Posterior Column-Medial Lemniscus

Type of Tract

GSA - general somatic afferent (somatosensation)

ascending tract

conscious - ends in the cortex 

* Modalities of the PCML * 

Proprioception

Discriminative Touch

Vibration Sense

Axons of the PCML

have very large diameter

are very heavily myleinated

are very rapidly conduction

A-alpha fibers 

PCML projects to ... 

Primary Somatosensory Cortex

Broadmann's Area 3,1,2

Somatotopy of PCML

the neurons are arranged in a "map" of the body 

very accurate localization 

Area's included in the PCML

C1 to S5

the head and face are not included in PCML - that information comes from the trigeminal nerve (cranial nerve V) 

General Proprioception

the ability to tell where your body is in space via your limbs

Proprioception:

Static Position Sense

the ability to feeel where your limbs are

General Proprioception:

Kinesthesia

the ability to sense movement

Difference between General Proprioception and Special Proprioception

General = ability to tell where body is in space

Special = comes from the understanding of vestibular input

Difference of Conscious Proprioception and Unconscious Proprioception

Conscious = via the PCML - cortex

Unconscious = via the cerebellum 

Receptors for Proprioception

Joint Receptors - joint angle and tension in joint capsule

Muscle Spindles - change in muscle length, provide info of when muscles move

Skin Mechanoreceptors - touch receptors that are in contact with surfaces

Golgi Tendon Orgons - tension in the tendons 

Discriminitive Touch

There is a high degree of accuracy in location 

It is integrated with movement

Graphesthesia

type of descriminative touch

ex. when you draw a letter in someone's hand and they guess what the letter is

Sterognosis

type of discriminative touch

feeling the 3-D shape of something

ex. putting a coin in someone's hand and having them feel the edges to figure out what it is

Vibration Sense

PCML

there are separate receptors for this

usually tested on a bony part of the body 

Sensory Ataxia

incoordination of sensory information to reach the brain 

Dysmetria

Form of sensory Ataxia - abnormal metering, movement is wrong in it's scope

Presented in: abnormal stance, stumbling, difficulty turning, truncaly sway (ataxia of the trunk), inaccuracy in voluntary movements 

fasciculus cuneateus

bundle of fibers in the posterior horn - fibers come from primary afferents from the cervical region (neck, upper truck, and arms)

located on the lateral side of the posterior horn

fasciculus gracilis

bundle of fibers associated with the primary afferents from receptors in the lumbar region (lower trunk and legs)

only seen above T6

located in the medial posterior funiculus 

Nucleus Gracilis

the Pre-thalamic Relay Nucleus in the Mid-medulla where the primary afferent from the lumbar region synapses onto the 2nd order neuron

Nucleus Cuneatus

The Pre-thalamic Relay Nucleus in the mid-medulla where the primary afferent synapses onto the second order neuron - from the cervical region

Internal Arcuate Fibers

where the two pathways (cervial and lumbar) decussate - cross midline in the mid-medulla

medial lemniscus

area where the cervical and lumbar ascending pathways (2nd order neurons) have decussated and are ascending the brainstem - is heavily myleinated 

VPL

Thalamic Relay Nucleus for the cervical and lumbar PCML pathways - where the 2nd order neurons synapse onto the 3rd order neurons