1. Sensory & Motor Pathways
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2. There is a continuous flow of information between the brain, spinal cord, and peripheral nerves. This information is relayed by sensory (ascending) and motor (descending) ‘pathways’.
Generally the pathways:
Consists of a chain of tracts, associated nuclei and varying number of relays (synapses)
Consist of two or three neurons
Exhibit somatotopy (precise spatial relationships)
Decussate
Involve both the brain and spinal cord
Are paired (bilaterally and symmetrically)

3. Somatic Sensory Pathways

4. Sensory Pathways
Monitor conditions both inside the body and in the external environment
Sensation-stimulated receptor passes information to the CNS via afferent (sensory) fibers
Most sensory information is processed in the spinal cord , thalamus, or brain stem. Only 1% reaches the cerebral cortex and our conscious awareness
Processing in the spinal cord can produce a rapid motor response (stretch reflex)
Processing within the brain stem may result in complex motor activities (positional changes in the eye, head, trunk)

5. Sensory Pathways
Contain a sequence of THREE neurons from the receptor to the cerebral cortex
First order neuron: Sensory neuron that delivers information from the receptor to the CNS.
Cell body located in the dorsal root ganglion. The Axon (central process) passes to the spinal cord through the dorsal root of spinal nerve gives many collaterals which take part in spinal cord reflexes runs ipsilaterally and synapses with second-order neurons in the cord and medulla oblongata 3 2 1

6. Second order neuron:
Has cell body in the spinal cord or medulla oblongata
Axon decussate &
Terminate on 3rd order neuron
Third order neuron:
Has cell body in thalamus
Axon terminates on cerebral cortex ipsilaterally

7. White Matter: Pathway Generalizations
Ascending and descending fibers are organized in distinct bundles which occupy particular areas and regions in the white matter
Generally long tracts are located peripherally in the white matter, while shorter tracts are found near the gray matter
The TRACT is a bundle of nerve fibers (within CNS) having the same origin, course, destination & function
The name of the tract indicates the origin and destination of its fibers
The axons within each tract are grouped according to the body region innervated

8. Tracts of the Spinal Cord
Tracts that serve to join brain to the spinal cord
Ascending
Descending
Fibers that interconnect adjacent or distant segments of the spinal cord
Intersegmental (propriospinal)

9. Intersegmental Tracts
Extensive fiber connections between spinal segments
Fasciculus proprius
Short ascending & descending fibers
Both crossed & uncrossed
Begin and end within the spinal cord
Participate in intersegmental spinal reflexes
Present in all funiculi adjacent to gray matter

10. Intersegmental Tracts
Dorsolateral tract of Lissauer: Primary sensory fibers carrying pain, temperature and touch information bifurcate upon entering the spinal cord. Their branches ascend and descend for several spinal segments in the dorsolateral tract, before synapsing in the dorsal horn
Intersegmental fibers, establishing connections with neurons in the opposite half of the spinal cord, cross the midline in the anterior white commissure

12. Ascending Spinal Tracts
Transmit impulses:
Concerned with specific sensory modalities: pain, temperature, touch, proprioception, that reach a conscious level (cerebral cortex)
Dorsal column funiculi
Spinothalmic tracts
From tactile and stretch receptors to subconscious centers (cerebellum)
Spinocerebellar tracts

13. Three major pathways carry sensory information
Posterior column pathway (gracile & cuneate fasciculi)
Anterolateral pathway (spinothalamic)
Spinocerebellar pathway

14. Ascending Spinal Tracts
Dorsal white column
Lateral spinothalamic
Anterior spinothalamic
Anterior spinocerebellar
Posterior spinocerebellar
Cuneocerebellar
Spinotectal
Spinoreticulr
Spino-olivary
Visceral sensory tracts

15. Dorsal Column
Contains two tracts, Fasciculus gracilis (FG) & fasciculus cuneatus (FC)
Carry impulses concerned with proprioception and discriminative touch from ipsilateral side of body
Contain the axons of primary afferent neurons that have entered cord through dorsal roots of spinal nerves
FG contains fibers received at sacral, lumbar and lower thoracic levels, FC contains fibers received at upper thoracic and cervical levels

16. Fibers ascend without interruption where they terminate upon 2nd order neurons in nucleus gracilis and nucleus cuneatus
The axons of the 2nd order neurons decussate in the medulla as internal arcuate fibers and ascend through the brain stem as medial lemniscus.
The medial lemniscus terminates in the ventral posterior nucleus of the thalamus upon 3rd order neurons, which project to the somatosensory cortex (thalamocortical fibers)

