NEUROPHYSIO CORD REFLEXES.pdf

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9/8/20

Motor Functions of the Spinal

Cord

and Cord Reflexes

Charlie E. Labarda, MD

ORGANIZATION OF

MOTOR FUNCTION BY

THE

SPINAL CORD

9/8/2011 School of Health Sciences

MOTOR UNITS

• a single motoneuron and the muscle fibers that it innervates

• number of muscle fibers innervated can vary from a few fibers to thousands of fibers, depending on the nature of the motor activity

• eye movements—motoneurons innervate only a few muscle fibers; postural muscles—motoneurons innervate thousands of muscle fibers


Size Principle

• States that as more motor units are

recruited, progressively larger

motoneurons are involved and greater

tension will be generated

• The force of contraction of a muscle is

graded by recruitment of motor units

• E.g. small vs large motoneurons




9/8/20

• Small motoneurons

innervate a few muscle fibers

because they have the lowest thresholds, they fire first

also generate the smallest amounts of force

• Large motoneurons

innervate many muscle fibers

have the highest thresholds to fire action potentials; thus, they fire last

they also generate the greatest amounts

of force9/8/2011 School of Health Sciences

Types of

Motoneurons


α Motoneuron

• innervate extrafusal skeletal muscle fibers

• Action potentials lead to action potentials in

the extrafusal muscle fibers they innervate,

which results in contraction


γ Motoneurons

• innervate specialized intrafusal muscle fibers, a component of the muscle spindles

• function of the muscle spindle: to sense muscle length;

• the function of the γmotoneuronsinnervating them is to adjust the sensitivity of the muscle spindles (so they respond appropriately as the extrafusal fibers contract and shorten)




9/8/20

• αMotoneurons and γmotoneurons are

coactivated (activated simultaneously) so

that muscle spindles remain sensitive to

changes in muscle length even as the muscle

contracts and shortens


Types of

Muscle Fibers

Extrafusal fibers

• constitute the majority of skeletal muscle

• innervated by αmotoneurons

• are used to generate force


Intrafusal fibers

• are specialized fibers that are innervated by γ

motoneurons and are too small to generate

significant force

• encapsulated in sheaths, forming muscle

spindles that run parallel to the extrafusal

fibers




9/8/20

MUSCLE

SPINDLES

• distributed among the extrafusal muscle

fibers

• especially abundant in muscles utilized for

fine movements (e.g., muscles of the eye)

• spindle‐shaped organs composed of

intrafusal muscle fibers and innervated by

sensory and motor nerve fibers

• are attached to connective tissue and

arranged in parallel with the extrafusalmuscle fibers


Intrafusal Muscle Fibers

of Muscle Spindles

• two types of intrafusal fibers present in

muscle spindles:

nuclear bag fibers‐‐are larger, and their nuclei

are accumulated in a central ("bag") region

nuclear chain fibers‐‐smaller, and their nuclei are

arranged in rows ("chains")

• Generally, both types of fibers are present in

every muscle spindle, but nuclear chain

fibers are more plentiful than nuclear bag

fibers9/8/2011 School of Health Sciences

Innervation of

Muscle Spindles: both

sensory (afferent) and

motor (efferent) nerves



9/8/20

Sensory Innervation

• group Ia afferent nerve

innervates both the nuclear bag fibers and

the nuclear chain fibers

among the largest nerves in the body

among the fastest conduction velocities

form primary endings in a spiral‐shaped

terminal around the central region of the

nuclear bag and nuclear chain fibers


• group II afferent nerves

innervate only the nuclear chain fibers

have intermediate diameters and

intermediate conduction velocities

form secondary endings on the nuclear

chain fibers


Motor Innervation

• consists of two types of γmotoneurons:

Dynamic

Static

Dynamic γmotoneurons synapse on nuclear

bag fibers in "plate endings."

Static γmotoneurons synapse on nuclear

chain fibers in "trail endings," which spread

out over longer distances


• γMotoneurons are smaller and slower than

the αmotoneurons that innervate the

extrafusal fibers

• the function of the γmotoneurons (either

static or dynamic) is to regulate the

sensitivity of the intrafusal muscle fibers they

innervate




9/8/20

Function of

Muscle Spindles

• stretch receptors

• Function: to correct for changes in muscle

length when extrafusal muscle fibers are

either shortened (by contraction) or

lengthened (by stretch)

