Copyright © 2010 Pearson Education, Inc. Marieb Chapter 13 Part A PNS.

Copyright © 2010 Pearson Education, Inc.

Marieb Chapter 13 Part A PNS


Peripheral Nervous System (PNS)

• All neural structures outside the brain

• Sensory receptors

• Peripheral nerves and associated ganglia

• Motor endings

Copyright © 2010 Pearson Education, Inc. Figure 13.1

Central nervous system (CNS) Peripheral nervous system (PNS)

Motor (efferent) divisionSensory (afferent)division

Somatic nervoussystem

Autonomic nervoussystem (ANS)

Sympatheticdivision

Parasympatheticdivision


Sensory Receptors

• Specialized to respond to changes in their environment (stimuli)

• Activation results in graded potentials that trigger action potentials

• Sensation (awareness of stimulus) and perception (interpretation of the meaning of the stimulus) occur in the brain


Classification of Sensory Receptors

• Based on:

• Stimulus type

• Location

• Structural complexity


Classification by Structural Complexity

1. Complex receptors (special senses)

• Vision, hearing, equilibrium, smell, and taste (Chapter 15; we won’t cover these)

• More than one type of cell that work together

2. Simple receptors for general senses:

• Tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sensation

• Unencapsulated (free) or encapsulated dendrites as sensors


Unencapsulated Dendrites

• Thermoreceptors

• Cold receptors (10–40ºC);more numerous, in superficial

dermis

• Heat receptors (32–48ºC); in deeper dermis

• Also located in muscles, liver,

hypothalamus, etc.


Unencapsulated Dendritic Endings

• Nociceptors (PAIN receptors!)• Respond to:

• Pinching/mechanical force

• Chemicals from damaged tissue (inflammation chemicals)

• Temperatures outside the range of thermoreceptors (extremes)

• Other chemicals [Capsaicin (hot pepper extract)]

• Located in skin, periosteum, joint capsules, tendons, meninges, blood vessel walls, etc.


Unencapsulated Dendrites

• Light touch receptors

• Tactile (Merkel) discs

• Hair follicle receptors


Encapsulated Dendrites

• All are mechanoreceptors

• Meissner’s (tactile) corpuscles— touch

• Pacinian (lamellated) corpuscles—deep pressure and vibration

• Ruffini endings—deep continuous pressure

• Muscle spindles—muscle stretch

• Golgi tendon organs—stretch in tendons

• Joint kinesthetic receptors—stretch in articular capsules (a proprioceptor)


Classification by Stimulus Type

• Mechanoreceptors—respond to touch, pressure, vibration, stretch, and itch

• Thermoreceptors—sensitive to changes in temperature

• Photoreceptors—respond to light energy (e.g., retina)

• Chemoreceptors—respond to chemicals (e.g., smell, taste, changes in blood chemistry)

• Nociceptors—sensitive to pain-causing stimuli (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals)


Classification by Location

1. Exteroceptors

• Respond to stimuli arising outside the body

• Receptors in the skin for touch, pressure, pain, and temperature

• Most special sense organs in this class



2. Interoceptors (visceroceptors)

• Respond to stimuli arising in internal viscera and blood vessels

• Sensitive to chemical changes, tissue stretch, and temperature changes



3. Proprioceptors

• Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles

• Inform the cerebellum and cortex of our position in space


From Sensation to Perception

• Survival depends upon sensation and perception

• Sensation: the awareness of changes in the internal and external environment

• Perception: the conscious interpretation of those stimuli


Sensory Integration

• Input comes from exteroceptors, proprioceptors, and interoceptors

• Input is relayed toward the head, but is processed along the way


Sensory Integration

• The signal can be processed and altered at three different levels:

1. Receptor level—the sensor receptors

2. Circuit level—ascending pathways

3. Perceptual level—neuronal circuits in the cerebral cortex


1

2

3

Receptor level(sensory receptionand transmissionto CNS)

Circuit level(processing inascending pathways)

