somatosensory system

Examples of somatosensory system in the following topics:

• General Organization of the Somatosensory System

  • The somatosensory system is composed of the neurons that make sensing touch, temperature, and position in space possible.
  • The somatosensory system is distributed throughout all major parts of our body.
  • It includes both sensory receptor neurons in the periphery (eg., skin, muscle, and organs) and deeper neurons within the central nervous system.
  • The somatosensory system functions in the body’s periphery, spinal cord, and the brain.
  • Describe how the somatosensory system is composed of neurons that make sensing touch, temperature and position in space possible

• Cutaneous Sensation

  • The somatosensory system is composed of the receptors and processing centers to produce the sensory modalities, such as touch and pain.
  • The somatosensory is the system of nerve cells that responds to changes to the external or internal state of the body, predominately through the sense of touch, but also by the senses of body position and movement.
  • Spread through all major parts of the body, it consists of sensory receptors and sensory neurons in the periphery (for example, skin, muscle, and organs), along with deeper neurons within the central nervous system.
  • Processing primarily occurs in the primary somatosensory area in the parietal lobe of the cerebral cortex.

• Somatic Sensory Pathways to the Cerebellum

  • A sensory system is a part of the nervous system responsible for processing sensory information.
  • A sensory system consists of sensory receptors, neural pathways, and parts of the brain involved in sensory perception.
  • It is part of the somatosensory system and runs in parallel with the dorsal spinocerebellar tract.
  • It is part of the somatosensory system and runs in parallel with the ventral spinocerebellar tract.
  • The major somatosensory pathways communicating with the cerebellum are the anterior and posterior spinocerebellar tracts.

• Somatic Sensory Pathways

  • The somatosensory pathway is composed of three neurons located in the dorsal root ganglion, the spinal cord, and the thalamus.
  • A somatosensory pathway will typically have three long neurons: primary, secondary and tertiary.
  • The axons of many of these neurons terminate in the thalamus, and others terminate in the reticular activating system or the cerebellum.
  • In the periphery, the somatosensory system detects various stimuli by sensory receptors, e.g., by mechanoreceptors for tactile sensation and nociceptors for pain sensation.
  • In the spinal cord, the somatosensory system includes ascending pathways from the body to the brain .

• Classification of Receptors by Location

  • Our brains commonly receive sensory stimuli from our visual, auditory, olfactory, gustatory, and somatosensory systems.
  • Remarkably, specialized receptors have evolved to transmit sensory inputs from each of these sensory systems.
  • Receptors are sensitive to discrete stimuli and are often classified by both the systemic function and the location of the receptor.

• Sensory Areas

  • The primary somatosensory cortex is across the central sulcus and behind the primary motor cortex configured to generally correspond with the arrangement of nearby motor cells related to specific body parts.
  • Olfaction is the only sensory system that is not routed through the thalamus.
  • For example, the right primary somatosensory cortex receives information from the left limbs, and the right visual cortex receives information from the left eye.
  • This somatotopic map has commonly been illustrated as a deformed human representation, the somatosensory homunculus, where the size of different body parts reflects the relative density of their innervation.

• Sensory and Motor Tracts

  • The spinothalamic tract is a somatosensory tract while the corticospinal tract conducts motor impulses from the brain.
  • Somatosensory organization is divided into the dorsal column–medial lemniscus tract (the touch/proprioception/vibration sensory pathway) and the anterolateral system, or ALS (the pain/temperature sensory pathway).
  • However, connections to the somatosensory cortex suggest that the pyramidal tracts are also responsible for modulating sensory information from the body.

• Mapping the Primary Somatosensory Area

  • This is the point-for-point correspondence of an area of the body to a specific point on the central nervous system.
  • Typically, the area of the body corresponds to a point on the primary somatosensory cortex (postcentral gyrus).
  • Areas such as the appendages, digits, and face can draw their sensory locations upon the somatosensory cortex.
  • The idea of the cortical homunculus was created by Wilder Penfield and serves as a rough map of the receptive fields for regions of primary somatosensory cortex.
  • The postcentral gyrus is located in the parietal lobe of the human cortex and is the primary somatosensory region of the human brain.

• Thalamus

  • The thalamus has a system of myelinated fibers that separate the different thalamic subparts.
  • These are all derived from the vertebrobasilar arterial system except the polar artery.
  • In particular, every sensory system (with the exception of the olfactory system) has a thalamic nucleus that receives sensory signals and sends them to the associated primary cortical area.
  • For the visual system, for example, inputs from the retina are sent to the lateral geniculate nucleus of the thalamus, which in turn projects to the primary visual cortex in the occipital lobe.
  • The ventral posterior nucleus is a key somatosensory relay, which sends touch and proprioceptive information to the primary somatosensory cortex.

• Cholinergic Neurons and Receptors

  • Acetylcholine (often abbreviated ACh) is an organic, polyatomic ion that acts as a neurotransmitter in both the peripheral nervous system (PNS) and central nervous system (CNS) in many organisms including humans.
  • Acetylcholine has functions both in the peripheral nervous system (PNS) and in the central nervous system (CNS) as a neuromodulator.
  • In the peripheral nervous system, acetylcholine activates muscles and is a major neurotransmitter in the autonomic nervous system.
  • In the central nervous system, acetylcholine and the associated neurons form the cholinergic system.
  • Phasic increases of ACh during visual, auditory, and somatosensory stimulus presentations have been found to increase the firing rate of neurons in the corresponding primary sensory cortices.