somatic nervous system - rnrausch.comrnrausch.com/25x/pdf/252/chapter_15all_2x.pdf · somatic...

Chapter 15 – Sensory Pathways, Somatic Nervous System!

1!

Chapter 15!Neural Integration I: Sensory Pathways and the Somatic Nervous System!

2

SECTION 15-2!Sensory receptors connect our internal and external environments with the nervous system!


2!

3

Sensation and Receptors!

Transduction!•  Translation of stimulus into action potential!•  No transduction, no sensation!

Perception!•  Conscious awareness of sensation!

Senses!General senses!•  Temperature, pain, touch, pressure, vibration,

proprioception!•  Receptors distributed throughout body!•  Receptors relatively simple in structure!

4

Sensation!

Special senses!•  Olfaction, vision, gustation, equilibrium, audition!•  Receptors found in special sense organs!•  Receptors more complex in structure!

Receptor specificity = “Law of specific nerve energies” or “Modality specificity”!

•  You can’t see with your ears.!Specificity results from:!

•  Structure of receptor cell and/or!•  Presence of accessory structures!


3!

5

Specificity Examples!

1. Pain receptors are free nerve endings!•  Respond to tissue damage from:!

Pressure, chemicals, heat!•  i.e., simple structure; somewhat non-specific!

2. Photoreceptors!•  Respond to light!•  Complex receptor cells!•  Surrounded by specialized pigment and CT

cells!•  i.e., complex structure; very specific!

6

Receptive Field!

Receptive field = area monitored by a single receptor!•  Smaller receptive field → more precise

localization!e.g. on finger tips!

•  Larger receptive field → less precise localization!

e.g. on back!!“Two-point discrimination test”!


4!

7

Receptors and Receptive Fields Figure 15-2!

8

Receptor and Generator Potentials – 1!

Stimulus transduced into action potentials!•  Stimulus causes a change in membrane

potential of receptor!•  Change = receptor potential, a graded

potential!Stronger stimulus → larger change!

Can be de- or hyperpolarization!A graded depolarization in a neuronal receptor is

called generator potential.!The greater the generator potential, the more

frequently action potentials are generated.


5!

9

Receptor (Generator) Potential!

Guyton and Hall, 1996!

Generator (receptor) potential

10

Interpretation of Sensory Information!Labeled line:!

•  Specific sensory info sent from specific receptor in specific part of body to specific area of cortex!

•  Neuronal pathway from receptor to cortex is the “labeled line”!

Cortex interprets:!1. Type of stimulus based upon labeled line!2. Location of stimulus based upon where in

cortex labeled line arrives (homunculus)!3. Intensity, duration of stimulus based upon

action potential frequency and pattern!


6!

11

General Types of Receptors!Rapidly-adapting (phasic) receptors!

•  Reduce firing rate during constant stimulation•  Fire rapidly at the beginning and end of

stimulation!•  Conveys information about changes in stimulus

intensity!•  E.g. Pacinian corpuscle, thermoreceptor!

Slowly-adapting (tonic) receptors!•  Fire at some level all the time!•  Continue firing during constant stimulation•  Conveys information about the duration of the

stimulus!•  E.g. pain receptors, muscle spindles!

12

Tonic and Phasic Receptors Figure 15-3!

Tonic Receptors!•  Slow adapting•  Pain receptors!•  Muscle spindle!•  Merkel disc!

Phasic Receptors!•  Rapid adapting•  Pacinian corpuscle!•  Thermoreceptor!•  Meissner’s corpuscle!


7!

13

Pacinian Corpuscle – Rapid Peripheral Adaptation!

14

Adaptation to a Stimulus!

Adaptation = ↓ sensitivity to constant stimulus1. Peripheral adaptation!

•  Receptor sensitivity changes!•  ↓ amount of info reaching CNS!

2. Central adaptation!Occurs within CNS, along sensory pathways!

A. Subconscious adaptation - e.g. olfaction!•  Brain signals inhibitory cells in olfactory bulb!

B.  Conscious adaptation!e.g., “tuning out” noise during an exam!


8!

