Lecture 6 Spinal Cord Cortex Reflexes

7/24/2019 Lecture 6 Spinal Cord Cortex Reflexes

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Lecture 6: Spinal Cord; Cortex; Reflexes

Spinal Cord and Pathways

Objectives

1.

Be able to identify the components of the spinal cord: dorsal, ventral and lateral grey horns,

anterior, posterior and lateral white columns, grey commissure and central canal. Describe theirfunctional significance and differences.

2.

Explain the functional significance of the dorsal root ganglia.

3.

Explain the roles of white matter and gray matter in processing and relaying sensory information

and motor commands.

4.

Identify and differentiate between the three somatic sensory pathways: Dorsal ColumnMedial

Meniscus Pathway, Anterolateral Spinothalamic Tracts, Spinocerebellar tracts.

5.

Identify somatic motor pathway and rationalize the types of function losses from spinal cord

injuries.

1. Be able to identify the components of the spinal cord: dorsal, ventral and lateral grey horns,

anterior, posterior and lateral white columns, grey commissure and central canal. Describe their

functional significance and differences.

Central canalis a canal filled with CSF. The gray (grey) commissureis a strip of gray matter that

connects left and right halves of the spinal cord and surrounds the central canal.

Posterior (dorsal) gray hornscarry sensory information.

Anterior (ventral) gray hornscontain bodies of somatic motor neurons.

Lateral gray hornscontain bodies of autonomic, presynaptic neurons.

Anterior (ventral) white column (funiculus) isthe white substance of the spinal cord lying on either

side between the anterior median fissureand the ventral root. Anterior white column contains both

ascending and descending tracts.


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Lateral white column (funiculus) is the white substance of the spinal cord lying between the dorsal and

ventral horns. It also contains both ascending (e.g. spinocerebellar tracts, lateral spinothalamic tract)

and descending tracts (e.g. lateral corticospinal).

Dorsal (posterior) white column (funiculus) is the white matter of the spinal cord lying between the

posterior median sulcus and the dorsal root. It contains only ascending (sensory) tracts: fasciculus

gracilis and fasciculus cuneatus.

Fasciculus gracilis

(slender one) brings

sensory signals from

the legs and lower part

of the body.

Fasciculus cuneatus

(wedge-shaped) brings

sensory information

from the arms and

upper body.

The fasciculus gracilis axons synapse in the gracile nucleus, and the cuneates axons synapse in the

cuneate nucleus.

2. Identify the parts of the spinal cord and location of different types of neurons in spinal cord,

spinal nerves and adjacent ganglia


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Identify: the parts of spinal cord: anterior (motor) and posterior (sensory) horns, ventral and dorsal

roots; the ganglia: sympathetic chain ganglionvs. dorsal root ganglion; and the types of neurons (e.g.

sensory, motor, unipolar, multipolar) in the picture above.

The bodies of the sensory neurons are residing in the dorsal root ganglia (please refer to the picture.)

There is one such ganglion for every spinal nerve. The sensory neurons, which are located in these

ganglia, are unipolar. The paravertebral sympathetic chain ganglia contain bodies of postganglionic

sympathetic neurons.

3. Explain the roles of white matter and gray matter in processing and relaying sensory information

and motor commands.

White matter

(myelinated axons)

tracts propagate

sensory impulses

from receptors to

the brain and motor

impulses from thebrain to the

effectors.Gray

matter(bodies of

neural cells) receives

and integrates

incoming and

outgoing

information.

4. Identify and differentiate between the three somatic sensory pathways: Dorsal Column Medial

Lemniscus Pathway, Anterolateral Spinothalamic Tracts, Spinocerebellar tracts.

The somatosensory systemis a division allowing the perception of different sensations from the body

(e.g. light touch, pain, pressure, temperature and proprioception). Different sensations are referred to

different areas of the brain via three different somatosensory (afferent) pathways:

1. Dorsal ColumnMedial Lemniscus Pathway carries the signals of discriminative touch, which is a

perception of touch, pressure, vibration and texture. It allows for the feeling of shape and texture of the

object.

2. Anterolateral Spinothalamic Tracts carry signals of pain, temperature, tickling and itching.3. Spinocerebellar tracts carry signals fromjoints, tendons and muscles (proprioception). It allows

feeling the position of the body parts (stretch, tension, movement) without looking at them. These

signals are referred to the cerebellum which receives second-to-second feedback about the movement

of the body.


