PNS Flashcards
By definition, what is a stimulus?
Changes in the environment
What types of stimuli are received and transduced by each of the following types of receptors: mechanoreceptors
- Respond to mechanical force such as touch (on skin surface), pressure (on skin surface and within organ walls, including blood pressure), vibration, and stretch (within organ walls)
- Helps with hearing and equilibrium
What types of stimuli are received and transduced by each of the following types of receptors:
thermoreceptors
- Respond to temperature changes
- Can detect internal temperatures
- Have free dendritic endings that can detect hot and cold
What types of stimuli are received and transduced by each of the following types of receptors:
nociceptors
- Respond to stimuli that could potentially cause mechanical or chemical damage to tissues
- In all tissues except brain
- Overstimulation of various receptor types can also be interpreted as pain by CNS
- Ex. searing heat, extreme cold, excessive pressure, and inflammatory chemicals
What types of stimuli are received and transduced by each of the following types of receptors:
photoreceptors
- Respond to light
- Only in retina
What types of stimuli are received and transduced by each of the following types of receptors:
chemoreceptors
- Respond to chemicals dissolved in body fluids (molecules smelled or tasted, or changes in blood or interstitial fluid chemistry)
- Ex. saliva, blood glucose levels
- Involved in homeostasis regulations
Describe the location and stimulus selectivity of exteroceptors
Location: at or near the body surface - in skin and mucous membranes and special sense organs
Stimulus: stimuli that arise outside the body; touch, pressure, pain, and temperature receptors as well as receptors of special senses (vision, hearing, equilibrium, smell, and taste)
Describe the location and stimulus selectivity of interoceptors
Location: in walls of the viscera and blood vessels
Stimulus: stimuli inside the body including chemical changes, tissue stretch, temperature, and homeostatic mechanisms; sometimes pain, discomfort, hunger, or thirst
Describe the location and stimulus selectivity of proprioceptors
Location: in skeletal muscles, tendons, joints, and ligaments and CT coverings of bone and muscles
Stimulus: respond to internal stimuli as well; provide info about body position, muscle tension, angle of joints and movements occurring at joints
Identify and describe the main difference between the two subcategories of simple receptors
Free dendritic endings
- Small diameter, unmyelinated nerve fibers
- Little knob-like bulbs on ends of dendrites (sensory terminals) that increase surface area for sensory input
Encapsulated dendritic endings
- Free dendritic endings surrounded by a CT capsule
- Virtually all encapsulated dendritic endings are mechanoreceptors, but they vary greatly in shape, size and distribution in the body
What is the function of free dendritic nerve endings? In what tissues is this type of receptor especially abundant?
- Respond to pain and temperature stimuli (nerve endings that respond to cold are located in superficial dermis and nerve endings that respond to heat are located in deeper dermis), some can respond to pressure, itch, tickling, etc
- Especially abundant in CT and ET
Identify and describe two specialized types of free dendritic endings.
