Sensory Tract
Neuroanatomy block-Anatomy-Lecture 3
Editing file Objectives
At the end of the lecture, students should be able to:
01 Define the meaning of a tract 02 Distinguish between the different types of tracts 03 Locate the position of each 04 Describe the sensory pathway 05 Identify the different sensory spinal tracts and their functions 06 Identify the course of each of these tracts Color guide
07 Know some associated lesions regarding the main tracts ● Only in boys slides in Green ● Only in girls slides in Purple ● important in Red ● Notes in Grey White matter of spinal cord Posterior funiculus
● The grey matter of the spinal cord is completely surrounded by the white matter. ● The White matter of the spinal cord consists of Ascending and Descending Nerve Fibers. ● It is divided into Dorsal, Lateral & Ventral Columns or Funiculi. Lateral funiculus White matter tracts ● Is Bundles or fasciculi of fibers that occupy more or less definite positions in the white matter. They have the same Origin, Termination and carry the same Function. Anterior ● They are classified (according to the length) into: funiculus
Short tracts Long tracts (intersegmental or propriospinal) Divided according to the function
Types 1. Ascending (sensory or Fibers occupy narrow band afferent) immediately peripheral to the grey matter (fasciculus proprius). 2. Descending (motor or Fasciculus efferent) proprius
They interconnect adjacent or distant spinal segments & Permit They serve to join the brain to the Function intersegmental spinal cord. coordination
3 Cerebral Cerebellum Ascending tracts hemisphere
● Carry impulses from pain, thermal, tactile, muscle and joint receptors to the brain.
● Some of this information eventually reaches a conscious level (at the cerebral cortex), while some is destined for subconscious centers (e.g at the cerebellum)
● Pathways that carry information to a conscious level (which we feel) share certain common characteristics: ● There is a sequence of Three Neurons between the peripheral receptors and the cerebral cortex.
Conscious level Subconscious level
4 Ascending tracts cont’
● The axons of the first-order neuron or primary afferent neuron: enters the spinal cord through the dorsal root 01 of a spinal nerve and its cell body lies in the dorsal root ganglion.
● The main fiber remains on the ipsilateral (uncrossed/same) side of the cord and terminates in 02 synaptic contact with the second neuron which lies either in the spinal grey matter or in the medulla oblongata of the brain stem.
● The axon of the second order neuron crosses over (decussates) to the opposite side of the CNS and ascends to the thalamus, where it terminates.
● The third-order neuron (ventral posterior nucleus) has 03 its cell body in the thalamus. Its axon passes to the somatosensory cortex of the parietal lobe of the cerebral hemisphere.
5 Sensory pathways
Fasciculus gracilis Dorsal (Posterior) column Dorsal
column Fasciculus Spinocerebellar ● Contain the axons of primary cuneatus afferent neurons that have entered Fasciculus cuneatus cord through dorsal roots of spinal (FC) nerves, from ipsilateral side of the body. contains fibers that are received ● They are concerned with: at upper thoracic and cervical 1. Discriminative touch: Ability to levels localise accurately the area of the body touched. Fasciculus gracilis 2. Two point discrimination: To be (FG) aware that two points are touched simultaneously even they are close contains fibers that are received together. at sacral, lumbar and lower thoracic 3. Proprioception: from muscles and levels joints for the movement & position of different parts of the body. Spinothalamic
Anterolateral (Spinothalamic)
Spinocerebellar
6 Dorsal column
The medial lemniscus 3 terminates in the ventral posterior nucleus of the thalamus (3rd order neurons), which project to the somatosensory cortex (thalamocortical fibers) 2 The axons of the 2nd order neurons decussate in the medulla as internal arcuate fibers, and ascend through the brainstem as Medial Fibers ascend without 1 Lemniscus. interruption where they nucleus gracilis terminate upon 2nd order neurons in nucleus gracilis and nucleus cuneatus.
- In the case of dorsal column, the 2nd order neuron is found in the medulla oblongata. - Whether to terminate in nucleus gracilis or cuneatus depends on which tract is ascending, each tract terminate in its corresponding nucleus (fasciculus gracilis > nucleus gracilis, fasciculus cuneatus > nucleus cuneatus).