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Dorsal ColumnMedian

Lemniscus Pathwayis presented

in the picture to the right. The

key to the picture is summarized

in the table below. Dorsal

columnMedial Lemniscus

pathway carries discriminative

touch signals from all the parts

of the body except face. These

types of signals from the face are

carried by the cranial nerves V

and VII.

The signals from the upper part

of the body including hand and

arm are going through the

fasciculus cuneatus of dorsal

column; the signals from lower

part of the body are comingthrough f. gracile. In medulla

axons synapse in corresponding

nuclei and the signals proceed

via medial lemniscus pathway,

which decussates in medulla.

Dorsal ColumnMedian Lemniscus Pathway consists of three neurons:1st order neuron (red in the

image)2

ndorder neuron (blue) 3

rdorder neuron

(violet)

Sensation comes via sensory

axon of unipolar neuron.

Receives signals from 1st Receives signals

from 2nd

Body is located in Dorsal Root

Ganglion.

Body of the neuron is located in

medulla: Nucleus gracile or cuneate

nuclus.

Body is located in

thalamus (clearing

house of sensory

info)

Primary afferent axonssynapse in medulla (gracile

fasciculus in gracile nucleus,

cuneate fasciculus in cuneate

nucleus)

Axon (secondary afferent) decussatecrosses to the opposite side of

medulla and then goes to synapse in

thalamus.

Axon synapse incerebral cortex


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Anteriolateral Spinothalamic pathway carries signals of pain and temperature.

Anterolateral Spinothalamic Tracts ( Pain and Temperature) consists of three neurons:

1st order neuron2

nd

order neuron 3rd

order neuron

Collect signals from receptorsof pain and temperature.

Receives signals from 1st

Receives signals from2nd

Body in dorsal root ganglia Body is in dorsal horns. Body is located in

thalamus (clearing

house of sensory info)

Synapse in dorsal horn of the

spinal cord.

Axon (secondary afferent)

crosses to the opposite side of

spinal cord to synapse in

thalamus.

Axon synapse in

cerebral cortex

Compare discriminative touch pathway to pain and temperature pathway in the image above.


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Spinocerebellar tracts carry signals of proprioception (position of parts).

Compare discriminative touch pathway to pain and proprioception pathway in the image above.


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Spinocerebellar Tracts (proprioception) consists of two neurons only:

1st order neuron 2nd

order neuron

Collect signals from muscles and tendons in

limbs and trunk

Receives signals from 1st

Body in dorsal root ganglia Body is in dorsal horns.

Synapse in dorsal horn of the spinal cord. Axon synapses in the cerebellum on the

same side .

IPSILATERAL

Please identify locations of the three sensory pathways (blue) in the image of spinal cord below.

In the summary:

Dorsal column - medial meniscus pathway decussates high in the medulla oblongata. Thalamo-cortical

afferents go through internal capsule to primary somatosensory cortex, postcentral gyrus.

Anterolateral Spinothalamic Tracts (Pain and Temperature) decussates low in the spinal cord.

Spinocerebellar Tracts do not decussate at all.

5. Identify somatic motor pathway and rationalize the types of function losses from spinal cord

injuries.

Somatic motor pathways (red in the image above) carry signals from the CNS to skeletal muscles.

Initiation of movement is controlled by the prefrontal cortex, supplementary motor area, premotor

cortex and basal ganglia in the brain. Then the signal (the command to do) is communicated to the


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motor cortex. Then the motor cortex sends the actual nervous impulses to the muscles required to

perform the action, as well as a copy to the cerebellum.

The axons of Upper Motor Neuronscarrying this information travel down through the brainstem and

spinal cord (primarily via the cortico-spinal tract, in the white matter) until they reach the spinal

segment corresponding to the muscle(s) they innervate. Most of these axons decussate high in the

brain before they reach the spinal cord.

The axons of upper motor neurons synapse with the cell bodies of their corresponding Lower Motor

Neurons(LMN) in the anterior or lateral grey horns of the spinal cord. The axons of LMN travel out

through the ventral roots to their destination, where they stimulate the muscle fibers to contract.

http://medchrome.com/wp-content/uploads/2010/07/UMN-and-LMN.jpg
http://medchrome.com/wp-content/uploads/2010/07/UMN-and-LMN.jpghttp://medchrome.com/wp-content/uploads/2010/07/UMN-and-LMN.jpghttp://medchrome.com/wp-content/uploads/2010/07/UMN-and-LMN.jpg


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Upper motor neurons (UMN)have their bodies in the motor cortex. Their axons synapse in the medulla

oblongata or in the spinal cord. UMN are completely inside the CNS. Damage to the UMN leads to

spasticity, increased muscular tone and exaggerated reflexes.