• Thus, muscle spindle reflexes operate to

return muscle to its resting length after it has

been shortened or lengthened

• To illustrate the function of the muscle

spindle reflex: consider the events that occur

when a muscle is stretched


Muscle is

stretched

Extrafusal muscle

is lengthened

Intrafusal muscle

also lengthened

Events that Occur when a Muscleis Stretched


detected by the sensory afferent fibers

Group Ia

afferent fibers

(nuclear bag and

nuclear chain fibers)

detect the velocity

of length change

Group II

afferent fibers

(nuclear chain fibers)

detect the length of the

muscle fiber

Activation of

Group Iα afferent

fibers

stimulates

α motoneurons

in the spinal cord

cause the muscle to

contract (shorten)the original stretch (lengthening) is

opposed when the reflex causes the

muscle to contract and shorten

innervate

extrafusal fibers in

the homonymous

(same) muscle

γ Motoneurons are

coactivated

ensures that the muscle

spindle will remain

sensitive to changes in

muscle length even during

the contraction9/8/2011 School of Health Sciences



9/8/20

SPINAL CORD REFLEXES

• are stereotypical motor responses to specific

kinds of stimuli, such as stretch of the muscle

• The neuronal circuit that directs this motor

response is called the reflex arc

• The reflex arc includes:

the sensory receptors;

the sensory afferent nerves—carry

information to the spinal cord

the interneurons in the spinal cord

Motoneurons‐‐direct the muscle to

contract or relax9/8/2011 School of Health Sciences

Stretch (Myotatic) Reflex

• the simplest of all spinal cord reflexes

• having only one synapse between sensory

afferent nerves (group Ia afferents) and

motor efferent nerves (α motoneurons)

• exemplified by the knee‐ jerk reflex





9/8/20

The Knee-Jerk Reflex


Tapping the

patellar tendon

Quadriceps muscle

stretch (muscle

spindle stretched)

group Ia afferent

fibers are

stimulated

synapse on and activate

α motoneurons in the

spinal cord

innervate and cause

contraction of the

quadriceps (the

muscle that originally

was stretched

Quadriceps

contract and

shortens

Knee

jerk

Golgi Tendon Reflex

• a disynaptic spinal cord reflex

• also called the inverse myotatic reflex

• Golgi tendon organ

is a stretch receptor found in tendons

senses contraction (shortening) of muscle and

activates group Ib afferent nerves

are arranged in series with the extrafusal muscle

fibers (contrasting the parallel arrangement of

muscle spindles in the stretch reflex)


Steps in the Golgi tendon Reflex

1. When the muscle contracts, the extrafusal

muscle fibers shorten, activating the Golgi

tendon organs attached to them. In turn,

the group Ib afferent fibers that synapse on

inhibitory interneurons in the spinal cord

are activated. These inhibitory

interneurons synapse on the α

motoneurons


Golgi Tendon Reflex




9/8/20

2. When the inhibitory interneurons are

activated (i.e., activated to inhibit ), they

inhibit firing of the αmotoneurons,

producing relaxation of the homonymous

muscle (the muscle that originally was

contracted)

3. As the homonymous muscle relaxes, the

reflex also causes synergistic muscles to relax

and antagonistic muscles to contract.


Flexor‐Withdrawal Reflex

• a polysynaptic reflex that occurs in response

to a painful or noxious stimulus

• Somatosensory and pain afferent fibers

initiate a flexion reflex that causes withdrawal

of the affected part of the body from the

painful or noxious stimulus

• The reflex produces flexion on the ipsilateral

side (i.e., side of the stimulus) and extension

on the contralateral side


Steps in the Flexor‐Withdrawal

Reflex

1. When a limb touches a painful stimulus, flexor

reflex afferent fibers (groups II, III, and IV) are

activated. These afferent fibers synapse on

multiple interneurons in the spinal cord (i.e.,

polysynaptic reflex).

2. On the ipsilateral side of the pain stimulus,

reflexes are activated that cause flexor

muscles to contract and extensor muscles to

relax. This portion of the reflex produces

flexion on the ipsilateral side 9/8/2011 School of Health Sciences

Flexor‐Withdrawal Reflex




9/8/20

3. On the contralateral side of the pain

stimulus, reflexes are activated that cause

extensor muscles to contract and flexor

muscles to relax. This portion of the reflex

produces extension on the contralateral side

and is called the crossed‐extension reflex.

Thus, if the painful stimulus occurs on the

left side, the left arm and leg will flex or

withdraw and the right arm and leg will

extend to maintain balance.


4. A persistent neural discharge, called an

afterdischarge, occurs in the polysynaptic

reflex circuits. As a result of the

afterdischarge, the contracted muscles

remain contracted for a period of time after

the reflex is activated.


Type ofReflex

(Example)

Number of

Synapses

Stimulus for

Reflex

SensoryAfferent

Fibers Responses

Stretch reflex

(knee jerk)

One Stretch

(lengthening)

of the muscle

Ia Contraction

of the muscle

Golgi tendon

reflex (clasp

knife)

Two Contraction

(shortening)

of the muscle

Ib Relaxation of

the muscle

Flexor-

withdrawal

reflex

(touching a

hot stove)

