Descending pathwaysDescending pathways

Types Types

• direct pathways• indirect pathways .

• Most of the indirect pathways, sometimes called the extrapyramidal system, are involved in less precise control of motor functions, especially those associated with overall body coordination and cerebellar function such as posture.

• Some indirect pathways, such as those from the basal nuclei and

cerebellum, help in fine control of the direct pathways.

Direct PathwaysDirect Pathways

• The direct pathways, also called the pyramidal (pi-rami-dal) system, are involved in the maintenance of muscle tone and in controlling the speed and precision of skilled movements.

• Direct pathways are so named because upper motor neurons in the cerebral cortex, whose axons form these pathways, synapse directly with lower motor neurons in the brainstem or spinal cord.

• They are also called the pyramidal system because the fibers of these pathways primarily pass through the medullary pyramids.

• They include the corticospinal tract, which is involved in direct cortical control of movements below the head,

• the corticobulbar tract, which is involved in direct cortical control of movements in the head and neck.

Indirect PathwaysIndirect Pathways

• The indirect pathways originate in upper motor neurons of the cerebrum and cerebellum whose axons synapse in some intermediate nucleus rather than directly with lower motor neurons.

• Axons from the upper motor neurons in these nuclei form the indirect pathways. They do not pass through the pyramids or through the corticobulbar tracts and, therefore, are sometimes called extrapyramidal.

• rubrospinal,• vestibulospinal• reticulospinal tracts

Parts of somatic nervous system

1- The sensory division It comprises all sensory

(afferent) nerves, the sensory ascending tracts, the thalamus and sensory cortex.

It serves the somatic sensations from the skin, muscles, tendons, ligaments and periosteum.

Parts of somatic nervous system – Cont.

2- The motor division It comprises of motor

cerebral cortex, the basal ganglia, and the thalamus, the reticular formation of the brain stem, the descending motor tracts and the motor (efferent) nerves.

This division controls the voluntary muscular activity.

Motor NeuronsMotor Neurons

• Motor neurons are split into two groups: Upper and Lower motor neurons.

• Upper motor neurons originate in the motor region of the cerebral cortex of the brain stem and carry motor information down to the final common pathway. The nerve fibers that descend in white matter from supraspinal levels are segregate into nerve bundles –descending tracts.

• The cell bodies of these neurons are some of the largest in the brain, approaching nearly 100μm in diameter.

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Figure 16.4b

Locations of major descending motor tracts that contain axons of upper

motor neurons

ANATOMICAL ORGANIZATIONANATOMICAL ORGANIZATION

• FIRST ORDER- CEREBRAL CORTEX

• SECOND ORDER(INTERNUNCIAL NEURON- ANTERIOR GRAY COLUMN

• THIRD ORDER(LOWERMOTOR NEURON)- ANTERIOR GRAY COLUMN

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Figure 16.3a

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Figure 16.3b

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Figure 16.5

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Figure 16.4a

a) Originates at the primary motor cortex – corticobulbar tracts end at the motor nuclei of CNs on the opposite side of the brain- most fibers crossover in the medulla and enter the lateral corticospinal tracts- rest descend in the anterior corticospinal tracts and crossover after reaching target segment in the SC

• corticospinal tracts– Axonsof pyramidal cells situated

in fifth layer of cerbral cortex.– One third –primary motor cortex– One third-secondary motor

cortex• Descending fibers converge in

corona radiata• Midbrain-basis pedunculi• Pons-transverse pontocerebellar

fibers• Medulla-pyramids• Junctionof medulla and spinal cord-

decussating of fibers• Lateral corticospinal tract

DESCENDING DESCENDING TRACTS OF TRACTS OF

MOTOR MOTOR FIBERSFIBERS

Motor neuronsMotor neurons

• The motor neurons situated in the anterior gray column send axon to innervate muscles through anterior roots of spinal nerve. A lower motor neuron's axon terminates on an effector (muscle).

• Lower motor neurons are classified based on the type of muscle fibre they innervate:

– Alpha motor neuronsAlpha motor neurons (α-MNs) innervate extrafusal muscle fibers.– Gamma motor neuronsGamma motor neurons (γ-MNs) innervate intrafusal muscle fibers,

Descending Pathway Lesions Descending Pathway Lesions

• An upper motor neuron lesion is a lesion of the neural pathway above the anterior horn cell or motor nuclei of the cranial nerves.

• This is in contrast to a lower motor neuron lesion, which affects nerve fibers travelling from the anterior horn of the spinal cord to the relevant muscle(s).

Corticospinal tract leisonsCorticospinal tract leisons

• Babinski s sign is present

• Superfficial abdominal reflexes absent

• Loss of fine skilled movements

• Great toe becomes dorsiflexed and other toes fan outward-positive

• Normally there is plantar flexion of all toes because corticospinal tract is intact

Upper motor neuron lesions (extra pyramidal tract)Upper motor neuron lesions (extra pyramidal tract) are are indicated by:indicated by:

– Spasticity, increase in tone in the extensor muscles (lower limbs) or flexor muscles (upper limbs) .As a result upperlimb is maintained in flexion and lower limb in extension

– Clasp-knife response where initial resistance to movement is followed by relaxation. When passive movement of a joint is attempted there is resistance owing to spasticity of muscle.

– Weakness in the flexors (lower limbs) or extensors (upper limbs), but no muscle wasting

• Exaggerated abdominal reflexes

Lower motor neuron leisonLower motor neuron leison

• Flaccid paralysis

• Atrophy of muscles

• Muscular fasiculations

• Muscular contracture

Indirect (Extrapyramidal) SystemIndirect (Extrapyramidal) System• Includes the brain stem, motor nuclei, and all motor

pathways not part of the pyramidal system• This system includes the rubrospinal, vestibulospinal,

reticulospinal, and tectospinal tracts• These motor pathways are complex and multisynaptic, and

regulate:– Axial muscles that maintain balance and posture– Muscles controlling coarse movement of the proximal

portions of limbs– Head, neck, and eye movement

• LMN are known as the ‘final common pathway’ because despite all the descending tracts they are the only neurons that actually physically contact the muscle fibers

• LMN’s get stimulated by corticospinal, vestibulospinal, reticulospinal, rubrospinal and tectospinal tracts, among others. The cerebellum exerts a substantial influence on many of them.

Questions?Questions?

• Draw labelled diagram showing ascending tracts in spinal cord.

• What will happen in leison of spinal leminiscus in medulla oblongata.

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Dorsal root

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Spinal cord organizationSpinal cord organization

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Spinal Cord Cross SectionSpinal Cord Cross Section

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Spinal Cord, Roots & Spinal Cord, Roots & NervesNerves

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