Lower motor neurons (LMN)receive the signals from UMNs and carry these signals to the skeletal

muscles via peripheral nerves (cranial or spinal).

About 90% of the axons of UMNs decussate to the contralateral side in the medulla oblongata; the

remaining 10% eventually cross over at the spinal cord level when they synapse with an interneuron or

LMN.

It is clinically important to distinguish between upper and lower motor neuron weakness. The signs of

UMN damage versus LMN damage are summarized in the table on the following page.

Lower motor neuron weakness (LMN) Upper motor neuron weakness (UMN)

Flaccid Spasticity

Decreased tone Increased tone

Decreased muscle stretch reflexes Increased muscle stretch reflexes

Profound muscle atrophy Minimal muscle atrophy

Fasciculations present Fasciculations absent

May have sensory disturbances May have associated sensory disturbances

http://www.neuroanatomy.wisc.edu/SClinic/Weakness/Weakness.htm

Fasciculationsare irregular contractions of a group of muscle fibers that belong to one fascicle.

Clinically, this appears as a small muscle twitch.

It is also helpful, to differentiate between the sites of the LMN or muscle damage: body of neuron in the

anterior horn, myelination of axon (neuropathy), NMJ (myestenia gravis) or muscle itself (myopathy).

Answer these practice questions:

Where would the sensory loss be, if you cut:

1). The left gracile fasciculus?2). The left dorsal columns (gracile & cuneate)?

3). The right medial lemniscus, in the medulla?

Where the motor loss will be if the damage is:

4). In right side of the brain?

5). In the right anterior grey horn?
http://www.neuroanatomy.wisc.edu/SClinic/Weakness/Weakness.htmhttp://www.neuroanatomy.wisc.edu/SClinic/Weakness/Weakness.htmhttp://www.neuroanatomy.wisc.edu/SClinic/Weakness/Weakness.htm


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Answers:

1) The left leg and lower left trunk. 2) The left side of the body below the level of the cut. 3) The entire

left body, from the neck down. 4) The left side of the body. 5). Muscles controlled by the peripheral

nerve at this level.

Cortex

Objectives

1.

Differentiate the major areas of the cerebral cortex: somatosensory cortex, motor cortex and

selected Brodmanns areas.

The brain cortexis a thin (2-

4mm thick) layer of gray matter

overlying white matter. This

layer of gray matter is the

cerebral cortexand the

cerebellar cortex.The cortex forms convex folds

(gyri) and concave grooves (sulci

or fissures).

A longitudinal fissure separates the left & right cerebral hemispheres. A big sulci divides each

hemisphere into 5 lobes: frontal, parietal, temporal, occipital and insula. The Centralsulcusseparates

the frontal and parietal lobes. The Parieto-occipitalsulcus separates the parietal and occipital lobes. The

Lateral sulcusseparates the temporal lobe from the frontal and parietal lobes. The Insula (or insular

lobe) is a portion of the cortex that sits in the fold of the lateral sulcus between the frontal and temporal

lobes.


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The precental gyrus(anterior to the central sulcus) is the primary motor area. The postcentral gyrus

(posterior to the central sulcus) is the primary sensory area.

The primary sensory area receives input from the thalamus which is a major relay station. The two

cerebral hemispheres usually receive information from the contralateral side of the body. The special

senses (vision and hearing) are referred to special areas of the cortex on the contaralateral side.

Early in the twentieth century, Dr. Brodmanncreated a map of the areas of the cerebral cortex. These

studies were based on electrical stimulation of differrent areas of the cortex during the surgery of


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epileptic patients and recording their physiological and behavioral reactions. Brodmann assigned

numbers to the areas with different functions. Now these areas are known as Brodmanns areas.

Please note the location and function of: Primary somatosensory area: postcentral gyrus = 1,2,3;

Primary visual area = 17, Primary auditory area = 41 and 42; Primary gustatory area = 43. Wernickes

area 39-40, speech interpretation.

Reflection of the body parts in primary

sensory area (postcentral gyrus). The

size of the area is proportional to the

number of receptors in the

corresponding part of the body rather

than the size of this body part: e.g. lips

are better represented than the leg.


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The motor areas of the cerebral cortexcontrol voluntary movements. This control is also contralateral.

