Peripheral nervous system: LN/R Flashcards
1
Q
What are sensory receptors?
A
-general sensory endings are scattered profusely throughout the body
-they are biological transducers, in which physical or chemical stimuli create action potentials in nerve endings
-the resulting nerve impulses, on reaching the CNS, produce reflex responses, awareness of the stimuli, or both
-these structures respond to stimuli for pain, temperature, touch and pressure
2
Q
What are afferent neurons?
A
Two broad categories of afferent neurons:
1.) Somatic afferents
- For the skin, bones, muscles, and connective tissue that makes up most of the mass of the body (soma)
2.) Visceral afferents
-for the internal organs of the circulatory, respiratory, alimentary, excretory and reproductive systems
3
Q
Describe somatic efferent neurons
A
-The cell bodies of somatic efferent neurons (also called motor neurons) are in the ventral grayhorns of the spinal cord and motor nuclei of cranial nerves
-The axons of ventral horn cells traverse (travel through) the ventral roots and spinal nerves and terminate in motor end plates on skeletal muscles fibres
4
Q
Describe the visceral efferent neurons (autonomic system)
A
- Has a special feature, in that at least 2 neurons participate in transmission from the CNS to smooth muscle, cardiac muscle, or secretory cells
5
Q
What are peripheral nerve fibres arranged in?
A
Bundles or fascicles (except the very small ones)
6
Q
What is the entire PNS surrounded by?
A
Epineurium (derived from mesodermal cells), this is composed of ordinary connective tissue, and it also fills the spaces between the fascicles
7
Q
What does undulations in the epineurial collagen fibres around each fascicle allow for?
A
Stretching of the nerve that accompanies flexion of joints and other movement
8
Q
What is the sheath that encloses each small bundle of fibres in a nerve?
A
Perineurium (derived from mesodermal cells), consists of several layers of flattened cells.
9
Q
What is within the Perineurium?
A
individual nerve fibres have a delicate covering of connective tissue called endoneurium (derived from mesodermal cells) (or sheath of Henle)
10
Q
What is within the endoneurium?
A
The axons are intimately ensheathed by neuroglial cells (Schwann cells), which are derived from the neural crest and constitute the neurolemma (or neurilemma) or sheath of Schwann
11
Q
What does the nerve fibre consist of?
A
-the axons
-the myelin sheath (of fibres in groups A and B)
-the neurolemma (sheath of Schwann)
12
Q
What does the cytoplasm of an axon (axoplasm) contain?
A
Neurofilaments, microtubules, patches of smooth-surfaced endoplasmic reticulum, and mitochondria (the plasma membrane of axon = axolemma)
13
Q
What are the components of schwann cells?
A
Neurolemma and myelin sheath
14
Q
What does neurolemma consist of?
A
The cytoplasm of Schwann cell, of the myelin sheath
15
Q
what is myelin sheath interrupted by at intervals?
A
Nodes of Ranvier, distance between nodes vary from 100 micrometres to 1 mm
16
Q
How many Schwann cells to each internode (part that’s myelinated)
A
1 schwann cell to each internode
17
Q
What does the myelin sheath?
A
electrically insulates the internodal parts of the axon
18
Q
Where are voltage-gated sodium channels present in the axolemma? and what does this arrangement allow? and what is this transmission called?
A
Only at the nodes, allows action potentials to skip electrically (instantaneously) from node to node –> called saltatory conduction
19
Q
What are the most rapidly conducting myelinated fibres?
A
Those with the largest diameters and the longest internodes
20
Q
how many unmyelinated axons do schwann cells envelop?
A
Up to 15, the cell and its included axons constitute a Remak fibre
21
Q
What are exteroceptors? (or exteroceptive endings)
A
-The superficially located sensory receptors, such as those in skin.
-respond to stimuli for pain, temperature, touch and pressure
22
Q
What are interoceptors?
A
Deep sensory receptors and can be categorised as:
-visceroceptors
-Proprioceptors
23
Q
What are visceroceptors?
A
Found in the viscera, are adapted to respond to changes to stretch, pressure, chemical composition and osmotic pressure
24
Q
What are the classes of cutaneous sensory endings?
A
-Non-encapsulated endings
-Encapsulated endings
25
What are non-encapsulated endings?
These are terminal branches of the axon that may either be closely applied to cells or lie freely in the extracellular spaces of connective tissue
26
What are encapsulating endings?
These have distinctive arrangements of non-neuronal cells that completely enclose the terminal parts of the axons
27
What is epidermis?
