Pyramidal system

Pyramidal system.Upper motor neuron,Corticospinal tract,
Corticobulbar tract,Giant pyramidal cells ,reticular formation ,
Babinski sign ,

1. Pyramidal system By- Dr.Ranjeet Singha,PT (MPT in Neurology) HAAD Licensed Associate Professor, College of Physiotherapy and Medical Sciences, Guwahati,Assam .

2. The pyramidal system is made up of three pairs of descending motor tracts: (1) the  corticobulbar tracts, (2) the lateral corticospinal tracts, and (3) the anterior corticospinal tracts (Fig. 103-1). The corticobulbar tracts find their origin in the primary  motor cortex  of the  cerebrum  and end at the  brainstem  motor nuclei of  cranial nerves  III, IV, VI, VII, IX, and XII, which are responsible for control of eye movements, the tongue, the muscles of facial expression, and the more superficial muscles of the neck and back.

3. consists of upper motor neurons extending from the cortex to the brainstem or spinal cord that make up two major pathways of voluntary movement: the corticospinal and corticobulbar tracts (sometimes called the pyramidal tracts).

4. Upper motor neuron: a neuron that extends from the  cerebral cortex  or  brainstem  to synapse with a  lower motor neuron  (usually in the  spinal cord ). Upper motor neurons control the activity of lower motor neurons, which control the activity of muscles to produce movement.

5. Corticospinal tract: primary pathway for producing voluntary movement, the corticospinal tract is a large collection of axons that travel from the cerebral cortex down to the spinal cord and synapse on neurons that can influence muscle activity. Many of the axons that enter the corticospinal tract originate in the primary motor cortex , although other motor areas also contribute to the pathway.

6. Corticobulbar tract: pathway involved with voluntary movement using muscles of the head, neck, and face. The corticobulbar tract travels from the  cortex  as part of the  pyramidal system  (along with the corticospinal tract), but it terminates on  cranial nerve nuclei  in the  brainstem  instead of continuing down to the  spinal cord .

7. Corticospinal The corticospinal tract represents the highest order of motor function in humans and is most directly involved in control of fine, digital movements. This tract arises in pyramidal neurons of layer V of the precentral gyrus , the "primary motor cortex." Betz cells are the largest of these pyramidal neurons.

8. There is a motor homonculus in this gyrus , with the feet represented near the superomedial part of the motor cortex and the leg, trunk, arm, hand and head represented progressively further inferior on the lateral side of the brain.

9. Axons arising from neurons in the precentral gyrus exit through the white matter and pass through the internal capsule where they are topographically arranged in the posterior limb.

10. The fibers controlling the lower extremity are posterior to those of the upper limb. Corticospinal fibers traverse the middle portion of the cerebral peduncle of the midbrain and then the basal pons . They enter the pyramids of the medulla (from whence they get their name).

11. Over 90% of the axons in the pyramids decussate just before reaching the upper cervical spinal cord (the pyramidal decussation ) and they enter the lateral funiculus of the spinal cord to become the (lateral) corticospinal tract

12. ). Most of these axons terminate in the intermediate gray matter of the cord, although some enter the dorsal horn (where they can have an effect on sensory transmission) and a few terminate directly on alpha motor neurons, contributing to rapid voluntary movement. Most of these fibers terminate on interneurons of the spinal cord.

13. These interneurons are responsible for reflexes and, therefore, most motor activity actually occurs by the regulation of reflex excitability in the spinal cord.

14. A few corticospinal axons descend the anterior funiculus of the spinal cord as the anterior (ventral) corticospinal tract. This is more involved in axial (trunk and neck) movements and terminates bilaterally.

15. Origin. Giant pyramidal cells (30K) in the precentral gyrus (cerebral cortex). Axon termination. Directly on skeletomotor (alpha) and fusimotor (gamma) motor neurons (55% in cervical region, 25% lumbosacral region).

16. Functional Significance 1. Important in individual finger flexor movements = fractionation of movements. 2. Important in movements that require speed, agility, adaptability. 3. Terminate contralaterally on lower motor neurons to distal or appendicular muscles, especially flexors of upper limb.

17. The course of the axon, which forms the tract, is as follows: 1. Pre-central gyrus (site of the upper motor neuron cell body) 2. Internal capsule (posterior limb, see below) 3. Cerebral peduncle ( crus cerebri ) middle 3/5s 4. Pons proper or basal pons 5 . Pyramid in the medulla 6. Pyramidal decussation (how the left brain controls the right body) 7. Spinal cord (a) The lateral corticospinal tract, crossed. -90% (b) The ventral or anterior corticospinal tract, uncrossed 10% to axial muscles.

18. Axons terminate on Ventral Horn Cells = Lower Motor Neurons. a. Crossed ( decussation ) axons terminate contralaterally on motor neurons to appendicular muscles (b. Uncrossed axons terminate ipsilaterally on axial muscles, they cross before terminating )

19. Corticobulbar Many projections from the cerebral cortex terminate in the brain stem (generically called corticobulbar projections). These projections have several functions including voluntary control over cranial nerves, relay to the cerebellum, activation of other descending pathways (i.e., "indirect corticospinal projections") and modulation of sensory processing.

20. Many cranial nerve nuclei receive direct and indirect (through the reticular formation) cortical input via nerve fibers arising from the motor cortex and traversing the genu of the internal capsule. Most corticobulbar connections are bilateral, meaning that unless both sides of the nervous system are affected, there is no loss of motor control.

