Lecture 10- Sensory Pathways.pdf

Sensory Pathways Dr. Kelly C. S. Roballo [email protected] VCOM-Main Building Room 341 Learn Objectives 1. Distinguish spinal cord segmental levels from spinal column vertebral levels. 2. Explain fully the anatomical relationships of the conus medullaris and cauda equina in relation to the lowe...

Sensory Pathways Dr. Kelly C. S. Roballo [email protected] VCOM-Main Building Room 341 Learn Objectives 1. Distinguish spinal cord segmental levels from spinal column vertebral levels. 2. Explain fully the anatomical relationships of the conus medullaris and cauda equina in relation to the lower portions of the vertebral column. 3. Distinguish the mixed peripheral nerve from the dorsal and ventral roots. 4. Understand the concept of neuronal order. 5. Define the somatotopic organization of the spinal cord tracts and nuclear columns. 6. Identify the anatomical and clinical relationships of the synaptic layers of the spinal cord dorsal horn. 7. Describe the function and anatomical relationships of these ascending tracts of the spinal cord: spinothalamic, fasciculus gracilis and fasciculus cuneatus, and the anterior and posterior spinocerebellar tracts. Chapter 11- basic clinical neuroscience Sensory Receptors, Fiber Types and Modalities Blumenfeld, Neuroanatomy Sensory Pathways • Almost all sensory information from the somatic segments of the body enters the spinal cord through the dorsal roots of the spinal nerves. • From the entry point into the cord and then to the brain the sensory signals are carried through one of two alternative sensory pathways: • (1) the dorsal column-medial lemniscal pathway, or • (2) the anterolateral pathway • These two systems come back together partially at the level of the thalamus. Decussation means that somatosensory information from one side of the body is received in the contralateral thalamus Costanzo, Physiology The first-order neuron in the somatosensory pathway is the primary afferent (information to the CNS) neuron. Primary afferent neurons have their cell bodies in dorsal root or cranial ganglia, and their axons synapse on somatosensory receptor cells • • The signal is transduced by the receptor and transmitted to the CNS by the primary afferent neuron. The second-order neuron is located in the spinal cord (anterolateral system) or in the brain stem (dorsal column system). The second-order neurons receive information from first-order neurons and transmit that information to the thalamus. Axons of the second-order neurons cross the midline, either in the spinal cord or in the brain stem and ascend to the thalamus. • • This decussation means that somatosensory information from one side of the body is received in the contralateral thalamus. • The third-order neuron is located in one of the somatosensory nuclei of the thalamus. The thalamus has a somatotopic arrangement of somatosensory information. • The fourth-order neuron is located in the somatosensory cortex. Higher-order neurons in the somatosensory cortex and other associative cortical areas integrate complex information. • The somatosensory cortex has a somatotopic representation, or "map," similar to that in the thalamus (somatosensory homunculus). SomatoSensory Cortex Two-point-test Blumenfeld, Neuroanatomy SomatoSensory Cortex Guyton & Hall, Medical Physiology Cortex ManHomunculus n n Within the layers the neurons are also arranged in columns Each column serves a specific sensory modality (i.e., stretch, pressure, touch) Blumenfeld, Neuroanatomy Layers of the Neocortex Blumenfeld, Neuroanatomy Main nuclear divisions of the Thalamus Blumenfeld, Neuroanatomy Blumenfeld, Neuroanatomy Posterior (Dorsal) Column Medial Lemniscal Pathway (vibration, proprioception, light touch) Anterolateral Pathway (pain, Temp, crude touch) Dorsal Column-Medial Lemniscal Pathway • Contains large myelinated nerve fibers for fast transmission (30-110 m/sec= rapid). • High degree of spatial orientation maintained throughout the tract. • Transmits information rapidly and with a high degree of spatial fidelity (i.e., discrete types of mechanoreceptor information). • Touch, vibration, position, fine pressure Dorsal ColumnMedial Lemniscal Pathway Blumenfeld, Neuroanatomy Dorsal Column-Medial Lemniscal Pathway • Touch sensations requiring a high degree of localization of the stimulus • Touch sensations requiring transmission of fine gradations of intensity (ex: intensity of touch while playing a piano) • Phasic sensations, such as vibratory sensations • Sensations that signal movement against the skin • Position sensations from the joints (your brain needs to know where your joins are in 3D at any giving point) • Pressure sensations having to do with fine degrees of judgment of pressure intensity *Clinical correlation Anterolateral Pathway • Smaller myelinated and unmyelinated fibers for slow transmission (0.5-40 m/sec= slow) • Low degree of spatial orientation • Transmits a broad spectrum of modalities • Pain, thermal sensations, crude touch and pressure, tickle and itch, sexual sensations. Antero-Lateral Pathway Blumenfeld, Neuroanatomy Somatotopic Organization of the Sensory Pathways Cross in SC Blumenfeld, Neuroanatomy Sensory and Motor Spinal Cord Pathways Dissociated Sensory Loss • Following a spinal cord hemisection at the 10th thoracic level on the left side • This pattern, together with motor weakness on the same side as the lesion is also referred as “Brown-Sequard Syndrome” • Clinical correlation T10 on the left side injury Hemisection lesion Full SC lesion • Clinical correlation Half SC lesion Blumenfeld, Neuroanatomy • Clinical correlation Blumenfeld, Neuroanatomy Question • Match each set of symptoms with one of the lesions illustrated below. Each lesion may be used once and only once. Note: the lists of symptoms may not be complete (i.e., there may be additional symptoms that are not listed). right left Different level C4/L1 • Paresis and hyperreflexia of the left limbs; analgesia and thermal anesthesia of the right limbs (a) (doesn’t say arms and legs) • Loss of proprioception and vibration sense in all four limbs (b) • Loss of vibration sense from the right leg; (upper motor neurons) paresis and hyperreflexia of the right leg (e) • Loss of proprioception and vibration sense in both legs (f) • Clinical correlation Patterns of Sensory Loss in Lesions of the Brain or Peripheral Nerves Primary Somatosensory Cortex: Deficit is contralateral to the lesion. Despite the depiction in the figure, sensory loss may not begin neatly at the midline, and various subregions may be differently affected, depending on the size and location. Discriminative touch and joint position sense are often most severely affected. Associated deficits from involvement of adjacent cortical areas may include upper motor neuron type weakness, visual field deficits, or aphasia. Thalamic VPL and VPM: Deficit is contralateral to the lesion. As with lesions of the primary somatosensory cortex, sensory loss does not begin neatly in the midline. Larger lesions may be accompanied by hemiparesis or hemianopia caused by involvement of the internal capsule. • Clinical correlation Patterns of Sensory Loss in Lesions of the Brain or Peripheral Nerves Lateral Pontine or medullary lesion: The lesion involves anterolateral pathways and the spinal trigeminal nucleus on the same side. It causes loss of pain and temperature sensation in the body opposite the lesion, and loss of pain and temperature sensation in the face on the same side as the lesion. • Clinical correlation Phantom Limbs and Phantom Pain • Clinical correlation Thank you