Please note the location and function of: Primary motor area (precentral gyrus = 4) . The primary motor

area controls voluntary contractions of skeletal muscles on the contralateral side; Motor speech

(Brocas) area = 44 & 45. Upper motor neurons are located in this area.

Reflection of the body parts in

primary motor area depends on the

number of motor units controlled

by this area

Blue Boxesin M&A pp. 589- 592

Medical Imaging of Head.


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Reflexes

Objectives

1.

Describe the steps in a neural reflex

2.

Classify the types of reflexes and explain the functions of each.

1. Describe the steps in a neural reflex

http://www.answers.com/topic/reflex

-arc"Reflex arc." The Oxford Dictionary

of Sports Science. Oxford University

Press, 1998, 2006, 2007. Answers.com

10 Dec. 2010.

A reflexis a specific motor reaction to

a specific stimulus. A reflex arc is the

pathway followed by nerve impulses

that produce a reflex. An ideal reflex

arc consists of 5 steps: 1) receptor,

2) afferent fiber, 3) interneuron(s),

4) efferent fiber and 5) effector.

1. Receptors convert external or

internal stimulus (e.g. light, heat,

pressure, chemical changes) into the

action potentials.

2. Afferent (sensory) fibers transmit

these signals to the interneurons in

CNS.

3. Interneurons integrate and process the signals; it may be one or few interneurons or no interneuron

at all.

4. Motor fibers transfer the AP from CNS to the effector.

5. Effectors (e.g. skeletal muscles, smooth muscles, glands) perform the response.

2. Classify the types of reflexes and explain the functions of each.

Reflexes can be classified by the site of processing: e.g. spinal, brain stem, cerebral, etc.

Spinal reflexesare processed at the level of the spinal cord. This allows the faster motor reaction

because the motor neuron is activated without sending signals to the brain and back. Although, the

brain still receives the signal while the motor reaction is occurring: e.g. myotatic stretch reflexes.

Brain reflexesare processed at the level of the brain. Their absence indicates brain death; e.g. pupillary,

pharyngeal, cough reflexes. Cough reflex is coughing in response to irritation of the airway linings.

Pupillary reflex is a contraction of the pupil in response to the light. Pharyngeal reflex is a contraction of

pharyngeal constrictor muscle in response to touching the back of the pharynx.
http://www.answers.com/topic/reflex-archttp://www.answers.com/topic/reflex-archttp://www.answers.com/topic/reflex-archttp://www.answers.com/topic/reflex-archttp://www.answers.com/topic/reflex-arc


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Reflexes can be classified by the number of synapses: polysynaptic vs. monosynaptic. Polysynaptic

reflexes involve more than one synapse. They are often complex and take a longer time. Monosynaptic

reflexes involve just one synapse. There are no interneurons involved and processing occurs in the

motor neuron. These reflexes are the most simple and fastest.

Myotatic stretch reflexesare an example of a

spinal monosynaptic reflex. A stretch reflex is the

contraction of the muscle in response to the

stretching of the muscle spindles. Muscle

spindles are receptors inside the muscle that lie

parallel to muscle fibers. When a muscle spindle

is stretched the sensory neuron sends the signal

to the motor neuron in the ventral horn of the

spinal cord which in turn sends the signal back to

the muscle and the muscle contracts. Stretch

reflexes are very important to maintain posture

and balance. For the testing purposes the

myotatic stretch reflexes can be checked by brisktaping of the tendon of the corresponding

muscle: knee jerk reflex (patellar ligament) ,

ankle jerk reflex (Achilles tendon), biceps reflex

(biceps tendon), brachioradialis reflex, jaw jerk,

finger jerk, etc.

http://thebrain.mcgill.ca/flash/d/d_01/d_01_cr/

d_01_cr_fon/d_01_cr_fon.html

Hyporeflexiaabsent or low response totapping (weak reflexes) usually indicate the damage of the motor neuron within the reflex arc (lower

neuron damage). Hyperreflexiarepeating or too strong response to tapping indicates damage to the

descending tracts of the corticospinal

pathway (upper neuron damage). Even

if the reflex is monosynaptic information

is actually sent to the cortex as a

reference.

The Golgi tendon organ, which is

another receptor, is attached between

the muscle and the tendon. When

muscle contracts Golgi tendon organ

sends the signal to the spinal cord and

synapses on an inhibitory interneuron.

