10/7 - ASCENDING AND DESCENDING PATHWAYS Flashcards
CONDUIT FUNCTION OF BRAINSTEM
Information is transmitted bidirectonally between spinal cord and cerebral cortex, often with relays in the brainstem.
For the nervous system to function properly, communication must be established between different parts of the CNS. For example, sensory information has to be relayed to the cerebral cortex for proper interpretation. Likewise, control of movement arising in the cerebral cortex must be transmitted to neurons in the spinal cord. The pathways used to transmit this information can be identified at each level of the brainstem.
General Rule for Ascending Pathways
These carry sensory information to the CNS.
Thus, the first order neurons are located within a peripheral ganglion – Dorsal Root Ganglion (DRG) for body, cranial nerve ganglion for head.
Primary Neuron
first order neuron
REPRESENTATIVE EXAMPLES OF ASCENDING TRACTS
Dorsal (Posterior) Column-medial Lemniscus
Spinothalamic
There are numerous tracts that carry information from the periphery to the CNS. For the most part, the name of the tract tells you where it originates and where it ends. For example, the spinothalamic tract begins in the spinal cord and ends in the thalamus. We will only focus on 2 of the many different ascending tracts: the dorsal column-medial lemniscus pathway which transmits general sensory information and the spinothalamic tract which conveys information on pain and temperature.
spinothalamic tract
conveys information on pain and temperature
dorsal column-medial lemniscus
transmits general sensory information
DORSAL (POSTERIOR) COLUMN – MEDIAL LEMNISCUS
CONSCIOUS PERCEPTION OF SENSORY INFORMATION
Modality:
1. Low Threshold Cutaneous Receptors for:
Touch, Pressure, Vibration, Fine Form, and Texture Discrimination. Form Recognition Of 3-dimensional Objects (Stereognosis).
- Joint And Muscle Receptors. Conscious Awareness Of Body Position (Proprioception), Limb Movement In Space (Kinesthesia)
Large heavily myelinated axons
Input From Lower Limb And Trunk (T6-S5) Forms a Bundle of Axons Called the Fasciculus Gracilis
Input From Upper Limb And Trunk (C1-T5) Forms a Bundle of Axons Called the Fasciculus Cuneatus
Fasciculus Gracilis
Input From Lower Limb And Trunk (T6-S5) Forms a Bundle of Axons Called the Fasciculus Gracilis
Fasciculus Cuneatus
Input From Upper Limb And Trunk (C1-T5) Forms a Bundle of Axons Called the Fasciculus Cuneatus
Stereognosis
Form Recognition Of 3-dimensional Objects
Kinesthesia
Limb Movement In Space
Proprioception
Conscious Awareness Of Body Position
DORSAL (POSTERIOR) COLUMN – MEDIAL LEMNISCUS
NEURON
1st ORDER NEURON: DORSAL ROOT GANGLIA CELLS.
Primary afferent axons enter via dorsal root to enter ipsilateral dorsal column. Below T5 only fasciculus gracilis. Above T5, 2 tracts – fasciculus gracilis (FG) and fasciculus cuneatus (FC).
2nd ORDER NEURONS IN CAUDAL MEDULLA IN N. GRACILIS AND N. CUNEATUS.
Axons arising from neurons in these nuclei cross the midline as the internal arcuate fibers and form the medial lemniscus (L: ribbon).
3rd ORDER NEURON IN THALAMUS (VENTRAL POSTERIOR LATERAL NUCLEUS).
Axons arising from these neurons project to primary sensory (parietal) cortex
FIRST ORDER NEURON IS IN A _____
FIRST ORDER NEURON IS IN A DRG
Dorsal root ganglion cells give rise to a 2 processes. One process projects to the periphery and other projects into the spinal cord.
The peripheral processes have different sizes and amounts of myelin. The larger the process, or the heavier the myelin, the faster action potentials are conducted to the CNS. For the dorsal column-medial lemniscus system, the axons are large and heavily myelinated. Those carrying proprioceptive information from muscles and joints are the largest diameter and thus will have the fastest conducation rate. Those carrying sensory information from cutaneous receptors are just slightly smaller and thus just a bit slower.
The central processes enters the spinal cord via the dorsal root. In the dorsal column-medial lemniscus pathway, these axons collect in the dorsal or posterior funiculus and ascend to the brainstem.