- Most superficial layer of skin primarily functions as a first line of defence for physical, chemical, viral and bacterial harm
- layered full of stratified squamous epithelium
-basal layer is made up of stem cells. Cells are constantly produced and the upper layers are constantly shedded
28
What is the dermis?
-Cushions the body from stress and strain. 2 layers.
-The dermal plexus is located in the deeper part of the dermis.
-the papillary plexus lies in the papillary layer of the dermis, immediately beneath the epidermis
29
What is hypodermis/ subcutaneous tissue?
-insulating fatty layer, also used for energy storage. Where the subcutaneous plexus is located (this lies in the loose connective tissue in skin)
30
What are free nerve endings?
-non-encapsulated sensory endings
-widely distributed throughout the body, including the epidermis, dermis and subcutaneous tissue
-the terminal end (receptor) is devoid of myelin. Group C fibres and unmyelinated group A fibres
-the afferent axons are either myelinated or non-myelinated
-these occur in the subcutaneous tissue and dermis, and some extend among the cells of the epidermis
-most detect pain, whilst some detect crude touch and pressure
-invested in Schwann cells and do not contact the extracellular fluid directly
31
What are merkel discs?
-non-encapsulated sensory endings
-located on hairless skin like fingertips and soles of feet, as well as between hair follicles
-these are found in the germinal layer (stratum basale) of the epidermis
-axonal branches end as flattened disc-shaped expansions, each being closely applied to the Merkel cell (epithelial cell)
-slow adapting receptor to sense the degree of pressure on the skin, such as when holding a pen
32
What are hair follicle receptors (peritrichial nerve endings)?
- non-encapsulated sensory nerve endings
-located in hairy skin
-nerve fibres that wind around the outer connective tissue sheath of hair follicles, below the sebaceous gland.
-fast adapting receptor to sense sensitive/light touch
-a single axons sends branches to many hair follicles... and a single follicle is supplied by 2- 20 axons
-bending of the hair stimulates the nerve. Whilst bent, the stimulation stops. When the hair is released, a further impulse is initiated
-an extreme form of these are found in whiskers
33
Wha is the ruffini corpuscle?
-encapsulated sensory endings
-typically, 1mm long and 20-20micrometres wide
-respond to mechanical stimuli that pull on the collagen fibres attached to its capsule, when pressure on or stretching of the skin causes movement in the subcutaneous tissue
-each corpuscle consists of branches from a single myelinated nerve. The branches are surrounded by a cellular capsule
-slow adapting mechanoreceptor, located deep in the dermis of hairy skin which respond to temperature
-no pain when there is a deep burn because these have been burned off
-some evidence they response to sustained pressure and stretch (mismatch of evidence)
-present in subcutaneous tissue and dermis of both hair and glabrous skin
34
What is the Pacinian corpuscle?
-Encapsulated sensory endings
-Functionally, Pacini corpuscles are representative of RA II-LTMRs (group A Beta-fibres type II low-threshold receptors)
-respond to pressure and vibratory stimuli between 20 and 1500 Hz, with a maximum sensitivity at 200-400 Hz
-sustained deformation causes a change of shape without mechanically disturbing the axon in the centre
-most rapidly adapting (attributed to the fluid between the many layers of corpuscle)
-consists of a single axon that loses its myeline sheath and is encapsulated by several layers of flattened cells with greatly attenuated cytoplasm
-ellipsoidal --> about 1mm long and 0.7mm wide
-present in the subcutaneous tissue and dermis of both hairy and glabrous skin
35
What is Meissner's corpuscle?
-Encapsulated sensory endings
-adapt rapidly
-sensitive to mechanical deformation
-they occur in large numbers in he dermal papillary ridges of the fingertips and are less abundant in other hairless regions
-Responsible for sensing low vibration frequencies and fine touch/movement
-identify with great accuracy the positions and movements of objects touching or moving across the surface of skin
-each Miessner's corpuscle is supplied by 3 or 4 myelinated axons whose terminal branches from a complicated knot that is enclosed in a cellular and collagenous capsule
-Meissner's corpuscle, are about 80 by 30 micrometres in size and are oriented with their long axes perpendicular to the skin's surface
36
What are End bulbs?
-encapsulated sensory endings
-respond to tactile stimuli
-They vary in size and shape, and several types have been described (e.g., end bulbs of Krause, Golgi-Mazzoniendings)
-they are commonly spherical, about 50 micrometres in length, with each containing a coiled, branching axonal terminal in a thin cellular capsule
-Most end bulbs occur in mucous membranes (mouth, conjunctiva, anal canal) and in the dermis of glabous skin close to orifices (lips, external genitalia)
37
What are proprioceptors?
