Physiology sheet # 36

Part2….

Somatic Sensation

Objectives:

  • List the type of mechanoreceptive sensations and its receptors
  • Describe the two pathways for its transmission(Dorsal column and anterolateral spinthalamic tracts)
  • Follow these pathway to the cortex
  • Differentiate the two pathways

Classification of Somatic Sensation:

1. Mechanoreceptive:

  • Tactile

-Touch

-Pressure

-Tickle and itch

-Vibration

Mechanoreceptive because it change the structure

This called structural deformation

  • Position or proprioceptive

-Static

-Dynamic (change in position )

Other kinds which is not mechanoreceptive sensation:

  • Thermodynamic ( sensation of temperature)
  • Nociceptive ( sensation pf pain )

These kinds are transmitted throw anterolateral spinthalamic pathway.

Tactile receptors:

  • Free nerve endings that are distributing all through the skin

-sensitive to touch,pain and temperature

-they usually connected to Aδ and C fibers which mean that they are small myelinated or unmyelinated fibers

  • Meissner's courpuscles (Aβ )

-any courpuscular usually they are adapting receptor

-found on nonhairy skin

-it is sensitive to touch,vibration ( low rate)

  • Merkel's discs (Aβ)

-tips  Iggo dom receptors

-forming too many receptor area under the skin

-sensitive to touch

-found in hairy and nonhairy skin

-fibers that connected to Aβ

- slowly adapting fiber

  • Hair end organ

-around hair shaft

-responsible for movement of the hair

  • Ruffini's end organ

-found in the dermis

-they are slowly adapting fibers

-respond to continual deformatiom of the joint and pressure

  • Pacinian corpuscle

-very rapidly adapting and is stimulated by rapid movement

-detects vibration ( highest rate) beside also pressure and other rapid changes in the skin

-found in dermis

-responsible for deep touch

-high frequency vibration

Meissner's corpuscles, hair receptor,Pacinian corpuscles and Ruffini's end organs transmit signals in type Aβ nerve fibers at 30-70 m per sec

 Free nerve endings transmit signals in type Aδ nerve fibers at 5-30 m per sec, some type C unmyelinated fibers at 0.5-2 m per sec

 the more critical the information the faster the rate of transmission because if it transmitted slowly the impulses will follow each other then you are not going to differentiate intensity or even location

 That’s why fast conducting fibers they are the fibers that specific for fast system

-which is dorsal column- medial lemniscal system (fine touch, fine pressure )

 The slow fibers are specific for less localization

The grades of intensity for the fasting fibers are too many, localization are higher

-Slow system  not differentiates the grades of intensity. Divergence that lose localization

 almost all sensory information enters the spinal cord through the dorsal roots of the spinal nerves

Two pathways for sensory information

-dorsal column- medial lemniscal system

-anterolateral system

Dorsal column pathway :

From the receptors  the first order neuron enters the dorsal horn or post. Horn  the cell body is found in the dorsal ganglia  then it ascends in the dorsal column that contains:

  • Graclile fasciculus ( medial) : from the lower half of the body

-Highly oriented ( most lower most medial )

  • Cuneate fasciculus ( lateral): low other part

Then it ascend to its corresponding nucleus :

Gracilus goes to gracilus nucleus

Cuneatus goes to cuneatus nucleus

Then it synapse there and second order neuron goes to the other side

-that’s why information from left part of the body lay down on the right part of the cerebral cortex

-right part of the body  left part of the cerebral cortex

 it goes to thalamus ( ventro postrio lateral part)

Then it goes to postcentral gyrus of the cerebral cortex where our part is representing here

Pathway:

Type of sensation this system transmitted:

-fine touch (well localized)

-fine pressure

-vibration

-proprioception

-two point discrimination

-weight discrimination

-stereognosis: to differentiate things by form or texture

ex: as you don’t use your visual sensation and by touch you know the object( smooth or rough)

*Note: every part of the brain our part is represented

This called Topographic represented

contain large myelinated nerve fibers for fast transmission

grades of intensity is very high

high degree of spatial orientation

high degree of spatial and temporal fidelity (faithfulness)

fine touch,vibration,position,fine pressure

Done by:

Aya Mahfouz