Lecture 15 - Somatic Motor System

what type of circuit does the somatic motor system use

descending convergent hierarchical circuits

what is descending

down the spinal cord to muscles

what is convergent

many neurons to one

path of motor control systems

upper motor neurons (motor cortex and brainstem centers) -> spinal cord and brainstem circuits (local circuit neurons to lower motor neurons) -> skeletal muscles

where do local circuit neurons get input from

sensory

where do local circuit neurons output

lower motor neurons

where do lower motor neurons input and output

input: local circuit neurons

output: skeletal muscles

main groups of motor neurons

upper motor neurons and lower motor neurons

what are upper motor neurons

localized in CNS

motor cortex -> interneuron circuits in brainstem or spinal cord

what are lower motor neurons

leave CNS

cranial nerve nuclei or spinal cord to muscle

what side of the body does the right motor cortex control

left

what side of the body does the left motor cortex control

right

which neurons cross the midline

upper motor neurons

where do upper motor neurons for the hand project to

directly to lower motor neurons

where do axons of the motor neurons in the corticospinal tracts synapse with

interneurons and lower motor neurons in the ventral horn of the spinal cord

know slice of spinal cord picture

hello

where do 90% of the pyramids cross over

pyramidal decussation

which tract do 90& of the pyramids travel through

lateral corticospinal tract

what tract do 10% of pyramids travel through

anterior corticospinal tracts

where do 10% of the upper motor neurons cross throguh

bottom of anterior corticospinal tracts to skeletal muscles

what are the 3 classes of lower motor neurons

alpha, beta and gamma

which class of lower motor neurons cause muscle contractions

alpha

which motor neuron classes project to muscle spindles

beta and gamma

which muscle fibers contract

extrafusal muscle fibers

proportion of muscle fibers and alpha motor neurons

many more fibers than neurons

each motor neuron innervates several fibers within SAME muscle

how many motor neurons innervate each muscle

one

what is a motor unit

one alpha motor neuron and all its postsynaptic fibers

what does the size of the alpha motor neuron depend on

size of motor neuron and the fibers it innervates

what are the 3 classes of motor units

fast fatigable (FF), slow (S), fast fatigue resistant (FFR)

characteristics of FF units

pale; few mitochondria; so very easily fatigued

"fast twitch"

highest force, fastest response

brief exertions (sprinting, jumping)

largest alpha motor neurons

characteristics of S motor units

smallest alpha motor neuons

small "red" slow fibers

high in mitochondria, myoglobin (red)

lowest force

most common in skeletal muscle

maintenance of upright posture

characteristics of FFR units

intermediate size + speed

intermediate in amt of ATP and force generated

intermediate in fatigue

walking, running

order of recruitment of motor units

S -> FFR -> FF

what does the force of muscle contraction depend on

- number of motor units activated

- type of motor unit activated

- rate of APs generated in the motor neurons

what does the neuromuscular junction (NMJ) contain

presynaptic boutons of the motor neuron and the postsynaptic end plate of the muscle fiber

what is the end plate

specialization of the postsynaptic fiber with membrane "pockets" called junctional folds

what do the vescicles in the active zone of the NMJ contain

cevtylcholine

what receptors do NMJs contain

postsynaptic acetylcholine receptor - "ligand-gated ion channel"

what channel is in the junctional folds to let in ions

post synaptic voltage gated na+ channel

what is the resting membane potential of the muscle fiber

-90 mV

how is the postsynpatic cell depolarize

ACh binds to the AChR and depolarizes t ocause an end plate potential

what voltage does the end plate potential recerse at

0 mV

what voltage does the end plate potential hyperpolarize at

55 mV

what ions are involved in EPPs

Na+ and K+

how does contraction happen

depolarize due to EPP -> Na+ channels activated -> AP -> Calcium channels activated -> contraction

Propogation of EPPs vs APs

EPPs decay with distance from NMJ

APs propogate without decrement from NMJ

what type of contraction

all or nothing

low AP firing rates in alpha neuron

low AP firing rates in muscle fibers -> single muscle twitches with low force

increasing alpha motor neuron AP rates

increased muscle fiber AP rates -> contractions summate and cause tetanus and increased force

tetanus (infection tho)

clostridium tetani

bacteria releases toxin to disinhibit alpha- motor neurons allowing them to fire at high rates and causing muscle spasms