Spinalcord

Cerebellum

Reticularformation

Pons

Musclespindle

Jointkinestheticreceptor

Free nerveendings (pain,cold, warmth)

Medulla

Perceptual level (processing incortical sensory centers)

Motorcortex

Somatosensorycortex

Thalamus


Processing at the Receptor Level

• Stimulus energy is converted into a graded potential called a receptor potential (don’t pay attention to the term generator potential- only used with special senses)

• In general sense receptors, it works like this:

stimulus

receptor potential in afferent neuron

action potential at first node of Ranvier


Processing at the Receptor Level

• In special sense organs:

stimulus

receptor potential in receptor cell

release of neurotransmitter

generator potential in first-order sensory neuron

action potentials (if threshold is reached)


Adaptation of Sensory Receptors

• Adaptation is a change in sensitivity in the presence of a constant stimulus

• Receptor membranes become less responsive

• So the receptor potentials decline in frequency or stop

•Why does this happen? Is it a good thing?


Adaptation of Sensory Receptors

• Phasic (fast-adapting) receptors adapt

• Examples: receptors for pressure, touch, and smell

• Tonic receptors adapt very slowly or not at all

• Examples: nociceptors and most proprioceptors


Adaptation - What Happens to Signaling?


Processing at the Circuit Level

• Ascending pathways of three neurons conduct sensory impulses to the appropriate brain regions

• First-order neurons

• Conduct impulses from the receptor level to the second-order neurons in the CNS

• Second-order neurons

• Transmit impulses to the thalamus or cerebellum

• Third-order neurons

• Conduct impulses from the thalamus to the somatosensory cortex (perceptual level)


1

2

3

Receptor level(sensory receptionand transmissionto CNS)

Circuit level(processing inascending pathways)

Spinalcord

Cerebellum

Reticularformation

Pons

Musclespindle

Jointkinestheticreceptor

Free nerveendings (pain,cold, warmth)

Medulla

Perceptual level (processing incortical sensory centers)

Motorcortex

Somatosensorycortex

Thalamus


Perception of Pain• Definition: an unpleasant sensory and emotional

experience associated with actual or potential tissue damage

• Warns that you are “at the edge of a cliff!”

• Stimuli include:

• extreme pressure

• extreme temperature

• histamine, K+, ATP, acids, and bradykinin (chemicals released during inflammation)

• Some pain impulses are blocked by inhibitory endogenous opioids

• Is pain necessary?


Referred Pain

• Visceral pain afferent fibers travel along the same pathway as somatic pain fibers

• Referred Pain = pain stimuli arising in the viscera are perceived as somatic in origin

• Examples:


Referred Pain

Heart

Lungs anddiaphragmLiver

Stomach

Kidneys

OvariesSmall intestine

Ureters

Urinarybladder

Colon

Pancreas

Liver

Heart

Appendix

Gallbladder


Pain

• Does everyone have the same pain threshold?

• Does everyone have the same pain tolerance?


Pain

• Pain tolerance is influenced by many factors:

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•

•

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How Is Pain Processed?


Analgesia

• Defined as “ “

• Major Analgesics

•

•

• Other agents that can act as pain relievers

• •

•

•

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•


Classification of Nerves

• Peripheral nerves classified as cranial or spinal nerves

• Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers

(carry sensory + motor = mixed nerves)

• Pure sensory (afferent) or motor (efferent) nerves are rare (which cranial nerves are purely sensory nerves?)

• Types of fibers in mixed nerves:

• Somatic afferent and somatic efferent

• Visceral afferent and visceral efferent


Regeneration of Nerve Fibers

• Mature neurons can’t divide

• If the soma of a damaged nerve is intact, its axon will regenerate

• Involves coordinated activity among

• Macrophages

• Schwann cells

• Axons

• CNS oligodendrocytes bear growth-inhibiting proteins that prevent CNS fiber regeneration (UGH!)

Copyright © 2010 Pearson Education, Inc. Marieb Chapter 13 Part A PNS.

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