21. However, the facial nucleus to the lower face receives only input from the contralateral motor cortex and, therefore, there will be weakness of voluntary movement of the lower face on the side opposite damage to corticobulbar neurons (with sparing of movements of the upper face).

22. The majority of corticobulbar projections terminate in the ipsilateral basal pontine nuclei. These nuclei relay to the cerebellar cortex via projections that decussate in the pons and enter the cerebellum through the middle cerebellar peduncle (see below). These represent, by far, the largest input to the cerebellum.

23. Bulbospinal projections There are several brain stem nuclei that project to the spinal cord. The cerebral cortex projects to most of these and, therefore, may affect them as "indirect corticospinal projections." These areas include the red nucleus, which gives rise to the rubrospinal tract that decussates in the midbrain and descends the lateral funiculus near to the location of the lateral corticospinal tract

24. The reticular formation gives rise to several descending pathways, one from the rostral pons that helps pattern locomotion, one from the caudal pons that can affect head movement to coincide with eye movement and one from the medulla that mostly inhibits reflex activity in the spinal cord.

25. This latter tract is excited by cortical input and, therefore, cerebral motor cortical output is mostly inhibitory to spinal cord reflexes via this indirect pathway. For this reason, interruption of corticobulbar projections typically increases reflexes.

26. Cerebral cortical projections also go to the superior colliculus , a region that gives rise to a tectospinal tract as well as projections to eye movement centers . The superior colliculus is mostly responsible for reflex head and eye movement toward novel stimuli. The cerebral cortical projections to the superior colliculus may effect movement via these projections.

27. Bulb: the medulla + pons + mesencephalon The corticobulbar fibers are similar to corticospinal fibers except instead of terminating in the ventral horn of the cord, they end in cranial nerve motor nuclei (EXCEPT THE EXTRAOCULAR NUCLEI III, IV, and VI to be discussed later). The terms upper motoneuron and pyramidal tract are often used collectively as a term for both corticospinal and corticobulbar axons. Hence we have upper motor neurons that end on cranial motor neurons.

28. . Origin -- Similar to that of the corticospinal axons except face region of cortex. Course 1. Internal capsule -- posterior limb (some books say genu ) 2. Cerebral peduncle -- the axons either leave the corticospinal fibers at this point or at a slightly more caudal level and make their way to the appropriate cranial nerve nuclei. Others travel more diffusely in the tegmentum . There is no visible corticobulbar tract as there is for the corticospinal tract.

29. . Termination: Examples. 1.Most muscles act together such as the pharynx, larynx. They get input from both hemispheres. 5. Termination on the hypoglossal nuclei motoneurons is mostly crossed and can be useful in localizing lesions in the acute state. Signs may disappear after a few days. 6. Muscles of facial expression

30. Corticobulbar tract to the facial nucleus lower motor neurons. 1. The lower motor neurons that innervate muscles of the lower face receive only crossed corticobulbar axons from the cortex of the opposite side. 2. The motor neurons that innervate muscle of the upper face receive both crossed and uncrossed corticobulbar axons. That is that both hemispheres send cortical fibers to the nuclei on both sides of the brainstem

31. BLOOD SUPPLY TO CORTICOSPINAL and CORTICOBULBAR SYSTEM A. Motor cortex -- middle & anterior cerebral arteries. B. Internal Capsule -- very variable 1. Anterior limb -- lenticulostriate arteries from middle cerebral artery. 2. Posterior limb -- middle cerebral artery, anterior choroidal artery and rarely branches from posterior cerebral artery. C. Blood Supply of Cerebral Peduncle -- posterior cerebral artery (variable). D. Blood Supply of Pons -- Basilar artery. E. Blood Supply of Pyramid and lateral cord -- Anterior spinal artery.

32. CLINICAL FINDINGS IN UPPER MOTOR NEURON DISEASE Most corticospinal lesions are in the internal capsule or cerebral cortex in the distribution of the middle cerebral artery and result in classic signs. When the tract is lesioned in the brain stem these signs are associated with cranial nerve signs A . Babinski sign (extensor plantar or dorsiflexion response or upgoing toe and fanning of the other toes) is abnormal and indicates damage to corticospinal tract. The big toe normally goes down. Hoffmann’s sign is a similar phenomenon for the hand.

33. B. Exaggerated tendon reflexes ( hyperreflexia or increased DTRs). This includes clonus , crossed adductor, jaw jerk. Early on flaccidity may be due to "spinal" shock. C. Spasticity: increased resistance to passive stretch. In upper extremity greater in flexors; in lower extremity greater in extensors. Clinical sign is the claspedknife response. D. Re-emergence of primitive reflexes, so-called Frontal (lobe) release signs: snout, grasp, suck, root, palmomental and glabellar

34. CONTRASTING UPPER MOTOR NEURON AND LOWER MOTOR NEURON DISEASE Lesions of the upper motor neuron system (Corticospinal-Corticobulbar System) produce a different constellation of signs than do lesions of the lower motor neuron system (anterior horn cells).

35. UPPER MOTOR NEURON LOWER MOTOR NEURON Initial weakness or paralysis of muscles of entire limb or side of body and reduced reflexes Spasticity of affected muscles, clasped knife Hyperactivity of deep tendon reflexes, clonus Weakness or paralysis muscles in discrete area Flaccidity of affected muscles Hypoactive or absent deep tendon reflexes

36. No muscle atrophy or very slight from disuse No muscle fasciculations Pathologic reflexes, Babinski , Prominent muscle atrophy Fasciculations present No pathologic reflexes present