Spinal Cord 1 Flashcards
1
Q
Formation of the vertebral canal and its contents
A
Formation - vertebral foramina form a bony tubular cavity, while other accessory structures add to the boundaries of this vertebral canal
Contents - the canal contain neural tissue and other structures associated with the protection and metabolic support of the neural tissue
2
Q
anterior boundary of vertebral canal
A
vertebral bodies, intervertebral discs, posterior longitundinal ligament
3
Q
posterior boundary of vertebral canal
A
vertebral laminae and the ligamentum flavum
4
Q
lateral boundaries of the vertebral canal
A
vertebral pedicles and the intervertebral foramina
5
Q
Contents of the vertebral canal
A
- spinal cord proper
- spinal nerve rootlets/roots
- spinal meninges
- epidural fat
- cerebral spinal fluid (CSF)
- bv
6
Q
spinal cord proper
A
includes gray and white matter
7
Q
spinal nerve rootlets/roots
A
L and R dorsal and ventral
8
Q
spinal meninges
A
pia mater, arachnoid mater, dura mater
9
Q
Spinal cord - external structure/dimensions
A
- superior end at foramen magnum, continuous superiorly with brainstem
- Inferior end (conus medullaris) at L1/L2 vertebral disc level (L3/4 in newborns)
- Shape - an irregular cylinder
10
Q
Conus medullaris
A
- differential growth rates for vertebral column and spinal cord
- knowledge importance for spinal taps - level of spinal cord and ligamentum flavum last to puncture to enter epidural space
- spinal cord is ~18 in long
11
Q
cervical enlargement
A
supplies upper limbs, spinal cord segments (C5-T1)
12
Q
lumbosacral enlargement
A
supplies lower limbs; spinal cord segements L1 - S3
13
Q
Longitudinal grooves help to
A
delineate L R and column sections
14
Q
different longitudinal grooves
A
- ventral (anterior) median fissure
- dorsal (posterior) median sulcus
- ventral lateral sulci and dorsal lateral sulci
15
Q
ventral (anterior) median fissure
A
wide groove on the anterior midline (a good landmark)
16
Q
dorsal (posterior) median sulcus
A
narrow groove on the posterior midline
17
Q
ventral lateral sulci and dorsal lateral sulci
A
lateral grooves at site of respective rootlet attachments (dorsal is much more prominent)
18
Q
internal structure of spinal cord
A
Spinal cord - rootlet - root - spinal nerve - rami
19
Q
3 primary features of spinal cord
A
white gray matter and central canal
20
Q
describe the white and gray matter in spinal cord
A
white is superficial, while the gray matter is found deeper, forming an H like shape
21
Q
describe the central canal
A
the most central within the middle of the gray matter and contains csf
22
Q
how is white matter formed
A
by neural fiber processes (axons) with minimal cell bodies. The axons are bundled to form tracts. Various tracts are organized within columns. The dorsal, lateral, and ventral columns are delineated by the external longitudinal grooves mentioned previously. The ascending tracts rely info to the brain, while descending tracts relay info to he periphery.
23
Q
what are dorsal columns formed by
A
primary ascending sensory fibers
24
Q
primary sensory neurons
A
start at the receptor in the skin and synapse with secondary sensory neurons
25
secondary sensory neurons
in spinal cord or medulla. They cross the midline leading to the 3
26
3 sensory neurons
end in the cerebral cortex
27
dorsal columns function to
transmit fine touch and proprioception on the same side of the spinal cord as souce (ipsilateral)
28
dorsal column connections
synapse with secondary fibers within nuclei of medulla, which then cross the midline and ascend to synapse with the tertiary neurons in the thalamus and eventually terminate in the contralateral brain.
29
what are secondary fibers typically
decussators (cross midline)
30
location of spinothalamic tract
found in both the lateral and ventral columns of white matter
31
formation of spinothalamic tract
bundles formed by secondary ascending gibers
32
function of spinothalamic tract
transmit pain and temperature on the opposite side of the spinal cord as source (contralateral)
33
what do unilateral lesions on spinothalamic tract cause
contralateral anesthesia (1-2 segments below lesion). Tested using pin pricks and knowledge of dermatomes
34
what does the spinothalamic tract pass through
the medulla without crossing, then secondary fibers terminate (synapse with the tertiary) in the thalamus before continuing to cortex
35
lateral corticospinal (pyramidal) tract found in
the lateral column of white matter
36
lateral corticospinal (pyramidal) tract formed by
descending upper motor neurons (UMN), which originate in the motor cortices of the cerebrum (brain0
37
lateral corticospinal (pyramidal) tract UMN fibers
cross the midline in the medulla at the pyramids, thus run contralaterally in the spinal cord (contra to brain) HOWEVER, this can be though of as "this tract is ipsilateral (same side of spinal cord) realtive to the muscles it innervates)
38
lateral corticospinal (pyramidal) tract synapse with
lower motor neurons in the ventral gray horn for eventual distribution to skeletal muscle for voluntary motor control
39
lesion of lateral corticospinal (pyramidal) tract
leads to paralysis below the level of damage
40
gray matter is primarily formed by
neural cell bodies, axon terminal branches, dendrites, and interneurons.
