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The Peripheral Nervous System (Reflex Activity (reflexes = rapid,…
The Peripheral Nervous System
Classification
Classification by
STIMULUS TYPE
Photoreceptors
- detect light energy, only present in the retina of eye
Chemoreceptors
- respond to chemicals in solution (molecules smelled or tasted, or changes in blood or interstitial fluid chemistry) --> internal regulation (glucose/sodium level
Thermoreceptors
- detect changes in temperature
free dendrites/nerve endings in skin and internally, specialized cold and hot
Nocioreceptors
- respond to potentially damaging stimuli that results in pain
Mechanoreceptors
- detect mechanical pressure or stretching (e.g. stretching in walls of abdomen)
Classification by
RECEPTOR STRUCTURE
Simple Receptors
Free Dendritic Endings
Merkel Cells
associate with free dendritic endings to form a
tactile disc
light touch receptors, slow adapting
abundant in finger tips, lips, and external genitalia
Hair Follicle Receptor
in the root hair plexus/spirals around hair follicle
light touch receptor, activated when hair bends
esp. abundant in ET and CT, small diameter, non-mylenated
Encapsulated Dendritic Endings
Pacinian corpuscles (aka lamellar corpuscles)
consists of a single dendrite, surrounded by many layers of flattened Schwann cells and enclosed in a CT capsule
deep in dermis and subcutaneous tissue
responds to deep pressure and stretch, rapidly adapting
Ruffini's endings (aka bulbous copuscles
consist of a spray of dendritic endings enclosed by a flattened capsule, located in dermis and subcutaneous tissue
respond to deep and continuous pressure
Found in skin, joint capsules, tendons, ligaments
Krause's end bulbs
mechanoreceptors, located in mucous membranes
Muscles Spindles
found in all skeletal muscles, fine motor control areas have more
Meisnner's corpuscles
found in the dermal papillae, especially of hairless skin (nipples, fingertips, soles of feet)
rapidly adapting light touch receptors - activated at the onset of a touch
Golgi tendon organs
Found in skeletal muscle tendons
Deal with deep tendon reflexes
Joint Kinesthetic Receptors
Proprioceptors that monitor articular capsules and provide joint movement info
Complex Receptors
sense organs
Classification by
LOCATION
Interoceptors
- located in walls of the viscera and blood vessels
detect stimuli inside the body e.g. glucose/blood chemistry, BP
Proprioceptors
- located in skeletal muscles, tendons, joints, ligaments, and CT coverings of bones and muscles
body position, muscle tension, angle and movement at joints
Exteroceptors
- most are at or near the body surface
detect stimuli that arrive outside the body e.g. touch in the skin, sound waves, light
Regeneration of Nerve Axons
CNS axons do not regenerate
oligodendrocytes (supporting cells of CNS) actively suppress CNS axon regeneration
studded with growth-inhibiting proteins
astrocytes at the site of injury form scar tissue that blocks axon regrowth
PNS axons can regenerate successfully
1) the cut axon ends seal themselves off - axon transport is interrupted causing the cut ends to swell
without access to the cell body, the axon (and its myelin sheath) begins to disintegrate distal to the injury = Wallerian degeneration
2)Schwann cells and macrophages clean out the dead axon distal to the injury
3) Axon filaments grow through a regeneration tube - Schwann cells line up along the tube of the remaining endoneurium, --> guides the regenerating axons across the gap to their original contacts
Schwann cells release growth factors and express cell adhesion molecules that encourage axon growth
4) The axon regenerates and a new myelin sheet forms
Sensory Integration
1) Receptor Level
The stimulus has to match the
specificity
of a receptor, and the stimulus has to be applied in the sensory receptor's
receptive field
The stimulus energy must be converted into the energy of a graded potential through
transduction
Receptor potential is usually a depolarizing potential, which results from an increased influx of Na or hyperpolarization, which is an increased efflux of K or influx of Cl
Generator Potential
- When the receptor is part of the sensory neuron
Receptor Potential
- when the receptor is a separate cell
2) Circuit Level
processing in ascending pathways
3) Perceptual Level
processing in cortical sensory centers
ability to identify and appreciate sensations depends on the location of the target neurons in the sensory cortex, not of the nature of the message
Adaption
- a change in sensitivity (and nerve impulse generation) in the prescence of a constant stimulus
Tonic Receptors
- provide a sustained response with little or no adaption
e.g. nocioreceptors and most proprioceptors (because of protective importance of their information)
Phasic Receptors
- fast adapting, giving bursts of impulses at the beginning and the end of the stimulus
e.g. Meissner's corpuscles (aka tactile), and Pacinian corpuscles (aka lamellar)
Reflex Activity
reflexes = rapid, automatic responses to a specific (threshold) stimuli, predictable and always the same
Somatic reflexes - activate skeletal muscles
Visceral reflexes - activate smooth/cardiac muscles or glands
Spinal reflexes - integration happens at level of spinal cord
Cranial reflexes - integration happens at cranial nerve nuclei
Reflex arc = the specific neural pathways/circuits involved in producing reflex behavior
1) Receptor - the site of stimulus action
2) Sensory neuron - transmits afferent impulses to the CNS
3) Integration center
may be
monosynaptic
(between motor and sensory neuron) or
polysynaptic
(synapses with an interneuron: sensory --> interneuron --> motor)
4) motor neuron - conducts efferent impulses form the integration center to an effector organ
5) Effector - muscle fiber or gland cell that responds to an efferent impulse (by contracting or secreting)
Stretch Reflex - monosynaptic, Stretching a muscle causes reflexive contraction
Receptor: intrafusal fibers of muscle spindle
Sensory neuron: associated neurons
Integration center: spinal cord
Motor neuron: alpha motor fibers
Effector: muscles
muscles spindles
alpha motor fibers - innervate extrafusal fibers (contracting fibers of muscle itself)
Gamma efferent fibers - innervate intrafusal fibers in contracting regions at ends of muscle spindle
Deep tendon reflex - polysynaptic, Increased muscle tension causes relaxation and lengthening
Golgi Tendon Oranges
Biceps: C5-C6, Triceps: C6-C7, Brachioradialis: C5-C6, Patella (stretch reflex): L2-L4, Calcaneal/Achilles: S1-S2