15

SECTION 15-3!General sensory receptors can be classified by the type of stimulus that excites them!

16

General Senses – Receptors!

•  Exteroceptors!•  Interoceptors!•  Proprioceptors!

Note: There are no proprioceptors in visceral organs of thoracic and abdominopelvic cavities. !Why is this a good thing?!


9!

17

Nociceptors (Pain Receptors)!

Slow-adapting Few are in deep tissues or viscera!Are free nerve endings of sensory neurons!Have large receptive fields!Subtypes based on sensitivity to:!

1.  Extreme temperature!2.  Mechanical damage!3.  Chemicals!4.  https://www.eurekalert.org/pub_releases/2016-11/ucl-

pss111016.php !A strong stimulus can excite all three types!

Neurotransmitters: Glutamate, Substance P (for pain)!

18

Nociceptors – 2 – Fiber Types!

1. Type A fibers!•  Myelinated - “fast pain” or prickling pain!•  Relayed to primary sensory cortex

Conscious, localized pain!e.g. deep cut, pin prick!

2. Type C fibers!•  Unmyelinated - “slow pain” = burning, aching!•  Relayed to thalamus and reticular formation

Conscious, but not precisely localized!e.g. uterine cramps!


10!

19

Nociceptors – 3!

Central adaptation to pain!•  May decrease perception of pain!

e.g. endorphins, enkephalins prevent release of substance P!

Part of the mechanism of pain!1.  Tissue damage →!2.  Arachadonic acid released from cell membranes!3.  Converted to prostaglandins by cyclo-oxygenase!4.  Prostaglandins stimulate pain receptors!

(Aspirin, Ibuprofen - block cyclo-oxygenase)!5. Info ascends in cord via lateral spinothalamic tract!

20

Thermoreceptors!

Are free nerve endings!Phasic (fast-adapting)!

•  But background firing rate depends on temperature!

There are 3 to 4 times more cold than warm receptors!

Information ascends in lateral spinothalamic tract (with pain information)!


11!

21

Thermoreceptors – 2 Handout!

Kandel, Schwartz and!Jessell, 2000!

Cold fiber!

Warm fiber!

Temperature!

Constant !temperature!

Changing temperature!

22

Tactile Receptors in the Skin Figure 15-4!


12!

23

Mechanoreceptors!

Have mechanically-gated ion channels!A.  Tactile receptors!

Touch, pressure (deep touch), vibration!•  Fine touch and pressure!

Precisely localized - small receptive fields!(Posterior column pathway)!

•  Crude touch and pressure!Poorly localized - larger receptive fields!

(Anterior spinothalamic tract)!

24

Tactile (Touch and Pressure) Receptors – 1!1. Free nerve endings!

•  Between epithelial cells; in corneal epithelium!•  Touch and pressure!•  Tonic (slow-adapting)!•  Small receptive fields!

2. Root hair plexus!•  Dendrites surround hair root!•  Movement of hair!•  Phasic (fast-adapting)!


13!

25

Touch and Pressure Receptors – 2!

3. Merkel’s (tactile) discs!•  Dendrites contact Merkel (epidermal) cells!•  Fine touch and pressure - very sensitive!•  Tonic (slow adapting)!•  Small receptive field!

4.  Meissner’s (tactile) corpuscles!•  Papillary layer of dermis!•  Capsule = modified Schwann cells!•  Fine touch and pressure, low freq. vibration!•  Phasic (fast adapting)!

26


5. Pacinian (lamellated) corpuscles!•  Dermis, joint capsules, fasciae, serous

membranes, some viscera!•  CT capsule!•  Deep pressure, high freq. vibration!•  Phasic (fast adapting)!•  Large size → large receptive field!


14!

27


6. Ruffini corpuscles (end organs)!•  Reticular layer of skin!•  Capsule covers dendrites innervating

collagen fibers of skin!•  Senses stretch, distortion of dermis!•  Tonic (very slowly adapting - if at all)!

28

Mechanoreceptors – Baroreceptors!

B.  Baroreceptors!•  Monitor pressure (actually monitor stretch

due to fluid pressure)!•  Free nerve endings within elastic CT!•  Stretch/recoil → change in AP frequency!•  Phasic (fast adapting), respond to change!•  Some locations:!

