1. Sensory pathway into the CNS by Adejare A. A
Sensory pathway into the CNS by Adejare A. A. Department of Physiology College of Medicine University of Lagos

2. Dorsal Column Medial Lemniscal Pathway
For sensory information that must be transmitted rapidly (30-120m/s)
And with high temporal and spatial fidelity
Information in it is limited to discrete types of mechanoreceptive sensations
The fibres are large and myelinated

3. The pathway carries: Torch sensations Vibratory sensations
Position sensations
Sensations that signal movement against the skin
Pressure sensations

4. The pathway

5. Cross section of the spinal cord: showing the anatomy of the cord gray matter and of ascending sensory tracts in the white columns of the spinal cord.

6. Branches Lateral branch:
Forms synapses and re-enter the dorsal column (15%)
Forms the spinocervical tract that also rejoins the dorsal column in the medulla
Others give rise to the spinocerebellar tract
Many of these elicit local cord reflexes

7. Medial branch
The 1st order neurones (fibers) pass uninterrupted up to the medulla where they synapse in the dorsal column nuclei (cuneate and gracile nuclei)
2nd order neurones decussate to the opposite side and continue to the thalamus through the medial lemnisci (bilateral brainstem pathways)
Fibers from the sensory nuclei of the trigeminal nerve (CN 5)(sensory function from head and neck) joins the medial lemniscus in the brainstem

8. All fibers terminate in the ventrobasal complex (nuclei) of the thalamus.
The complex consists of
Ventral posterolateral nucleus receiving medial lemniscal fibers
Ventral posteromedial nucleus receiving fibers fron trigeminal nuclei and
Posterior thalamic nuclei receicing fibers from the anterolateral system

9. From the VBC of the thalamus,
3rd order nerve fibers project to the postcentral gyrus of the cerebral cortex (SSA 1)
Because of the crossing of the medial lemnisci in the medulla, the left side of the body is represented on the right side of the thalamus while the right side is represented on the left side of the thalamus

10. DCMLP

11. Projection of the dorsal column–medial lemniscal system through the thalamus to the somatosensory cortex.

12. Somatic sensory cortex
The cerebral cortex is divided into about 50 areas called (Brodmann areas)
The large central fissure (central sulcus) separates the cerebral cortex into anterior and posterior halves.
Sensory signals from all modalities of sensation terminate in the cerebral cortex posterior to the central fissure

13. somatosensory areas I and II.

14. Representation of the different areas of the body in somatosensory area I of the cortex

15. Functions of SSA 1 (Brodmann 3, 1, 2)
To localize discretely the different sensations from the body parts
Judge critical degrees of pressure against the body
Judge the weight of objects
Judge shapes of forms of object (inability: astereognosis)
Judge texture of materials

16. Functions of somatic sensory association areas (Brodmann areas 5 & 7)
To decipher the sensory information that enters the SSAreas
Removal of these association areas causes amorphosynthesis

17. Anterolateral pathway
The pathway transmits: pain, heat, cold, crude tactile, tickle, itch & sexual sensations.
Pain: is a protective mechanism for the body. It shows that a tissue is being damaged and the pain stimulus has to be removed
Types of pain
Fast: sharp, pricking, acute, electric pain
Stimuli: mechanical , thermal
Not felt in the visceral

18. Causes of visceral pain
Ischemia of visceral tissue
Spasm of the smooth muscle in a hollow viscus
Distension of a hollow viscuss
Stretching of the ligaments
Transmitted in C-fibres

19. Slow pain: aching, throbbing, nauseous, chronic
Stimuli: mechanical, thermal and chemical (bradykinin, 5HT, Histamine, K+, ACh, Substance P)

20. Pain receptors Free nerve endings
Location: skin, periosteum, arterial walls, joint surfaces, falx and tentorium of the cranial vault
Pain impulses are transmitted to the CNS by:
Aδ fibers: small, myelinated, 2-5µm dia, 12-30m/s
Terminate on neurones in lamina I and V
Secrete glutamate
C-fibers: unmyelinated, µm dia., 0.5-2m/s
Terminate on neurones in laminas I and II
Secrete substance P
Both fibers end in the dorsal horn

21. Transmission of both “fast-sharp” and “slow-chronic” pain signals into and through the spinal cord on their way to the brain.