Tactile/Merkel discs
- In superficial structure of dermis
- Merkel cell associates with a free dendritic ending to form tactile/merkel disc
- Light touch receptors
- Slow adapting
- Esp abundant in fingertips, lips, palm of hands, and external genitalia
Hair follicle receptor/Root hair plexus
- Spirals around outside of hair follicle
- Light touch receptors - detects movement/bending of hair follicles
Locate and describe the structure and function of the encapsulated dendritic endings: Meissner’s corpuscles (tactile corpuscles)
Structure
- Small receptors in which a few spiraling sensory terminals are surrounded by Schwann cells and then by a thin egg-shaped CT capsule
Function
- Receptors for discriminative touch and play role in sensing light touch in hairless skin
Location
- Dermal papillae and especially numerous in sensitive and hairless skin areas (e.g. nipples, fingertips, and soles of the feet)
Locate and describe the structure and function of the encapsulated dendritic endings: Krause’s end bulbs
- Mechanoreceptors
- Located in mucous membranes
Locate and describe the structure and function of the encapsulated dendritic endings: Pacinian corpuscles (lamellar corpuscles)
Structure
- A single dendrite surrounded by many layers of flattened Schwann cells and enclosed in a CT capsule
- Relatively large
Function
- Mechanoreceptor stimulated by deep pressure and stretch, only respond when the pressure is first applied so they are best suited to monitoring vibration
Location
- Scattered deep in dermis and subcutaneous tissue
Locate and describe the structure and function of the encapsulated dendritic endings: Ruffini’s endings (bulbous corpuscles)
Structure
- Spray of receptor endings enclosed by a flattened capsule
- Spindle appearance
Function
- Respond to deep and continuous pressure
Location
- In dermis, subcutaneous tissue, joint capsules, tendons, and ligaments
Locate and describe the structure and function of the encapsulated dendritic endings: muscle spindles
Structure
- Spindle shaped
- Bundle of modified skeletal muscle fibers (intrafusal fibers) enclosed in a CT capsule
Function
- Detect muscle stretch and initiate a reflex that resists the stretch
Location
- All skeletal muscles
Locate and describe the structure and function of the encapsulated dendritic endings: golgi tendon organs (GTO)
Structure
- Consist of small bundles of tendon (collagen) fibers enclosed in a layered capsule with sensory terminals coiling between and around the fibers
Function
- Initiates a reflex that causes the contracting muscle that activated the GTO to relax
Location
- In tendons close to where the muscle tissue fibers end
Locate and describe the structure and function of the encapsulated dendritic endings: joint kinesthetic receptors
- Proprioceptors that monitor stretch in articular capsules that enclose synovial joints
- Contains lamellar corpuscles, bulbous corpuscles, free nerve endings, and receptors resembling GTO
- Together they provide info on joint position and motion
Describe the energy conversion that occurs in transduction
- Stimulus energy is converted into the energy of a graded potential
- The graded potential can be either depolarizing or hyperpolarizing (similar to EPSP and IPSP)
- Stimulus creates a graded potential in the receptor cell → altered membrane → most of the time leads to depolarization → influx of Na+, can lead to increase/decrease of NT → AP
What is a receptor potential? How does a receptor potential produce an AP in a first- order afferent neuron?
When receptor is a separate cell, the graded potential= receptor potential. This requires receptor potential and a Neurotransmitter (NT). The receptor potential changes the amount of NT released by the receptor cell onto the sensory neuron. The NT then generates graded potentials in the sensory neurons. Must reach threshold to open sodium channels on axon and nerve impulses to be generates and propagated to CNS.
What is a generator potential? How does a generator potential produce an AP in a first- order afferent neuron?
When the receptor is part of a sensory neuron (as with free dendrites or encapsulated receptors of most general sense receptors), the graded potential= generator potential. The entire process uses generator potential that can sum up to reach threshold and AP is fired.
Define the term receptor specificity.
Receptors are selective about the type of stimuli they respond to
Define the term receptive field.
The area the receptor monitors
What is sensory adaptation?
A change in sensitivity (and nerve impulse generation) in the presence of a constant stimulus
What is the difference between phasic and tonic receptors?
Phasic receptors
- Fast adapting
- Often give bursts of impulses at the beginning and the end of the stimulus
- Report changes in internal or external environment
- Provide sensations of pressure, touch, and smell
Ex. lamellar and tactile corpuscles
Tonic receptors
- Provide sustained response with little or no adaptation
- Nociceptors and most proprioceptors
- Slow adapting receptors that produce a repetitive discharge of APs in response to an unchanging stimulus
- Receptors involved in detecting pain, body position, and certain chemicals in the blood
Identify and describe the three main levels of neural integration in the somatosensory system.
