Sensory pathways/ somatic nervous system

Question 1

What are first order neurons?

Answer 1

Sensory neurons that delivers sensations to CNS

Question 2

What are second order neurons?

Answer 2

Interneuron in spinal cord or brainstem that receives information from first-order neuron
Crosses to opposite side of CNS (decussation)

Question 3

What are third order neurons ?

Answer 3

Neuron in thalamus that must receive information from second-order neuron
-For the sensation to reach our awareness

Question 4

1. What do somatic sensory pathways do?

2. What are the major somatic sensory pathways?

Answer 4

1.Carry sensory information from skin and muscles of body wall, head, neck, and limbs to CNS

2. Spinothalamic pathway
   Posterior column pathway
   Spinocerebellar pathway

Question 5

What does the spinothalamic pathway do?

Answer 5

• Carries sensations of crude touch, pressure, pain, and temperature
• First-order neurons enter spinal cord and synapse within posterior horns
• Second-order neurons cross to opposite side of spinal cord before ascending
• Third-order neurons in ventral nuclei of thalamus
• After sorting and processing, sensations are sent to primary somatosensory cortex

Question 6

What are the two tracts of the spinothalamic pathway and what do they sense?

Answer 6

1.-Anterior spinothalamic tract
Crude touch and pressure
2. -Lateral spinothalamic tract
Pain and temperature

Question 7

What is the result of an abnormality in the spinothalamic pathway?

Answer 7

Painful sensations that are not produced where they are perceived to originate
Example: phantom limb syndrome (continued feeling of pain in amputated limb)

Question 8

What is referred pain and what is an example of it?

Answer 8

Feeling pain in an uninjured part of body when pain originates at another location
Visceral pain can manifest as pain in body surface
Example: a heart attack is frequently felt in the left arm

Question 9

1. What does the posterior column pathway do?

2. What spinal tracts are involved?

Answer 9

1.-Carries sensations of fine touch, vibration, pressure, and proprioception
2. -Spinal tracts involved
Left and right gracile fasciculus
Left and right cuneate fasciculus
-After second-order neurons of gracile and cuneate nuclei decussate,
Their axons enter the medial lemniscus (tract)

Question 10

1. Where do the second order neurons of the posterior column pathway synapse?
2. Where are they processed and how is localization determined?

Answer 10

1.
Second-order neurons synapse on third-order neurons in ventral nuclei of thalamus
Nuclei sort arriving information according to
Nature of stimulus
Region of body involved
2.
Processing in thalamus
Determines how a sensation is perceived
Localization of sensation depends on where it arrives in primary somatosensory cortex

Question 11

What is the sensory homunculus?

Answer 11

Functional map of primary somatosensory cortex
Area devoted to a particular body region is
Proportional to density of sensory neurons
Not proportional to region’s size

Question 12

What does the spinocerebellar pathway do?

Answer 12

• Conveys information about positions of muscles, tendons, and joints from spinal cord to cerebellum
• This information does not reach our awareness

Question 13

What kind of tracts are there in the spinocerebellar tract?

Answer 13

1.Posterior spinocerebellar tracts
Axons do not cross to opposite side of spinal cord
Travel through inferior cerebellar peduncle
2.Anterior spinocerebellar tracts
Sensations reach cerebellar cortex via superior cerebellar peduncle

-Many axons cross over twice
Once in spinal cord
Once in cerebellum

Question 14

How is the information in visceral sensory pathways collected?

What other kinds of interoreceptors are there?

Answer 14

1.
Visceral sensory information is collected by interoceptors monitoring visceral tissues and organs
(Primarily within thoracic and abdominopelvic cavities)

2.
Interoceptors include nociceptors, baroreceptors, thermoreceptors, tactile receptors, chemoreceptors
Not as numerous as in somatic tissues

Question 15

What nerves are included in the visceral sensory pathways?

Answer 15

Cranial nerves V, VII, IX, and X

Carry sensory information from mouth, palate, pharynx, larynx, trachea, esophagus, etc.

Question 16

What is a solitary nucleus?

Answer 16

• Large nucleus on each side of medulla oblongata
• Major processing and sorting center for visceral sensory information
• Extensive connections with cardiovascular and respiratory centers and reticular formation

Question 17

What does the somatic nervous system (SNS) control?

Answer 17

Contractions of skeletal muscles

Question 18

Somatic motor neurons involve which motor neurons?

Answer 18

upper motor neuron and lower motor neuron

Question 19

The upper motor neurons are found where and participate in what?

