KIN 313 Lecture 2 - Motor units: Morphology

afferent neuron direction

from periphery to CNS

efferent neurons direction

from CNS to periphery

motor units exit the spinal cord how?

from anterior horn via ventral root

these are efferent axons

how does sensory info enter the spinal cord?

comes through dorsal route ganglion, down the dorsal routeand into the spinal cord

it enters the grey matter and can either talk directly to motor neurons (via interneurons) or go to the brain via sensory axons

where are sensory axons in spinal cord

posterior aspect of grey matter

why is grey matter grey

packed with cell bodies

posterior aspect of spinal cord contains

mostly sensory axons going to the brain

ventral side of spinal cord contains

mostly motor pathways going down/out to periphery

morphological features/anatomy of a motor neuron

• large dendritic tree (branches of collateral, spherical, going out in all directions like a dandelion)

• soma (cell body)

• axonal hillock (where APs are generated)

• axon (heavily myelinated, APs travel down this to reach muscle)

• presynaptic terminal

motor neuron pool

all the motor neurons innervating one particular muscle

every muscle has its own MN pool

MN pool exists in the brainstem and spinal cord

the axon of a motor neuron biforcates to form…

collaterals… other neurons can come synapse onto these collaterals to make connections

Neuromuscular synapse is modified specifically to…

release ACh onto the surface of the muscle over the zone of overlap of the fillaments

neuronal cell bodies for a muscle are found..

housed together in the spinal cord, but across several segments of the spinal cord

all motor neurons go out the respective ventral route of their segment to eventually reach their muscle

motor neuron pools crossing several segments: implications for spinal cord injuries

if you get an injury in a place where there are still some motor neurons for a particular muscle leaving from higher segments, then you will still have some form of functionality of the muscle

spinal cord enlargement based on location?

the fact that the grey matter of the spinal cord is bigger around the arms (cervical) and legs (lumbar) because there are a lot of muscles in these areas that need to be controlled

mediolateral distribution of spinal cord

location of motor neurons withing the spinal cord is indicative of…

the muscle they are innervating

motor neurons for proximal muscles are found i nthe medial portion of the spinal cord

motor neurons for distal muscles are found in the lateral portion of the spinal cord

the motor unit

the cell body and dendrites of a motor neuron, the multiple branches of its axon (collaterals), and the muscle fibres that it innervates 

muscle unit

all the muscle fibres belonging to a motor unitm

motor unit vs muscle unit

motor unit includes all parts of a motor neuron and the muscle fibres that it innervates.

muscle unit only refers to the muscle fibres that belong to a motor unit

how many motor neurons can innervate a muscle fibre

only one motor neuron can speak to a muscle fibre. there is not competition

path of an AP from motor neuron in spinal cord to the muscle

MotN sends AP, which goes down axon, down each collateral, until it reaches the end plates, leading to a contraction of the muscle

implications of one motor neuron innervating a lot of muscle fibres

one AP causes ALL those muscle fibres to contract at the same time

the muscle unit contracts together

path of motor neurons going from spinal cord  to the muscle

they leave the spinal cord through the ventral route of whichever segment they are in, then join together as the nerve leaving that segment, then join with the motor neurons from the other segments to form plexi before eventually spliting off to form the nerves for specific muscles and then splitting off further to innervate their respective muscle fibres

distribution of a muscle unit in a muscle

why?

the muscle fibers are not all packed together, but spread out throughout a compartment of a muscle

dispersing reduces twitchy movements

innervation ratio

the number of muscle fibres innervated by a single motor neuron

relationship between force production and innervation ratio

high innervation ratio = high force production (gastroc = 1:1900)

lower innervation ratios = lower force production (eye muscles = 1:15)

relationship between innervation ratio and control/precision

aka implications of innervation ratio for motor control

high innervation = high force production but low control/precision

low innervation = low force production but high control/precision

gastroc = high = strong = low precision

FDI = medium

eye = low = weak = high precision

innervation ratio and aging

Innervation ratio increases

# of MotUs decreases, so other MotUs have to pick up the slack

nervous system sends collaterals from healthy neurons to connect with the muscle fibres that are no longer receiving signals so they can work again

this results in an increased innervation ratio

Amyotrophic Lateral Sclerosis (ALS)

death of motor neurons

healthy neurons send out collaterals to try to pick up slack

death outpaces help until there are no MotUs left in that muscle

Signs and Symptoms: muscle fibres that are abandoned contract out of sync = squirming sensation

