PHS 3105 - Ch 15

Chapter 15 - Sensory Pathways and the Somatic Nervous System

Sensory pathways

A series of neurons that relay sensory information from sensory receptors to the CNS

Sensory receptors

Specialized cells or neuron processes that monitor specific conditions inside or outside the body

• Stimulated: generates action potential that are sent along sensory pathway

Afferent division (of NS)

Somatic and visceral sensory pathways

• Somatic sensory: Information to the cerebral cortex

• Visceral sensory: Information to the brainstem and diencephalon

Efferent division (of the NS)

Somatic motor pathways (control peripheral effectors)

• Motor commands: From motor in brain along somatic motor pathway

  • Modified by higher-order function in brain

    • Effectors are skeletal muscles

What is… Sensation

Sesnroy information arriving in the CNS

What is… Perception

Conscious awareness of a sensation

What is… Transduction

Conversion of an arriving stimulus into an action potential by a sensory receptor

What are… General senses

• Temperature

• Pain

• Touch

• Pressure

• Vibration

  • Proprioception (body position)

What are… Special senses

• Olfaction (smell)

• Gustation (taste)

• Vision (sight)

• Equilibrium (balnace)

• Hearing

Special sensory receptors

Provide sensations of the special senses.

• Location: Sense organs such as the eye or ear

Detection of stimuli

• Receptor specifitiy

• Receptive field

• Receptor specificity

  • Each receptor has a characteristic sensitivity

• Receptive field

  • The area monitored by a single receptor cell

  • The larger the field, the more difficult it is to localize a stimulus

    • Ex/ Backside v. tip of finger

Receptor potential (Detection of stimuli)

The stimulus changes the receptor membran potential

• Depolarizing: Generator potential, brings membrane closer to threshold

• Hyperholarizing: Brings membrane further away from threshold

• Size of receptor potential depends on strength of stimulus

Tonic receptors (Sensory receptors)

Always active

• Increase or decrease in stimulation causes increase or decrease in frequency of action potentials

Phasic receptors (Sensory receptors)

Normally inactive, only get activated by stimulus

• Provide information on intensity and rate of change of stimulus

What is… Adaptation

(+ 2 types of adaptation)

• Reduction of receptor sensitivity in the presence of a constant stimulus.

  • NS quickly adapts to painless, constant stimuli

• Most incoming sensory information is processed in centers along the spinal cord, brain stem, or thalamus before it reaches the cerebral cortex

  • Conscious and subconscious

  • Ex/ Tuning out background noise/listening carefully

• Fast-adapting receptors

• Slow-adapting receptors

Fast-adapting receptors (Adaptation)

• Respond strongly (intense) at first but then activity decreases (phasic receptors)

  • Ex/ Temperature

• Needs strong stimulus, responds and decreases very quick

Slow-adapting receptors (Adaptation)

• Show little peripheral adaptation (tonic receptors)

  • Ex/ Pain

Identify the types of sensory receptors

• Exteroreceptors

• Proprioreceptors

• Interoreceptors

• Nociceptors

• Termoreceptors

• Mecanoreceptors

• Chemoreceptors

Exoreceptors

Provide information about the external environment

Proprioceptors (Mechanoreceptors)

Provide information about the position of skeletal muscles and joints

• Only a somatic sensation, NONE in visceral organs

• Types of proprioceptors:

  • Muscle spindles: Monitor skeletal muscle length and trigger stretch reflexes

  • Golgi tendon organs: At the junction between skeletal muscle and its tendon and monitor tension during muscle contraction

  • Receptors in joint capsules: Free nerve endings that detect pressure, tension, and movement at the joint

Ex/ What butt-cheek you’re sitting on has more weight put on it.😁

Interoreceptors

Provide information about visceral organs and functions

Ex/ Knowing when you need pee

Nociceptors

Detect pain

• Free nerve endings w/ large receptive fields

• Tonic receptors

  • Location: Superficial skin, joint capsules, periosteum of bones, around walls of blood vessels

• Ex/ Temperature extremes (burning/freezing), mechanical damage, or dissolved chemicals (released by injured cells)

• Endorphins and enkephalins inhibit pain pathways in the CNS

Thermoreceptors

Detect temperature

• Phasic receptor

  • Location: Dermis, skeletal muscles, liver, and hypothalamus

Mechanoreceptors ★

(+ 3 classes)

Detect physical stimuli that distortion their plasma membranes

• Contain mechanically-gated ion channels

• Classes

  • Tactile receptors

  • Baroreceptors

  • Proprioceptors

Ex/ Touch, stretching, compression, twisting

Chemoreceptors

Detect chemical concentration. Respond to water-soluble and lipid-soluble substances that are dissolved in body fluids

• Fast peripheral adaptation

• Monitor pH, carbon dioxide, and oxygen levels in arterial blood inside body

• Carotid bodies: Near the origin of the internal carotid arteries (CN, IX)