PERIPHERAL PROCESS of a DRG:
Carry information from cutaneous receptors (touch, vibration)as well as muscle spindles/joint receptors (proprioception).
CENTRAL PROCESS of a DRG:
Enters through dorsal root and courses directly to posterior column where they ascend to brainstem
Everything you need to know about the dorsal column-medial lemniscus pathway
How general sensory information is transmitted from the periphery to the cerebral cortex.
The modalities carried by this pathway include low threshold cutaneous receptors which detect touch, pressure and vibration. Information on the fine form and texture discrmination are also carried by these receptors. Putting several pieces of information together, form recognition, that is the 3-dimensional shape of an object can be detected (stereognosis). For example, putting your hand in your pocket and feeling an object that you identify as a key, even without seeing it, based on shape, texture, etc. In addition, sensory information from muscle and joint receptors are carried in this pathway. These axons relay information on where the body is in space (proprioception) or how the limbs are moving in space (kinesthesia).
Sensory information from the lower limb is carried in a tract called the fasciculus gracilis. This tract is formed by the central processes of dorsal root ganglion neurons located between T6 and S5. Sensory information from the upper limb is carried in a tract called the fasciculus cuneatus which is formed from the central processes of DRG neurons located between C1 and T5. The first order neuron, aka the cell of origin, are dorsal root ganglion neurons. The central processes or axons of these neurons enter the spinal cord through the dorsal root and gather in the dorsal or posterior column where they ascend to the medulla. Below T5 only the fasciculus gracilis is present. Above T5, both the fasciculus gracilis and the fasciculus cuneatus are present. Gracilis is medial to cuneatus. These axons synapse on neurons in the nucleus gracilis or nucleus cuneatus. These nuclei are located in the caudal medulla and are defined as the second order neuron. The axons of neurons located in these two nuclei leave and form another pathway that arcs ventrally and medially and are called internal arcuate axons. They all gather together on the midline, just medial to the inferior olivary nuclei (described previously) to form the Medial lemniscus tract. These axons ascend on the contralateral side of the medulla, pons and midbrain until they reach the thalamus in the diencephalon where another synapse occurs on neurons in a subdivision of the thalamus called the ventral posterior lateral nucleus (VPL). Axons arising from VPL project to areas of the forebrain that are specialized for processing sensory information.
DORSAL COLUMN – MEDIAL LEMNISCUS
SPINAL CORD
Series of cross sections from S4 to C3. Sensory input arising from the lower limb and lower trunk form the fasciculus gracilis. Between S5 and T6 only the fasciculus gracilis is present and it occupies the entire dorsal funiculus. Above T6 the fasciculus gracilis is still present, but it is pushed medially as Sensory input from the upper limb and upper trunk is added on laterally forming the fasiculus cuneatus. At C5 and C3, the fasiculus gracilis occupies the medial part of the dorsal funiculus. Axons added on as the fasciculus cuneatus are located laterally.
As Axons Enter From The Upper Limb, Those Present In The Dorsal Funiculus From The Lower Limb Are Shifted Medially.
INPUT FROM T6-S5 (LOWER LIMB)– FORMS FASCICULUS ____ (LATIN for ____)
INPUT FROM T6-S5 (LOWER LIMB)– FORMS FASCICULUS GRACILIS (LATIN for SLENDER)
INPUT FROM ___ (_____ limb)– FORMS FASCICULUS CUNEATUS (LATIN for ____)
INPUT FROM C1-T6 (UPPER LIMB)– FORMS FASCICULUS CUNEATUS (LATIN for WEDGE)
CAUDAL MEDULLA- DORSAL
Dorsal surface of the caudal medulla. We will focus on the dorsal or tegmental region of the brainstem. We still see the Nucleus gracilis medially, the nucleus cuneatus lateral to it, and the spinal nucleus and tract of V, most lateral.
The major change is that the nuclei are getting larger and the fiber tracts are getting smaller as they terminate in their respective nuclei. The fasciculus gracilis is almost completely gone, whereas there are still some axons remaining in the fasciculus cuneatus.
The Spinal Tract of V is actually getting larger as we go rostrally due to the fact that it enters the brainstem at the level of the pons and descends to the medulla. Thus as we move, rostrally, we will encounter more and more of these axons.
DORSAL COLUMN – MEDIAL LEMNISCUS
2nd ORDER NEURONS
1st order fibers arising from DRG ascend to TERMINATE in the ipsilateral N. Cuneatus and N. Gracilis in caudal medulla.
