Mod 9 V2 - Lower Motor Neurons Flashcards
What are the 2 brain regions contained in the lowest level of the motor system hierarchy?
Spinal cord (laminai VIII & IX) and brainstem (CN somatic motor nuclei).
Which CN (cranial nerve) somatic motor nuclei are involved in the brainstem?
- Oculomotor (CN III)
- Trochlear (CN IV)
- Trigeminal (CN V)
- Abducens (CN VI)
- Facial (CN VII)
- Nucleus ambigus
- Spinal accessory (CN XI)
- Hypoglossal (CN XII)
What are the 2 locations of LMN?
- Spinal (cord) anterior horn (laminae VIII & IX)
- Brainstem (CN somatic motor nuclei)
LMNs are modulated by what 2 sources?
- Descending control
- Peripheral sensory input
Explain what neuromuscular junction (aka motor end plate) is.
The site where axons of LMN contact skeletal muscle.
What are 2 types of LMNs and their functions?
- Alpha motor neurons
Role: Innervate extrafusal fibers: tension-generating fibers of skeletal muscle. - Gamma motor neurons
Role: Innervate intrafusal fibers (contractile ends of the muscle fiber): special type of skeletal muscle fiber found within the muscle spindle; smaller in size.
What are intermotor neurons?
Axons distributed locally in the spinal gray matter regulate the activity of alpha and gamma motor neurons; they can excite or inhibit these motor neurons.
Explain what neuromuscular junction (motor end plate) is.
- The site where axons of LMNs synapse with skeletal muscle fibers.
- It is a 1-to-1 relationship between the firing of the motor
neuron action potential and the activation of the muscle fiber
unique chemical synapse - AKA ENDPLATE POTENTIAL
What are the 6 steps that occur in the neuromuscular junction?
- Alpha motor neuron & the motor end plate
- Synaptic cleft
- Active zones: contain large numbers of docking proteins
located in close proximity to voltage gated Ca2+ channels
& large numbers of neurotransmitter (Acetylcholine
ACh )) filled - Subjunctional folds: contain clusters of nicotinic
acetylcholine receptors ( AChRs ) which are ionotropic
receptors selective for cations. - Activation of the receptor causes influx on NA+
(predominantly) which depolarizes the muscle
membrane - Ach is removed from synaptic space by enzymatic breakdown:
acetylcholinesterase.
What is a motor unit?
A single alpha motor neuron and the muscle fiber it innervates.
What 3 factors determine the motor unit size?
- Size of muscle mass
- Speed of contraction needed
- Type of control
What are the 2 types of motor units?
Type I
SLOW twitch: generate LOWER levels of TENSION but for LONGER PERIOD of time (fatigue resistant) - postural control muscles.
Type II
FAST twitch: generate HIGHER levels of FORCE but for SHORTER PERIODS of time IIa : Fast twitch oxidative; IIb : Fast twitch glycolytic
What are the 2 ypes of TYPE II motor units?
IIa : Fast twitch oxidative
IIb : Fast twitch glycoly
Regarding grading force of muscle contractions, what is the SIZE PRINCIPLE?
Smaller motor units are recruited first followed by larger units.
Regarding grading force of muscle contractions, what is RATE CODE?
as the need for greater force and speed increases, synaptic input increases and the firing rate of the neurons increases.
Lecture Question 1
Lower motor neurons are the final motor pathway, allowing a neural signal to be converted into a mechanical event.
A. True
B. False
A. True
Explanation:
This statement is true: lower motor neurons are the final motor pathway. The action potential of a motor neuron will cause a muscle fiber to depolarize setting off cross bridge cycling.
Lecture Question 2
Where are lower motor neurons that innervate limb muscles located?
A. Anterior horn of the spinal cord
B. Trigeminal motor nucleus
C. Facial motor nucleus
A. Anterior horn of the spinal cord
Explanation:
Motor neurons that innervate the muscles of the trunk and limbs are located in the anterior horn of the spinal cord. Motor neurons that innervate muscles of the head and neck are located in the cranial nerve somatic motor nuclei (like the trigeminal and facial motor nuclei).
Lecture Question 3
Which of the following neurons innervates the contractile ends of the muscle spindle?
A. alpha motor neurons
B. gamma motor neurons
C. interneurons
B. gamma motor neurons
Explanation:
Gamma motor neurons will innervate the infrafusal fiber (contractile poles of the muscle spindle) and will be important in maintaining the tension in the muscle spindle. Alpha motor neurons innervate the extrafusal fibers which are the tension generating fibers of skeletal muscle. Interneurons (local circuit) neurons will integrate sensory information from the periphery and descending control signals from higher brain regions and they will use this information to regulate the activity of the lower motor neurons.
Lecture Question 4
Which neurotransmitter is released at the neuromuscular junction?
A. Acetylcholine
B. Glutamate
C. Glycin
A. Acetylcholine
Explanation:
Acetylcholine is the neurotransmitter released at the neuromuscular junction. It will bind the nicotinic acetylcholine receptors allowing Na+ to enter the cell resulting in the depolarization of the muscle fiber.
What is the topographical arrangement of LMN?
The longitudinal arrangement between LMN pools and the muscles they supply in the spinal cord.
What are the three regions of the spinal cord in which LMN pools are arranged, and what areas of the body do they innervate?
Cervical Region - innervate the UL (upper limbs/extremities).
Thoracic Region - innervate the trunk.
Lumbar Region - innervate LL (lower limbs/extremities)
Regarding the organization of LMN in the spinal gray matter schematic, what is the Flexor-Extensor Rule?
FLEXOR LMNs are located posteriorly
EXTENSOR LMNs are located anteriorly
Regarding the organization of LMN in the spinal gray matter schematic, what is the Proximal-Distal Rule?
LMNs innervating PROXIMAL muscles are
located MEDIALLY (ie. axial muscles)
LMNs innervating DISTAL muscles are
located LATERALLY (ie. limb muscles)
Local circuit neurons are organized _______.
somatotopically
