Lower Motor Neurons and Muscles and Reflex

What is the Function of Lower Motor Neurons (LMNS)

They Receive commands from the Higher Motor Neurons and they directly control muscles and organs

• They are efferent meaning they leave the CNS and to their target muscles

LMNS can be two types of neurons what are they?

1. They Can be motor neurons in the spinal cord ventral horn

2. Autonomic Ganglion Neurons (Pre and Post, thus forming two LMNS)

In Simple terms what is the function of the LMNS in the motor neurons in the spinal cord ventral horn

Their axons travel in the ventral roots of the spinal cord and innervate their target muscles '

• This is the Somatic Motor System

What is the Function of the Autonomic Ganglion Neurons

• They are Pre-Ganglionic and Post-Ganglionic, thus forming two LMNS

• Their function is too innervate the smooth muscles, organs and glands

• They form the Autonomic Nervous System

What are the Main Motor Neurons that are found in the Somatic Motor System (The Ventral horn of the Spinal Cord)

These are the Alpha Motor Neurons

What is a Motor Unit

This comprises of a Motor Neuron and all the muscles fibers or Muscle Cells it will innervate

• One Motor Neuron + the Muscle cells it innervates

What is a Motor Neuron pool

This is all the Motor Neurons that are needed to innervate a single muscle (Bicep)

• All the Motor Neurons → One Muscle (Bicep)

The Ventral Root/Horn is the motor part of the spinal cord

yes this is true and important

The Ventral Horn of the Spinal Cord contains what type of representation

It contains a somatotopic Representation

• Distal Muscles such as the Digits are represented in the peripheral areas of the Ventral Horn

• The Axial and central muscles such as the core are represented in the more central area of the Ventral Horn

What are the two type of muscles

1. Smooth Muscles

2. Striated Muscles

What are some examples of Smooth Muscle

Muscles of the digestive tract, arteries and related structures

What are some examples of striated muscle

Cardiac muscle, skeletal muscles.

• Striated due to myosin and actin filaments

What are the three kind of skeletal muscle fibers

• Red Fibers (Type I)

• White Fibers (Type IIB)

• Intermediate Fibers (Type IIA)

What are some Traits of the Red Fibers (Skeletal Muscle)

• They are red due to their abundance in mitochondria

• They are thin

• They contract weakly and slowly

  • Thus they are able to sustain contractions for a longer period of time

• They are composed of slow motor units

• Example - Long Distance Runners

What are some Traits of the White fibers

• They are larger

• They use aerobic metabolism

• They contract in brief and powerful twitches:

• They fatigue rapidly

• They are composed of fast motor units

• example - A boxer, sprinter

What are the Intermediate fibers for (Skeletal Muscle)

They share properties between red and white fibers

Axial Muscles pertain to

The Core, The trunk and the abs

Distal Muscles pertain to

Hands, feet, digits

Proximal muscles pertain to

Shoulder, Elbow, pelvis (areas that are the first point of joint connection)

In Simple Terms what is Excitation-Contraction Coupling (You will come back to this)

1. Alpha motor neurons will release ACH

2. The Muscle receptors will receive this and will open ACH Receptors

3. This will cause the opening of Ion Channels

4. Thus an influx of Na+ and Ca+

5. This will depolarize the muscle allowing for the opening of V-G Ca2+ Channels'

6. this will further cause activation of the SR (Holds more intracellular Ca2+)

7. This is what causes the sliding mechanism for the Actin and Myosin filaments (Contraction)

What is a reflex

It is an involuntary, specific, stereotyped motor response to a sensory input

Most reflexes are very simple and will involve what?

1 Sensory neuron from the DRG (afferent) 1 Efferent Motor Neurons, and 1 or more interneurons (for reciprocal inhibition)

The Simplest reflex is what? what does this mean

The Simplest reflex are monosynaptic, meaning they involve two neurons (one sensory and one motor)

What are Supra-Spinal Mechanisms, what is their job

This is how reflexes can be suppressed

• e.g. - food that you cooked is super hot and naturally, or by reflex you will drop is, however since you made it for a loved one it will not be instantly dropped but with a certain time

Myotatic Reflex / Stretch Reflex / Tendon Reflex

Traits'

