1. Sensory & Motor Pathways
faizan

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)

10. 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

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

12. Ascending Spinal Tracts
Dorsal white column
Lateral spinothalamic
Anterior spinothalamic
Anterior spinocerebellar
Posterior spinocerebellar
Cuneocerebellar
Spinotectal
Spinoreticulr

13. Dorsal Column
Contains two tracts, Fasciculus gracilis (FG) & fasciculus cuneatus (FC)
Carry impulses concerned with proprioception and discriminative touch from ipsilateral side of body

14. Lateral Spinothalamic Tract
Carries impulses concerned with pain and thermal sensations.

15. Anterior Spinothalamic Tract
Carries impulses concerned with non- discriminative touch and pressure

16. Spinocerebellar Tracts
The spinocerebellar system consists of a sequence of only two neurons
Two tracts: Posterior & Anterior
Carry information derived from muscle spindles, Golgi tendon organs and tectile receptors to the cerebellum for the control of posture and coordination of movements

17. Somatic Motor Pathways

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

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

25. 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

26. 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

27. Rubrospinal Tract
Controls the tone of limb flexor muscles, being excitatory to motor neurons of these muscles
Origin: Red nucleus
Cortico-rubro-spinal pathway (Extrapyramidal)

28. Tectospinal Tract
Mediates reflex movements of the head and neck in response to visual stimuli
Origin: Superior colliculus

29. Vestibulospinal Tracts
Lateral Vestibulospinal Tracts
Origin: lateral vestibular (Deiter’s) nucleus
Has excitatory influences upon extensor motor neurons, control extensor muscle tone in the antigravity maintenance of posture

30. Vestibulospinal Tracts
Medial vestibulospinal tract
Origin: medial vestibular nucleus
Involved in movements of the head required for maintaining equilibrium

31. Reticulospinal Tracts
Influence voluntary movement, reflex activity and muscle tone by controlling the activity of both alpha and gamma motor neurons
Are involved in control of breathing
Origin: pontine & medullary reticular formation