11/4 - Peripheral Muscle Receptors and Spinal Cord Reflexes Flashcards
Spinal Nerve Pathway
PERIPHERAL PROCESS >>> DORSAL ROOT >>> DORSAL ROOT GANGLION >>> DORSAL HORN >>> VENTRAL HORN >>> MOTOR NEURON >>> EFFERENT AXON >>> VENTRAL ROOT >>> SPINAL NERVE >>>
ALPHA MOTOR NEURONS
Innervate ____ muscle fibers: _____ portion of muscle
ALPHA MOTOR NEURONS Innervate extrafusal muscle fibers: Regular contractile portion of muscle
ORGANIZATION OF MOTOR NEURONS IN SPINAL CORD
MOTOR NEURONS FORM FUNCTIONAL CLUSTERS IN THE VENTRAL HORN
Motor Neurons are arranged in longitudinal columns.
All the neurons in one column go to the same one muscle.
Each column contains the motor neurons that innervate one muscle.
Column usually extends through more than 1 spinal cord segment.
TOPOGRAPHIC ORGANIZATION:
FLEXORS
DORSAL
TOPOGRAPHIC ORGANIZATION:
EXTENSORS
VENTRAL
TOPOGRAPHIC ORGANIZATION:
DISTAL LIMB Mucles
LATERAL
TOPOGRAPHIC ORGANIZATION:
PROXIMAL LIMB & TRUNK Muscles
MEDIAL
Skeletal muscle is made up of bundles of cylindrical ____ that vary in length from 1-40 mm and in diameter from 50–100 um.
Skeletal muscle is made up of bundles of cylindrical Muscle Fibers that vary in length from 1-40 mm and in diameter from 50–100 um.
Each muscle fiber is a _____ .
Each muscle fiber is a single, multinucleated cell
MOTOR UNIT
Defined as a single alpha motor neuron and ALL THE MUSCLE FIBERS it innervates
Each muscle fiber receives input from a _____ which synapses at the ______.
Each muscle fiber receives input from a single motorneuron which synapses at the single motor end plate.
A ____ may innervate more than one ____ .
A motor axon may innervate more than one muscle fiber.
Motor End-Plate
Site where axons make synaptic contact with muscle fiber.
The size of motor units varies from small (10–100 fibers/motor neuron) to large (600–several thousand fibers/motor neuron).
The size of motor units varies from small (10–100 fibers/motor neuron) to large (600–several thousand fibers/motor neuron).
Small motor units
Small motor units provide more Precise Control of motor activity.
These would be found in
muscles that control individual Digits or muscles that control movements of the Eye.
MUSCLE CONTRACTION
The force of contraction of individual muscle fibers is determined by the firing
frequency of the motor neuron
Total force of contraction of a muscle is determined by number of alpha motor neurons that are active.
The force of contraction of individual muscle fibers is determined by ______ .
The force of contraction of individual muscle fibers is determined by the firing
frequency of the motor neuron
Total force of contraction of a muscle is determined by ______.
Total force of contraction of a muscle is determined by the number of alpha motor
neurons that are active.
Tetany
a sustained muscular contraction caused by a series of stimuli repeated so rapidly that the individual muscular responses are fused.
The maximal force a muscle can generate.
Temporal summation.
TETANUS
- Binds to peripheral nerve terminals and transported within the axon to CNS.
- Binds to proteins at presynaptic inhibitory motor nerve endings and taken up into the neurons.
- Effect is to block release of inhibitory neurotransmitters (GABA, glycine).
- Results in uncontrolled firing of motor neurons resulting in muscular spasms.
- Acts by selective cleavage of a protein required for neurotransmitter release, synaptobrevin II
Clostridium tetani
A grampositive rod-shaped bacterium that is found worldwide in soil.
It is usually in its dormant form, spores, and becomes the rod-shaped bacterium when it multiplies.
AFFERENT SIDE OF SPINAL LOOP:
PROPRIOCEPTORS
Proprioceptors within the muscle that continually provide information about the Current State of muscle Contraction and Rate of Change in muscle length and tension.