18. Spinothalamic Tracts
Located lateral and ventral to the ventral horn
Carry impulses concerned with pain and thermal sensations (lateral tract) and also non- discriminative touch and pressure (medial tract)
Fibers of the two tracts are intermingled to some extent
In brain stem, constitute the spinal lemniscus
Fibers are highly somato-topically arranged, with those for the lower limb lying most superficially and those for the upper limb lying deeply
Information is sent to the primary sensory cortex on the opposite side of the body

19. Lateral Spinothalamic Tract
Carries impulses concerned with pain and thermal sensations.
Axons of 1st order neurons terminate in the dorsal horn
Axons of 2nd order neuron (mostly in the nucleus proprius), decussate within one segment of their origin, by passing through the ventral white commissure & terminate on 3rd order neurons in ventral posterior nucleus of the thalamus
Thalamic neurons project to the somatosensory cortex

20. Anterior Spinothalamic Tract
Carries impulses concerned with non- discriminative touch and pressure
Axons of 1st order neurons enter cord terminate in the dorsal horn
Axons of 2nd order neuron (mostly in the nucleus proprius) may ascend several segments before crossing to opposite side by passing through the ventral white commissure & terminate on 3rd order neurons in ventral posterior nucleus of the thalamus
Thalamic neurons project to the somatosensory cortex

21. Spino-reticulo-thalamic System
The system represents an additional route by which dull, aching pain is transmitted to a conscious level
Some 2nd order neurons terminate in the reticular formation of the brain stem, mainly within the medulla
Reticulothalamic fibers ascend to intralaminar nuclei of thalamus, which in turn activate the cerebral cortex

22. Spinocerebellar Tracts
The spinocerebellar system consists of a sequence of only two neurons
Two tracts: Posterior & Anterior
Located near the dorsolateral and ventrolateral surfaces of the cord
Contain axons of the second order neurons
Carry information derived from muscle spindles, Golgi tendon organs and tectile receptors to the cerebellum for the control of posture and coordination of movements

23. Posterior Spinocerebellar Tracts
Present only above level L3
The cell bodies of 2nd order neuron lie in Clark’s column
Axons of 2nd order neuron terminate ipsilaterally (uncrossed) in the cerebellar cortex by entering through the inferior cerebellar peduncle

24. Ventral Spinocerebellar Tracts
The cell bodies of 2nd order neuron lie in base of the dorsal horn of the lumbosacral segments
Axons of 2nd order neuron cross to opposite side, ascend as far as the midbrain, and then make a sharp turn caudally and enter the superior cerebellar peduncle
The fibers cross the midline for a second time within the cerebellum before terminating in the cerebellar cortex
Both spinocerebellar tracts convey sensory information to the same side of the cerebellum

25. Spinotectal Tract
Ascends in the anterolateral part in close association with spinothalamic system
Primary afferents reach dorsal horn through dorsal roots and terminate on 2nd order neurons
The cell bodies of 2nd order neuron lie in base of the dorsal horn
Axons of 2nd order neuron cross to opposite side, and project to the periaquiductal gray matter and superior colliculus in the midbrain

26. Spino - olivary Tract
Indirect spinocerebellar pathway (spino-olivo-cerebellar)
Impulses from the spinal cord are relayed to the cerebellum via inferior olivary nucleus
Conveys sensory information to the cerebellum
Fibers arise at all level of the spinal cord

27. Spinoreticular Tract Originates in laminae IV-VIII
Contains uncrossed fibers that end in medullary reticular formation & crossed & uncrossed fibers that terminate in pontine reticular formation
Form part of the ascending reticular activating system

28. Somatic Motor Pathways

29. Motor Pathways
CNS issues motor commands in response to information provided by sensory systems, sent by the somatic nervous system (SNS) and the autonomic nervous system (ANS)
Conscious and subconscious motor commands control skeletal muscles by traveling over 3 integrated motor pathways
The corticospinal pathway – voluntary control of motor activity
Corticobulbar tracts
Corticospinal tracts
The medial and lateral pathways – modify or direct skeletal muscle contractions by stimulating, facilitating, or inhibiting lower motor neurons

30. Motor Pathways
Contain a sequence of TWO neurons from the cerebral cortex or brain stem to the muscles
Upper motor neuron : has cell body in the cerebral cortex or brain stem, axon decussates before terminating on the lower motor neuron
Lower motor neuron: has cell body in the ventral horn of the spinal cord, axon runs in the ipsilateral ventral root of the spinal nerve and supply the muscle.
UMN
LMN

