Peripheral Nervous System & Reflex Activity

Part A

Prepared by Janice Meeking & W. Rose.Figures from Marieb & Hoehn 8th , 9th ed.

Portions copyright Pearson Education

Figure 13.1

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

Motor (efferent) divisionSensory (afferent)division

Somatic nervoussystem

Autonomic nervoussystem (ANS)

Sympatheticdivision

Parasympatheticdivision

Peripheral Nervous System (PNS)All neural structures outside the brain• Sensory receptors• Peripheral nerves and associated ganglia• Motor endings

Sensory ReceptorsSpecialized structures which respond to changes in

their environment (stimuli)Some receptors are simply ends of sensory nerve fibers.

Other receptors are cells adjacent to sensory nerrve fibers. Other receptors are sensory nerve fiber endings plus specialized supporting cells and/or extracellular material

Receptor activation results in graded potentials that may trigger action potentials

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

Classifying receptorsReceptors have been classified according to:

– The type of stimulus they detect (best)– Receptor location in the body– Structural complexity of the receptor

Classification by Stimulus TypeMechanoreceptors—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 LocationExteroceptors

Respond to stimuli arising outside the body: receptors in the skin for touch, pressure, pain, and temperature; also most special sense organs (eyes, ears, etc)

Interoceptors (visceroceptors)

Respond to stimuli arising in internal viscera and blood vessels: chemical environment, tissue stretch, temperature

Proprioceptors

Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles; inform the brain of one’s movements

Classification by Structural ComplexityComplex receptors: Special sense organs

Vision, hearing, equilibrium, smell, taste (ch. 15)

Simple receptors: General sensationTactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense

Unencapsulated (free) dendritic endings

Encapsulated dendritic endings

Table 13.1

Unencapsulated (free) dendritic endings

Table 13.1

Encapsulated dendritic endings

From Sensation to Perception• Survival depends upon sensation and

perception

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

• Perception: conscious interpretation of those stimuli

Figure 13.2

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•Different receptors respond to different stimuli

Light, pressure, temperature, chemicals, etc.

•Receptive field Physical region over which each receptor can detect stimuli

•Transduction occursStimulus energy is converted into a graded potential called a receptor potential

Receptor processing in general sense receptors (receptors in which there are not separate receptor cells)

Stimulus

Receptor Potential (graded potential) at distal end of afferent neuron

Action Potential(s) (if threshold is reached)

Receptor processing in special sense receptors

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 ReceptorsChange in response in the presence of a constant

stimulus• Receptor membranes become less responsive

• Receptor potentials decline in frequency or stop

Phasic (fast-adapting) receptors signal beginning or end of stimulus

• Examples: receptors for pressure, touch, smell

Tonic receptors adapt slowly or not at all• Examples: nociceptors; most proprioceptors

Processing at the Circuit Level• 3-neuron pathway conducts sensory impulses

upward to appropriate brain regions

• First-order neurons (cell body in DRG or other peripheral ganglion): conduct impulses from receptor to second-order neurons in the CNS

• Second-order neurons (in CNS): transmit impulses to thalamus or cerebellum

• Third-order (thalamic) neurons: conduct impulses from the thalamus to the somatosensory cortex (perceptual level)

Processing at the Perceptual Level• Identification of the sensation depends on the

specific location of the target neurons in the sensory cortex

• Aspects of sensory perception• Stimulus detection: requires multiple impulses

• Magnitude estimation: intensity coded by frequency of APs & number of neurons active

• “Spatial” discrimination: identify the site or pattern of stimulus (e.g. two-point discrimination test)

Further Processing at the Perceptual Level• Feature abstraction—identification of more

complex aspects and several stimulus properties

• Quality discrimination: identification of submodalities of a sensation (e.g., sweet or sour tastes)

• Pattern recognition: identification of familiar or significant patterns in stimuli (face, melody, etc.)

Figure 13.2

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

Nerve Structure• Bundle of myelinated and unmyelinated

peripheral axons enclosed by connective tissue

• Connective tissue coverings, from inside to outside:– Endoneurium, perineurium, epineurium

Figure 13.3b

Bloodvessels

Fascicle

Epineurium

Perineurium

Endoneurium

AxonMyelin sheath

(b)

Classification of Nerves• Most nerves are mixed: afferent and

efferent fibers and somatic and autonomic (visceral) fibers

• Pure sensory (afferent) or motor (efferent) nerves are rare

• Cranial and spinal nerves (12 pair cranial, Roman numerals; 31 pair spinal, named for the nearby vertebra, e.g. C5 or L4)

Ganglion (plural: ganglia) • A group of neuron cell bodies outside

the CNS (analogous to nuclei inside the CNS)

• associated with nervesExamples• dorsal root ganglia (sensory, somatic;

ch. 12)• autonomic ganglia, such as sympathetic

trunk ganglia (motor, visceral; ch.14)