MUSCLE SPINDLE
INTRAFUSAL FIBERS
3–12 miniature muscle fibers anchored in parallel to connective sheath of extrafusal muscle fiber.
Central part is non-contractile and acts as stretch receptor.
Primary afferents innervate spindle by wrapping around central part.
Primary ____ innervate spindle by wrapping around ____ .
Primary afferents innervate spindle by wrapping around central part.
Central part is non-contractile and acts as _____ .
Central part is non-contractile and acts as a stretch receptor.
NUCLEAR BAG FIBERS
nuclei cluster in center of fiber.
Static bag fiber
Responds to changes of muscle length
Dynamic bag fiber
Responds to rate of change of muscle length
NUCLEAR CHAIN FIBERS
nuclei line up in a row with no enlargement of central
region
Respond to changes in muscle length
INNERVATION OF MUSCLE SPINDLE
Primary afferent (Ia/Aα) loop around equatorial region of nuclear bag and chain fibers forming annulospiral ending.
Activated by stretching of muscle spindle which opens ion channels on terminals resulting in influx of ions and depolarization.
Transmit information on length of muscle (static nuclear bag and nuclear chain fibers) as well as rate of change in length (dynamic nuclear bag fibers).
annulospiral ending
Primary afferent (Ia/Aα) loop around equatorial region of nuclear bag and chain fibers forming annulospiral ending.
MUSCLE TONE
During contraction of the muscle, the spindle relaxes which should decrease afferent input and thus motor output to extrafusal fibers.
Aα
the axons of largest diameter and highest conduction.
Proprioceptors of skeletal muscle.
intrafusal muscle fibers
fusus is Latin for “spindle,” so intrafusal means “inside the spindle”
The ends of the intrafusal fibers are attached to extrafusal fibers, so whenever the muscle is stretched, the intrafusal fibers are also stretched.
The central region of each intrafusal fiber has few myofilaments and is noncontractile, but it does have one or more sensory endings applied to it.
When the muscle is stretched, the central part of the intrafusal fiber is stretched, mechanically sensitive channels are distorted, the resulting receptor potential spreads to a nearby trigger zone, and a train of impulses ensues at each sensory ending.
Intrafusal fibers are too small and too few to contribute to the strength of a muscle, and firing all the gamma motor neurons to a muscle does not generate significant force.
The smaller motor neurons supplying the contractile portions of intrafusal fibers are the gamma motor neurons (or fusimotor neurons).
the smaller axons innervating intrafusal muscle fibers are in the Aγ category.
extrafusal muscle fibers
“outside the spindle”
The large motor neurons that supply extrafusal muscle fibers are the alpha motor neurons,
The large axons innervating the extrafusal fibers of skeletal muscle are in the Aα category.
PROPRIOCEPTIVE PATHWAY:
Modality
Low Threshold Cutaneous, Joint & Muscle Receptors.
Touch, Pressure, Vibration, Fine Form and Texture Discrimination.
Form Recognition of 3-dimensional objects (Stereognosis).
Conscious Awareness of Body Position (Proprioception).
Limb Movement In Space (Kinesthesia).
Input From Lower Limb And Trunk (T6-S5) Forms _____ .
Fasciculus Gracilis
Input From Upper Limb And Trunk (C1-T5) Forms _____ .
Fasciculus Cuneatus
PROPRIOCEPTIVE PATHWAY:
1ST ORDER NEURON
DORSAL ROOT GANGLIA CELLS.
Primary afferent axons enter via dorsal root to enter ipsilateral posterior column.
Below T5 only fasciculus gracilis. Above T5, 2 tracts fasciculus gracilis (FG) and fasciculus cuneatus (FC).
PROPRIOCEPTIVE PATHWAY:
2nd ORDER NEURON
IN CAUDAL MEDULLA IN N. GRACILIS & N. CUNEATUS.
Axons arising from neurons in these nuclei cross the midline
and form the MEDIAL LEMNISCUS.
PROPRIOCEPTIVE PATHWAY:
3rd ORDER NEURON
IN THALAMUS VPL (VENTRAL POSTERIOR LATERAL NUCLEUS).