Receptor level
- Sensory reception and transmission to CNS
- Stimulus matches specificity of the receptor and is in receptive field
- Stimulus energy is converted into the energy of a graded potential ← transduction
- Graded potential is either a generator
potential or a receptor potential
- Information about a stimulus’s strength, duration, and pattern are encoded in the frequency of the nerve impulses - the greater the frequency, the stronger the stimulus
Circuit level
- Processing in ascending pathways
- The task is to deliver impulses to the appropriate region of the cerebral cortex for localization and perception of the stimulus - first, second, and third-order neurons
Perceptual level
- Processing in cortical sensory areas
- The ability to identify and appreciate sensations depends on the location of the target neurons in the sensory cortex, not on the nature of the message
- Each sensory fiber is analogous to a “labeled line” that tells the brain “who” is calling (a taste bud, a pressure receptor, etc) and from “where”. The brain always interprets the activity of a specific sensory receptor (“who”) as a specific sensation, no matter how it is activated
Explain how the receptive field influences discriminative ability.
The smaller the receptive field, the greater the ability of the brain to accurately localize the stimulus site - if two stimuli are in the same receptive field, then they will be perceived as one stimulus whereas if two stimuli are in two different receptive fields, they will be perceived as two separate stimuli
Describe the discriminative sensation techniques utilized to test the ability of the sensory cortex to correlate, analyze and interpret sensations.
Stereogenesis
- Identify a familiar object (e.g. coin, paperclip) in the palm of your hand with your hands/fingers while your eyes are closed
Graphethesia
- Have the patient close their eyes and draw a number on the palm of their hand. Have them identify what the number is.
Two-point discrimination
- Have the patient close their eyes and use an opened paper clip and touch the points a finger pad area, alternating between single and double points to find if patient can distinguish 1 and 2 touches
Point localization
- eyes closed, touch a place on the patient’s skin, ask them to open eyes and point to where they felt the touch, useful on trunk and legs
- extinction- touch both sides of the body simultaneously, ask patient to point to areas where they felt touch
monofilament-LEAP test
- using the monofilament on the patient’s foot to detect neuropathies, prevent amputations
Identify and describe the connective tissue sheaths which are associated with a nerve.
Endoneurium
- A delicate layer of loose connective tissue that encloses each axon/nerve fiber and its associated Schwann cells
Perineurium
- Coarser CT wrapping that binds groups of axons into bundles called fascicles
Epineurium
- A tough fibrous sheath that encloses all the fascicles to form the nerve
What is a mixed nerve?
A nerve that contains both sensory and motor fibers and transmit impulses both to and from the central nervous system
What type of fibers is present in the dorsal root?
Sensory fibers (that arise from sensory neurons in the dorsal root ganglia and conduct impulses from peripheral receptors to the spinal cord)
What type of fibers is present in the ventral root?
Motor fibers (that arise from ventral horn motor neurons and extend to and innervate the skeletal muscles)
Name and identify the general function of each of the initial three branches of a spinal nerve.
Dorsal ramus
- Supply the posterior body trunk; mixed
Ventral ramus
- Supply the rest of the trunk and the limbs; mixed
Meningeal branch
- Reenters the vertebral canal to innervate the meninges and blood vessels within; mixed
Explain the difference between the roots and rami associated with the spinal cord.
Roots: lie medial to and from the spinal nerves. Each root is strictly sensory or motor
Rami: lie distal to and are lateral branches of the spinal nerves. Like spinal nerves, rami carry both sensory and motor fibers
How is the dorsal trunk region innervated?
The dorsal trunk region is innervated by dorsal rami
How are the anterolateral thorax and the abdominal wall innervated?
The anterolateral thorax and the abdominal wall are innervated by the intercostal nerves (ventral rami of T1 - T12)
How will you remember what nerve supplies each synovial joint?
Hilton’s law: any nerve serving a muscle that produces movement at a joint also innervates the joint and the skin over the joint
Ex. the quads, gracilis, and hamstring muscles all cross the knee. The nerves to these muscles are the femoral nerve anteriorly and branches of the sciatic and obturator nerves posteriorly. Consequently, these nerves innervate the knee joint as well.