-they are located in the capsules of joints, muscles, and tendons
-they furnish the CNS with information required for the performance of properly coordinated movements through reflex action
-proprioceptive information reaches consciousness so that there is awareness of the position of body parts and of their movements (kinesthetic sense or conscious proprioception)
38
What are nociceptors?
-pain that arises in muscles, tendons, ligaments, and bones detected by these free nerve endings in connective tissue
-nociceptive endings respond to physical injury and to local chemical changes such as those causes by inflammation or ischemia
39
What are muscle spindles/neuromuscular spindles?
-the neuromuscular spindles (muscles spindle) are the proprioceptive organs in skeletal muscles
-they are innervated by both sensory and motor neurons
-size-fraction of a mm wide and up to 6 mm long
-they lie in the long axis of the muscle, and their collagenous capsules are continuous with the fibrous septa that separate the muscle fibres
-sense muscle stretch, cause muscle activation
40
What are Golgi tendons?
-proprioceptors
-Golgi tendon organs (neurotendinous spindles ) are most numerous near the attachments of tendons to muscles
-Each receptor has thin capsule of connective tissue that encloses a few collagenous fibres of the tendon
-the axon of the A-beta or Ib (there may be more than one) divides into myelinated terminal branches after entering the spindle, and the branches end as varicosities on the intrafusal tendon fibre
41
How do Golgi tendons work?
-in contrast to the muscle spindle, the golgi tendon is stimulated by tension in the tendon, which responds to changes in the length of the region containing the sensory nerve endings
-afferent signals from golgi tendon organs reach interneurons in the spinal cord
-these interneurons have an inhibitory effect on alpha motor neurons, thus, they cause relaxation of the muscle to which the particular tendon is attached
-sense tendon stretch, cause muscle inhibition
42
What is the important of Golgi tendons?
-the different functions of the neuromuscular and neurotendinous spindles are in balance in the total integration of spinal reflex activity
-the golgi tendon organs act as constant monitors of tension, and they also provide protection against damage that might result from an excessively strong muscular contraction
43
What are thought to be principal kinestheic receptors? (conscious proprioception)
muscles spindles
44
The motor end plates, or neuromuscular junction, or myoneural junctions (on extrafusal and intrafusal fibres of skeletal striated muscles) are synaptic structures comprising of...?
-a motor axon (the endings)
-The subjacent (below) part of the muscle fibre which it innervates
45
What are ganglia?
Swellings of gray matter outside the CNS
46
What are spinal ganglia? (dorsal root ganglia/sensory ganglia)
-swellings on the dorsal roots of spinal nerves
-contain the cell bodies of primary sensory neurons (these conduct signals from sensory endings).
-The centre of the ganglion is occupied by the proximal parts of the neurites. They are located in the intervertebral foramina, just proximal to the union of dorsal and ventral root
-first bipolar, but the 2 neurites soon unite to form single process
-neurites divide into peripheral (terminates in sensory endings) and central branches (enters spinal cord through a dorsal root)
-diameter from 20 to 100 micrometres in diameter
-axons have varying size from small unmyelinated fibres in group C to the largest myelinated fibres in group A
47
What are autonomic ganglia?
-include those of the sympathetic trunks along the sides of the vertebral bodies, collateral or prevertebral ganglia in plexuses of thorax and abdomen (e.g., the cardiac, celiac, and mesenteric plexuses), and certain ganglia near viscera
-contain principal cells and interneurons (these have short dendrites that are postsynaptic to the preganglionic axons and presynaptic to dendrites of principal cells)
47
Where are the cell bodies of primary sensory neurons located?
in dorsal root ganglia (or in an equivalent cranial nerve ganglion)
48
On entering the spinal cord, the dorsal root fibre...
divide into ascending and descending branches --> distributed for responses (such as reflexes) and for transmission of sensory data to the brain
49
The sensory axon reaches the sympathetic trunk through a white communicating ramus and continues to a viscus in a branch of the sympathetic trunk. For which receptors are these properties for?
receptors concerned with pain
50
some sensory axons may follow similar courses from that of pain receptors, and they have their cell bodies in cranial nerve ganglia and are connected centrally with the
brain stem. For which receptors are these properties for?
