The Nervous System 15 Sensory and Motor Tracts of the Spinal Cord

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Ascending and Descending tracts

Ascending tracts are sensory
• Deliver information to the brain
Descending tracts are motor
• Deliver information to the periphery

Naming the tracts

• If the tract name begins with "spino" (as in spinocerebellar), the tract is a sensory tract delivering information from the spinal cord to the cerebellum (in this case)
• If the tract name ends with "spinal" (as in vestibulospinal), the tract is a motor tract that delivers information from the vestibular apparatus (in this case) to the spinal cord

There are three major sensory tracts

• The posterior column tract
• The spinothalamic tract
• The spinocerebellar tract

• The three major sensory tracts involve
chains of neurons

First order neuron
Second-order neuron
Third-order neuron

First-order neuron

• Sensory neuron
- Delivers sensations to the CNS
• The cell body is in the dorsal or cranial root ganglion

Second-order neuron

• acts upon first order neuron synapses
• An interneuron with the cell body in the spinal cord or brain

Third-order neuron

• synapses with 2nd order neuron in thalamus
• Transmits information from the thalamus to the cerebral cortex
- still carrying sensory information

Neurons in the sensory tracts are arranged
according to three anatomical principles

Sensory modality
• Fine touch sensations are carried in one sensory tract
Somatotopic
• Ascending tracts are arranged according to the site of origin
Medial-lateral rule
• Sensory neurons that enter a low level of the spinal cord are more medial within the spinal cord
• Sensory neurons that enter at a higher level of the spinal cord are more lateral within the spinal cord

Posterior Column tract consists of

Fasciculus gracilis
• Transmits information coming from areas inferior to T6
Fasciculus cuneatus
• Transmits information coming from areas superior to T6

Spinothalamic tract

• Transmits pain and temperature sensations to the thalamus and then to the cerebrum

Spinocerebellar tract

• Transmits proprioception sensations to the
cerebellum

Motor tracts

• CNS transmits motor commands in response to sensory information
• Motor commands are delivered by the:
-Somatic nervous system (SNS): directs contraction of skeletal muscles
-Autonomic nervous system (ANS): directs the activity of glands, smooth muscles, and cardiac muscle

• These are descending tracts
• There are two major descending tracts
-Corticospinal tract: Conscious control of skeletal muscles
-Subconscious tract: Subconscious regulation of balance, muscle tone, eye, hand, and upper limb position

The Corticospinal Tracts

Consists of three pairs of descending tracts
• Corticobulbar tracts: conscious control over eye, jaw, and face muscles
• Lateral corticospinal tracts: conscious control over skeletal muscles
• Anterior corticospinal tracts: conscious control over skeletal muscles

The Subconscious Motor Tracts

Consists of four tracts involved in monitoring the subconscious motor control
• Vestibulospinal tracts
• Tectospinal tracts
• Reticulospinal tracts
• Rubrospinal tracts

Vestibulospinal tracts


• Send information from the inner ear to monitor position of the head
• Vestibular nuclei respond by altering muscle tone, neck muscle contraction, and limbs for posture and balance

Tectospinal tracts

Send information to the head, neck, and upper limbs in response to bright and sudden movements and loud noises
• The tectum area consists of superior and inferior colliculi
• Superior colliculi: receives visual information
• Inferior colliculi: receives auditory information

Reticulospinal tracts


• Send information to cause eye movements and activate respiratory muscles

Rubrospinal tracts


• Send information to the flexor and extensor muscles

Summary of somatic motor control

• Cerebral cortex initiates voluntary movement
• Information goes to the basal nuclei and cerebellum
• These structures modify and coordinate the movements so they are performed in a smooth manner
• Information goes from the basal nuclei and
cerebellum back to the cerebral cortex to constantly monitor position and muscle tone

Summary of somatic motor control

Thalamus
• Controls reflexes associated with visual and
auditory stimuli
Hypothalamus
• Responds to hunger, thirst, and sexual activity
Pons
• Regulates the rhythmic breathing patterns
Medulla oblongata
• Alters the breathing patterns
Brain stem
• Controls simple reflexes
Spinal cord
• Controls simple reflexes

sensory modality arrangement

sensory fibers are arranged within the spinal cord according to the type of sensory info carried by individual neuron

somatopic arrangement

ascending sensory fibers are arranged within individual tracts according to origin within the body

medial-lateral rule

most sensory nerves entering the spinal cord at more inferior levels travel more medially within sensory tract than a sensory nerve entering the cord at a more superior level (would be more lateral on cord)

3 major somatosensory tracts

1) posterior columns
2) spino thalamic tract
3) spinocerebellar

posterior columns 1st order

-also dorsal column or medial leminscal pathway
- localized info about skin, muscular an proprioception
- "what" "where" and "when"
- axons of first order neurons through dorsal roots to spinal nerves and sensory roots of cranial nerves
- axons from dorsal roots that enter spinal cord below T6 ascend within Fasiculus Gracilis
- enter above T6, ascend within Fasiculus cuneatus
- then synapse with medulla oblongata
- 2nd order:

posterior columns 2nd order

- cross over to opposite side brain along tract called medial lemniscus
- as it travels towards the thalamus, it incorportates other sensory infomation from V VII IX + X
- sensory information in posterior colums is integrated by the ventralposlateral nucleus of thalamus, which soirts the data
- then projected to primary sensory cortex where the individual becomes aware

sensory honmunculus

a map of the sensory pathways
little man
more areas dedicated to face, fingers, than to back

spinothalamic tract

carries sensations about pain, temperature and "crude" touch and pressure
- first order synapse with posterior grey horns
- 2nd order cross to opposite side of spinal cord before ascending in the
- anterior and lateral spinothalamic tracts
- converge in thalamus
- projection fibers of 3rd order neurons carry info to sensory cortex
- both sides of spinal cords have anterior and lateral tracts

