Neuroglia, Astrocyte lecture note

The dorsal column–medial lemniscus pathway

The dorsal column–medial lemniscus pathway (DCML) (also known as the posterior column-medial lemniscus pathway, PCML) is a sensory pathway of the central nervous system that conveys sensations of fine touch, vibration, two-point discrimination, and proprioception(position) from the skin and joints. It transmits information from the body to the primary somatosensory cortex in the postcentral gyrus of the parietal lobe of the brain.

 The pathway receives information from sensory receptors (missner’s corpuscle, pacinian corpuscle etc.) throughout the body, and carries this in nerve tracts in the white matter of the dorsal columns of the spinal cord to the medulla, where it is continued in the medial lemniscuses, on to the thalamus and relayed from there through the internal capsule and transmitted to the somatosensory cortex.

The name dorsal-column medial lemniscus comes from the two structures that carry the sensory information: the dorsal columns of the spinal cord, and the medial lemniscus in the brainstem.

There are three groupings of neurons that are involved in the pathway: first-order neurons, second-order neurons, and third-order neurons.

 The first-order neurons are sensory neurons located in the dorsal root ganglia, that send their afferent fibers through the two dorsal columns – the gracile fasciculus, or gracile tract, and the cuneate fasciculus, or cuneate tract. The first-order axons make contact with second-order neurons of the dorsal column nuclei (the gracile nucleus and the cuneate nucleus) in the lower medulla.

The second-order neurons send their axons to the thalamus.

The third-order neurons are in the ventral nuclear group in the thalamus and fibres from these ascend to the postcentral gyrus.

Sensory information from the upper half of the body is received at the cervical level of the spinal cord and carried in the cuneate tract, and information from the lower body is received at the lumbar level and carried in the gracile tract. The gracile tract is medial to the more lateral cuneate tract.

The axons of second-order neurons of the gracile and cuneate nuclei are known as the internal arcuate fibers and when they cross over the midline, at the sensory decussation in the medulla, they form the medial lemniscus which connects with thalamus; the axons synapse on neurons in the ventral nuclear group which then send axons to the postcentral gyrus in the parietal lobe. All of the axons in the DCML pathway are rapidly conducting, large, myelinated fibers.

 

Lateral spinothalamic tract

 

The lateral spinothalamic tract, also known as the lateral spinothalamic fasciculus, is an ascending pathway located anterolaterally within the peripheral white matter of the spinal cord. It is primarily responsible for transmitting pain and temperature as well as coarse touch. 

The anterior spinothalamic tract (discussed separately), in contrast, primarily transmits coarse touch and pressure. 

First-order neurons, whose cell bodies are in the dorsal root ganglion and whose axons extend from peripheral receptors, enter the cord via dorsal nerve roots. 

First-order neurons synapse with second-order neurons whose bodies are located in the ipsilateral substantia gelatinosa of dorsal horn of the spinal cord.

The lateral spinothalamic tract then ascends in the lateral funiculus, just medial to the ventral spinocerebellar tract. Fibers of this tract are somatotopically organized for their entire course.

Upon reaching the brainstem, the lateral and anterior spinothalamic tracts combine to form the spinal lemniscus, which runs lateral to the medial lemniscus

 The spinal lemniscus terminates in the ventral posterior nucleus of the thalamus. Here, it synapses with third-order neurons, which project to the primary somatosensory area on the ipsilateral side of the cerebral cortex.

Lesion : 

Contra-lateral loss of pain & thermal sensibilities below the level of the lesion

Unable to sense pin prick or recognize hot or cold objects 

Summary of Somatosensory pathways: lateral & anterior spinothalamic tract and tract of gall and cuneatus

Summary of Somatosensory pathways

The somatosensory system is a part of the sensory nervous system.

It  is a complex system of sensory neurons and neural pathways that responds to changes at the external environment or inside the body.

The axons (as afferent nerve fibers) of sensory neurons has two part peripheral and center part. Terminal portion of peripheral portion of axon of sensory neuron is converted into receptors.

 Sensory receptors are found all over the body including the skin, epithelial tissues, muscles, bones and joints, internal organs, and the cardiovascular system.

Touch is a crucial means of receiving information. This photo shows tactile markings identifying stairs for visually impaired people.

Somatic senses are sometimes referred to as somesthetic senses, with the understanding that somesthesis includes the sense of touch, proprioception (sense of position and movement), and (depending on usage) haptic perception

The mapping of the body surfaces in the brain is called somatotopy. In the cortex, it is also referred to as the cortical homunculus. This brain-surface ("cortical") map is not immutable, however. Dramatic shifts can occur in response to stroke or injury.

Our nervous system deal two types of sensation conscious and unconscious . this article summarized the main somatosensory pathways 

Topics	Lateral spino-thalamic tract	Anterior spinothalamic tract	Tract of gracilis & cuneatus
Sensation	Pain and temperature	Light touch and pressure	Discriminative touch, vibration , conscious muscle joint sense
Receptor	Free nerve ending	Free nerve ending	Meissners corpuscles, pacinian corpuscles, muscle spindles , tendon organ
1st order neuron	Posterior root ganglion	Posterior root ganglion	Posterior root ganglion
2nd order neuron	Substantia getalinosa	Substantia getalinosa	Nuclei gracilis and nuceatus
3rd order neuron	Ventral posterior lateral nucleus of thalamus	Ventral posterior lateral nucleus of thalamus	Ventral posterior lateral nucleus of thalamus
Related with Spinal lamniscus	Present	Present	Absent
Related with Medial lamniscus	Absent	Absent	Present
Destination	Post central gyrus	Post central gyrus	Post central gyrus
Lesion	Contra-lateral loss of pain and thermal sensibilities below the level of the lesion Unable to sense pin prick or recognize hot or cold objects	Contra-lateral loss of light touch and pressure  sensibilities below the level of the lesion. Discriminative touch will be present but unable to feel light touch of cotton placed against the skin	Ipsilateral loss of muscle and joint sensation, two point discrimination and vibration  below the level of lesion but sense of light touch is unaffected Movement are jerky or ataxic