Axons arising from these neurons project to primary sensory (parietal) cortex
PROPRIOCEPTIVE PATHWAY:
Modality: Low Threshold Cutaneous, Joint & Muscle Receptors.
Touch, Pressure, Vibration, Fine Form & Texture Discrimination, Form Recognition of 3-dimensional objects.
Conscious Awareness of Body Position.
Limb Movement In Space.
1ST ORDER NEURON: DORSAL ROOT GANGLIA CELLS.
Primary afferent axons enter via dorsal root to enter ipsilateral posterior column. Below T5 only fasciculus gracilis. Above T5, 2
tracts – fasciculus gracilis (FG) and fasciculus cuneatus (FC).
2ND ORDER NEURONS IN CAUDAL MEDULLA IN N. GRACILIS & N. CUNEATUS.
Axons arising from neurons in these nuclei cross the midline
and form the MEDIAL LEMNISCUS.
3RD ORDER NEURON IN THALAMUS (VENTRAL POSTERIOR LATERAL NUCLEUS).
Axons arising from these neurons project to primary sensory (parietal) cortex
GAMMA MOTOR NEURONS
Motor innervation to the muscle fibers located on either side of the nuclear area (INTRAFUSAL muscle).
smallest motor neurons in ventral horn.
Innervate contractile portion of intrafusal muscle fibers:
INTRAFUSAL muscle
the muscle fibers located on either side of the nuclear area.
Aα Axon from DRG Neuron
vs.
Axon of Gamma Motor Neurons
Aα AFFERENTS + DESCENDING AFFERENTS
------> α MOTOR NEURON ------> CONTRACTION EXTRAFUSAL MUSCLE ------> DECREASE TENSION ON MUSCLE SPINDLE ------> DECREASE ACTIVITY IN Aα AFFERENTS ------> MUSCLE RELAXATION
Gamma Aγ Process
γ MOTOR NEURONS ------> CONTRACTION INTRAFUSAL MUSCLE ------> MAINTAINS TENSION ON SPINDLE ------> MAINTAINS ACTIVATION OF Aα AFFERENTS ------> MAINTAINS STIMULATION OF Aα MOTOR NEURONS ------> MAINTAINS CONTRACTION EXTRAFUSAL MUSCLE
Good for posture
SPINAL REFLEXES
Efferent motor neuron discharge following a sufficient afferent input.
Independent of supraspinal control; they function even if spinal cord is transected.
Supraspinal input is usually inhibitory in nature; thus, in the absence of descending control, reflexes are usually exaggerated.
COMPONENTS OF SPINAL REFLEX
Peripheral afferents contact both Alpha motor neurons and interneurons
ROLE OF INTERNEURONS
Integrate peripheral afferent input with descending signals
There are both excitatory & inhibitory interneurons
They contact alpha & gamma motor neurons
MONOSYNAPTIC STRETCH (EXTENSOR) REFLEX
Examples:
Triceps tendon, Patellar tendon, Achilles tendon.
STRETCH REFLEX
Latency between tap and response: 30 msec
____ synapse directly on alpha motor neurons that project back to the same muscle.
Aα afferents synapse directly on alpha motor neurons that project back to the same muscle.
Afferents also synapse on _____ that synapse on Alpha motor neurons that project to
antagonist muscles.
Afferents also synapse on inhibitory interneurons that synapse on Alpha Motor Neurons that project to
antagonist muscles.
FLEXOR/WITHDRAWAL REFLEX
All are polysynaptic involving one or more spinal interneurons
Input may be from distal nociceptor; goal is to withdraw limb from harm
Perceived as critical for survival and pre-empt all other reflexes
FLEXOR REFLEX AFFERENT to EXCITATORY INTERNEURONS
EXCITATORY INTERNEURONS ------> ALPHA MOTOR NEURONS ------> INDUCING CONTRACTION & WITHDRAWAL
FLEXOR REFLEX AFFERENT to INHIBITORY INTERNEURONS
INHIBITORY INTERNEURONS ------> ALPHA MOTOR NEURONS ------> ANTAGONIST MUSCLES ------> INDUCING RELAXATION
smallest motor neurons in ventral horn.
GAMMA MOTOR NEURONS
