NP2 Long Tracts of the CNS1

How many pairs of spinal nerves are there in the body?

31 (62 nerves)

Sensory inputs: Dermatome

Area of skin innervated by a single dorsal root

How many dermatomes?

62 dermatomes (62 nerves = 62 dermatomes)

Each nerve covers:

Only particular area of skin, not the entire skin.

1 motor neuron innervates:

Multiple skeletal muscle cells

Many, many neurons innervate:

The entire muscle

α-motor neurons innervate:

Muscle cells

Motor neuron pool

All neurons innervating 1 single muscle

Gastrocnemius muscle is innervated by:

7 spinal nerves (7 segments; L4-S3)

Soleus muscle is innervated by:

5 nerves (5 segments; L4-S1)

In sensory system, there are how many neurons?

3

In sensory system, what are the neurons called?

1. Primary neuron

2. Secondary neuron

3. Tertiary neuron

In motor system, how many neurons?

2

In motor system, what are the neurons called?

1. Corticospinal (upper motor) neuron

2. α-motor (lower motor) neuron

What are the 3 long tracts called?

1. Dorsal column-medial lemniscal tract => sensory

2. Spinothalamic tract (aka. anterolateral system) => sensory

3. Corticospinal tract (aka. pyramidal tract) => motor

Dorsal column-medial lemniscal tract

1. Begin in skin (sensory information: Touch).

2. Cell body of 1st order neuron (primary) is located in dorsal root ganglion (DRG) outside CNS.

3. Axons ascend to caudal medulla. This is ipsilateral (on same side of body).

4. 2nd order neuron (secondary): caudal medulla. Axons cross midline and continues to thalamus.

5. 3rd order neuron (tertiary): Ventral posterior lateral (VPL) nucleus of thalamus. Sends its projections to somatosensory cortex.

6. Cortex has sensory representation of all sensory systems.

Spinothalamic tract (anterolateral system)

1. Sensory information is passed by this tract: Pain & Temperature.

2. 1st order neuron: also located in dorsal root ganglion (DRG).

3. Axon of 1st order neuron ascends ipsilaterally in same segment as spinal cord.

4. 2nd order neuron: dorsal horn of spinal cord (crosses midline here)

5. Axon of 2nd order neuron crosses midline & continues up to thalamus => contralateral.

6. 3rd order neuron: Ventral posterior lateral (VPL) nucleus of thalamus.

7. Sends 3rd order neuron projections to somatosensory cortex.

Dorsal column-medial lemniscal tract and Spinothalamic tract

Different types of information but arrives at same area of cortex.

Corticospinal tract

1. Begins in cortex => 1st neuron: Corticospinal (upper motor) neuron.

   *Remember: only 2 neurons in motor tract.

2. Crosses midline in same area Dorsal column-medial lemniscal tract cross: caudal medulla.

3. Continues contralaterally to spinal cord => 2nd neuron: α-motor (lower motor) neuron.

4. Then, innervated skeletal muscle.

All 3 Long tracts are:

In the same spinal nerve on both sides of spinal cord

31 pairs mean:

31 nerves on each side of spinal cord = 62 nerves in the body total. Each nerve is organized with the 3 long tracts.

Sensory information of dorsal solemn-medial lemniscal tract

Touch

Sensory information of Spinothalamic tract

Pain & temperature

Corticospinal tract is a:

Motor pathway

Right body is represented:

Contralaterally in the left cortex

Left body is respresented:

Contralaterally in the right cortex

Both sensory & motor (3 long tracts) eventually:

Crosses midline

In Dorsal column-medial lemniscal tract:

There are also neurons that express different types of neurons that are not sensitive to touch. They are sensitive to pressure.

Dorsal column-medial lemniscal tract: Proprioception

Body’s ability to sense its own position, movement, and balance — sometimes called the “sixth sense.”

• Along with vibrations

Ex: With closed eyes, you can tell where is your arms and legs are.