• Aortic bodies: Between the major branches of the aortic arch (CN, X)

• Medulla oblongata: Sensitive to changes in pH, O₂ and CO₂ in CSF

Ex/ Oxygen in body

Tactile receptors (Mechanoreceptors) ★

(+ 6 types of tactile receptors in skin)

Detect touch (shape or texture), pressure (degree of mechanical distortion), and vibration (pulsing pressure)

• Fine touch and pressure receptors provide detailed information about: source, location, shape, size, and direction

  • Extremely sensitive

  • Narrow receptive fields

• Crude touch and pressure receptors provide poor localization, little information about stimulus

  • Large receptive fields

    • Ex/ Backside

• Types of tactile receptors

  • Free nerve endings

  • Root hair plexus

  • Tactile discs

  • Bulbous corpuscles

  • Lamellar corpuscles

  • Tactile corpuscles

Baroreceptors (Mechanoreceptors)

Detect pressure changes in blood vessels and in portions of the digestive, respiratory, and urinary tracts

• Free nerve endings branch with elastic tissue in wall of distensible organ

  • Ex/ Blood vessel

• Respond immediately to change in pressure, adapt rapidly

• Baroreceptors in the body:

  • Carotid sinuses and aortic arch

  • Lung

  • Digestive tract

  • Colon

  • Bladder wall

Types of pain (2)

• Fast pain: Prickling pain

  • Carried by myelinated Type A fibers

  • Ex/ Injection or deep cut

• Slow pain: Burning and aching pain

  • Carried by unmyelinated Type C fibers

  • Become aware of pain only with a general idea of area affected

  • Ex/ Soreness

Sensory neurons (3)

1. First-order neuron

   • Delivers sensations from the periphery (receptors) to the CNS

   • Cell body located in a spinal or cranial nerve ganglion

2. Second-order neuron

   • Interneuron in the spinal cord or brainstem

   • Crosses to the opposite side of the CNS (decussation)

3. Third-order neuron

   • Neuron in the thalamus

Somatic sensory pathways

(+ 3 pathways)

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

1. Spinothalamic pathway

2. Posterior column pathway

3. Spinocerebellar pathway

Made up of symmetrical pairs of spinal tracts. All axons in tract share common origin and destination

☆: Like different highways. Right goes to left, left goes to right.

Sensory Pathways and Ascending Tracts in the Spinal Cord

A. —

1. —

2. —

B. —

3. —

4. —

C. —

5. —

6. —

A. Posterior Column Pathway

1. Gracile fasciculus

2. Cuneate fasciculus

B. Spinocerebellar Pathway

3. Posterior spinocerebellar tract

4. Anterior spinocerebllar tract

C. Spinothalamic Pathway

5. Lateral spinothalamic tract

6. Anterior spinothalamic tract

Spinothalamic pathway (Sensory pathway)

Carries sensations of crude touch, pressure, pain, and temperature

• Ability to detect location of stimulus depends on the thalamus sending information to appropriate area of somatosensory cortex

• Painful sensations that are not produced where they are not perceived to originate, may be felt

  • Ex/ Phantom limb syndrome

• Referred pain

• Tracts:

  • Anterior spinothalamic tract: Crude touch and pressure

  • Lateral spinothalamic tract: Pain and temperature

    • Ex/ Post-menopause hot flashes

☆: #-o.n. = #-order neuron

3-order neurons in the spinothalamic pathway

• First-order neurons

  • Enter spinal cord and synapse with 2nd-o.n. with posterior horns

• Second-order neurons

  • Cross to opposite side of the spinal cord and then ascend to synapse with 3rd-o.n.

• Third-order neurons

  • In the ventral nuclei of the thalamus

  • Sort and process sensations and then carry information to neurons in primary somatosensory cortex

Referred pain (Spinothalamic pathway)

Visceral pain can manifest as body surface pain

Ex/ Heart attack frequently felt as pain in the left arm

Anterior Spinothalamic Tract

☆: INFORMATIONAL DIAGRAM FLASHCARD

Lateral Spinothalamic Tract

☆: INFORMATIONAL DIAGRAM FLASHCARD

Posterior column pathway (Sensory pathways)

Carries sensations of fine touch, vibration, pressure, and proprioception (body positioning)

• Tract:

  • Left and right gracile fasciculus: Axons that carry sensation from the inferior half of the body and synapse in gracile nucleus of the medulla oblongata

  • Left and right cuneate fasciculus: Axons that carry sensation from the superior half of the body and synapse in cuneate nucleus of the medulla oblongata

  • Medial lemniscus: Axons of 2nd-o.n. after they decussate

☆: Drugs and alcohol numb this pathway

3-order neurons in the posterior column pathway

• First-order neuron

  • Reach the CNS and ascend grouped according to the region they innervate—synapse with 2nd-o.n. in the medulla oblongata

• Second-order neuron

  • Decussate in the brain stem and ascend to the thalamus to synapse with 3rd-o.n.

