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sensory and motor tracts
The spinal cord is a pathway for ascending tracts (afferent and sensory neurons) and
descending tracts (efferent and motor neurons). A tract is a collection of axons in the
CNS. The white matter of the spinal cord contains both motor and sensory tracts.
The name of the tract often indicates its location in the white matter and where it
begins and ends. There are four major somatic sensory pathways:
1. The posterior column-medial lemniscus tract, also called the dorsal
column-medial lemniscus tract.
2. The anterolateral (spinothalamic) tract.
3. The trigeminothalamic tract. *
4. The anterior and posterior spinocerebellar tract.
-
Gracile
- cuneate
*
*
Anterolatera
*
"
*
The posterior column-medial lemniscus pathway
The posterior column is divided into two areas:
1. Gracile Fasciculus: receives sensory information from the lower half
of the body. It is found on the medial aspect of the posterior column.
2. Cuneate fasciculus: receives sensory information from the upper half
of the body. It is found on the lateral aspect of the posterior column.
Sensations detected by the receptors will enter the posterior horn (root) in the
spinal cord, and then will ascend through the posterior column-medial lemniscus
, tract, then synapses with another neuron in the medulla oblongata. The neurons at
this stage will cross to the other side- a process called decussation. This neuron
will carry the signal to the thalamus, and then synapses with another neuron that
will carry the signal to the cerebral cortex.
rtex
neuron neuron .
NOTE: Any sensory signals must enter the posterior horn and any motor signals
must enter the anterior horn.
The medulla oblongata contains a
nucleus that is divided into two parts:
gracile nucleus and cuneate nucleus.
The first order neuron (first neuron)
will synapse with the second order
neuron (second neuron) in the nucleus
of the medulla, and then decussate. The
second order neuron will synapse with K
the third order neuron in the thalamus
and will transmit signals to the cerebral
cortex. Therefore, we need 3 neurons to
transmit a signal through the posterior
column.
Decussation of the second order neuron shows that the right part of the brain
controls the left part of our body and vice versa. This tract conveys fine touch,
which means that receptors are well-localized, fine pressure, vibration, weight
discrimination, proprioception (sense of position), and stereognosis from the limbs,
spot the sensation is coming from.
Stereognosis is when you identify the form, texture, and shape of an object while
closing your eyes. It is well-developed in blind people since they rely mainly on
touch. The posterior column-medial lemniscus also allows us to differentiate
between two points. We can discriminate the two points because each point hits a
different receptor. Discrimination of two points on the body depends on the density
- -
of the receptors. More receptors are found on the fingertips, the face, and the lips;
-
therefore, we can clearly discriminate between two points in these regions.
sensory and motor tracts
The spinal cord is a pathway for ascending tracts (afferent and sensory neurons) and
descending tracts (efferent and motor neurons). A tract is a collection of axons in the
CNS. The white matter of the spinal cord contains both motor and sensory tracts.
The name of the tract often indicates its location in the white matter and where it
begins and ends. There are four major somatic sensory pathways:
1. The posterior column-medial lemniscus tract, also called the dorsal
column-medial lemniscus tract.
2. The anterolateral (spinothalamic) tract.
3. The trigeminothalamic tract. *
4. The anterior and posterior spinocerebellar tract.
-
Gracile
- cuneate
*
*
Anterolatera
*
"
*
The posterior column-medial lemniscus pathway
The posterior column is divided into two areas:
1. Gracile Fasciculus: receives sensory information from the lower half
of the body. It is found on the medial aspect of the posterior column.
2. Cuneate fasciculus: receives sensory information from the upper half
of the body. It is found on the lateral aspect of the posterior column.
Sensations detected by the receptors will enter the posterior horn (root) in the
spinal cord, and then will ascend through the posterior column-medial lemniscus
, tract, then synapses with another neuron in the medulla oblongata. The neurons at
this stage will cross to the other side- a process called decussation. This neuron
will carry the signal to the thalamus, and then synapses with another neuron that
will carry the signal to the cerebral cortex.
rtex
neuron neuron .
NOTE: Any sensory signals must enter the posterior horn and any motor signals
must enter the anterior horn.
The medulla oblongata contains a
nucleus that is divided into two parts:
gracile nucleus and cuneate nucleus.
The first order neuron (first neuron)
will synapse with the second order
neuron (second neuron) in the nucleus
of the medulla, and then decussate. The
second order neuron will synapse with K
the third order neuron in the thalamus
and will transmit signals to the cerebral
cortex. Therefore, we need 3 neurons to
transmit a signal through the posterior
column.
Decussation of the second order neuron shows that the right part of the brain
controls the left part of our body and vice versa. This tract conveys fine touch,
which means that receptors are well-localized, fine pressure, vibration, weight
discrimination, proprioception (sense of position), and stereognosis from the limbs,
spot the sensation is coming from.
Stereognosis is when you identify the form, texture, and shape of an object while
closing your eyes. It is well-developed in blind people since they rely mainly on
touch. The posterior column-medial lemniscus also allows us to differentiate
between two points. We can discriminate the two points because each point hits a
different receptor. Discrimination of two points on the body depends on the density
- -
of the receptors. More receptors are found on the fingertips, the face, and the lips;
-
therefore, we can clearly discriminate between two points in these regions.