7 Diseases associated with dorsal column lesion
Subacute Combined Degeneration Tabes dorsalis Multiple sclerosis (venereal disease) of the spinal cord
1. A late manifestation of syphilitic infection 1. A systemic disease results from B12 1. An immune disease affects on the CNS deficiency specifically fasciculus cuneatus 2. Affects the lumbosacral dorsal spinal roots 2. It produces Sensory Ataxia (there will be upper limb (carried by fasciculus gracilis that’s why 3. Lateral columns are also affected manifestations) of the cervical the lower limbs are affected & the upper (combined) causing weak and spastic region. limbs are intact) and dorsal columns of the limbs 2. Leads to loss of proprioception in spinal cord 4. It is completely recovered by proper hands and fingers (astereognosis 3. Leads to loss of proprioception which is treatment with B12 = inability to determine the shape manifested by a high Steppage & unsteady of an object through touch) gait (Sensory Ataxia/loss of sensation with 3. Its signs include intact muscles) Musculoskeletal: weakness,spasms, ataxia Sensation: pain, hypoesthesia, paraesthesias
Steppage gait 8 Spinothalamic tract
Contains axons of second-order neurons, the cell bodies of which lie They terminate in the 1 in the contralateral dorsal horn, then decussate and ascend as ventral posterior nucleus of the thalamus spinal lemniscus
2 Located lateral and ventral to the ventral horn.
● Lateral spinothalamic tract: carry impulses with pain and thermal sensation. 3 At the level of medulla ● Anterior spinothalamic tract: carry ½ non-discriminative they fuse together forming a single bundle touch and pressure. called spinal lemniscus. *from contralateral side So the first order neurons terminate as soon as they enter to the 4 In the brain stem the two tracts constitute the spinal lemniscus. spinal cord in area called substantia gelatinosa , once it terminate the second order neurons will start and they will cross through the anterior white The information is sent to the primary sensory cortex on the commissure to the 5 contralateral side and opposite side of the body. then ascend
9 Spinothalamic tract cont,
Lateral spinothalamic tract Anterior spinothalamic tract
Carries pain and temperature to thalamus Carries crude touch (non-discriminative) Function and sensory area of the cerebral and pressure to thalamus and sensory cortex.(conscious level) cortex.
1- Neuron I : small cells in the dorsal root 1- Neuron I : medium sized cells in the ganglia. dorsal root ganglia. 2- Neuron II : cells of Substantia gelatinosa 2- Neuron II : cells of main sensory nucleus of Rolandi in the (contralateral) posterior or (nucleus proprius; laminae IV,V,VI, and horn.(in the spinal cord) I). Neurons After 2nd order neuron the fibers Fibers arising from Substantia gelatinosa decussate then ascend and called lateral and Nucleus proprius, decussate in the spinothalamic tract. anterior white commissure , then ascend 3- Neuron III : cells of ventral posterior as anterior spinothalamic tract. nucleus of the thalamus. Then continue till 3- Neuron III : cells of ventral posterior the somatosensory cortex of the cerebral . nucleus of thalamus.
Because along the Arrangement of fiber: spinal cord as we ascend the neurons sacral are more superficial then lumber, are added to the tract thoracic and lastly cervical which are closer to so first will be the sacral then comes the the gray matter lumbar medially and so on 10 Spinocerebellar tracts ● The spinocerebellar system consists of a sequence of only two neurons. (subconscious) Neuron I : large cells of dorsal ganglia. Neuron II : cells of the nucleus dorsalis; Clark’s nucleus(column). ● Two tracts : dorsal (posterior) and ventral (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 tactile receptors, joints, skin, and subcutaneous tissue to the cerebellum for the control of posture and coordination of movements. ● Cerebellum controls the same side of the body.
Ventral (Anterior) spinocerebellar tract Dorsal (Posterior) spinocerebellar tract
● The cell bodies of 2nd order neuron lie in base of ● Present only above L3. the dorsal horn of the lumbosacral segments. ● The cell bodies of 2nd order neuron lie in ● Axons of 2nd order neuron cross to opposite side, clark’s column. ascends as far as the midbrain then make a ● Axons of 2nd order neuron terminate sharp turn caudally and enter the superior ipsilaterally (uncrossed) in the cerebellar cerebellar peduncle cortex by entering through the inferior ● the fibers cross the midline for a second time cerebellar peduncle. within cerebellum before terminates in the ● Posterior spinocerebellar tract convey cerebellar cortex. sensory information to the same side of the ● So ventral spinocerebellar tract conveys sensory cerebellum information to the same side of the cerebellum.
