Spinal Cord › Major Projection Pathways

Sensory System

Notes

Sensory System

Sections






Overview

Overview

  • Here, we will address the neurobiology of sensation.
  • To begin, start a table.
  • Let's start with some important definitions.
  • Denote that we can divide sensation into four steps:

Transduction

  • The conversion of a stimulus into an electrical impulse: the stimulus triggers an action potential on the sensory receptor membrane, which kicks off its transmission along the neuroaxis.

Transmission

– The propagation of the electrical impulse from the tissues to the cerebrum.

Modulation

  • The alteration of impulse transmission along the neuroaxis.

Perception

– Impulse interpretation: it includes not only sensory impulse reception but also the consolidation of many other psychological inputs (emotional and subjective), as well.

Transduction

Overview

  • To begin, let's address sensory transduction in the periphery.
  • Start a table.

List the three major physiological types of sensory receptors:

  • Mechanoreceptors, which detect mechanical deformation
  • Nociceptors, which detect pain
  • Thermoreceptors, which detect temperature

Sensory Receptors

  • There are six major structural types of sensory receptors, which are, from superficial to deep:
    • Free nerve endings
    • Merkel cells
    • Meissner corpuscles
    • Ruffini corpuscles & Krause end bulbs
    • Pacinian corpuscles
  • Let's generate a detailed diagram of the various sensory receptors, based on their location within the skin, and let's indicate some of their key functions.
  • First, draw a section of cutaneous tissue from superficial to deep: epidermis, dermis, and hypodermis where we show vasculature.
  • Draw a nerve bundle.

Now, add the sensory receptors from superficial to deep:

Free Nerve Endings

  • In the epidermis, draw free nerve endings at the end of a nerve fiber.
    • These are unmyelinated nerve endings that detect pain and temperature; they are nociceptors. To learn more about nociceptor fibers projections, see our nociception tutorial.

Merkel Cell

  • In the basal epidermis, draw a Merkel cell.
  • Show that they comprise Merkel discs (aka disks) at the ends of an unencapsulated nerve fiber.
    • They detect tactile sensation: light touch/hair movement.

Meissner corpuscle

  • Now, in the superficial dermis, draw a Meissner corpuscle (aka tactile corpuscle)
  • Show that they are elliptical-shaped and that they lie perpendicular to the epidermis, within the dermal papillae.
    • They detect light touch; hence, they are referred to as "tactile corpuscles".

Ruffini Ending

  • Next deeper in the dermis, draw a Ruffini ending (aka corpuscle), which detect pressure.

Krause End Bulb

  • And draw a Krause end bulb, which detects pressure and low-frequency vibration.

Pacinian Corpuscle

  • Finally, in the deep dermis & hypodermis, draw a Pacinian corpuscle (aka lamellated corpuscle).
  • Show that Pacinian corpuscles are large and oval-shaped with a very characteristic concentric, lamellated histological appearance.
    • They are encapsulated mechanoreceptors that detect touch, pressure, and vibration. Capsule distortion amplifies the signal of a mechanical stimulus.

Transmission

Peripheral Nerve

  • Next, let's address the transmission of the stimulus.
    • Pay attention to where the impulses cross sides (decussate) along the neuroaxis, as this is critical for sensory deficit localization.
  • To start, draw an outline of a hemi-cord axial section.

Next, draw the:

