At the end of the class, Student should be able to:• Classify descending tracts. a) Pyramidal tract b) extrapyramidal tract• Describe origin, course termination of the descending tracts.• Functions of the descending tracts.• Differences between pyramidal & extrapyramidal tract• Differences between lower & upper neuron lesion
Functions of pyramidal tractsTracts : Lateral corticospinal tractsFunction: Controlling the voluntary movements- fine, precise movements of the fingers and hands to carry out skilled work.Tracts : Anterior corticospinal tractsFunction: Control of muscles of trunk & proximal portions of the limbs to carry postrual adjustments and gross movements
• They form a part of the pathways for superficial reflexes such as cremasteric, abdominal and plantar reflexes.• Some corticospinal fibers end at excitatory synapses on α and γ-motor neurons, whereas other end on interneurons that may excite or inhibit the α-motor neurons. Thus the effect of the corticospinal pathway on the α-motor neurons may be excitatory or inhibitory.• Some fibers transmit information from the brain to ‘afferent’ neurons and so can effect afferent system, they do this by ending either: (a) Presynaptically on the axon terminals of afferent neurons as these fibers enter the CNS; or
• Corticospinal fibers arising from the somatic sensory area (I and II) and parietal lobe association cortex are concerned with sensory-motor coordination. For example, aiming the hands towards it, hand-eye coordination etc. lesion of these areas causes defects in motor performance that are characterized inability to execute learned sequences f movements such as eating with a knife and fork.
Functions of corticobulbar (corticonuclear) tracts• These are responsible for voluntary control of muscles of larynx, pharynx, palate, upper and lower face, jaw, eye etc. Pseudobulbar Palsy is a condition resulting in paralysis or weakness of the muscles which control swallowing, talking, tongue and lip movements due to bilateral lesion of these tracts.
EXTRAPYRAMIDAL TRACTS• Extrapyramidal system is made up of those areas the CNS (other than the pyramidal and cerebellar system) that are concerned with muscular movements and posture. Its fibers have many synapses in their descending path with cells of the nuclear masses on the way which include: nuclei of the cerebral cortex, basal ganglia, hupothalamus and nuclei of the reticular formation in the brain stem. In the spinal cord the fibers form separate groups according to their site of origin.
FUNCTIONS OF EXTRAPYRAMIDAL TRACTS• Corticobulbar (corticonuclear) fibers control the movements of the eye balls.• They are responsible for control of tone, posture and equilibrium (rubrospinal- for tone and posture; tectospinal- for visuospinal reflex; vestibulospinal -for the equilibrium).• They control complex movements of the body and limb such as coordinated movements of arms and legs during walking.• They exert tonic inhibitory control over the lower centres. Their damage increases rigidity of the muscles, called release phenomenon.• If the pyramidal tracts are damaged, they can carry out voluntary movement to some extent.
Descending pathways that contribute to the extrapyramidal systemTRACTS DESCRIPTION(ORIGIN AND COURSE) Main function1. Rubrospinal It originates from the Red nucleus Facilitatory influencetract (nucleus Magnocellularis i.e. large over flexor muscle tone. nucleus ) located in the mid brain; crosses immediately to the opposite side, some fibers end in the cerebellum. The tract does not extend below the thoracic region.2. Tectospinal It originates from the superior Mediate reflex posturaland tectobulbar colliculus (which is an optic movements in responsetract centre); crosses at once to the to visual and auditory opposite side. The tract descends stimuli. upto the lower cervical region.
3. Reticulospinal Origin from neurons of the (i) Facilitate or inhibittract reticular formation in pons and voluntary medulla. movements, mainly (ii)Medial division i.e. fibers influence γ–motor from the pontine reticular neurons. formation are mainly crossed (ii) Alteration in muscle (iii)Lateral division i.e. tone, respiration medullary reticular fibers and blood pressure descend uncrossed.4. Vestibulospinal Origin: from lateral vestibular Facilitatory influencetract nucleus located at the junction upon reflex activity in the of pons and medulla. It receives spinal cord and upon the fibers from the vestibular mechanism which division of VIII nerve. control muscle tone Both lateral and medial division (mainly extensor group descend uncrossed throughout i.e. antigravity group of the entire length of the spinal muscles). cord
5. Medial Origin: from the medial Coordinator of reflexlongitudinal vestibular nucleus, reticular ocular movements andfasiculus formation, superior colliculus integration of eye and(orbundle) and interstitial nucleus of Cajal; neck movements. the tract descend uncrossed upto upper cervical region
Major differences between pyramidal exrapyramidal tractsPyramidal tracts Extrapyramidal tracts1. Origin Origin2. Its axons pass without relay to the They have many synapses in theirspinal segmental levels where they form descending path with cells of nuclei ofsynapses with either interneurons in the the striatum (caudate and putamen), thedorsal horn or directly with the motor globus pallidus, the hypothalamus andneurons themselves. the nuclei of the reticular formation.3. They have grater influence over motor They are more involved with coordinationneurons that control muscles involved in of the large muscle groups used in thethe fine movements, particularly those of maintenance of upright posture, inthe fingers and hand. locomotion, and in head and body movements when turning towards a specific stimulus.4. Lesion of this tract produces ‘spasticity’ Its lesion produces ’rigidity’ of thein the muscles involved. involved muscles.
Differences between lower and upper motor neuron lesionLower motor neuron lesion(LMNL) Upper motor neuron lesion(UMNL)1. It is due to lesion of the lower It is due to lesion of the upper motorneurons (LMNs) i.e. the spinal and neurons (UMNs) i.e. the neurons in thecranial motor neurons that directly brain and spinal cord that can influenceinnervate the muscles the activity of LMNs; major cause being lesion of pyramidal tracts.2. Here usually single or individual It involves a group of muscles.muscle is affected.3. Muscle becomes completely Affected muscles due to pyramidal tractparalysed (flaccid paralysis). This is due lesion become hypertonic (spasticto complete loss of muscle tone which paralysis). This is due to:depends on integrity of the reflex arc.` (ii)Release phenomenon i.e. loss of higher inhibitory control; and (iii)Denervation hypersensitivity of centres below the transection.
4. Disuse atrophy of the muscle occurs i.e. The Muscle atrophy is not severe (ifshrinkage of muscle fiber which is finally present, very mild) because musclesreplaced by fibrous tissue(fibrous muscle). though not used in the voluntary movements are continuously in action to maintain posture by ‘reflexes’.5. All reflexes (superficial or deep) are (i) Deep reflexes are hyperactiveabsent (lost) as the motor pathway is (accentuated) because of increased γ-damaged motor discharge; and ii) Superficial reflexes; only abdominal, cremastric and anal reflexes are lost.6. Babinski plantar response: Babinski sign Babinski sign: positive (abnormal). Innot elicited. UMNL, stroking outer edge the sole of the foot with firm tactile stimulus produces first an upward movements (dorsiflexion) of the great toe and fanning out (abduction) of the small toes. This is due to contraction of extensor hallucis longus (Anatomists misleadingly call it an ‘extensor response’), physiologically a ‘flexor’ (withdrawal response. (note: All the muscles which contract during a flexor response are called physiological flexors.)