Many Pain;

temperature

II, III, and IV Flexion on

ipsilateral

side;

extension on

contralateral

side9/8/2011 School of Health Sciences

CONTROL OF POSTURE AND

MOVEMENT BY THE BRAIN

STEM



9/8/20

• Descending motor pathways (i.e., those

descending from the cerebral cortex and

brain stem) are divided among the pyramidal

tract and the extrapyramidal tract

• Pyramidal tracts are corticospinal and

corticobulbar tracts that pass through the

medullary pyramids and descend directly

onto lower motoneurons in the spinal cord

• All others are extrapyramidal tracts


The extrapyramidal tracts originate in the

following structures of the brain stem:

The rubrospinal tract

originates in the red nucleus and projects to

motoneurons in the lateral spinal cord

Stimulation of the red nucleus produces

activation of flexor muscles and inhibition of

extensor muscles


The pontine reticulospinal tract

originates in nuclei of the pons and projects

to the ventromedial spinal cord

Stimulation has a generalized activating

effect on both flexor and extensor muscles,

with its predominant effect on extensors


The medullary reticulospinal tract

originates in the medullary reticular

formation and projects to motoneurons in

the spinal cord

Stimulation has a generalized inhibitory

effect on both flexor and extensor muscles,

with the predominant effect on extensors






9/8/20

CEREBELLUM

• "little brain," regulates movement and posture

and plays a role in certain kinds of motor

learning

• helps control the rate, range, force, and

direction of movements (synergy)

• Damage results in lack of coordination

• located in the posterior fossa just below the

occipital lobe

• connected to the brain stem by three cerebellar

peduncles, which contain both afferent and

efferent nerve fibers9/8/2011 School of Health Sciences

3 main divisions of the cerebellum:

the vestibulocerebellum‐‐ dominated by

vestibular input and controls balance and eye

movements

Spinocerebellum‐‐ dominated by spinal cord

input and controls synergy of movement

Pontocerebellum‐‐ dominated by cerebral

input, via pontine nuclei, and controls the

planning and initiation of movements


Layers of the Cerebellar Cortex

The granular layer

the innermost layer

contains granule cells, Golgi II cells, and

glomeruli

In the glomeruli, axons of mossy fibers from

the spinocerebellar and pontocerebellar tracts

synapse on dendrites of granule and Golgi

type II cells




9/8/20

The Purkinje cell layer

the middle layer

contains Purkinje cells, and its output is

always inhibitory


The molecular layer

the outermost layer

contains outer stellate cells, basket cells,

dendrites of Purkinje and Golgi II cells, and

axons of granule cells

The axons of granule cells form parallel

fibers, which synapse on the dendrites of

Purkinje cells, basket cells, outer stellate

cells, and Golgi type II cells.


Layers of the Cerebellar Cortex


Input to the Cerebellar Cortex

• Two systems provide excitatory input to the

cerebellar cortex:

the climbing fiber system

the mossy fiber system

• Each system also sends collateral branches

directly to deep cerebellar nuclei, in addition

to their projections to the cerebellar cortex

• Excitatory projections from the cerebellar

cortex then activate secondary circuits,

which modulate the output of the cerebellar

nuclei via the Purkinje cells9/8/2011 School of Health Sciences



9/8/20

Climbing fibers

• originate in the inferior olive of the medulla

and project directly onto Purkinje cells

• make multiple synaptic connections along

the dendrites of Purkinje cells, although each

Purkinje cell receives input from only one

climbing fiber

• These synaptic connections are very

powerful!


• A single action potential from a climbing

fiber can elicit multiple excitatory bursts,

called complex spikes, in the dendrites of the

Purkinje cell

• It is believed that climbing fibers "condition"

the Purkinje cells and modulate their

responses to mossy fiber input

• Climbing fibers also may play a role in

cerebellar learning.


Mossy fibers

• constitute the majority of the cerebellar input

• These fibers include vestibulocerebellar,

spinocerebellar, and pontocerebellar afferents

• Mossy fibers project to granule cells, which

are excitatory interneurons located in

collections of synapses called glomeruli

• Axons from these granule cells then ascend to

the molecular layer, where they bifurcate and

give rise to parallel fibers9/8/2011 School of Health Sciences

• Parallel fibers from the granule cells contact the

dendrites of many Purkinje cells, producing a

"beam" of excitation along the row of Purkinje cells

• The dendritic tree of each Purkinje cell may receive

input from as many as 250,000 parallel fibers!

• In contrast to the climbing fiber input to the

Purkinje dendrites (which produce complex spikes),

the mossy fiber input produces single action

potentials called simple spikes

• These parallel fibers also synapse on cerebellar

interneurons (basket, stellate, and Golgi II).




9/8/20

Interneurons of the Cerebellum

• The function of cerebellar interneurons is to

modulate Purkinje cell output

• With the exception of granule cells, all of the

cerebellar interneurons are inhibitory

• Granule cells have excitatory input to basket cells,

stellate cells, Golgi II cells, and Purkinje cells

• Basket cells and stellate cells inhibit Purkinje cells

(via parallel fibers)

• Golgi II cells inhibit granule cells, thereby reducing

their excitatory effect on Purkinje cells



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NEUROPHYSIO CORD REFLEXES.pdf