  • Dorsal horn (which receives sensory innervation)
  • Draw the other two regions for context: the intermediate zone (aka intermediate gray), which contains both autonomic and cerebellar pathway neurons, and the ventral horn, which produces motor innervation.
  • Next, the dorsal root, which receives sensory fibers.
  • Specify the dorsal root ganglion, which houses the primary sensory neuron.
    • It is pseudounipolar, meaning it sends fibers both towards the spinal cord and also toward the periphery.
  • Now, draw the ventral root which emits motor fibers.
    • Remind ourselves that the rami are not the roots, so we don't confuse them when we add them in next.
  • Show that where the roots merge, they form the mixed spinal nerve, which divides into the:
  • Anterior ramus (ventral ramus), which innervates the limbs and anterior torso.
  • Posterior ramus (dorsal ramus), which innervates the back.
  • Indicate that motor fibers emerge from an anterior horn neuron, pass along the ventral (anterior) root, join the mixed spinal nerve and then pass down the anterior ramus and posterior ramus.
  • Then, show that both rami carry sensory fibers sensory fibers that join the mixed spinal nerve, pass along the dorsal root and synapse in the dorsal horn.
    • Indicate that these fibers can pass up the spinal cord within the posterior column, as the posterior column pathway (PCP) or cross within the spinal cord to the contralateral side as the spinothalamic tract (STT).
  • At the dorsal root ganglion, include a pseudounipolar neuron.
  • Note that sensory reception not only emanates from the skin, but also from vasculature and musculoskeletal regions of the limbs and anterolateral trunk.
  • Specifically indicate that the posterior ramus carries fibers from the vertebrae, notably the articular facet joints, which lie between vertebral levels.
    • This innervation is important to pain management because clinicians purposefully desensitize the posterior ramus to treat painful facet joints. Treatments include blocking the posterior ramus via denervation, anesthesia, and electrical stimulation.

Projection Pathways

Overview

  • Now, let's draw the somatosensory pathway projections.
  • Draw a coronal view of the brain (just the hemisphere opposite to the origin of the sensory stimulus).
  • Then, include the ventroposterior nucleus of the thalamus, the brainstem, the upper cervical spinal cord, and the lumbar cord.

As background, there are three main somatosensory pathways we'll address:

  • The posterior column/ medial lemniscus carries input from the sensory fibers from the body, which transmits vibration and joint position sensory information along with select other large fiber modalities, including two-point discrimination.
  • The anterolateral system (which, notably comprises the spinothalamic tract), which carries input from the small sensory fibers from the body, which transmit pain, itch, and thermal sensory information.
  • The trigeminothalamic tract, which projects sensory information from the face (CN 5).
  • All three pathways ascend the brainstem separately and then bundle within the thalamus before they project to the somatosensory cortex.

Spinothalamic Tract

  • The position of the spinothalamic tract is largely unchanged throughout its spinal cord and brainstem ascent, so start with it.
  • Show leg fibers synapse within the lumbar spinal cord and then decussate and ascend the anterolateral cord and brainstem to the thalamus.
  • Then, show arm fibers synapse within the cervical spinal cord, and, also decussate and ascend the anterolateral cord and brainstem to the thalamus.

Posterior Column Pathway

  • Next, let's draw the posterior column/medial lemniscus tract as it enters the brainstem.
  • Show that the arm fibers synapse laterally, and the leg fibers synapse medially.
  • Show that the arm fibers ascend the brainstem medial to the leg fibers as the medial lemniscus to the thalamus and indicate with a cross that the fibers switch medial/lateral positions, opposite of their position within the spinal cord..
    • This switch occurs in the internal arcuate decussation:
    • The leg fibers emerge from the gracile nucleus, medially, and pass anteriorly and then laterally.
    • The arm fibers emerge from the cuneate nucleus, laterally, and pass posterior to the legs and then medially.
    • Thus, the arm fibers ascend the brainstem medial to the leg fibers: this is the opposite of their position within the spinal cord.

The Trigeminothalamic Tract

  • Now, show the trigeminothalamic facial fibers decussate within the pons and ascend the brainstem, medially, to the thalamus.
    • We simplify the originations of the trigeminothalamic tracts for our purposes, here.