11 Spinothalamic tracts lesion Spinocerebellar tracts lesion
1 It is a selectively damaged in syringomyelia Friedreich's ataxia: The central canal becomes enlarged forming a cavity 1 An inherited degenerated disease 2 compressing the adjacent nerve fibers (while the fibers decussate through the white commissure, the lateral spinothalamic will be closer to the central canal then the anterior) 2 Affecting the spinocerebellar tracts Fibers serving pain and temperature are damaged as they 3 decussate in the ventral white commissure close to the central canal causing selectively loss of pain and temperature in the 3 Leading to incoordination of arms, intense upper limbs (dissociation sensory loss) tremor, wide base reeling gait ataxia. 4 Light touch and proprioceptive sensations are retained. Because it’s carried by the dorsal column 4 It begins in childhood. 5 Joints of the limbs become disorganized without discomfort (Charcot’s joint) 5 Wheelchair is bound by 20 years of age
12 Other spino-tracts Spinotectal tract Spinoreticular tract Spinotectal tract Spinoreticular tract
● Ascends in the anterolateral ● Originates in the dorsal horn, and part, in close association with ascends in the ventrolateral region of spinothalamic system. the cord. ● Primary afferents reach dorsal ● Contains: horn through dorsal roots and 1- uncrossed fibers that end in terminate on 2nd order neuron. medullary reticular formation. ● The cell bodies of 2nd order 2- crossed & uncrossed fibers that neuron lie in base of the dorsal terminates in pontine reticular horn. formation. ● Axons of 2nd order neuron cross ● Then to brain stem reticular formation to opposite side, and project to the finally to the thalamus , that activates periaqueductal gray matter and the cerebral cortex. superior colliculus in midbrain. ● Forms part of the ascending reticular ● Involved in reflexive turning of activating system. head and eyes toward a point of ● Involved in perception of dull aching cutaneous stimulation. (slow pain). (spino-visual reflex)
Spino-olivary tract Spino-olivary tract ● Indirect spinocerebellar pathway. ● Impulses from the spinal cord are relayed to the cerebellum via inferior olivary nucleus. ● Conveys sensory information to the cerebellum. ● Fibers arise at all levels of the spinal cord. ● Contribute to movement coordination association primarily with balance.
Males’ doctor note: you only need to know the names and the function for these three 13 Practice
Q1: Fibers occupy narrow band immediately peripheral to the grey matter: Q5: spinotectal tract in close association with: A. Fasciculus cuneatus A. Spinothalamic tract B. Fasciculus gracilis B. Spinocerebellar tract C. Fasciculus proprius C.spino-olivary tract D. Spinocerebellar D. Spinoreticular tract Q2: The third-order neuron has its cell body in: Q6: lateral spinothalamic tract carry: A. Thalamus A.temperature B. Dorsal root ganglion B.pain C. Spinal grey matter C.pressure D. Medulla oblongata D.A&B Q3: The dorsal column carry impulses which are concerned of all of the following Q7: spinal lemniscus constitute ………………….. when they enter the brainstem EXCEPT: A.lateral spinothalamic tract + anterior spinothalamic tract A. Proprioception B. lateral spinothalamic tract + posterior spinothalamic tract B. Pain C. lateral spinothalamic tract + spinotectal tract C. Discriminative touch D.non of the above D. Crude touch Q8 : charcot’s joint seen in: Q4: A late manifestation of syphilitic infection on the CNS: A. Spinothalamic tract lesion A. Multiple sclerosis B.spinocerebellar tract lesion B. Subacute combined degeneration of the spinal cord C. Dorsal column lesion C. Friedreich's ataxia D. Posterior column lesion D. Tabes dorsalis
Answers: Q1(C) Q2(A) Q3(B) Q4(D) Q5(A) Q6(D) Q7(A) Q8(A) 14 Members board
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