Sensory pathways

1. Sensory pathways By Dr Farah Amir Ali

2. GTOs helps to control muscle tension Has both a dynamic response and static response Golgi tendon organ

5. Difference b/w muscle spindle and GTOs • Muscle spindles detects muscle length and change in muscle length • Whereas golgi tendon organs detects muscle tension as reflected by the tension in its self

6. Sensory pathways for transmitting signals into CNS • Almost all the information from somatic segment of body enters the spinal cord through dorsal roots of spinal cord. • The sensory signals are carried by one of two alternative sensory pathways i) Dorsal column-medial lemniscal system ii) Anterolateral system

8. DCML Carries impulses in dorsal column (same side) Synapse in medulla Then 2nd order to thalamus via medial lamniscus. ALS Immediately after entering spinal cord they synapse. Then 2nd order cross to opposite site. Through anterior and lateral white column reaches all levels of lower brain stem and in the thalamus. Difference b/w dorsal column lamnsical pathway and anterlateral system

10. DCML . 1. Highly localized Touch sensations 2. Touch sensations requiring transmission of fine gradations of intensity 3. Phasic sensations, such as vibratory sensations 4. Position sensations from the joints 5. Pressure sensations ALS 1. Pain 2. Thermal sensations, 3. Crude touch and pressure sensations 4. Tickle and itch sensations 5. Sexual sensations Difference b/w dorsal column lamnsical pathway and anterlateral system

11. DCML It is composed of large myelinated nerve fibers (30-110m/sec) The dorsal system is limited to discrete types of mechanoreceptive sensations ALS It is composed of smaller myelinated nerve fibers (up to 40m/sec) The anterolateral system has ability to transmit a broad spectrum of sensory modalities— pain, warmth, cold, and crude tactile sensations; Difference b/w dorsal column lamnsical pathway and anterlateral system

12. Pathways in somatosensory system are formed by 3 neurons which ultimately reach sensory cortex 1st order neuron: from receptors (A α,β,σ and C fibers) Cell bodies: in dorsal rootganglion. Central process constitute the dorsal nerve root of spinal nerve Dorsal column Medial lamniscal pathway

13. 2nd order neuron: Arise from nucleus gracilus and nucleus cuneatus located in medulla oblongata. The fibers of medial lemnisci terminate in the ventral posterior lateral nucleus of thalmus (VPL) Dorsal column Medial lamniscal pathway

14. 3rd order neuron: From ventral posterior lateral nucleus of thalmus (VPL) ascend to thalmic radiations Dorsal column Medial lamniscal pathway

15. Dorsal column Medial lamniscal pathwaySpatial orientation of nerve fibers • In the dorsal column fibers coming from lower parts are in the centre (medially) while from higher segmental level form successive lateral layer

16. Dorsal column Medial lamniscal pathwayFunctions : • Component of sense of touch: Deep touch and pressure Fine touch Tactile localization: ability to localize , exactly the part of skin touched. Tactile discrimination: Ability to recognize as separate two points on the skin that are touched simultaneously Stereognosis: ability to recognize the shape of known objects by touch with eyes close

17. Dorsal column Medial lamniscal pathwayFunctions : • Proprioceptive impulses: sense of position of different parts of body under static conditions as well as rate of change of movement of different parts during body movements. • Sense of vibration

18. Two-Point Discrimination • A method frequently used to test tactile discrimination is to determine a person’s so-called “two-point” discriminatory ability. • In this test, two needles are pressed lightly against the skin at the same time, and the person determines whether two points of stimulus are felt or one point. On the tips of the fingers, a person can distinguish two separate points even when the needles are as close together as 1 to 2 millimeters. However, on the person’s back, the needles must usually be as far apart as 30 to 70millimeters before two separate points can be detected.The reason for this difference is the different numbers of specialized tactile receptors in the two areas.

20. Effect of Lateral Inhibition (Also Called Surround Inhibition) to Increase the Degree of Contrast in the Perceived Spatial Pattern. • The importance of lateral inhibition is that it blocks lateral spread of the excitatory signals and, therefore, increases the degree of contrast in the sensory pattern perceived in the cerebral cortex

22. Effects of damage to Dorsal column Medial lamniscal pathway • Sensations of fine touch, tactile discrimination, vibration, joint and position sense is carried by DCML. • Therefore, damage to this pathway will produce : Sensory ataxia: it is difficult for a person to detect the position without help of visual apparatus in standing position (Romberg’s +ive) Loss of sensations of fine touch, tactile discrimination and vibration from affected side

25. 1st order neuron: They are formed by Aσ and C fibers which enter the spinal cord through dorsal nerve root. Aσ terminate on neuron in lamininae I and V, whereas C fibers terminate on neurons in laminae I and II (thermo and noci fibers) Aσ (mechano fibers) terminate in III and IV layers. Anterolateral pathway

26. Anterolateral pathway2nd order neuron • They originate mainly in I, IV, V,VI laminae . The anterolateral fibers cross immediately in the anterior commissure of the cord to the opposite anterior and lateral white columns, where they turn upward toward the brain by way of the anterior spinothalamic ( crude touch, tickle ,itch) and lateral spinothalamic tracts (pain & temp).

27. Anterolateral pathway2nd order neuron • The upper terminus of the two spinothalamic tracts is mainly twofold: • (1) throughout the reticular nuclei of the brain stem • (2) in two different nuclear complexes of the thalamus, the ventrobasal complex and the intralaminar nuclei.

28. Anterolateral pathway2nd order neuron • In general, the tactilesignals are transmitted mainly into the ventrobasal complex, terminating in some of the same thalamic nuclei where the dorsal column tactile signals terminate • Conversely, only a small fraction of the pain signals project directly to the ventrobasal complex of the thalamus. Instead, most pain signals terminate in the reticular nuclei of the brain stem and from there are relayed to the intralaminar nuclei of the thalamus

29. Anterolateral pathway3rd order neuron • From thalamus, the signals are transmitted to the somatosensory cortex along with the signals from the dorsal columns.

31. Post central gyrus Axons of 3rd order neurons Medullaoblongata Spinal cord

32. Characteristics of transmission in anterolateral pathway 1) The velocities of transmission are only one third to one half those in the dorsal column–medial lemniscal system, ranging between 8 and 40 m/sec; (2) the degree of spatial localization of signals is poor; (3) the gradations of intensities are also far less accurate, most of the sensations being recognized in 10 to 20 gradations of strength, rather than as many as 100 gradations for the dorsal column systemand (4) the ability to transmit rapidly changing or rapidly repetitive signals is poor