Sensory pathways/ somatic nervous system Flashcards
What are first order neurons?
Sensory neurons that delivers sensations to CNS
What are second order neurons?
Interneuron in spinal cord or brainstem that receives information from first-order neuron
Crosses to opposite side of CNS (decussation)
What are third order neurons ?
Neuron in thalamus that must receive information from second-order neuron
-For the sensation to reach our awareness
- What do somatic sensory pathways do?
2. What are the major somatic sensory pathways?
1.Carry sensory information from skin and muscles of body wall, head, neck, and limbs to CNS
- Spinothalamic pathway
Posterior column pathway
Spinocerebellar pathway
What does the spinothalamic pathway do?
- 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
What are the two tracts of the spinothalamic pathway and what do they sense?
1.-Anterior spinothalamic tract
Crude touch and pressure
2. -Lateral spinothalamic tract
Pain and temperature
What is the result of an abnormality in the spinothalamic pathway?
Painful sensations that are not produced where they are perceived to originate
Example: phantom limb syndrome (continued feeling of pain in amputated limb)
What is referred pain and what is an example of it?
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
- What does the posterior column pathway do?
2. What spinal tracts are involved?
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)
- Where do the second order neurons of the posterior column pathway synapse?
- Where are they processed and how is localization determined?
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
What is the sensory homunculus?
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
What does the spinocerebellar pathway do?
- Conveys information about positions of muscles, tendons, and joints from spinal cord to cerebellum
- This information does not reach our awareness
What kind of tracts are there in the spinocerebellar tract?
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
How is the information in visceral sensory pathways collected?
What other kinds of interoreceptors are there?
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
What nerves are included in the visceral sensory pathways?
Cranial nerves V, VII, IX, and X
Carry sensory information from mouth, palate, pharynx, larynx, trachea, esophagus, etc.
What is a solitary nucleus?
- 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
What does the somatic nervous system (SNS) control?
Contractions of skeletal muscles
Somatic motor neurons involve which motor neurons?
upper motor neuron and lower motor neuron
The upper motor neurons are found where and participate in what?
Cell body lies in a CNS processing center
Synapses on lower motor neuron
Activity may facilitate or inhibit lower motor neuron
The lower motor neurons are found where and participate in what?
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
What do conscious and subconscious motor commands do?
What pathways do they travel over?
- Control skeletal muscles by traveling over three integrated motor pathways
Corticospinal pathway
Medial pathway
Lateral pathway
What does the corticospinal pathway do and where does it begin?
What kind of descending tracts do they have?
-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
Corticobulbar tracts allow what and innervate what?
- Allow conscious movement of eyes, jaw, face, and some muscles of neck and pharynx
- Innervate motor centers of medial and lateral pathways
Corticospinal tracts synapse where? Are they visible?
- 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.
Where do lateral and anterior corticospinal tracts decussate?
Lateral-Contain axons that decussate at pyramids.
Anterior- Contain axons that cross over at targeted spinal segment in anterior white commissure.
What is a motor homunculus?
-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
What can the centers in cerebrum, diencephalon, and brainstem do in response to subconscious processing?
What pathways do they use?
-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
What does the medial pathway control?
Where are the upper motor neurons located?
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
Vestibular nuclei do what?
What do the descending fibers form?
- 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
Superior and inferior colliculi are located where?
What does the superior and inferior colliculi receive?
Located in tectum (roof of midbrain)
- Superior colliculi receive visual sensations
- Inferior colliculi receive auditory sensations
Where do the axons of the upper motor neurons descend too?
superior/inferior colliculi
Axons of upper motor neurons descend in tectospinal tracts
-Decussate immediately, before descending to synapse on lower motor neurons
Reticular formation is what?
where do the axons of the upper motor neurons descend too?
-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
Lateral pathways do what?
Where do the axons of the upper motor neurons in red nuclei dessucate and descend into?
- 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
What is the function of basal nuclei?
What do those axons do?
- 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
What does the cerebellum monitor?
- 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