spinocerebellar tract

carry proprioceptive information concerning the position of muscles, tendons + joints to the cerebellum,
- responsible for fine coordination
- axons of first order sensory neurons synapse with 2nd order in posterior grey horns of spinal cord.
- axons of 2nd order neurons ascend in anterior or posterior spinocerebellar tracts

spinocerebellar tract

- axons that corss over to opposite side ascend to cerebellar cortex by way or superior cerebellar peduncle
these fibers then cross a second time to finish ipsilateral cerebellum
- posterior spinocerebellar tract carries axons that do not cross over, these axons are brought to cerebellar cortex by the inferior cerebella peduncle
- bc they dont synapse with the thalamus, info is processed at subconscious level, unlike posterior columns

motor tracts

somatic NS (skeletal) has + ANS (visceral) has
- always involved 2 motor neurons or more
- upper motor neurons + lower motor neurons

- upper motor neurons + lower motor neurons

- upper motor neurons - cell bodies lie in CNS
+
- lower motor neurons - bodies lie in motor nucleus of brain stem or spinal cord
- activity in upper motor neuron can excite or inhibit lower neuron
- only axon in lower neuron cam extend to skeletal muscles
- destruction to lower causes flaccid paralysis
- damage to upper causes rigidity, flaccidity or uncoordinated contractions

Ganglionic + Preganglionic neurons

- Pre: cell body lies in CNS
- Ganglionicin peripheral
- higher centers in hypothalamus and brain stem by stimulate or inhibit PRE.

Conscious and Subconscious descending motor tracts

- corticospinal: prymidal tracts:
- works indriectly
* corticobulbar tracts
* anterior and lateral corticospinal tracts
* motor Homunculus
- Subconscious motor pathways:
* vestibulospinal
* tectospinal
*reticulospinal help gross movements of trunk + proximal limbs
*rubrospinal helps distal limbs be more precise

corticospinal tracts

provide conscious control over skeletal muscles
- beings at pyramidal cells of primary motor cortex
- axons descend from brain stem and spinal cord to synapse with lower motor neurons that control skeletal muscles.
- Direct
- indirectly innervates subconscious tracts
corticobulbar tracts, anterior, and lateral corticospinal tracts, which decend from white matter or internal capsule into brain stem

corticobulbar tracts

axons synapse on lower motor neurons in motor nuclei of crainial nerves III, IV, V, VI, IX, + XII
- provide conscious control over muscles that move eye, jaw face + some in neck + pharynx

anterior, and lateral corticospinal tracts

synapse on lower motor neurons in anterior grey horns of spinal cord
- visible as thick bands along medulla called Pyramids
- along the pyramids, 85% of axons cross midline to descending LATERAL corticospinal tracts
- 15% stay on same side (contralateral) as anterior corticospinal tracts
- anterior cross over on same side as anterior white commissure.
upper motor neuron then synapses with lower in anterior grey horns of cervical and superior thracic regions of spinal cord

Motor Homunculus

- activity in pyramidal cells in primary motor cortex results contraction of specific peripheral muscles
- identities of stimulated muscles depends on region of cortex that is active
- areas that control specific parts of the body have been mapped out - homunculus
- proportions of homunculus are very differnt from actual body, bigger area fore more sensitive or fine/complex movement

Subconscious motor pathways

control of muscle tone and gross movements in neck, trunk and proximal limbs are transmited by
- vestibulospinal,
- tectospinal,
- reticulospinal tracts
- upper motor neurons of these tracts are located in
-vestibular nuclei (vestibulospinal)
-superior and inferior colliculi (tectospinal)
- reticular formation, (reticulospinal )
respectively

vestibulospinal tract

- receive info from inner ear that moniters position + movement of head.
- change orientation of head, alter muscle tone, position of neck, eyes, head and limbs.
-primary goal is to maintain posture and balance
- descending fibers of spinal cords are vestibulospinal tracts

tectospinal tract

- superior and inferior colliculi are in mesencephalon.
- receive visual (superior) and auditory (inferior) sensations
- coordinate responses
- superior relays info to inferior + upper motor neurons in superior descend into tectospinal tracts
- axons to opposite side immediately before descending to synapse on lower neurons in brain stem/spinal cord
- change position in neck/head + upper limbs in response to bright lights, sudden movements or loud noises

reticulospinal tract

- reticular formation extends throught brain stem. receives info from almost every ascending and descending tract
- axons of upper motor neurons descend to reticulospinal tracts without crossing to oppsite side.
-effects of reticular formation stimulation are determined by the region that was stimulated. eye movements are different from respiratory muscles

rubrospinal tract

- distal portions of upper limbs are transmitted here
- commands facilitate flexor muscles and inhibit extender muscles
- come from red nuclei of mesencephalon
- axons of upper motor neurons cross to opposite side of brain and decend into spinal cord in rubrospinal tract
- small in humans
- good back up if lateral cortiocospinal tracts are damaged

Amyotropic Lateral Sclerosis

demeylination of motor neurons (and some sensory neurons)
- ALS
- loss of motor control
- no loss of sensation or brain function
- symptoms appear around 40. 3-5 cases in 100, 000
- more common in males
- disease progresses rapidly, muscles atrophy
- 3-5 year prognosis
- Stephen Hawking and Lou Gehrig (baseball player)
- 5-10% genetic, rest is mutation that codes fro an enzyme sthat protects the cell from harmful chemicals generated during metabolism.
- underlying probably is post synaptic membranes of motor neurons
- treated with riluzole, a drug that suppresses glutamate delays onset of respirator paralysis and extends life

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