Describe the process of nerve fiber regeneration in the PNS.
- The axon fragments
- 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
- Degeneration of the distal end of the cut axon (Wallerian degeneration) spreads down the axon - Schwann cells and macrophages clean out the dead axon distal to the injury
- Surviving Schwann cells engulf the myelin fragments and secrete chemicals that recruit macrophages
- Macrophages help dispose of the debris and release chemicals that stimulate Schwann cells to divide - Axon filaments grow through a regeneration tube
- Schwann cells release growth factors and express cell adhesion molecules (CAMs) that encourage axon growth
- Schwann cells line up along the tube of remaining endoneurium, forming a regeneration tube that guides the regenerating axon “sprouts” across the gap to their original contacts - The axon regenerates and a new myelin sheath forms
- The Schwann cells protect and support the regenerating axon and ultimately produce a new myelin sheath
What factors inhibit the process of nerve fiber regeneration in the CNS?
- Oligodendrocytes have growth inhibiting proteins that actively suppress CNS axon regeneration
- Astrocytes at the site of injury form scar tissue that blocks axon growth
What is a dermatome?
An area of skin with sensory innervation supplied by the fibers of a single pair of spinal nerves
Regarding assessment of dermatome T4 and T10: what is the location and significance of each?
T4 - nipples
T10 - umbilicus
These points are used postoperatively to test sensation. Anesthesia/sedation needs to be moving down not up postoperatively as the patient recovers because if it moves up to C3-5 it could impact our ability to breathe. So sensation must be examined to make sure that it is in fact moving down and wearing off. When the patient can feel sensation at T10 level, they can leave the PACU.
Where does somatic pain arise? Where does visceral pain arise?
somatic pain arises from skin, joints, and muscles
visceral pain results from stimulation of nociceptors in the organs of ventral body cavity.
Identify the three levels in the hierarchy of motor control and describe the activities performed at each level.
Segmental level
- Lowest level
- Consists of reflexes and spinal cord circuits that control automatic movements
- Segmental circuit activates a network of ventral horn neurons in a group of cord segments, causing them to stimulate specific groups of muscles
- Contains central pattern generators (CPGs) - circuits that control locomotion and other specific and repeated motor activities, consist of networks of oscillating inhibitory and excitatory neurons which set crude rhythms and alternating patterns of movement
Projection level
- Directly controls spinal cord
- Consists of neurons acting through the direct and indirect motor pathways
- Upper motor neurons of the motor cortex initiate the
direct pathways. Axons of direct pathway neurons
produce discrete voluntary movements of the
skeletal muscles
- Brain stem motor nuclei oversee the indirect
pathways. Axons of these pathways help control
reflex and CPG-controlled motor actions, modifying and controlling the activity of the segmental apparatus
- Conveys instructions to spinal cord motor neurons and sends a copy of that information to higher levels
Precommand level
- Highest level
- Regulate motor activity - precisely start or stop movements, block unwanted movements, coordinate movements with posture, and monitor muscle tone
- Control the outputs of the cortex and brain stem motor centers
What parts of the CNS are associated with each level of the motor control hierarchy?
Segmental level - spinal cord
Projection level - primary motor cortex and brain stem nuclei
Precommand level - cerebellum and basal nuclei
Describe the difference between a reflex and a reflex arc.
Reflexes are rapid, automatic responses to specific (threshold) stimuli. They are predictable and uniform.
Reflex arcs are the specific neural pathways/circuits involved in producing reflex behavior
The reflex arc is an example of what type of processing –serial or parallel?
Serial
What are the five components of all reflex arcs?
Receptor: site of the stimulus action
Sensory neuron: transmits afferent impulses to the CNS
Integration center: in simple reflex arcs, the integration center may be a single synapse between a sensory neuron and a motor neuron (monosynaptic reflex). More complex reflex arcs involve multiple synapses with chains of interneurons (polysynaptic reflex)
Motor neuron: conducts efferent impulses from the integration center to an effector organ
Effector: muscle fiber or gland cell that responds to the efferent impulses (by contracting or secreting)
Where is the integration center for the reflex arc always located?