2ND ORDER NEURONS in N. Gracilis and Cuneatus give rise to axons that course ventrally in an arc = INTERNAL ARCUATE FIBERS. CROSS MIDLINE
2nd order axons collect on either side of midline to form MEDIAL LEMNISCUS (L. Ribbon or band) in the medullary tegmentum. This tract ascends through medulla, pons, midbrain to terminate in thalamus
DORSAL COLUMN – MEDIAL LEMNISCUS
2nd ORDER NEURONS
in other words
The junction of the medulla and spinal cord. 3 nuclei are evident on the dorsal surface. These are the nucleus gracilis, the nucleus cuneatus and the spinal nucleus of V.
Axons ascending in the fasciculus gracilis leave their respective fiber tracts and synapse on neurons in the nucleus gracilis. Likewise, axons ascending in the fasciculus cuneatus leave the tract and synapse on neurons in the nucleus cuneatus. These neurons are the first relay of sensory information. These are the 2nd order neurons. Axons arising from these neurons leave their respective nucleus and course ventrally and medially forming an arc. This fiber bundle is called the Internal Arcuate Fibers. These axons cross the midline just dorsal to the pyramids and the inferior olivary nucleus (IOC). After crossing the midline, these axons coalesce to form a large fiber tract called the Medial Lemniscus. Within the medial lemniscus, axons from the nucleus gracilis are ventral to those arising from the nucleus cuneatus.
One way to remember this is to think about a person standing on top of a pyramid (for the pyramidal tract). However, our little man doesn’t have a head because axons from the spinal trigeminal nucleus do not join this bundle. This is important!! Although they are anatomically related to each other, the nucleus gracilis and nucleus cuneatus carry general sensory information. The neurons in the spinal trigeminal nucleus relay information on pain and temperature from the face and follow a different pathway from the spinal cord.
DORSAL COLUMN – MEDIAL LEMNISCUS
Rostral Medulla
Medial Lemniscus REMAINS AS A VERTICAL COLUMN IN THE TEGMENTUM IMMEDIATELY ADJACENT TO THE MIDLINE AS IT ASCENDS THROUGH THE MEDULLA
Cross section through the rostral/open medulla. A key landmark here is the presence of the large inferior olivary nucleus as well as several cranial nerve nuclei on the dorsal surface of the brainstem including the hypoglossal nucleus, the dorsal motor nucleus of the vagus, the nucleus solitarius and the caudal end of the vestibular nuclei.
Nothing really changes with respect to the medial lemniscus. It remains on the midline, just above the pyramids with the same somatotopic organization. The one minor change is the addition of a new fiber tract that is located just dorsal to the medial lemniscus. This is called the medial longitudinal fasciculus or MLF. This tract arises from neurons located in the vestibular nuclei. This tract is involved in the vestibular system. The axons of the MLF and ML are difficult to separate. Just know that once the vestibular nuclei appear, the most dorsal part of the large midline fiber tract are part of the medial longitudinal fasciculus. The portion of the fiber tract located more ventrally is the medial lemniscus. You will not be asked to define the border between these two tracts.
DORSAL COLUMN –
MEDIAL LEMNISCUS
MID PONS
As ventral brainstem expands to form basilar pons, the Medial Lemniscus is “pushed” dorsally and assumes a more horizontal orientation.
UL remains close to midline while lower
limb “swings” laterally.
DORSAL COLUMN –
MEDIAL LEMNISCUS
MID PONS
summary
This section is at the level of the mid pons. Several previously described structures are visible in this section including the facial motor nucleus and facial nerve, the motor, main sensory, and mesencephalic nuclei of the trigrminal nerve, and the abducens nucleus.
The basilar pons has expanded ventrally and basically pushes the medial lemniscus dorsally into the tegmentum. As the basilar pons pushes the medial lemniscus into the tegmentum, it also “knocks it over”. Instead of a vertical orientation, the tract assumes a horizontal orientation at this level. The somatotopic organization follows this shift. Here is our figure of the man standing on the pyramid. As the basilar pons expands, think of the body being knocked over medially. The lower limb gets kicked out sideways and laterally and knocks our figure over in such a way that it grasps the midline for stability. Thus, the upper limb remains near the midline, holding on for dear life and the lower limb is moved laterally.
