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spinal chord: white matter: contains, fn

the white matter of the spinal cord is composed mainly of myelinated axons
that carry information back and forth between different parts of the spinal cord and the brain.

funiculi: def, contain

- Bundles of nerve fibers are called tracts or funiculi (funiculus, singular).

funiculi: attributes:list

1. tracts
2. 3 columns
3. directios of columns/tracks/funiculi
4. fn of each directional track/column


- The fasciculi contain bundles of axons with similar destinations and functions.

funiculi groups

- The fibers are grouped into 3 white matter columns on the right and left side of the cord also known as funiculi (= long ropes).
- Each column contains bundles of nerve fibers which have a common origin or are going to the same region.

funiculi: list

Each funiculus is named according to its position in the cord:
dorsal (posterior), lateral and ventral (anterior) columns.

direction of funiculi

The fibers run in three directions:
1. ascending tracts
2. descending tracts
3. tracts that run transversely.

ascending tracts

ascending tracts carry sensory input from peripheral receptors to higher centers of the spinal cord and brain.

descending tracts

descending tracts carry motor commands down to the cord from the brain or within the cord to a lower level/ periphery.

transverse tracts

tracts that run transversely cross from one side of the cord to the other.

commissural fibers

fibers that cross the midline to connect one side to the other are commissural fibers
(like corpus callosum that connects the right and left cerebral hemispheres).

spinal tracts: -neuron?, arrangement

- All of the major spinal tracts are part of multineuron pathways that connect the brain to the body's periphery.
- Finally, all pathways and tracts are paired and symmetrically arranged in the right and left sides of the spinal cord.
Note the bilateral symmetry of both types of tracts.


In addition, most pathways exhibit somatotopy, which is a precise spatial relationship among the tract fibers that reflects the orderly mapping of the body (like the homunculus of the cerebral cortex).


Most fiber tracts or pathways cross from one side of the cord to the other at some point along their journey.
This is called decussation
(eg the decussation of the pyramids in the brain).

naming convention of the tracts.

- They are named based on their relative position in the spinal cord (dorsal or posterior, lateral, ventral or anterior),
and where they are coming from and going to.
- Tracts are named based on their position in the spinal cord and where they begin and end.

posterior spinocerebellar : naming

the posterior spinocerebellar tract indicates that this bundle of fibers is a ascending tract (spino-) carrying sensory information brought into the spinal cord to the cerebellum for processing.

lateral corticolspinal tract: naming

the lateral corticospinal tract is a descending tract because it is coming from the cortex and going to the spinal cord,
and you know it is carrying voluntary motor commands because descending tracts are motor.

Sensory (Ascending) Tracts: list, wat

Ascending pathways conduct (integrate) sensory impulses (information) upward typically through chains of three neurons designated,
first-, second-, and third order neurons.

type of neuron: sensory tracts

Both second and third order neurons are interneurons.

first order tracts: residence

First-order neurons have their cell bodies in dorsal root or cranial ganglia and conduct impulses from cutaneous receptors and proprioceptors to the spinal cord or brainstem where they synapse with second-order neurons.

second order tracts: residence

The cell bodies of 2nd order neurons reside in the dorsal horn of spinal cord or in medullary nuclei;
they transmit impulses to the third-order neurons in the thalamus that relays the impulses to the somatosensory cortex.

how is somatosensory info conveyed

somatosensory information is conveyed via three main pathways,
1. spinocerebellar,
2. medial lemniscal
3. spinothalamic pathways.

conscious pathways

two of the pathways (medial lemniscal and spinothalamic) carry information that is relayed in the thalamus to the somatosensory cortex of the brain.
These pathways carry sensory information that we become consciously aware of.

spinocerebellar pathway

Now look at the spinocerebellar pathway (indicated on the left side of diagram (a) by the light blue line). This pathway never reaches the thalamus or somatosensory cortex of the brain.
The spinocerebellar pathway transmits proprioceptive (muscle or tendon stretch) information only to the cerebellum, so it is subconscious.
The cerebellum uses the information to coordinate skeletal muscle activity.

dorsal column-medial lemniscal pathway

Examine the right half of diagram (a). This is the dorsal column-medial lemniscal pathway.
The term lemniscal comes from a word that means ribbon.
It transmits discriminative touch and vibration, as well as conscious proprioception (position sense) to the cerebral cortex
which allows us to determine precisely where on the body the sensation originated from.
This pathway is a little more complex—follow this pathway which is indicated by the dark blue line.

dorsal column-medial lemniscal pathway: list, explain

The dorsal column-medial lemniscal pathway is divided into two tracts that are named for the nuclei in the medulla oblongata where the first-order neuron synapses with the second-order neuron.
1. fasiculus gracilis
2. fasiculus cuneatus

fasiculus gracilis

The fasiculus gracilis carries discriminative tactile input from receptors located in the lower limbs and inferior body trunk up through the lumbar spinal cord and ascending to the nucleus gracilis of the medulla.
The fasiculus cuneatus is a shorter pathway because its first-order neurons carry sensory information from the upper limbs, upper trunk and neck.
The first order neurons synapse in the nucleus cuneatus of the medulla.
From the medulla, the second order neurons of both tracts decussate (cross over) and continue their ascent to the thalamus in the medial lemniscal tract.
Once in the thalamus, the second-order neurons synapse with the third order neurons which ascend to the appropriate region of the somatosensory cortex (post-central gyrus).