Receptors concerned with the function regulation of internal organs
51
Describe sensory endings in viscera (internal organs)
-These consist of non-encapsulated terminal branches of axons. except for Pacinian corpuscles which are in the mesenteries
-Visceral afferent function in physiological visceral reflexes; in the sensations of fullness of stomach, rectum, and bladder; and in pain caused by visceral dysfunction or disease
-afferent fibres for pains generally travel in different nerves from those involved in functional control and they have different connections in the CNS
52
Describe the motor effector endings in viscera
-apart from muscle fibres, the NS also acts on non-neural secretory cells such as glands
-the control of these non-neural cells is effected by a mechanism similar to that of chemical synaptic transmission between neurons (i.e., neuroendocrine synapses)
-generally, at the neuroeffector endings, axons terminate in relation to skeletal, cardiac, and smooth muscle fibers and to the cells of exocrine and endocrine glands
53
What are autonomic endings?
-Parasympathetic: long preganglionic neurons (like vagus nerve), short postganglionic neuron
-sympathetic -short preganglionic neurons (from spinal cord to sympathetic chain), long postganglionic neuron
54
What are postganglionic autonomic endings?
- The presynaptic effector nerve endings on autonomic structures (e.g., smooth muscle, cardiac muscle, and secretory cells) are swellings, commonly called
varicosities.
- They are found along the courses and at the tips of unmyelinated axons, which are closely applied to the effector cells.
- They contain accumulations of mitochondria and clusters of synaptic vesicles (no post-synaptic structural specializations).
- Noradrenergic terminals of the sympathetic nervous system contain electron-dense synaptic vesicles, while cholinergic terminals (parasympathetic) contain small
electron-lucent vesicles.
55
What is glabrous skin?
Skin that lacks hair, present on the palmer surfaces of hands and fingers, the soles, and parts of the face and external genitalia
56
Where do the branches of each plexus send their branches?
Into the adjacent tissues
57
What are modalities?
-The types of sensation consciously perceived from the skin
-5 modalities:
*fine (discriminative) touch
*vibration
*light touch
*temperature (warmth or cold)
*pain
-sensations of each modality have qualities (e.g., pain may have aching or burning quality)
58
What is adaption in any receptor?
An important physiological property, which is reduced response to continued stimulation
59
What is slowly adapting receptors?
reports continuously on the stimulus that activated it
60
What is a rapidly adapting receptors?
receptor reports changes in the stimuli it receives
61
describe low-threshold mechanoreceptors
-receptors for tactile sensation are low threshold mechanoreceptors
-includes all encapsulated and some free nerve endings
62
what do high-threshold mechanoreceptors respond to
only to mechanical stimuli such as stretching and cutting (noxious stimuli)
63
what do polymodal nociceptors respond to?
respond to both mechanical and thermal (>45 degree Celsius) stimuli and to chemical mediators released from injured cells
64
what is hyperalgesia?
lowered pain threshold
65
What does each muscle spindle consist of? and what do they do?
-a capsule of connective tissue, with 2 to 14 intrafusal muscle fibres.
-simplest role of muscle spindle is that of a receptor for the stretch reflex
66
how do intrafusal fibres differ from extrafusal fibres (standard skeletal muscle fibres)?
-intrafusal fibres are considerably smaller than the extrafusal fibres
-the equatorial region lacks cross-striations and contains many nuclei that are not in the subsarcolemmal position characteristic of mature striated muscle
67
What is a muscle spindle supplied by?
2 sensory axons. A-alpha fibre (aka Ia fibre) and A-beta (aka II fibre)
68
Describe the A-alpha fibre that supplies the muscle spindle
The axon loses its myelin sheath as it pierces the capsule, and then it winds spirally around the midportions of the intrafusal muscle fibres in the form of an annulospiral ending
69
Describe the A-beta fibre that supplies the muscle spindle
-slightly smaller than A-alpha fibres
-branches terminally and ends as varicosities on the intrafusal muscle fibres some distance from the midregion
-terminals called flower spray endings
70
what are annulospiral and flower spray terminals also known as?
primary and secondary sensory ends, respectively, of spindles
71
What are extrafusal fibres innervated by?
innervated by large motor cells --> alpha motor neurons, axons are of A-alpha size
72
What are intrafusal muscle fibres within spindle innervated by?
innervated by small motor cells --> gamma motor neurons, with A-gamma axons
73
What is the stretch reflex?
-Slight stretching of a muscle lengthens the intrafusal fibres, and the sensory endings are stimulated.
-action potentials are conducted to the spinal cord, where terminal branches of the sensory axons synapse with alpha motor neurons that supply the main mass of the muscle
-The latter thereupon contracts in response to stretch through two-neuron reflex arc.