Progression: 2-5 years

three types of skeletal muscel fibres

slow oxidative (SO)

fast oxidative-glycolytic (FOG)

fast glycolitic (FG)

Characteristics of Slow MotorNeurons

low innervation ratio

low CSA

conducts signals slower than others

high input resistance = easier to excite (takes less current to trigger AP) → low rheobase

long afterhyperpolarization period (long refractory period, longer time between APs being fired)

Innervates Slow Oxidative muscles

high vascularity and mitochondria → can make lots of ATP → slow to fatigue

Characteristics of Fatigue Resistant Motor Neurons

Hybrid

medium innervation ratio

medium CSA

medium conduction speed

medium input resistance = medium current needed to trigger AP

medium rheobase

medium refractory period

Innervate FOG muscles

medium vascularity and mitochondria → makes some ATP → fatigue resistant

more blood supply than FG, more stored pyruvate than SO

Characteristics of Fast Fatiguable Motor Neurons

high innervation ratio

high CSA

fast conduction speed

Low input resistance (large current needed to trigger AP) → high rheobase

low refractory period (can fire another AP quickly after the last one)

innervates fast glycolytic muscle fibres

low vascularity, low mitochondria → fatiguable

high pyruvate storage → fast ATP making but runs out fast

high force production with contraction but very fatiguable

how motor neurons can be classified

• who they connect to (fibre type)

• muscle they innervate

• innervation ratio

relationship between input resistance and size

large motor neuron = low input resistance (harder to excite)

small motor neuron = high input resistance (easier to excite)

small have high resistance because after putting in a little bit of current, its already hard to put in more. they are easier to excite because then they are already full and at threshold

opposite applies to large motor neurons

equation for Voltage (needed to fire AP)

Voltage = Current x Resistance

if you have a large input resistance, how much current do you need to reach voltage required to trigger an AP

small current

Rheobase

how much current is required to excite the motor neuron enough to trigger an action potential 

low rheobase level in small Motor Neurons because their resistance is so high

absolute refractory period

where the polarization of a neuron goes super negative, too negative to be able to fire another AP

what is a twitch and how to calculate

a force time response of a motor unit to a single input/stimulation

twitch = change in force divided by time

contraction time

time from start of force build up to peak force

temporal summation

one motor neuron firing repeatedly so that the motor unit doesnt get to fully reset between twitches

unfused tetanus

oscillating muscle force due to summation of impulses

fused tetanus

APs firing so fast there is no oscilation in force production

physiologically doesnt happen naturally bc the muscle would burn out

would happen if you get shocked by electricity

Half relaxation time (HRT)

time for a motor unit to go from peak force to 50% force

in order for temporal summation to work, must hit the unit again before it hits HRT

peak, contraction time, and HRT of a slow contracting fatigue resistant motor unit

low peak, long CT, long HRT

peak, contraction time, and HRT of a fast contracting fatigue resistant motor unit

medium peak, meadium CT, medium HRT

peak, contraction time, and HRT of a fast contracting fast fatiguing motor unit

big peak, short CT, short HRT

why is it hard to fatigue a slow oxidative muscle

high vascularity

the whole metabolic system is there to take waste out and bring atp in, so it can generate force much longer

why fast glycolytic muscles fatigue so fast

low vascularity, rely on the pyruvates already in muscle

use this up quickly, then theres none left = fatigued = cant keep generating significant force even though it is stimulated

why the best powerlifter cant become the best marathon runner no matter the training

born with what you have

training can change muscle fibre size, but cannot change innervation ratio, vascularity

behaviour of muscle fibres can change, but not dramatically

you cant change their physiology