skeletel muscle?

excitable, contractile, extensile, elastic

what are the steps of initiating a muscle contraction?

1.Action potential

2.ACh binds to sarcolemma

3.Signal reaches T-tubule

4.SR releases Ca2+

5.Ca2+ binds to troponin

6.Conformational change moves tropomyosin

7.Cross bridge binding site exposed on actin

8.Myosin heads bind

9.Myosin swivels, pulls actin towards center of sarcomere

how does myosin bind?

- ATP is dephosphorylated (loses Pi) producing hydrolysis and energy to myosin for power stroke to occur

- ADP detaches, new ATP bind and

how does myosin detach?

after power stroke, ADP detachines and a new ATP binds and myosin head detaches from actin

when we lengthen our muscle, what happens to myosin and actin?

myosin head straightens out but still catches every actin binding site (just slower and no powerstroke)

aka powerstroke in reverse

breakdown the parts of a skeletal muscle

Muscle fiber ---> myofibrils ---> sarcomeres

what is a sarcomere?

functional unit of muscle

what makes up a sarcomere?

structural and contractile components

structural components of sarcomere

m and z lines, titin

contractile components of sarcomere

actin, myosin, troponin, tropomyosin

the more elongated a muscle its, what happens to its resistance to stretch?

it increases

what determines a muscles total resistance to stretch?

- active contraction (descending motor commands, proprioception/reflexes)

- titin

- connective tissue

- weak actin-myosin bonds

what is muscle tone?

resistance to stretch in a resting muscle

how is muscle tone clinically assessed?

via passive stretch

what indicates normal muscle tone using passive stretch?

- Titin and weak actin-myosin bonds functioning normally (no swivel)

- Signals from somatic NS acting normally

resistance to a muscle stretch increases after periods of

prolonged contraction and periods of immobility

ex. gripping a tiny object, then slowly releasing

ex. hamstring tightness after sitting for a long time

why is it that after prolonged contraction or periods of immobility it takes a second for those muscles to lengthen and relax?

this is because cross bridges need to break and the muscles need to fight against weak actin-myosin cross bridges that were formed during contracting

what are examples of neuro conditions that impact muscle tone?

1. CP

2. MS

3. PD

what is CP?

damage to motor areas of the cerebral cortex can lead to increased or decreased muscle tone

what is MS?

loss of myelination of axons can lead to increased or decreased muscle tone

what is PD?

loss of dopamine-producing neurons can lead to increased or decreased muscle tone

what determines a joints resistance to movement?

elastic and contractile forces of muscles

how can joint resistance be increased?

co contraction meaning agonist and antagonist muscles contract @ same time

ex. wrist when painting, writing, LE when balancing on wobble board, learning a new motor skill

co contraction provides joint

stability

where do lower motor neurons (LMN) convery info? what type of info do they convey?

Convey efferent signals to extrafusal and intrafusal muscle fibers

where are cell bodies of LMN located?

ventral horn of SC

what path do LMN axons take? where do they develop from?

exit ventral root of SC

develop from neural tube

explain relationship of LMN cell bodies and location in ventral horn

Cell bodies of nerves innervate the same muscle cluster in ventral horn(motor pools)

what are the parts of the ventral horn that LMN cell bodies innervate? what are the muscles?

•Medial part = axial, proximal muscles

•Lateral part = distal muscles

•Anterior part = extensors

•Posterior part = flexors

*pools may extend several spinal cord levels

what part of ventral horn are abductors, adductor, and rotator muscles innervated by?

LMN cell bodies will lie more medial because they drive motion more at proximal joints

what are myotomes?

group of muscles innervated by single spinal nerve

c1 myotome

cervical rotation

c2 myotome

cervical flexion, shoulder shrug

c3 myotome

shoulder shrug

c4 myotome

shoulder shrug

c5 myotome

should abduction

c6 myotome

elbow flexion, wrist extension

c7 myotome

elbow extension, wrist flexion

t1 myotome

finger adduction or little finger abduction

l1 myotome

hip flexion

l2 myotome

hip flexion

l3 myotome

knee extension (2 degree hip adduction)

l4 myotome

dorsiflexion

l5 myotome

- great toe extension (2 degree hip abduction)

- functional heel walking

s1 myotome

- eversion, plantar flexion (2 degree knee flexion, hip extension)