Digestive tract, urinary bladder!Carotid sinus and aortic sinus!Lungs!


15!

29

Proprioceptors!

Do not adapt to constant stimulation•  Muscle spindles (chapter 13)!•  Golgi tendon organs (chapter 13)!

Free nerve endings!Sense tension in tendon!

•  Joint capsule receptors!Free nerve endings!Pressure, tension, movement of joint!

30

Chemoreceptors!

Respond to water- and lipid-soluble substances!I.e. must be dissolved to be sensed!

•  Adapt rapidly (seconds)!General chemosensation:!•  Send info to brain stem (not sensory cortex)!

e.g. CNS sensors in pons, medulla![H+], [CO2] in CSF!

e.g. Peripheral sensors![H+], [CO2], [O2] in blood!Info to brain via CN IX and X!


16!

31

SECTION 15-4!Separate pathways carry somatic and visceral sensory information!

32

Organization of Sensory Pathways!

1. First-order neuron – connected to receptor!•  Cell body in dorsal root ganglion or cranial nerve

ganglion!•  Synapses with second-order (interneuron) in brain

stem or cord!2. Second-order neuron !

•  Crosses over to contralateral side!•  Usually synapses with third-order neuron in thalamus!

3. Third-order neuron!•  Synapses in post-central gyrus (primary sensory

cortex)!•  Does not cross over!


17!

33

Sensory/Ascending Tracts/Pathways Fig. 15-5!

Labeled line:Neuronal pathway from receptor to cortex !

34

Posterior Column Pathway Spotlight 15-6c!

First-order!neuron!

Second-order!neuron!

Third-order!neuron!

•  Fine touch!•  Vibration!•  Pressure!•  Proprioception!(from contralateral!side of body)!

Medulla!

Midbrain!


18!

35

Posterior Column – Medial Lemniscus – 1!

Sensations!•  Fine (highly localized) touch, pressure,

vibration, conscious proprioception!Tracts!

•  Fasciculus gracilis! Lower half of body - trunk and lower limbs!

•  Fasciculus cuneatus! Upper half of body - neck, arms, chest!

36

Posterior Column – Medial Lemniscus - 2!

1. First-order neuron!•  Receptor → synapse in nucleus gracilis or

cuneatus in medulla!2. Second-order neuron!

•  Cross over to other side in medulla!•  Enter medial lemniscus (“ribbon”)!•  Synapse in thalamus!

3. Third-order neuron!•  Thalamus → somatosensory cortex!•  Thalamus processes info according to stimulus

type and location!


19!

37

Anterior Spinothalamic Tract Spotlight 15-6a!

Third-order!neuron!

Crude touch and pressure!(from other side of body)!!“crude” = poorly localized!


First-order!neuron!

Medulla

Thalamus

Midbrain

38

Lateral Spinothalamic Tract Figure 15-5b!

Pain and Temperature!(from other side of

body)!Third-order!

neuron!


First-order!neuron!

Medulla

Thalamus


20!

39

Anterior and Lateral Spinothalamic Tracts!

Sensations!A. Anterior spinothalamic tract!•  Crude (poorly-localized) touch and pressure!

!B. Lateral spinothalamic tract !•  Pain and temperature!

!!

40

Anterolateral Pathways!

BOTH Anterior and Lateral Pathways!1. First-order neurons!

•  Synapse in dorsal horn!2. Second-order neurons!

•  Cross over in spinal cord!•  Synapse in thalamus!

3. Third-order neurons!•  Synapse in primary sensory cortex!


21!

41

The Spinocerebellar Pathway Spotlight 15-6d!

First-order!neuron!


Unconscious proprioception!

!No third-order

neuron

42

Spinocerebellar Pathways!

Sensation!Unconscious proprioception (cerebellum!)!Input from Golgi tendon organs, muscle

spindles, joint capsules!Pathway!

1.  First-order neurons!•  Synapse in dorsal horn!

2. Second-order neurons!•  Synapse with Purkinje cells in cerebellum!

3. NO third-order neurons!


22!