fasiculus cuneatus

The fasiculus cuneatus is a shorter pathway because its first-order neurons carry sensory information from the upper limbs, upper trunk and neck.
The first order neurons synapse in the nucleus cuneatus of the medulla.
From the medulla, the second order neurons of both tracts decussate (cross over) and continue their ascent to the thalamus in the medial lemniscal tract.
Once in the thalamus, the second-order neurons synapse with the third order neurons which ascend to the appropriate region of the somatosensory cortex (post-central gyrus).

spinothalamic tract

Figure b in the diagram shows the spinothalamic tract.
This lateral pathway carries pain and temperature information into the dorsal horn of the spinal cord where the first-order neuron synapses with second-order neurons.
The fibers of the second order neurons decussate (cross over to the other side) before ascending to the thalamus where they synapse with the third order neurons.
These carry the information to the appropriate region of the somatosensenory cortex.

descending pathways: fn, use, list

This figure shows three major descending pathways by which the brain regulates movement of the body.
Descending pathways differ from ascending pathways in that they can use only two neurons to transmit information instead of three.
In these two neuron pathways, the neurons are referred to as the upper and lower motor neurons.

upper motor neurons

The pyramidal cells of the motor cortex as well as the neurons in subcortical motor nuclei (basal nuclei, etc.) are called upper motor neurons.

lower motor neurons

The ventral horn motor neurons which innervate the skeletal muscles (their effectors) are called lower motor neurons.

pyramidal tracts: def, fn

= direct or corticospinal pathways
- the upper motor neurons are the large pyramidal cells of the somatomotor cortex (pre-central gyrus).
Direct pathways = pyramidal tracts (=lateral and anterior corticospinal tracts) carry motor information to skeletal muscles.

decussation of pyramidal neurons, directions

The axons of the pyramidal neurons comprising the lateral corticospinal tract decussate in the pyramids on the ventral surface of the medulla oblongata.
From there, they descend to the appropriate level of the spinal cord where they synapse either with short ventral horn interneurons, or directly with the large multipolar neurons of the ventral horn which send impulses out the spinal nerve to skeletal muscle.

importance of pyramids decussation

Because of the decussation of the pyramids, the right cerebral cortex controls skeletal muscles on the left side of the body and vice versa.

decussation of anterior corticospinal tracts

Unlike the lateral tracts, the anterior corticospinal tracts do not decussate in the medulla, but cross over at the spinal cord level to synapse directly with the ventral horn motor neuron.

rubrospinal tract: def

The other motor tract show in diagram b, is the rubrospinal tract.
This tract is an example of an extrapyramidal or indirect pathway so named because the origin of their neuronal nuclei where thought to be outside of the pyramidal tracts.
Recent evidence suggests that this may not be entirely accurate, but the term is still widely used in clinical settings.

indirect pathway: list

vestibulospinal tract
reticulospinal tract
tectospinal tract
subcortical motor nuclei

vestibulospinal tract

indirect pathway
regulates axial muscles
maintain balance n posture

reticulospinal tract

muscles controlling coarse limb movements

tectospinal tract

head, neck,
eye movements that follow objects in the visual field

subcortical motor nuclei

controls reflex activity

wat helps regulate muscle tone?

rubrosppinal pathway
vestibulospinal pathway
tectospinal pathway
reticulospinal pathway

rubrospinal tract: directions

In the rubrospinal tract, the origin of the upper motor neuron is in the red nucleus of the midbrain.
Their fibers decussate just inferior to the red nucleus.
From there, they continue through the brain and down the spinal cord.
Upon reaching the correct level, the fibers synaspe on a short interneuron which synapses with the ventral motor cells.

rubrospinal tracts fn

The rubrospinal tracts are concerned with muscle tone in flexors on the opposite side of the body.

types of paralysis: cause, ex

Although the spinal cord is elastic and stretches when we bend or move our trunk, it is delicate and sensitive to pressure.
Damage to the spinal cord or its roots can lead to some sort of functional loss.
Paresis and paralysis are varying degress of motor function impairment.


Paresis pertains to muscle weakness, whereas paralysis is more severe.


Paresthesias is loss of sensation.

flaccid paralysis

There are different types of paralysis.
If the cell bodies of the ventral horn or the ventral root are severely damaged,
then the patient develops flaccid paralysis of the muscles that were being served at the level of injury.
Flaccid paralysis results in a loss of voluntary control of the affected skeletal muscles, no reflex activity, the muscle(s) remain limp and will eventually atrophy.
This condition is also known as lower motor syndrome.

spastic paralysis

If however, only the upper motor neurons coming from the primary motor cortex are damage,
the individual develops spastic paralysis.
What happens is that the motor neurons remain intact and the muscles continue to be stimulated.
However, the stimulation is irregular and non-voluntary because it lacks central control.
Exaggerated reflexes because the integration that occurred along the way at various levels via association fibers was removed
The muscles remain healthy longer, but their movements are spastic.
Frequently, this leads to a permanent shortening of the muscles.

trasacted spinal cord

If the spinal cord is transected (severely damaged, there may be both motor and sensory loss.
Transection of the spinal cord at any level result in total motor and sensory loss in the body regions inferior to the site of damage.

motor loss: list

Motor loss include:


If the transection occurs between T1 and L1, both lower limbs are affected resulting in paraplegia.


In the injury occurs in the cervical region, all four limbs are affected and the result is quadripelgia.


Hemipelgia is paralysis of one side of the body.
This is usually the result of brain injury like from a stroke.

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