Somatosensory Systems

Home , Nuclear bag fiber, Nuclear chain fiber, Pacinian corpuscle

Sue Keirstead, Ph.D. Assistant Professor Dept. of Integrative Biology and Physiology Stem Cell Institute

Tel: 612 626 2290 Class 9: Somatosensory System (p. 292-306)

1. Describe the 3 main types of somatic sensations: 1. tactile: light touch, deep pressure, vibration, cold, hot, etc., 2. pain, 3. Proprioception. 2. List the types of sensory receptors that are found in the skin (Figure 9.11) and explain what determines the optimum type of stimulus that will activate each. 3. Describe the two different modality-specific ascending somatosensory pathways and note which modalities are carried in each (Figure 9.10 and 9.13). 4. Describe how it is possible for us to differentiate between stimuli of different modalities in the same body part (i.e. fingertip). Consider this at the level of 1) the sensory receptors and 2) the neurons onto which they synapse in the ascending sensory systems. 5. Explain how one might determine the location of a spinal cord injury based on the modality of sensation that is lost and the region of the body (both the side of the body and body part) where sensation is lost (Figure 9.18). 6. Describe how incoming sensory inputs from primary sensory axons can be modified at the level of the spinal cord and relate this to the mechanism of action of some common pain medications (Figure 9-18). 7. Describe the homunculus and explain the significance of the size of the region of the somatosensory cortex devoted to a particular body part. Cerebral cortex

Interneuron

Thalamus

Interneuron

4 Integration of sensory Stimulus input in the CNS

1 Stimulation Sensory Axon of sensory of sensory receptor neuron receptor

Graded potential Action potentials

2 Transduction 3 Generation of of the stimulus action potentials

1. Tactile: deep pressure, temperature, light touch, vibration, tickle 2. Pain, itch 3. Proprioception: muscle length, velocity of stretch, muscle tension Free nerve endings Merkel disc (itch, pain, temperature, (slowly adapting, touch & pressure) tickle) Meissner corpuscle Skin: (rapidly adapting, touch & Epidermis low frequency vibration)

Dermis

Ruffini corpuscle (slowly adapting, stretch & pressure)

Hair root plexus (rapidly adapting, touch, i.e. movement of hair)

Pacinian corpuscle (rapidly adapting, high Subcutaneous layer frequency vibration)

Multilayered capsule

Extracellular fluid

Ca2+ Cation channel Cation channel Plasma Na+ closed membrane open

Influx of Na+ and Ca2+ causes a depolarizing receptor potential Cytosol

A pacinian corpuscle at rest Transduction in a pacinian corpuscle

Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. Pacinian corpuscle = Rapidly Adapting Receptor - receptor includes connective tissue structure that cushions the end of the axon - the end of the axon contains mechanically-gated transduction channels From “Principles of Neural Science” by Kandel and Schwartz - the rapid adaptation is due to the connective tissue cushioning which redistributes the pressure among the layers of the cushion - if the connective tissue is stripped away the receptor becomes slowly adapting

Modality of Sensory Receptors ⁃ type of stimulus that optimally activates the receptor ⁃ Pacinian corpuscle modality is vibration ⁃ determined by the type of channels on the receptor membrane (mechanosensitive) plus the overall structure and location of the receptor in the tissue. From “Exploring the Brain” by Bear Receptor potentials are graded potentials that can sum over time and space.

8 From http://michaeldmann.net/mann4.html, Michael D. Mann, Ph.D. Proprioception – awareness of the position of your body.

Muscle spindle sensory axons (transmit information about Gamma motor neuron to muscle length or stretch) intrafusal muscle fibers

Alpha motor neuron to extrafusal muscle fibers Muscle spindle capsule

Sensory axon

Secondary (flower-spray) Tendon ending Primary (annulospiral) ending

Nuclear bag fiber Intrafusal muscle Nuclear chain fiber fibers Extrafusal muscle fibers

Golgi tendon organ (muscle tension)

Caliper

Receptive field

Skin surface Convergence of primary sensory axons onto a single Primary secondary sensory neuron will sensory reduce resolution. neurons