43

Referred Pain!

Mechanisms of Pain!Cleveland Clinic Journal of Medicine, 74(1): 2007:!•  Pain of visceral origin is referred to somatic

regions that are innervated from the same spinal segments as the heart.!

•  The pain is generally referred to proximal, but not distal, somatic structures.!

•  The referred pain is experienced as deep (slow) pain.!

•  Slides 44, 45, and 46 are FYI only

44

Neuroscience at University of Texas!

1. Common dermatome hypothesis!•  Structures develop from same embryonic

segment (dermatome)!2. Convergence hypothesis!

•  Axons from visceral (heart) and somatic (skin) receptors synapse with same neurons in cord!

3. Facilitation hypothesis !•  Signals from viscera “facilitate” perception of

pain from skin by higher centers!4. Learned phenomenon hypothesis!

•  The brain is not “used to” receiving visceral pain signals so interprets them as being somatic!

FYI Only!


23!

45

Phantom Pain!1. Peripheral mechanism!

Damage → changes inputs to cord!•  “deafferentation” and neuroma growth!•  Increased Na+ channel synthesis!•  Increased sensitivity → pain!

2. Central mechanism!a. Central sensitization!•  New growth of proximal end of cut neuron!•  New growth of non-pain neurons into pain

areas of DRG!•  New connections - increased receptive field!

FYI Only!

46

Phantom Pain – 2!

2b. “Windup effect”!•  Increased (upregulation) of pain pathway receptors!

2c. Rewiring may cause loss of inhibitory signals from higher centers.!

3. Brain changes mechanism!a. “Cortical reorganization”!

•  Areas formerly responsible for amputated limb taken over by neurons from other areas!

b. Changes in “body schema”!•  Disconnect between intended and actual movements

upsets body schema!http://www.hindawi.com/journals/prt/2011/864605/!

FYI Only!


24!

47

Principal Sensory Pathways – 1 Table 15-1!

48

Principal Sensory Pathways – 2 Table 15-1!


25!

49

SECTION 15-5!The somatic nervous system is an efferent division that controls skeletal muscles!

50

Descending (Motor) Tracts Figure 15-8!

Note: Names indicate direction.!


26!

51

Motor Pathways!Pathways involve at least 2 neurons!1. Upper motor neuron (UMN)!

•  Cell body in CNS (e.g. primary motor cortex)!•  Synapses with lower motor neuron!

(Excitatory or inhibitory synapse)!•  Stays in CNS!

2. Lower motor neuron (LMN)!•  Cell body in brain stem or cord!•  Innervates skeletal muscle fibers of a motor

unit (Always excitatory synapse)!•  Called “Final Common Pathway”!

52

Corticospinal Pathways – 1!

A.K.A. “Pyramidal System” or “Direct Pathways”!Conscious control of skeletal muscle!

1. Corticobulbar tract!•  (“bulbar” refers to the brain stem)!•  UMN cell bodies in primary motor cortex!•  Synapse in cranial nerve nuclei•  Conscious control of skeletal muscle!

e.g. eye, jaw, facial muscle movements!


27!

53

Corticospinal Pathways – 2!

2.  Lateral corticospinal tract !•  (85% of motor corticospinal pathway neurons)!•  UMN in primary motor cortex!

Cross over at decussation of pyramids in medulla!

•  Synapse on LMN in cord!•  Conscious control of skeletal muscle!

54

Corticospinal Pathways - 3!

3.  Anterior corticospinal tract!•  15% of corticospinal neurons!•  UMN in primary motor cortex!•  Synapses on LMN in cord!•  Cross over in spinal cord (not medulla)!•  Conscious control of skeletal muscle!


28!

55

The Corticospinal Pathway Figure 15-9!

Anterior corticospinal tract ≈ 15%Lateral corticospinal tract ≈ 85%

Decussation of the pyramids

Parts of Corticobulbar

tract

Final Common Pathway

56

Principal Motor Pathways Table 15-2!

somatic nervous system - rnrausch.comrnrausch.com/25x/pdf/252/chapter_15all_2x.pdf · somatic...

Documents