• Third-order neuron

  • In ventral nuclei of the thalamus

  • Sort and process sensations, then carry information to neurons in the primary somatosensory cortex

☆: #-o.n. = #-order neuron

Posterior Column Pathway

☆: INFORMATIONAL DIAGRAM FLASHCARD

Sensory homunculus (Sensory pathway)

Functional map of the primary somatosensory cortex (cerebrum)

• Corresponds with specific regions of the body

• Area devoted to a particular body region is proportional to the density of sensory neurons in that region, not to region’s size

• Ex/ Larger areas fro lips and tongue; smaller area for backside

Spinocerebellar pathway (Sensory pathway)★

Carries information about the position of muscles, tendons, and joints; coordination

• Tracts:

  • Posterior spinocerebellar tracts: Travel through inferior cerebellar peduncle

  • Anterior spinocerebellar tracts: Travel via superior cerebellar peduncle

3-order neurons in the spinocerebellar pathway

• First-order neurons

  • Reach the CNS and synapse with 2nd-o.n. in the posterior horn of the spinal cord

• Second-order neurons

  • Ascend to the cerebellum and often decussate twice (in spinal cord and cerebellum)

  • Information arrives in cerebellum; coordination (Purkinje cells or the cerebellar cortex) and does not reach our awareness

☆: #-o.n. = #-order neuron

Spinocerebellar Pathway

☆: INFORMATIONAL DIAGRAM FLASHCARD

Visceral sensory pathways (Sensory pathway)★

Information collected by interoceptors monitoring the visceral tissues and organs within the thoracic and abdominopelvic cavities

• ★ Interoceptors include…

  • Nociceptors

  • Baroreceptors

  • Thermoreceptors

  • Tacticle receptors

  • Chemoreceptors

3-order neurons in the visceral sensory pathways

• FIrst-order neurons

  • From the sensory portion of cranial nerves, V, VII, IX, and X and the posterior roots of spinal nerves T₁-L₂ and S₂-S₄

• Secon-order interneurons

  • Ascend within the spinothalamic pathway and deliver the information to the solitary nuclei of the medulla oblongata

  • Solitary nuclei: Extensive connections with cardiovascular and respiratory centers and the reticular formation

Ascending (Sensory) Pathways

★: KNOW THIS CHART FOR THE EXAM

Somatic nervous system (SNS)

Controls skeletal muscles

• Somatic motor pathways

  • Upper motor neuron

  • Lower motor neuron

Upper motor neuron (SNS)

Cell body lies in a CNS processing center

• Primary motor cortex or premotor cortex and axon synapses on lower motor neuron

Lower motor neuron (SNS)

Cell body lies in a nucleus of the brain stem or spinal cord

• Axon extends outside of the CNS to innervate a single motor unit in a skeletal muscle

Somatic motor pathways

1. —

2. —

3. —

Carry conscious and subconscious motor commands

• The basal nuclei and the cerebellum monitor and adjust these pathways

1. Corticospinal pathway

2. Medial pathway

3. Lateral pathway

Descending (Motor) Tracts in the Spinal Cord

A. —

1. —

2. —

A. Corticospinal pathway

1. Lateral corticospinal tract

2. Anterior corticospinal tract

Corticospinal pathway (Somatic motor pathways)

(Pyramidal system)

Upper motor neurons are the pyramidal cells of the primary motor cortex

• Axons descend into brainstem and spinal cord and synapse on lower motor neurons that control skeletal muscles

• Tracts:

  • Corticobulbar tracts

  • Lateral corticospinal tracts

  • Anterior corticospinal tracts

Corticobulbar tracts (Corticospinal pathway)

Axons of this tract synapse with lower motor neurons in the motor nuclei and cranial nervs III→VII, IX and XII

• Provide conscious control of movement of the eyes, jaw, face, and some muscles of neck and pharynx

• Innervate the motor centers of the medial and lateral pathways

Corticospinal Pathway

Sensory: Ascending

Premotor: From brain; down and out

Motor homunculus (Somatic motor pathways)

Functional map of the primary motor cortex

• Corresponds with specific regions of the body

• Proportions similar to those of sensory homunculus

• Medial pathway

  • Controls muscle tone and gross movements of the trunk and proximal limb muscles

• Lateral pathway

  • Control muscle tone and movements of the distal limb muscles that perform precise movements

Descending (Motor) Pathways

☆: INFORMATIONAL DIAGRAM FLASHCARD

Basal nuclei and the cerebellum (Somatic motor pathway)

Responsible for conscious or subconscious coordination and feedback control over muscle contractions

• Basal nuclei

  • Provide the background patterns of movement involved in voluntary motor activities

• Cerebellum

  • Monitors proprioceptive (body positioning) sensations, visual information from eyes and vestibular (balance) sensations from internal ear to adjust movement accordingly

☆ Damage to these = no voluntary motor activity/coordination