41
how is gray matter divided into
portions (horns) which vary in size/shape throughout spinal cord levels
42
ventral (anterior) horns
found at all levels of spinal cords and contains somatic motor neuron cell bodies (GSE, aka lower motor neurons (LMN) that innervate skeletal muscle)
43
GSE
general somatic efferents
44
somatic refers to
voluntary skeletal muscle and skin of limbs and torso wall
45
efferent refers to
going away from, thus mtor from CNS to muscle, viscera
46
Dorsal (posterior) horns
found at all levels of the spinal cord. It contains central processes of sensory neuron axons (GSA) and interneurons (Ia - interneurons) which integrate various sensory and motor neurons
47
GSA
general somatic afferents
48
lateral horns (or intermediolateral cell columns)
found only at spinal cord segments T1 to L2. It contains preganglionic sympathetic neuron cell bodies (GVE) this autonomic fibers of the sympathetic system
49
GVE
general visceral efferent
50
visceral refers to
involuntary, smooth muscle, organs
51
gray commissure
found at all levels of spinal cord. It is the deepest/central region of spinal cord, contains central canal. Connects the left and right gray columns (area of decussation)
52
Central canal
continuous with the 4th ventricle of the brain, found through entire length of spinal cord. It is the most central feature found within the gray commissure. It contains the csf, but may be occluded in the aged or diseased spinal cord.
53
Ventral nerve roots
a series of ventral (motor) roots respective to each spinal cord segment, individually are a collection of motor axons going to the periphery
54
how is a single ventral root formed
by a series of ventral rootlets from a single spinal cord segment level; is symmetrical bilaterally (L and R ventral roots)
55
a ventral rootlet is formed
along the ventral lateral sulcus and is a collection of axons from motor neurons in the lateral (GVE) and ventral gray horns (GSE). They contain motor fibers only
56
Dorsal nerve roots
a series of dorsal (sensory) roots respective to each spinal cord segment, individually are a collection of sensory axons going to the spinal cord
57
a single dorsal nerve root is
a collection of central axonal processes from pseudounipolar neuron cell bodies located in the dorsal root (spinal) ganglion,
58
the spinal ganglion is found
near the intervertebral foramina (it is important to note that all spinal nerve sensory neuron cell bodies are found within these ganglia)
59
a single dorsal root splits into
a series of dorsal rootlets which enter the spinal cord at the dorsal lateral sulcus; symmetrical bilaterally (L and R dorsal roots)
60
dorsal nerve roots contain
sensory fibers only
61
Typical spinal nerve description
formed near the intervertebral foramen and distal to the spinal ganglion. It is formed by the joining of a dorsal nerve root and a ventral nerve root, each relative to the same spinal cord segment level. It contains both motor (efferent) and sensory (afferent) fibers
62
how many spinal nerves
```
31L
8 cervical (C1-C8)
12 Thoracic (T1-T12)
5 Lumbar (L1-L5)
5 Sacral (S1-S5)
1 coccygeal (Co)
```
63
how do the spinal nerves exit the vertebral canal (cervical)
by passing superior to the vertebra of the corresponding number.
a. C1-C7 spinal nerves exit ABOVE C1-C7 vert, respectively
b. C8 spinal nerve exits BELOW the C8 vertebra, thus above the T1 vertebra.
c. relatively short spinal nerves with a more horizontal trajectory to exit.
64
how do the spinal nerves exit the vertebral canal (thoracic and lumbar)
exit the vertebral canal inferior to the pedicles of the vertebra with the same number.
a. ex: the right T6 spinal nerve exists below the right pedicle of the 6th thoracic vertebra
b. spinal nerves are successfully longer the further inferior, thus having an oblique to vertical trajectory to exit; due to the spinal cord being shorter than the vertebral column with relatively the same number of segments.
65
how do the spinal nerves exit the vertebral canal (sacral)
the sacral spinal nerves S1-S4 are found within the sacral canal and do not exit as a spinal nerve. Each must branch into their dorsal and ventral rami to exit through dorsal and ventral sacral foramina, respectively
66
where do S5 and Co1 spinal nerves exit
through sacral hiatus
67
spinal cords ends at
L1/L2 vertebral level, thus the dorsal and ventral roots of levels L2 to Co continue within the vertebral canal to their respective exit points. This collection of rootlets/roots is terms the cauda equina.
68
where is the cauda equina located
in the area of the thecal/dural sac called the lumbar cistern, a significant space filled with csf
69
size of spinal nerve
relatively short ~2cm
70
spinal nerve divisions
divides into a dorsal ramus and a ventral ramus
71
what do rami contain
both afferent (sensory) and efferent (motor) fibers
72
dorsal rami does what
innervate the skin, muscles, and joints of the vertebral column and nearby structures
73
ventral rami does what
innervate all the structures lateral and anterior to the vertebral column and including the limb structures/embryological derivatives. Thus, the ventral rami are much Larger than the dorsal rami.