-stimulation of the spindles ceases when the muscle contracts because the spindle fibres, in parallel with the other muscle fibres, return to their original lengths
-the stretch reflex is in constant use in the adjustment of muscle tone
-also forms the basis of tests for tendon reflexes, such as the knee jerk
74
What does the gamma reflex loop consist of?
The gamma motor neuron, muscle spindle, sensory neuron, and alpha neuron supplying extrafusal muscle fibres
75
Describe the role of spindles in muscle action that results from the activity of the brain
The motor fibres the descend from the brain into the spinal cord influence both alpha and gamma motor neurons in the ventral gray horns by synapsing with them directly and through the mediation of interneurons. Contraction of the intrafusal muscle fibres in response to stimulation by gamma motor neurons lengthens the midportions and starts a volley of impulses in the sensory axons. this causes contraction of the regular muscle fibres
76
describe the gamma reflex loop
it is an adjunct to the more direct control of muscles by descending fibres from the brain that control the alpha motor neuron. Activation of the gamma reflex loop can set the length of a muscle before the initiation of a movement
77
Where are Golgi tendon organs (aka neurotendinous spindles) most numerous?
Near the attachments of tendons to muscles
78
What does each golgi tendon organ receptor have?
A thin capsule of connective tissue that encloses a few collagenous fibres of the tendon
79
what responds to the cessation and initiation of movement?
around the capsules of the synovial joints, there are small Pacinian corpuscles (cessation) and formations similar to Ruffini cutaneous endings (initiation) --> these respond
80
receptors identical to Golgi tendon organs are present in _______ they mediate reflex inhibition of muscles when excessive strain is placed on join
-articular ligaments
81
what is the purpose of the abundance of free nerve endings in the synovial membrane, capsule and periarticular connective tissues?
respond to potentially injuries mechanical stresses and to mediate the pain that arises in diseases or injured joints
82
What does a motor unit consist of?
one motor neuron and the muscle fibres that it innervates --> number of muscles fibres in motor unit varies from fewer than 10 to several hundred depending on size and function of the muscle --> small muscles less, big muscles more
83
Each branch of the motor nerve fibre gives up its myelin sheath on approaching a...
...muscle fibre and ends as several branchlets that constitute the neural component of the end plate
84
Describe the end plate
-typically 40-60 micrometres in diameter
-usually located midway along the length of the muscle fibre
85
What is the neurotransmitter in motor end plates?
acetylcholine
86
What does each axonal branchlet occupy?
A groove or "synaptic gutter" on the surface of the muscle fibre
87
What comprises the sarcolemma of muscle fibre?
The plasma membrane and associated basement membrane --> they have a wavy outline where they appose the nerve terminal with the irregularities known as junctional folds
88
What is the folded region of the sarcolemma?
The subneural apparatus --> is demonstrable histochemically by its content of acetylcholinesterase
89
Where does the acetylcholine that is released from the synaptic vesicles bind to?
To acetylcholine receptor (AChR) molecules in the folded sarcolemma of the subneural apparatus
90
What are the large, intermediate and small neurons used for in dorsal root ganglia?
-The large neurons are for proprioception and discriminative touch
-intermediate are for light touch, pressure, pain and temperature
-smallest neurons transmit impulses for pain and temperature
91
What is each cell body closely invested by a layer of in dorsal root ganglia?
satellite cells that are continuous with the Schwann cell sheath that surrounds the axon
92
What are the principal cells of autonomic ganglia?
-principal cells are multipolar neurons 20 to 45 micrometres in diameter, cell body surrounded by satellite cells similar to those of spinal ganglia --> several dendrites extend and branch outside the capsule of satellite cells and receive synaptic contacts from preganglionic axons --> the thin, unmyelinated axons (group C fibres) of principals cells leaves the ganglia and eventually supply smooth muscle an gland cells in some viscera, cardiac muscle, the enteric plexuses, blood vessels throughout the body, and sweat glands and arrector pili muscle in the skin
93
A nerve root within the vertebral canal does not have an epineurium, why?
This ensheathing layer is acquired as the nerve pierces the dura mater on its way through an intervertebral foramen
94
Where do the cells of all three connective tissue layers of peripheral nerves get derived from (i.e., epineurium, perineurium and endoneurium)
Derived from the mesodermal cells
95
What are the incisures of schmidt-lanterman?
-funnel-shaped clefts in myelin sheath
-can be seen by light microscopy in longitudinal sections of nerves
-in electron micrographs, these incisures are shown to be zones in which there are spaces between the layers, with occasional retention of Schwann cell cytoplasm between the membrane
-this may aid the passage of materials through the myelin sheath to the axon