The inhibitory interneuron synapses on

an alpha motor neuron which goes to an

antagonist muscle and causes relaxation

of the antagonist. Information is also

conveyed from these receptors to the

cerebellum and cortex.13-16

Illustration of the Stretch Reflex


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Reflexes can be classified by their development: innate vs. acquired. Inborn or innate reflexes are

involuntary and unlearned. Withdraw reflex is pulling your body part away from pain (e.g. hand away

from a hot surface when touched). Palatine reflex is a swallowing in response to stimulation of the

palate. Grasp reflex is a flexion of the fingers (grasping) in response to stimulation of the palm (in

infants). Rooting reflex; when an infant's cheek is stroked, the baby responds by turning his or her head

in the direction of the touch and opening their mouth for feeding. Learned or acquired reflexes are often

complex, learned motor patterns acquired during the lifetime; e.g. walking on two extremities, biking,

swimming, driving the car, reading, writing, speech, languages, following the traffic rules, etc.

Reflexes can be classified by the effector organs: somatic vs. autonomic (visceral). Somatic Reflexes:

involve contractions of the skeletal muscles. Examples: Knee Jerk reflex, biking. Babinski's

reflex dorsiflexion of the big toe results from firmly stroking the lateral outer margin of the sole. In

adults positive Babinski is a sign of the lesions in the cortex or in the pyramidal tract, although it is a

normal reflex in infants till one year old.

Autonomic or visceral reflexes: involve glands, smooth and cardiac muscles, and generally are not

consciously perceived. The main integrating centers for most autonomic reflexes are located in the

hypothalamus and brain stem. Some autonomic reflexes, e.g. for urination and defecation, have

integrating centers in the spinal cord. Peristaltic reflex, when a portion of the intestine is full (stretched

and irritated), the area just proximal contracts and the area just distal relaxes. Pilomotor reflex, when

stroking or tickling the skin causing the activation of arrector pili with formation of goose bumps.

Baroreceptor reflex, the response to stimulation of baroreceptors of the carotid sinus and aortic arch,

regulating blood pressure by controlling heart rate, strength of heart contractions, and diameter of

blood vessels. The steps of the visceral reflex arch are essentially the same as somatic, but visceral has

two neurons in the motor branch: preganglionic and postganglionic; while somatic reflex has only one

neuron in the motor branch.

Compare and contrast

steps of somatic and

visceral reflex in the image

to the right. Label each step

and structure.

http://people.eku.edu/ritch

isong/301notes2b.html
http://people.eku.edu/ritchisong/301notes2b.htmlhttp://people.eku.edu/ritchisong/301notes2b.htmlhttp://people.eku.edu/ritchisong/301notes2b.htmlhttp://people.eku.edu/ritchisong/301notes2b.htmlhttp://people.eku.edu/ritchisong/301notes2b.html


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To perform Babinskis test, the sole of the foot must be firmly stroked on the lateral side in the direction

from heel to toes as it shown in the image.

http://medical-

dictionary.thefreedictionary.com/Babinski+reflex

A and B normal adult plantar reflex causes aflexion of the hallux and toes ( Negative Babinskis

sign).

C is positive Babinskis sign, when the big toe moves

toward the dorsum of the foot and the other toes

fan out in response to the foot stroke.

Positive Babinskis is normal in babies. Positive

Babinskis after age 2 is a sign of damage to the nerve paths connecting the spinal cord and the brain

(the corticospinal tract).

Underlying causes of positive Babinskis test may be head trauma, stroke, meningitis, multiple sclerosis,

brain tumor etc. A Babinski's reflex can occur on one side or on both sides of the body. Patients withpositive Babinskis may complain of poor coordination and muscle spasms or weakness.

http://what-when-how.com/neuroscience/the-upper-motor-neurons-motor-systems-part-3/
http://medical-dictionary.thefreedictionary.com/Babinski+reflexhttp://medical-dictionary.thefreedictionary.com/Babinski+reflexhttp://medical-dictionary.thefreedictionary.com/Babinski+reflexhttp://what-when-how.com/neuroscience/the-upper-motor-neurons-motor-systems-part-3/http://what-when-how.com/neuroscience/the-upper-motor-neurons-motor-systems-part-3/http://what-when-how.com/neuroscience/the-upper-motor-neurons-motor-systems-part-3/http://medical-dictionary.thefreedictionary.com/Babinski+reflexhttp://medical-dictionary.thefreedictionary.com/Babinski+reflex

Lecture 6 Spinal Cord Cortex Reflexes

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