22. Gating of pain
The synaptic junctions between the peripheral nociceptor fibers and the dorsal horn cells in the spinal cord are the sites of considerable plasticity where pain impulses can be modified.
The dorsal horn is thus referred to as a gate

23. The pathway
The anterolateral fibers originate mainly in laminae I, IV, V and VI
The fibers then cross in the anterior commissure of the cord to the opposite anterior and lateral white columns, where they turn upward toward the brain.
As they ascend, the fibers form the following divisions
Anterior division called anterior spinothalamic tract for fast pain
Lateral division called lateral spinothalamic tract for fast pain
Spinoreticular pathway to the reticular substance of the brainstem for slow pain
Spinotectal tract to the tectum of the mesencephalon

24. Anterior and lateral divisions of the anterolateral sensory pathway.

25. Termination The fast fibers terminate in:
Reticular nuclei of the brainstem (most fibers): where they are relayed to the intralaminar nuclei of the thalamus
the VBC, majorly in the ventral postero lateral and medial thalamic nuclei where they project to the somatosensory cortex for crude tactile signals and intralaminar nuclei of the thalamus

26. The slow pain fibers terminate in:
The reticular nuclei of medulla, pons and mesencephalon and intralaminar nuclei of thalamus
The tectal area of the mesencephalon deep to the superior and inferior colliculi
The periaqueductal gray region surrounding the aqueduct of Sylvius

27. Xtics of transmission in the anterolateral pw:
Velocity of transmission is reduced (8-40m/s) compared to DCMLPW
The degree of spatial localization is poor
The gradations of intensities are far less accurate
The ability to transmit rapidly repetitive signals is poor

28. Role of the thalamus in somatic sensation
Helps to relay tactile information to the cortex
Play a major role along with brainstem in discrimination of pain and temperature
Receives regulatory corticofugal fibers (majorly inhibitory) from the cortex
Role of the cortex
Interpretation of the quality of pain

29. Applied physiology Thalamic syndrome:
Cause: blockage of posterior cerebral that supplies posteroventral portion of the thalamus by thrombosis.
The nucleus does degenerate
Effects
Loss of pain sensation from the opposite side of the body
Ataxia: inability to control body mvmts precisely
The threshold for sensory perception becomes higher after some weeks
Perception of many affective sensations of extreme unpleasantness or pleasantness
Enhanced sensitivity to pain

30. Hyperalgesia: hypersensitivity to pain
Causes:
Primary hyperalgesia:
excessive sensitivity of the nociceptors eg release of histamine ff heat burn
Secondary hyperalgesia:
facilitation of sensory transmission due to lesion in the spinal cord or thalamus

31. Brown-Sequard Syndrome
Occurs when one half of the spinal cord is transected on a single side
All motor functions are blocked on the side of the transection in all segments below the level of the transection
The sensations of pain, heat and cold are lost on the opposite side (contralateral side) of the body in 2-6 segments below the level of transection
Kinesthetic, position, vibration sensations, light touch, discrete localization, two-point discrimination are lost on the side (ipsilateral side) of the transection in all segments below

32. Analgesia: pain suppression system in the brain and spinal cord
This refers to the capability of the brain to suppress the input of pain signals to the NS by activating a pain control system called an analgesia system

33. Cellular pathways of pain transmission and modulation
Cellular pathways of pain transmission and modulation. Painful stimulation releases substance P from afferent fibers in the spinal cord
(a). Substance P release is blocked (b) by a descending analgesic system using axo-axonic synapses on the afferent neuron.

34. The system consists of
The periaqueductal gray and periventricular areas of the mesencephalon and upper pons surrounding the aqueduct of Sylvius and adjacent to portions of the 3rd & 4th ventricles. These neurones project to
The raphe magnus nucleus located in the lower pons & upper medulla and the nucleus reticularis paragigantocellularis in the medulla. These also project to
A pain inhibitory complex located in the dorsal horns of the SC
Periventricular nuclei in the hypothalamus
Medial forebrain bundle
Most of the fibers involved in the transmission secrete enkephalins and serotonin

35. Analgesia system of the brain and spinal cord, showing (1) inhibition of incoming pain signals at the cord level and (2) presence of enkephalin-secreting neurons that suppress pain signals in both the cord and the brain stem.

36. Brains opiate system
Morphine-like agents, mainly opiates cause analgesia when injected into any part of the analgesia system.
This effect is mediated through activation of opiate receptors present in them.
The opiate substances include β-endorphin, met-enkephalin, leu-enkephalin, dynorphin

37. Thermal sensations Gradations of cold & heat
Freezing cold: -50°C-13°C, sensed by nociceptors
Cold:
Cool: sensed by cold receptors
Indifferent:
Warm: sensed by warm receptors
Hot:
Burning hot: 45°C >, sensed by nociceptors

38. The cold & warm receptors
Have a diameter of 1mm
There are 3-10 times as many cold receptors as warm receptors
Total no of receptors: lips (15-25 cold spots) > finger (3-5coldspots/cm2) > trunk(<1 cold spot)
Warm receptors use C-fibers while cold receptors use Aδ fibers
Thermal signals are transmitted in the ALPW
The signals travel in the Tract of Lissauer and then terminate in laminae I, II & III

39. The fibers also cross to the opposite side then ascend in the ALPW
The fibers terminate
The reticular areas of the brainstem
The VBC of the thalamus
Some to the SSA (Area 1)