31. Descending Spinal Tracts
Originate from the cerebral cortex & brain stem
Concerned with:
Control of movements
Muscle tone
Spinal reflexes & equilibrium
Modulation of sensory transmission to higher centers
Spinal autonomic functions

32. The motor pathways are divided into two groups
Direct pathways (voluntary motion pathways) - the pyramidal tracts
Indirect pathways (postural pathways), essentially all others - the extrapyramidal pathways

33. Direct (Pyramidal) System
Regulates fast and fine (skilled) movements
Originate in the pyramidal neurons in the precentral gyri,
Impulses are sent through the corticospinal tracts and synapse in the anterior horn
Stimulation of anterior horn neurons activates skeletal muscles
Part of the direct pathway, called corticobulbar tracts, innervates cranial nerve nuclei

34. Indirect (Extrapyramidal) System
Complex and multisynaptic pathways
The system includes:
Rubrospinal tracts: control flexor muscles
Vestibulospinal tracts: maintain balance and posture
Tectospinal tracts: mediate head neck, and eye movement
Reticulospinal tracts

35. Descending Spinal Tracts
Pyramidal
Corticospinal
Extrapyramidal
Rubrospinal
Tectospinal
Vestibulospinal
Olivospinal
Reticulospinal
Descending Autonomic Fibers

36. Corticospinal Tracts
Concerned with voluntary, discrete, skilled movements, especially those of distal parts of the limbs (fractionated movements)
Innervate the contralateral side of the spinal cord
Provide rapid direct method for controlling skeletal muscle

37. Origin: motor and sensory cortices
Axons pass through corona radiata, internal capsule, crus cerebri and pyramid of medulla oblongata
In the caudal medulla about 75-90% of the fibers decussate and form the lateral corticospinal tract
Rest of the fibers remain ipsilateral and form anterior corticospinal tract. They also decussate before termination

38. Distribution:
55% terminate at cervical region
20% at thoracic
25% at lumbosacral level
Termination: Ventral horn neurons (mostly through interneurons, a few fibers terminate directly)
Corticobulbar tracts end at the motor nuclei of CNs of the contralateral side

39. Rubrospinal Tract
Controls the tone of limb flexor muscles, being excitatory to motor neurons of these muscles
Origin: Red nucleus
Axons course ventro-medially, cross in ventral tegmental decussation, descend in spinal cord ventral to the lateral corticospinal tract
Cortico-rubro-spinal pathway (Extrapyramidal)

40. Tectospinal Tract
Mediates reflex movements of the head and neck in response to visual stimuli
Origin: Superior colliculus
Axons course ventro-medially around the periaqueductal gray matter, cross in dorsal tegmental decussation, descend in spinal cord near the ventral median fissure, terminate mainly in cervical segments
Cortico-tecto-spinal pathway (Extrapyramidal)

41. Vestibulospinal Tracts
Lateral Vestibulospinal Tracts
Origin: lateral vestibular (Deiter’s) nucleus
Axons descend ipsilaterally in the ventral funiculus
Terminate on ventral horn cells throughout the length of spinal cord
Has excitatory influences upon extensor motor neurons, control extensor muscle tone in the antigravity maintenance of posture

42. Vestibulospinal Tracts
Medial vestibulospinal tract
Origin: medial vestibular nucleus
Axons descend bilaterally in the ventral funiculus, with the medial longitudinal fasciculus
Most of the fibers end in the cervical region, some reaching upper thoracic segments
Involved in movements of the head required for maintaining equilibrium

43. Reticulospinal Tracts
Influence voluntary movement, reflex activity and muscle tone by controlling the activity of both alpha and gamma motor neurons
Mediate pressor and depressor effect on the circulatory system
Are involved in control of breathing
Origin: pontine & medullary reticular formation
Medial (pontine) reticulospinal tract descends ipsilaterally
Lateral (medullary) reticulospinal tract descends bilaterally
Both tracts located in the ventral funiculus

44. Descending Autonomic Fibers
The higher centers associated with the control of autonomic activity are situated mainly in the hypothalmaus
The fibers run in the reticulospinal tracts
Terminate on the autonomic neurons in the lateral horn of thoracic & upper lumbar (sympathetic) and sacral segments (parasympathetic) levels of the spinal cord

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