• It will cause the stretch of muscles and tendons

• Muscle spindles that detect muscle length will notice stretch and will send info afferently to the ventral horn

  • This will activate the alpha motor neurons to efferently cause the same group of muscles to contract

    • The muscle that stretches will be contracted (via Extrafusal muscle fibers

• This is a simple reflex (Monosynaptic)

• Examples

  • Patellar Tendon Reflex

  • Achilles Tendon Reflex

The Extrafusal muscle fibers used are innervated by the Alpha Motor Neurons

The Muscle spindles used are innervated by the Gamma (y) motor neurons

Reciprocal Inhibition during the myotatic stretch reflex

The same sensory info that is sent to the ventral horn will go to a different set of Alpha Motor neurons via inhibitory interneurons that will produce inhibitory signals to these new alpha motor neurons to control the antagonistic muscles to relax

• If the synergist muscle contracts, the antagonist muscle must contract

Pathway for Stretch Reflex

1. Stretch of Muscle Spindle

2. Activate Type IA and type II sensory fibers

3. Info carried to spinal cord

4. excitatory synapse to Alpha Motor Neurons

5. Carried to extrafusal muscle fibers, causing synergist to contract

Reverse/Inverse Myotatic Reflex (Autogenic Inhibition)

This shows how muscle tension can lead to inhibition of motor neurons

Traits

• This is done through the Golgi Tendon Organs which are specialized to signal information on muscle tension

  • They are inhibitory to the muscles they innervate

  • They send info via IB afferent sensory fibers and result in feedback control to prevent excessive stretch

• Regulate Muscle tension within an optimal range

Damage to this causes Clasp-Knife response -kind of like a army knife, hard to push knife down initially, but suddenly goes right back into place

Flexor Reflex/Withdrawal Reflex

It is often based on painful stimuli that can elicit coordinated withdrawal reflexes

Traits:

• The flexor Reflex is initiated by cutaneous receptors and involves a whole limb, and thus transduces through multiple spinal cords (thus polysynaptic reflex)

• In the Stretch Reflex the Alpha motor neurons will interact with extensor muscles, in this case, they will interact with flexor muscles

Example - Left side noxious stimuli

• causes the left side to be activated (the left leg will contract)

• The extensor for this leg is inactive

It can be inhibited by the supra-spinal mechanism

Reciprocal inhibition and crossed effects in reflex

Reciprocal inhibition occurs in all types of reflexes

• Reflex activity in a given muscle produces similar activity in its ipsilateral synergists and the opposite activity in its ipsilateral antagonists

This is important in the Crossed Extensor Reflex

• This is important for walking and maintaining balance

• A flexor reflex on one side will cause the opposite pattern activity in the contralateral limb

• ex - Stepping on a sharp object with the left foot  left foot withdrawal

  (Flexor reflex), while the right leg extends to support the body (Crossed

  extension reflex).

Crossed-extensor reflex: Activation of extensor muscles and inhibition of flexors on the opposite side

What is the sarcoplasimic reticulum

The sarcoplasmic reticulum is a specialized endoplasmic reticulum in muscle cells that stores calcium ions (Ca2+).

During excitation-contraction coupling, the sarcoplasmic reticulum releases stored Ca2+ into the muscle cell's cytoplasm following depolarization. This influx of calcium initiates muscle contraction by enabling the sliding mechanism between actin and myosin filaments.

Excitation-Contraction Coupling further

The  motor neurons release Acetylcholine (ACh). ACh produces large EPSP in the post- synaptic muscle fiber (aka. End- Plate Potential; EPP) The summation of EPPs, or when an EPP overcomes the threshold, then EPP evokes muscle action potential. Muscle Action potential: a. cause Ca2+ influx through plasma voltage- gated calcium channels; then b. triggers Ca2+ release from SR (sarcoplasmic reticulum). Muscle fiber (cell) contracts. Ca2+ reuptake then the fiber relaxes.

What is the molecular basis for Muscle Contraction (Z line stuff)

• Z Lines - a division of myofibril into segments by disks

• Sarcomere: Two Z lines and the myofibril

• Thin Filaments are a series of bristles

• Thick Filaments are found between and among thin filaments

• The Binding of Ca2+ to Troponin is what causes the myosin to bind to actin (Contraction-sliding)