In summary, Dorsal column-medial lemniscal tract =>

Vibration, proprioception, touch

(Vid: 1:13:00)

Sensory input from the face is communicated by:

Trigeminal nerve (CN V)

The sensory information that is passed through axon of CN (V) is:

The same: pain, temperature, vibration, pressure, touch, and proprioception (like jaw position). Sensory information is sent by different axons to the CNS.

Ex: With closed eyes, you can determine the position of you jaw: if it’s closed or open.

Trigeminal system (CN V) pathway

• Main sensory nucleus of the middle pons. Receives projections from large diameter neurons, which are associated with receptors which are sensitive to touch. 2nd order neurons are located in main sensory nucleus (in middle pons).

• Small diameter neurons pass pain and temperature information. Located in spinal nucleus. 2nd order neurons cross midline, go up to thalamus (ventral trigeminal-thalamic pathway), then VPM (Ventroposterior medial of thalamus).

  • *VPL => dorsal column-medial lemniscal or Spinothalamic

  • *VPM => Trigeminal

• 3rd order nuerons: send projections to face region of somatosensory cortex.

• Somatosensory cortex: there is a region which is dedicated to analysis of sensory information (touch, pain, temperature) coming from face.

Corticospinal (pyramidal) tract:

Control voluntary movements (motor)

Corticospinal (pyramidal) tract

1. Begins in cortex

2. Goes down to innervate α-motor neurons

Lumbar spinal cord: Dorsal horn:

Sensory axons & 2nd order neurons of the Spinothalamic tract

Lumbar spinal cord: Ventral horn:

α-motor neurons

Lumbar spinal cord: Intermediate region:

Preganglionic neurons of sympathetic nervous system (SNS), located between T1 and L3

Lumbar spinal cord structures

• Gracile Fasciculus (DC/ML) - lower body

• Lateral corticospinal tract

• Spinothalamic tract

What is not located in the Lumbar Spinal Cord?

Cuneate Fasciculus (Instead, it is located up in the cervical spinal cord)

Cervical spinal cord structures

• Cuneate Fasciculus (DC/ML)

• Gracile Fasciculus (DC/ML) - lower body

• Lateral corticospinal tract

• Spinothalamic tract

Lesions produce:

Characteristic deficits AT AND BELOW the level of the lesion

Lesion of the left hemisection: Touch

Ipsilateral => loss of touch sensation at the level of injury and below

Contralateral => NO loss of touch sensation

Lesion of left hemisection: Motor Control

Ipsilateral => loss of motor control at the level of lesion and below

Contralateral => NO loss of motor control

Lesion of left hemisection: Pain & Temp

Ipsilateral => Loss of pain and temperature sensation only from one spinal cord segment

• Because 2nd order neurons crosses midline. Sensory information from 1 dermatome will disappear. All other signals still continue and not be interrupted.

Contralateral => Loss of pain and sensation at level of injury and below

Lesion above the medulla:

Full loss of sensory information and motor control on the contralateral side

Caudal Medulla

• 2nd order neuron of the Dorsal Column-Medial Lemniscus (DC-ML) tract (touch information)

• Level of brainstem where DC-ML crosses midline

• Level where Corticospinal tract (Motor information) also across the midline

Where is decussation for the Spinothalamic tract? (Decision = crosses)

In same segment of spinal cord

Rostral Medulla

Pain and temperature information, not touch

Structures of Lumbar Spinal Cord (*ON EXAM*)

• Gracile Fasciculus (DC/ML) - lower body

• Lateral Corticospinal tract

• Spinothalamic tract

Structures of Cervical Spinal Cord (*ON EXAM*)

• Gracile Fasciculus (DC/ML) - lower body

• Cuneate Fasciculus (DC/ML) - upper body

• Lateral Corticospinal tract

• Spinothalamic tract

Structures of Middle Medulla (*ON EXAM*)

• Spinal trigeminal nerve & tract (CN V)

• Corticospinal tract

• Medial Lemniscus

• Hypoglossal nerve (CN XII)