Postsynaptic To higher One point of touch perceived neuron parts of the because only one secondary brain (secondary sensory neuron is being sensory neuron) activated. The size and density of receptive fields influences spatial resolution (the ability to distinguish between two stimuli)

Caliper

Receptive field

Skin surface

Primary Two points of touch perceived sensory because each stimulus activates neurons a separate sensory pathway

To higher parts of the Postsynaptic brain (i.e. thalamus) neurons (secondary sensory neuron) Sensory neurons with overlapping receptive fields

Stimulus

Skin surface Receptive field

Somatic sensory neuron

Higher frequency of Lower frequency of action potentials action potentials Skin

Action potentials

Sensory neurons Inhibitory (Primary interneurons sensory neurons) –

– – – –

Somatosensory cortex

Third-order neuron Thalamus (thalamus)

Dorsal column nuclei Brain stem Second-order neuron (secondary sensory neuron) First-order neuron (primary sensory neuron)

Modality-specific pathways Spinal nerve • Touch • Pressure • Vibration • Proprioception Spinal cord

Sensory receptors

Primary somatosensory cortex (SSC) Homunculus

The size of the region in the SSC that receives information about a Leg specific body part is proportional Foot

Shoulder Toes to the density of innervation of Genitals the body part (not to the size of the body part).

Nociceptor First-order (primary sensory)neuron

Interneuron

Skin

Spinal cord

Motor neuron Skeletal muscle

Thalamus Hypothalamus

Reticular formation

First-order Nociceptor neuron

Skin Second-order (secondary sensory) Second- neuron Substance P order neuron Spinal cord

First-order neuron Release of substance P

Midbrain

Medulla

Second-order neuron First-order neuron SPINOTHALAMIC TRACT Modality-specific Receptors for Spinal nerve pathways pain, temperature, itch, and tickle Spinal cord

Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved. Dorsal column pathway Anterolateral (spinothalamic) pathway RIGHT SIDE LEFT SIDE RIGHT SIDE LEFT SIDE OF BODY OF BODY OF BODY OF BODY Primary somatosensory cortex

Third-order Third-order neuron neuron Thalamus Thalamus

Midbrain Midbrain

Second-order Dorsal neuron Column Nuclei Medulla Medulla

First-order neuron Second-order neuron First-order neuron DORSAL COLUMN SPINOTHALAMIC Receptors for Receptors for TRACT touch, Spinal pain, temperature, pressure, nerve itch, and tickle vibration, and Spinal cord Spinal cord proprioception LO 5: Explain how one might determine the location of a spinal cord injury based on the modality of sensation that is lost and the region of the body (both the side of the body and body part) where sensation is lost (Figure 9.18). Convergence of somatic and visceral first-order neurons on the same second-order neuron

First-order neurons

To brain

Skin

Second- order neurons

Visceral organ (heart) Spinal cord

Lung and Liver and diaphragm Liver and gallbladder gallbladder Heart Stomach

Pancreas Liver and Gallbladder gallbladder Small intestine Stomach Ovary Ovary Kidney Kidney Colon Urinary Appendix bladder Ureter

Periaqueductal gray matter

Midbrain

Nucleus raphe magnus Medulla First-order neuron To brain Nociceptor

Spinal cord Second-order neuron Pre- and post- Neuron from synaptic inhibition by nucleus raphe descending opioid magnus pathways Inhibitory interneuron First-order Endogenous sensory opioid neuron

Opioid receptor Second-order neuron

Inhibitory postsynaptic Substance P potential Substance P release is reduced or blocked LO 6. Describe how incoming sensory inputs from primary sensory axons can be modified at the level of the spinal cord ... Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved Perception involves many areas of the brain, including association cortices, limbic system, hippocampus (memory), etc.

PRIMARY SOMATOSENSORY CORTEX (postcentral gyrus) SOMATOSENSORY ASSOCIATION AREA

COMMON INTEGRATIVE BROCA’S AREA AREA WERNICKE’S AREA

VISUAL ASSOCIATION AREA

PRIMARY VISUAL CORTEX

PRIMARY AUDITORY CORTEX

AUDITORY ASSOCIATION AREA