 

Anatomy of sphenoid

Anatomy of sphenoid 

 Anatomical points

1. Body lies centrally
2. superior surface of body lies horizontally
3. Two greater wings & two lesser wings project laterally from the sides of the body
4. Superior surfaces of greater wings are deeply concave
5. Two pterygoid processes are directed downwards from adjoining parts of the body and greater wings Morphological type of bone: pneumatic irregular bone

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Parts of sphenoid bone 

Body 

Two greater wings 

Two lesser wings 

Two pterygoid processes 

Body:  It has six surfaces : superior , inferior , anterior , posterior and two lateral surfaces 

Shape : cuboid 

It contains two large air sinuses which are separated from each other by a septum 

Superior or cerebral surface 

Articulates with ethmoid bone anteriorly and basilar part of occipital bone posteriorly. It shows: 

 Ethmoidal spine : it is articulate with posterior border of cribriform plate of ethmoid bon

  Jugum sphenoidale: smooth area which is the part of anterior cranial fossa . this part is related with gyri recti of cerebrum and olfactory tracts  

Sulcus chiasmaticus: it is a groove which connects both optic canal . it lodges optic chiasm

  Tuberculum sellae: round elevation it bears middle clinoid processe

 Sella turcica: deep depression behind the tuberculum sellae . deepest part of sella turcica is 

known as hypophyseal fossa

 Dorsum sellae: square shape bone which form posterior boundary of sella turcica . it bear

 posterior clinoid processes 

 Clivus: slopping area behind dorsum sellae . it continue with basilar part of occipital bone this part is related with pons 

Inferior surface 

Rostrum of sphenoid 
 Sphenoidal conchae 
Vaginal processes of medial pterygoid plate 

Anterior surface 

Sphenoidal crest articulates with the perpendicular plate of ethmoid leading to formation of a part of the septum of nose. 

Posterior surface: Basilar part of occipital bone 

Lateral surface

• It is united with the greater wing and medial pterygoid plate ,
  Carotid sulcus is present in the lateral surface which is extend from superior orbital fissure to foramen lacerum s lodging cavernous sinus and internal carotid
  
• Development:
• up to rostral half of sella turcica is developed from neural crest
• caudal half of sella turcica is developed from paraxial mesoderm

 

 

Difference between arteriole and venule

Arteriole	Venule
Arterioles distribute blood to capillary beds, the sites of exchange with the body tissues.	Blood of capillaries lead back to small vessels known as venules that flow into the larger veins and eventually back to the heart.
·         The lumina of arteriole are much smaller than corresponding venule	·         The lumina of venules are much larger  than corresponding arterioles.
·         It is also known as resistance vessel	It is also known as capacitance vessel
·         The walls of the arteriole are much thicker  than venule	·         The walls of the veins are much thinner than arterioles
Arteriole provides 50% peripheral resistance	·         Veins and venules can withstand a much lower pressure from the blood flowing through them. Their lumens are correspondingly larger in diameter to allow more blood to flow with less vessel resistance.

Arteriole : lecture note

 

Arteriole

The greatest change in blood pressure and velocity of blood flow occurs at the transition of arterioles to capillaries. An arteriole is a small-diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillaries

Arteriole is also known as resistance vessels . Most arterioles can dilate 60% to 100% from their resting diameter, and they can maintain as much as 40% constriction for a long time. Therefore, a large decrease or increase in vascular resistance has a direct effect on distribution of blood flow and systemic arterial pressure.

Arteriole About 10 percent of the total blood volume is in the systemic arterial system at any given

If all the arteries, veins, and capillaries were laid end to end, the total length would be  100,000 km

The diameter of arterioles  are in between  100  to 300 µm.

Arteriole: Like any other blood vessels arteriole is made by three concentric layers (within outward )

Tunica intima ,Tunica media and Tunica adventitia

Tunica intima of arteriole  contain

1. endothelium & its basement membrane

2. Subendothelial connective tissue is formed by loose connective tissue which contain smooth muscle cells

3. Large arteriole contain internal elastic lamina but small arteriole contain no  internal elastic lamina

Tunica media of arteriole contain Circularly arrange smooth muscle cells layers(1-2 cells layer)  with variable amounts of elastin, reticular fibers, and proteoglycans are interposed between the smooth muscle cells

Arteriole  has tunica adventitia which  is composed of collagenous & elastic connective tissues, whose thickness approaches that of the tunica media.

Large arteriole contain external elastic lamina but small arteriole contain no  external elastic lamina  

Arterioles control blood flow to capillary networks by contraction of the smooth muscle cells. Arterioles serve as flow regulators for the capillary beds. In the normal relationship between an arteriole and a capillary network, contraction of the smooth muscle in the wall of an arteriole increases the vascular resistance and reduces or shuts off the blood going to the capillaries.

Arteriole is also known as resistance vessels . Most arterioles can dilate 60% to 100% from their resting diameter, and they can maintain as much as 40% constriction for a long time. Therefore, a large decrease or increase in vascular resistance has a direct effect on distribution of blood flow and systemic arterial pressure.