peripheral nervous system & reflex activity part a prepared by janice meeking & w. rose....
TRANSCRIPT
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)