- functional toe walking

s2 myotome

bowel/bladder problems

types of LMN

alpha and gamma

characteristics of alpha motor neurons

•Cell bodies in ventral horn

•Axons leave ventral root

•Large cell bodies

•Large, myelinated axons that branch into numerous terminals

•Extrafusal muscle fibers (Force production)

characteristics of gamma motor neurons

•Cell bodies in ventral horn

•Axons leave ventral root

•Smaller cell bodies

•Smaller, myelinated axons

•Intrafusal muscle fibers (Sensation, muscle stretch)

alpha neurons contract

extrafusal fibers

gamma neurons contract

intrafusal fibers

how do excitatory signals to alpha motor neurons reach gamma motor neurons?

via collateral projections

what is a motor unit?

alpha motor neuron (AMN) + all muscle fibers it innervates

when a signal is sent _____ muscle fibers contract (MU)

all

muscle fibers can be classified as _____ or _____ twitch

slow; fast

characteristics of slow AMN

example?

• smaller diameter, slower conducting speed

• ex. Muscle of posture, e.g., soleus

characteristics of fast AMN

example?

•larger diameter, faster conducting speed

• ex. Muscles of powerful movement, e.g., gastrocnemius

in most movements what muscle fibers are recruited first? why?

slow twitch because they take less time time depolarize/depolarize fast

what is the henneman size principle?

AMN recruited in order from smallest to largest (can depend on task; e.g. high intensity movement and fatigue)

motor units vary in the number of fibers per neuron. why is this?

some muscles perform fine movements (precise control) vs large movements

ex. eye (2.5 muscle fiber/neuron)

ex. gastroc (2000 muscle fibers/neuron)

in the spinal cord, what are used to help coordinate signals?

•Somatosensory information

•Spinal interneurons

•Descending motor commands

what coordinated functions of the spinal cord contribute to smooth movement?

•Reciprocal inhibition

•Muscle synergies

•Proprioceptive input

•Stepping pattern generators

what is reciprocal inhibition?

inhibition of antagonist muscles during agonist contraction

how is reciprocal inhibition achieved?

internous

what do interneurons connect in the spinal cord?

•Agonist somatosensory afferents (via muscle spindle)

•Alpha motor neurons (inhibitory signal sent to antagonist muscle)

when is reciprocal inhibition triggered? ex?

during voluntary and reflexive muscle contractions

ex: flexion and extension of knee

what are muscle synergies?

coordinated muscle action

in muscle synergies, somatosensory afferent signals from one muscle are sent from what 3 things?

•Muscle spindles

•Joint receptors

•Cutaneous touch/pressure receptors

somatosensory afferent signals connect with interneurons. what does this trigger?

LMN from other joints/muscles which prepares and triggers other agonists/antagonists

ex. picking up and eating something

what is proprioceptive input?

spinal cord generates a complete proprioceptive image (schema) of the body in time and space

how does the spinal cord generates a complete proprioceptive image (schema) of the body in time and space? (3 things)

uses

•Joint capsule and ligament receptors

•Muscle spindles

•Golgi tendon organs

proprioceptive helps us ____ and ____ movements

example?

plan; adapt

ex. hitting a baseball

proprioceptive function of GTO

detects tension in msucle tendons

how do GTO impact LMN firing?

ex?

either facilitate or inhibit fiting

ex: stance vs swing, plantar flexors firing vs relaxed

what are stepping patter generators (SPGs)?

Adaptable networks of spinal interneurons that activate LMNs to cause alternating flexion, extension of the hips, knees

does each limb have a dedicated SPG?

- yes. when one triggers flexion, other triggers extension

- interpret proprioceptive input, predict next steps

what are spinal reflexes?

involuntary motor response to an external stimulus

pathway of spinal reflexes

Receptor ---> afferent ---> efferent (LMN) ---> effector

spinal reflexes can operate without

brain input

how can spinal reflexes be triggered?

in response to musculoskeletal or cutaneous receptors

what is the phasic stretch reflex: muscle spindles?

also called deep tendon reflex or myotatic reflex

Muscle contraction in response to quick stretch

what does phasic stretch reflex: muscle spindles trigger?

alpha motor neurons of same muscle being stretched

what is the sequence of events of stretch reflex?

1. stimulus (ex. mallet hitting knee)

2. Quick stretch of tendon activates muscle spindle

3. Afferents send signal to spinal cord

4..Synapse with alpha motor neurons, triggering efferents to muscle

5. Muscle contracts

what is the cutaneous reflex: withdrawal reflex?

cutaneous stimulation triggers reflexes usually in repsonse to a painful stimulus

process of cutaneous/withdrawal reflex

1. painful stimulus

2. afferent neuron

3. brain process

4. alpha motor neuron triggers reflex