CNS
Identify and describe the two functional types of reflex.
Somatic reflexes - activate skeletal muscle
Autonomic (visceral) reflexes - activate visceral effectors (smooth or cardiac muscles or glands), at unconscious level
Describe the structure and function of the muscle spindle.
Structure
- 3 - 10 intrafusal muscle fibers (modified skeletal muscle fibers) enclosed in extrafusal muscle fibers (a CT capsule)
- Extrafusal muscle fibers are contractile fibers
- The central regions of intrafusal fibers lack myofilaments and are noncontractile - these regions are the receptive surfaces of the muscle spindle - 2 types of afferent endings
- Anulospiral endings (primary sensory endings) -
endings of large axons that wrap around the spindle
center. Stimulated by both the rate and degree of
stretch
- Flower spray endings (secondary sensory endings) -
formed by smaller axons that supply the spindle
ends. They are stimulated only by degree of stretch.
- Intrafusal muscle fibers have contractile regions at their ends, which are the only areas containing actin and myosin myofilaments. These regions are innervated by gamma efferent fibers
Function
- Measure muscle length and relay this information to the cerebral cortex and cerebellum
What is the function of alpha motor efferents? of gamma motor efferents?
Gamma motor efferents - innervates intrafusal fibers in the contracting regions at each end of the muscle spindles, provides motor innervation to muscle spindle
Alpha motor efferents - stimulate the extrafusal muscle fibers to contract
Describe the function of the primary sensory ending of the muscle spindle.
Anulospiral endings (primary sensory endings) - endings of large axons that wrap around the spindle center
Stimulated by both the degree of length/stretch and the rate of change in length/stretch of the muscle
What is the role of alpha-gamma co-activation in ensuring that the muscle spindle functions effectively?
Descending fibers of motor pathways synapse with both alpha and gamma motor neurons, and motor impulses are simultaneous sent to the extrafusal fibers and to muscle spindle intrafusal fibers. Stimulating the intrafusal fibers maintains the spindle’s tension (and sensitivity) during muscle contraction, so that the brain continues to be notified of changes in the muscle length.
Without this alpha-gamma coactivation, information on changes in muscle length would cease to flow from contracting muscles
Identify the sensory receptor thought to be involved in the deep tendon reflex arc.
Golgi tendon organs
Describe the phenomenon of reciprocal activation.
Muscle tension increases and reaches the tendon organ threshold. Then, afferent impulses transmit to the spinal cord and cerebellum where info is used to adjust muscle tension. At the same time, spinal cord motor neurons are inhibiting the contracting muscle and stimulating the antagonist muscle. RECIPROCAL ACTIVATION - the contracting muscle is relaxed as the antagonist muscle is activated.
Understand how to rate a patient, using the scale for grading reflexes.
4+: very brisk, hyperactive, with clonus (rhythmic oscillations between flexion and extension)
3+: brisker than average; possibly but not necessarily indicative of disease
2+: average; normal
1+: somewhat diminished; low normal
0: no response
Identify 4 deep tendon reflexes and the 1 stretch reflex (often known as a DTR) and tell which segmental level is tested by each.
Biceps reflex: C5 - C6 Triceps reflex: C6 - C8 Brachioradialis (supinator) reflex: C5 - C6 Patella reflex (stretch reflex): L2 - L4 Achilles/calcaneal reflex: S1 - S2
What spinal nerve branches from the intercostal nerves?
Ventral rami of T1 - T12 (subscripted numbers)
What spinal nerve fibers form the various nerve plexuses?
Cervical plexus: C1 - C5
Brachial plexus: C5 - C8 most of T1 ramus; often receives fibers from C4 and/or T2
Lumbarsacral plexus: L1-S4
- Lumbar plexus L1-S4 - Sacral plexus L4-S4
