FUNCTIONAL ANATOMY: INTRODUCTION

ABC SYNAPSE IN SOMATOSENSORY SYSTEM

 

Textbook Module 7.2 Mechanical Systems -Somatosensory

Somatosensory Systems
The somatosensory system monitors the sensations of the body and its movements and include multiple systems.

STIMULUS
The different types of mechanical stimuli are listed in the text in Table 7.1 (pg 205). The stimuli include pain (generated by cell injury); heat or cold (on either side of physiological cell injury range)
Movement of hairs on the skin surface; sudden displacement of skin; light touch; skin stretch or also stretch or injury to joints and muscles.

 What these all have in common is that they are associated with types of receptors that are sensitive to them.

TRANSDUCTION
Receptors associated with the types of stimuli listed above are also listed in Table 7.1 They are
Free nerve endings; hair follicle receptors Meissner's corpuscles, Pacinian corpuscles, Merkel's disks, Ruffini endings and Krause end bulbs.  These receptors are located in the  skin and in the joints and muscles.  Stimulation of a touch receptor opens sodium channels in the axon and thereby starts an actionl potential.

TRANSMISSION
Somtosensory information from touch receptors on the head enter the CNS through the cranial nerves, such as Cranial Nerve V. . Somatosensory information  from receptors below the head enter the spinal cord via the dermatones.   Dermatomes were discussed previously in Module 2.1- cells of the Nervous System on this blog.

Transmission Pathway for Fine Touch and Vibratory Sense-  Dorsal Column Medial Lemniscus
see unit 9.5-9.6 of the Coloring Book (pg 142-145)

• The transmission pathway for touch is carried into the spinal cord via the dorsal roots (see Unit 1.4 Coloring Book pg 10) and then without synapsing ascend ipsilaterally in the dorsal part of the spinal cord to the dorsal column nuclei.  Sensory input from the legs synapse in the nucleus gracilis and input from the arms synapse in the nucleus cuneatus. 
• Axons from the dorsal column nuclei decussate in the medulla and ascend to the the thalamus via the dorsal column medial lemniscus.
• Axons from the dorsal column medial lemniscus ascend to the thalamus.  Fibers from Cranial Nerve 5 join the medial lemniscus enroute to the thalamus
• Most of the axons of the medial lemniscus synapse on neurons in the ventral posterior nucleus of the thalamus.
• Axons from the VPN project to the primary somatosensory cortex (Coloring book unit 7.6 pg 120).
• The primary somatosensory cortex is located on the post central gyri (see coloring book unit 7.1 pg 119) of the parietal lobes (CB unit 7.2 pg 112).

• The output of the primary somtosensory cortex is mostly projected to the secondary somatosensory cortex located just inferior to the primary somatosensory cortex located in the ventral portion of the postcentral gyrus and hidden by the lateral fissure
• Output from the sencodary somatosensory cortex is sent to the posterior parietal association cortex.

The somatosensory cortex is laid out somatopically, a map of the body is on the brain. (Just like there is a retinotopic representation of the visual image represented on the occiptial lobe and a tonatopic representation of frequency represented in the cochlea and auditory cortex).
The somatosnesory body map is called a homonculus. (CB unit 8.6 pg 144).


Transmission Pathway for Pain, Temperature, Crude Touch -Spinothalamic Tracts
Pain and Temperature (lateral spinothalamic tracts) Crude Touch (anterior spinothalamic tract)
 see Textbook figure 7.15 and 7.16 (pg 208-209).

Why is pain information conveyed so slowly to the brain?
What are the neurotransmitters involved in conveying pain in the spinal cord?
How does the pain transmission pathway differ from the touch transmission pathway?

Pain information crosses to the contralateral side of the spinal cord at once. The pathways then continue to the brain via the spinothalamic tract. This axonal pathway begins in spine then ends in thalamus- thus the name spine -o- thalamic.



 

• The spinothalamic tract conveys pain and temperature (lateral spinothalamic tract) and crude touch (anterior spinothalamic tract).
• Primary sensory nerves that register pain have  cell bodies that lie in the posterior dorsal root ganglion. 
• They synapse in in the posterior horn of the dorsal root in the spinal cord.
• Secondary neurons cross the spinal cord contralaterally and ascend to the brain via the  lateral spinothalamic tract in a somatotopic arrangement. 
• Secondary neurons synapse in the VPN of the thalamus,
• From the VPN of the thalamus, tertiary neurons ascend via the internal capsule to the primary sensory cortex  
• These cells also branch off axon collaterals to the limbic system, hypothalamus, and other portions of the CNS such s the amygdala, hippocampus and prefrontal cortex.  These areas react not to the sensation of pain but  to its emotional associations.

