11/4 - Peripheral Muscle Receptors and Spinal Cord Reflexes Flashcards Preview

Neuro 3050: Structure & Function of the Nervous System > 11/4 - Peripheral Muscle Receptors and Spinal Cord Reflexes > Flashcards

Flashcards in 11/4 - Peripheral Muscle Receptors and Spinal Cord Reflexes Deck (60):
1

Spinal Nerve Pathway

PERIPHERAL PROCESS >>> DORSAL ROOT >>>
DORSAL ROOT GANGLION >>>
DORSAL HORN >>>
VENTRAL HORN >>>
MOTOR NEURON >>>
EFFERENT AXON >>>
VENTRAL ROOT >>>
SPINAL NERVE >>>

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ALPHA MOTOR NEURONS

Innervate ____ muscle fibers: _____ portion of muscle

ALPHA MOTOR NEURONS Innervate extrafusal muscle fibers: Regular contractile portion of muscle

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ORGANIZATION OF MOTOR NEURONS IN SPINAL CORD

MOTOR NEURONS FORM FUNCTIONAL CLUSTERS IN THE VENTRAL HORN

4

Motor Neurons are arranged in longitudinal columns.

All the neurons in one column go to the same one muscle.

5

Each column contains the motor neurons that innervate one muscle.

Column usually extends through more than 1 spinal cord segment.

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TOPOGRAPHIC ORGANIZATION:

FLEXORS

DORSAL

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TOPOGRAPHIC ORGANIZATION:

EXTENSORS

VENTRAL

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TOPOGRAPHIC ORGANIZATION:

DISTAL LIMB Mucles

LATERAL

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TOPOGRAPHIC ORGANIZATION:

PROXIMAL LIMB & TRUNK Muscles

MEDIAL

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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.

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Each muscle fiber is a _____ .

Each muscle fiber is a single, multinucleated cell

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MOTOR UNIT

Defined as a single alpha motor neuron and ALL THE MUSCLE FIBERS it innervates

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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.

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A ____ may innervate more than one ____ .

A motor axon may innervate more than one muscle fiber.

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Motor End-Plate

Site where axons make synaptic contact with muscle fiber.

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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).

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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.

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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.

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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

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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.

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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.

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TETANUS

1. Binds to peripheral nerve terminals and transported within the axon to CNS.

2. Binds to proteins at presynaptic inhibitory motor nerve endings and taken up into the neurons.

3. Effect is to block release of inhibitory neurotransmitters (GABA, glycine).

4. Results in uncontrolled firing of motor neurons resulting in muscular spasms.

5. Acts by selective cleavage of a protein required for neurotransmitter release, synaptobrevin II

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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.

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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.

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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.

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Primary ____ innervate spindle by wrapping around ____ .

Primary afferents innervate spindle by wrapping around central part.

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Central part is non-contractile and acts as _____ .

Central part is non-contractile and acts as a stretch receptor.

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NUCLEAR BAG FIBERS

nuclei cluster in center of fiber.

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Static bag fiber

Responds to changes of muscle length

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Dynamic bag fiber

Responds to rate of change of muscle length

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NUCLEAR CHAIN FIBERS

nuclei line up in a row with no enlargement of central
region

Respond to changes in muscle length

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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).

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annulospiral ending

Primary afferent (Ia/Aα) loop around equatorial region of nuclear bag and chain fibers forming annulospiral ending.

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MUSCLE TONE

During contraction of the muscle, the spindle relaxes which should decrease afferent input and thus motor output to extrafusal fibers.

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the axons of largest diameter and highest conduction.

Proprioceptors of skeletal muscle.

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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.

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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.

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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).

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Input From Lower Limb And Trunk (T6-S5) Forms _____ .

Fasciculus Gracilis

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Input From Upper Limb And Trunk (C1-T5) Forms _____ .

Fasciculus Cuneatus

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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).

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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.

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PROPRIOCEPTIVE PATHWAY:

3rd ORDER NEURON

IN THALAMUS VPL (VENTRAL POSTERIOR LATERAL NUCLEUS).

Axons arising from these neurons project to primary sensory (parietal) cortex

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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

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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:

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INTRAFUSAL muscle

the muscle fibers located on either side of the nuclear area.

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Aα Axon from DRG Neuron

vs.

Axon of Gamma Motor Neurons

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Aα AFFERENTS + DESCENDING AFFERENTS

------>
α MOTOR NEURON
------>
CONTRACTION EXTRAFUSAL MUSCLE
------>
DECREASE TENSION ON MUSCLE SPINDLE
------>
DECREASE ACTIVITY IN Aα AFFERENTS
------>
MUSCLE RELAXATION

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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

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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.

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COMPONENTS OF SPINAL REFLEX

Peripheral afferents contact both Alpha motor neurons and interneurons

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ROLE OF INTERNEURONS

Integrate peripheral afferent input with descending signals

There are both excitatory & inhibitory interneurons

They contact alpha & gamma motor neurons

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MONOSYNAPTIC STRETCH (EXTENSOR) REFLEX

Examples:
Triceps tendon, Patellar tendon, Achilles tendon.

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STRETCH REFLEX

Latency between tap and response: 30 msec

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____ 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.

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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.

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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

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FLEXOR REFLEX AFFERENT to EXCITATORY INTERNEURONS

EXCITATORY INTERNEURONS ------>
ALPHA MOTOR NEURONS
------>
INDUCING CONTRACTION
& WITHDRAWAL

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FLEXOR REFLEX AFFERENT to INHIBITORY INTERNEURONS

INHIBITORY INTERNEURONS
------>
ALPHA MOTOR NEURONS
------>
ANTAGONIST MUSCLES
------>
INDUCING RELAXATION

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smallest motor neurons in ventral horn.

GAMMA MOTOR NEURONS