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)

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)

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

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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

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

Tracts that serve to join brain to the spinal Tracts that serve to join brain to the spinal cordcord› AscendingAscending› DescendingDescending

Fibers that interconnect adjacent or distant Fibers that interconnect adjacent or distant segments of the spinal cordsegments of the spinal cord› IntersegmentalIntersegmental (propriospinal) (propriospinal)

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

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

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

Three major pathways carry sensory informationPosterior column pathway (gracile & cuneate

fasciculi)Anterolateral pathway (spinothalamic) Spinocerebellar pathway

Dorsal white column Lateral spinothalamic Anterior spinothalamic Anterior spinocerebellar Posterior

spinocerebellar Cuneocerebellar Spinotectal Spinoreticulr Spino-olivary Visceral sensory tracts

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

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)

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

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

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

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

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

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

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

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

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

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

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

• 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

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

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

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

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

Pyramidal› Corticospinal

Extrapyramidal› Rubrospinal› Tectospinal› Vestibulospinal › Olivospinal › Reticulospinal

Descending Autonomic Fibers

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

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

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

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)

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)

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

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

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

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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