SENSATION PERCEPTION COGNITION

Descending Analgesic Circuit
Textbook Figure 7.18 pg 210 and unit 8.7 of the Coloring Book (pg 146)

A centrifugal pathway is a pathway that exerts either a facilliatory or inhibitory influence on a nerve signal.  One example of a centrifugal pathway is the descending analgesia circuit.  During times of intense emotion it is possible to feel little or no pain, even when there is a grave injury.

What is a centrifugal pathway?

The inhibition of pain information in the brain is due to the activation of natural opioids or endorphins in the midbrain structure, the periaqueductal gray (PAG)- gray area surrounding the cerebral aqueduct in the midbrain.

Axons from the PAG synapse back to the spinal cord  and decreases pain signals in the spinal cord. Therefore, even though pain receptors may be firing the signal to the brain has been blocked and therefore the injured individual does not experience pain.

See figure 7.18 in the textbook

• "Certain kinds of painful and other stimuli" send emotional pain information from the cortex to the PAG via endorphine neurotransmitters.  
• The PAG projects to the "area in the rostral part of the medulla" and excites cells that release more endorphine neurotransmitter into the spinal cord. 
• The endorphine neurotransmitter inhibits the primary sensory nerve which is releasing substance P as a neurotransmitter and therefore stops "areas of the spinal cord that receive pain messages" from sending ascending pain messages to the brain
• 'The injured person experiences no pain

More recent research about this pathway can be found here.



 

Textbook Modules 6.1 & 6.2 Vision

I. THE STIMULUS
        A. Getting the image to the retina
Most of the structures of the eyeball are involved in preparing the image that is reflecting visible light into the eye. The visible light range of the electromagnetic spectrum is the frequency of approximately 350-700nm. Humans perceive the shortest visible wavelengths as violet, medium short wavelenths is green; medium long wavelength is perceived as yellow and long wavelenght perceived as red. (see Figure 6.8 on page 160). Once the particular range of the electromagnetic energy is reflected off the image, the pattern of the reflected image enters the eyeball.
  
1. What role do the iris, pupil, lens and cornea (structures of the eyeball) play in getting the pattern of the reflection onto the photoreceptors of the retina?  (pag 156)

 2. In the illustration below draw in the placement of horizontal cells and amacrine cells.(page 169)
 3. Which cell axons leave the eyeball and what is this collection of axons called. Also, why is there a blindspot?






FRONT OF EYE BALL                                                                                BACK of EYE BALL

4. What is the role of the fovea? (pg 157) 
5. What is the functional significance of a midget ganglion cell?
6. What is convergence?


II. TRANSDUCTION (Retinal Processing)
  


IIIa. TRANSMISSION PATHWAYS IN THE EYE
   Ganglion Receptive Fields  (see lecture notes & pg 172)

IIIb. TRANSMISSION PATHWAYS IN THE BRAIN
Transmission of visual information actually begins in the retinal layers once the photoreceptors are stimulated. 
1. Trace the transmission pathway from when retinal cell axons leave the eyeball to the destination synapse of MOST of those cells in the LGN .(see pg 168 textbook and pg 134 Coloring Book)

 Lateral Geniculate Nucleus is located in the Thalamus.  (See PAGE 98 of Coloring Book)

IV. SENSATION, PERCEPTION and COGNITION

a. Receptive Fields (Sensation and Perception)
Use this site to help your understand the concept of Receptive Fields. You should understand the sections labeled
THE RETINA
RECEPTIVE FIELDS FROM THE RETINA TO THE CORTEX
You are not responsible for the third section- THE CELLULAR STRUCTURE OF THE VISUAL CORTEX.

b. Cogntion
Shape,
Color Perception
Motion Perception 

V. PUT IT ALL TOGETHER- Stimulus, Transduction, Transmission, Sensation, Perception & Cognition

If you are really understand this material, you should be able to easily read the following webites. Once you are comfortable reading the websites below, your next assignment will be to upload your version of the "Sensory Stories," in which you will be able to explain this sensory modality from start to finish  on a YouTube video.   More about the Sensory Stories assignment in class.

Put it all together site for Vision
 The following terms from this link will help you "put it all  together" in the story of the sensation of Vision. Read the website paying careful attention to the following terms. IN ADDITION PLEASE USED THE MARKED (*) TERMS IN THE CLASS SCHEMA HOMEWORK THAT IS DUE APRIL 3, 2014

THE EYE
350-700 nm
*cornea
lens
amacrine, horizontal, bipolar,
*photoreceptor layer
rods and cones
*pupil
*iris
*ganglion cells
first visual relay in the brain

THE TARGETS OF THE OPTIC NERVE
optic disk,
optic nerve
ganglion cell axons
optic chiasm
*lateral geniculate nucleus of the thalamus
receptive fields of the LGN


THE VARIOUS VISUAL CORTICES and TARGETS

receptive fields of the cells of the retina
occipital lobe
*ventral pathway
dorsal pathway
*superior colliculi
*fusiform gyrus of the inferior temporal cortex
posterior inferior temporal cortex
middle temporar cortex
*medial superior temporal cortex