Thalamocortical Projections

  • Next, specify that within the thalamus, the facial fibers project to the ventroposterior medial nucleus (VPM) and the body fibers (arms and legs) project to the ventroposterior lateral nucleus (VPL).
  • Finally, draw the twisting thalamocortical sensory projections to the cerebral cortex.
  • In their ascent, show that the sensory fibers again reverse their orientation: the leg fibers project medially; they terminate in the posterior paracentral gyrus.
  • The arm fibers project lateral to the legs; they terminate in the upper convexity of the postcentral gyrus.
  • The facial fibers project lateral to both of them; they terminate in the lower lateral postcentral gyrus.

Perception (& Modulation)

Cerebrum

  • Indicate that the cerebrum is the site of sensory perception.
  • As mentioned at the beginning, perception involves more than just the reception of a sensory impulse, there are affective (psychological components) that impact it as well.
  • These components involve many brain and brainstem regions not shown here, such as the secondary somatosensory regions, anterior cingulate gyrus, amygdala, insula and various pools brainstem neurons.
  • Note that we omit modulation, here, but we address it in detail in our nociception (pain) tutorial.

Dermatomes: Localization

Overview

  • One of the key elements to localization involves detection of sensory origins via their dermatomal landmarks (peripheral nerve anatomy and patterns are also important, and we address them in detail elsewhere).
  • Pay attention that two key dermatomal maps are still widely used, one from 1933 (based on the work of O. Foerster) and the other from 1948 (based on that of Keegan and Garrett); their differences are insubstantial, so either is acceptable.

Anterior

  • First, draw an outline of one half of the anterior head and body.

Upper Extremity & Head

  • Split the arm down the middle but create a fork in the transection that encapsulates the middle finger.
  • Show that C7 covers the middle finger.
  • Then, show that C8 covers the little finger and medial hand.
  • Next divide the upper extremity at the elbow.
  • Show that C6 covers the thumb, lateral hand, lateral forearm.
  • Then, show that C5 covers the upper lateral arm.
  • Next that T1 covers the medial forearm.
  • And that T2 covers the medial upper arm.
  • Show that C2 covers the jaw.
  • Show that C3/C4 covers the neck

Trunk & Pelvis

  • Next, draw the pectoralis, trunk, lower extremity, and groin region.
  • Show that T4 covers the nipple line.
  • T10 covers the navel
  • T12 covers the suprapubic area.
  • L1 covers the inguinal region
  • S2, S3 cover the genitalia.

Lower Extremity

  • Now, show the sloping dermatomal coverage of the anterior lower extremity.
  • Show that L2 begins its descent from the superolateral anterior lower extremity.
  • Followed by L3.
  • Then, show that L4 crosses the patella.
  • L5 covers the great toe.
  • Then, that S1 covers the ankle and little toe.

Posterior

Pelvis

  • Next, draw an outline of one half of the posterior head and body.
  • First show that the coccygeal nerve covers the anus and then show the dermatomal rings that surround it: the innermost is S5, then going outward is S4, then S3, and then show that S2 encircles S3 but also extends down the posteromedial lower limb.

Lower Extremity

  • Next, show that S1 extends down the posterolateral lower limb and covers the Achilles.
  • Now, show that L5 covers the lateral posterior thigh and lower back and that it covers the great toe along with the medial plantar foot.

Head & Neck

  • Show that C2 covers the back of the head
  • C3/C4 covers the posterior neck

Trunk

  • T2–L5 cover the upper back to the buttocks.

Upper Extremity

  • Now, show the posterior upper extremity coverage in the exact same way we showed the anterior coverage.
  • Split the arm down the middle but create a fork in the transection that encapsulates the middle finger.
  • Show that C7 covers the middle finger.
  • Then, show that C8 covers the little finger and medial hand.
  • Next divide the upper extremity at the elbow.
  • Show that C6 covers the thumb, lateral hand, lateral forearm.
  • Then, show that C5 covers the upper lateral arm.
  • Next that T1 covers the medial forearm.
  • And that T2 covers the medial upper arm.

Clinical Challenge Question

Now that you have a good knowledge of the anatomy of the sensory system, see if you can answer this clinical neurology question about a patient with an acute onset of sensory loss.