Answer 19

Cell body lies in a CNS processing center
Synapses on lower motor neuron
Activity may facilitate or inhibit lower motor neuron

Question 20

The lower motor neurons are found where and participate in what?

Answer 20

Cell body lies in a nucleus of brainstem or spinal cord
-Only the axon extends outside CNS
Innervates a single motor unit in a skeletal muscle
-Activation triggers a contraction in innervated muscle
-Damage eliminates voluntary and reflex control over innervated motor unit

Question 21

What do conscious and subconscious motor commands do?

What pathways do they travel over?

Answer 21

• Control skeletal muscles by traveling over three integrated motor pathways

Corticospinal pathway
Medial pathway
Lateral pathway

Question 22

What does the corticospinal pathway do and where does it begin?

What kind of descending tracts do they have?

Answer 22

-Provides voluntary control over skeletal muscles
-Begins at pyramidal cells of primary motor cortex
In which axons descend into the brainstem and spinal cord and synapse on lower motor neurons that control skeletal muscles.

Corticobulbar tracts
Lateral corticospinal tracts
Anterior corticospinal tracts

Question 23

Corticobulbar tracts allow what and innervate what?

Answer 23

• Allow conscious movement of eyes, jaw, face, and some muscles of neck and pharynx
• Innervate motor centers of medial and lateral pathways

Question 24

Corticospinal tracts synapse where? Are they visible?

Answer 24

• Axons synapse on lower motor neurons in anterior horns of spinal cord
• Yes, they are visible along anterior surface of medulla oblongata as a pair of thick bands, the pyramids.

Question 25

Where do lateral and anterior corticospinal tracts decussate?

Answer 25

Lateral-Contain axons that decussate at pyramids.

Anterior- Contain axons that cross over at targeted spinal segment in anterior white commissure.

Question 26

What is a motor homunculus?

Answer 26

-Functional map of primary motor cortex
-Corresponds with specific regions of the body
-Indicates degree of fine motor control available
Hands, face, and tongue appear large
Trunk is relatively small
-Proportions are similar to those of sensory homunculus

Question 27

What can the centers in cerebrum, diencephalon, and brainstem do in response to subconscious processing?

What pathways do they use?

Answer 27

-May issue somatic motor commands in response to subconscious processing

Medial pathway
Helps control gross movements of trunk and proximal limb muscles

Lateral pathway
Helps control distal limb muscles that perform precise movements

Question 28

What does the medial pathway control?

Where are the upper motor neurons located?

Answer 28

Controls muscle tone and gross movements of neck, trunk, and proximal limb muscles

Upper motor neurons are located in
Vestibular nuclei
Superior and inferior colliculi
Reticular formation

Question 29

Vestibular nuclei do what?

What do the descending fibers form?

Answer 29

• Receive information from vestibulocochlear nerve (VIII) regarding position and movement of head
• Primary goal is to maintain posture and balance
• Descending fibers form vestibulospinal tracts

Question 30

Superior and inferior colliculi are located where?

What does the superior and inferior colliculi receive?

Answer 30

Located in tectum (roof of midbrain)

• Superior colliculi receive visual sensations
• Inferior colliculi receive auditory sensations

Question 31

Where do the axons of the upper motor neurons descend too?

superior/inferior colliculi

Answer 31

Axons of upper motor neurons descend in tectospinal tracts

-Decussate immediately, before descending to synapse on lower motor neurons

Question 32

Reticular formation is what?

where do the axons of the upper motor neurons descend too?

Answer 32

-Loosely organized network of neurons that extends throughout brainstem.

• Axons of upper motor neurons descend into medial and lateral reticulospinal tracts
• Without crossing to opposite side

Question 33

Lateral pathways do what?

Where do the axons of the upper motor neurons in red nuclei dessucate and descend into?

Answer 33

• Controls muscle tone and precise movements of distal parts of limbs
• Axons of upper motor neurons in red nuclei decussate in brain
• Descend into spinal cord in rubrospinal tracts

Question 34

What is the function of basal nuclei?

What do those axons do?

Answer 34

• Provide background patterns of movement involved in voluntary motor activities
• Some axons extend to premotor cortex, which directs activities of primary motor cortex
• Alters instructions carried by corticospinal tracts
• Other axons alter excitatory or inhibitory output of reticulospinal tracts

Question 35

What does the cerebellum monitor?

Answer 35

• Proprioceptive (position) sensations
• Visual information from eyes
• Vestibular (balance) sensations from internal ear

Patterns of cerebellar activity are learned by trial and error over many repetitions
Fine-tuning of complex movements improves with practice