74
how is a spinal cord segment defined
by the spinal nerve it produces
75
31 spinal nerve pairs means
31 spinal cord segments
76
with a shorter spinal cord,
the segments of the spinal cord do not directly correspond anatomically with the same named vertebra, particularly in the lower spinal cord
77
all 5 lumbar segments location
found inside the T11 T12 vertebrae
78
all sacral/coccygeal segments are found
inside the L1 vertebra
79
Dermatome
the area of skin innervated by a single spinal cord segment
80
sensory bodies for each dermatome are located
in the dorsal root ganglia of the paired spinal nerves
81
dermatome pattern of the torso
consistently banded with some overlap.
82
segmentation of intercostal nerves (thoracic spinal nerves)
they are segmental, in that they provide sensory ad motor function for that one segment of the torso
83
T4 innervates
nipples
84
T10 innervates
umbilicus
85
C6 innervates
pad of thumb
86
C8 innervates
pinky
87
S5 innervates
anus
88
why is the dermatome pattern of the limbs much more complex
because the ventral rami of the cervical and lumbosacral regions form plexuses, which feed to the upper and lower limbs
1. allowing nerve fibers form various spinal cord segments to form the peripheral nerves
2. thus, the dermatome pattern of the limb does not match the cutaneous nerve distribution
3. with no plexus in the torso region, the dermatomes and the cutaneous nerve distributions are very similar, if not identical
89
what happens to the limbs during development
they rotate
90
general rule for segmental muscle innervations, particularly of the limbs,
that most muscles are innervated via 2 or more spinal nerves and that 1 spinal nerve helps to supply many muscles
91
spinal meninges
3 layers of connective tissue covering the spinal cord
92
describe the spinal dura mater
1. is the most exterior and densest meninx
2. forms a sac which closes inferiorly at sacral vertebral level 2 (S2)
3. is open superiorly, continuous with the cranial dura mater at the foramen magnum, enclosing the entire CNS (dural sac or thecal sac, sheath)
4. continues laterally at sleeve like projections (dural sheaths) around the dorsal and ventral roots of the spinal nerves, and is continuous with the coverings of the spinal nerves (epineurium)
5. bony anchors - attaches to the bony framework of the foramen magnum and the intervertebral foramen
6. continues beyond the sac at S2 as the external filum terminale (coccygeal ligament), which anchors the sac to the coccyx
93
meningitis
a viral, bacterial, or other microorganism based infection causing inflammation of the meninges. This is considered life threatening. A lumbar puncture to extract CSF is used to diagnose.
94
Spinal arachnoid mater
1. is the intermediate layer, more delicate than dura mater, lines the dura mater
2. lines the entire dural sac down to its termination at S2, as well as extending into the sleeves surrounding the dorsal and ventral roots (follows dura mater everywhere)
3. along with the deeper lying pia mater, forms fine strands of tissue (trabeculae) between these 2 layers resulting in a web like (arachnoid) appearance
95
Spinal pia mater
1. is the most internal and most delicate layer of the meninges
2. envelops the spinal cord like a glove (its skin), its associated bv, the rootlets, and dorsal/ventral roots, finally merging with the epineurium of the spinal nerves
3. forms the denticulate ligament, reinforced with denser connective tissue. This is an expanse of tissue found between the dorsal and ventral rootlets, bilaterally. These tooth like projections pierce the arachnoid into the dura at about 21 locations on each side. This attachment helps to stabilize the cord within the center of the dural sac
4. ends inferiorly at the conus medullaris except for its prolongation, the internal filum terminale, which extends from the tip of the conus medullaris.
96
epidural (extradural) space is located
external to the dural sac.
97
contents of the epidural sac
a cushioning layer of adipose tissue (fat) and an extensive plexus of veins (the internal vertebral venous plexus)
98
subdural space
a potential space located between the spinal dura mater and arachnoid mater. Normally these 2 meningeal layers are separated only by a very thin layer of lubricant and this subdural space does not really exist (potential space). however, this potential space can become a space when, for instance, blood invades this area (subdural hematoma)
99
subarachnoid space is located
between the arachnoid and pia mater layers. It is filled with CSF
100
CSF is primarily produced by
the choroid plexuses of the ventricles in the brain.
101
how do the cranial and spinal subarachnoid spaces communicate
freely, as does the flow of the CSF
102
what is below the conus medullaris
an expanded area of the subarachnoid space termed the lumbar cistern which contains CSF surrounding the cauda equina. The higher volume of CSF combined with the absence of the spinal cord results in an excellent location to extract CSF or inject anesthetics (lumbar puncture)
103
epidural v spinal anaesthesia
1. epidural needs larger dose 2. epidural has a longer onset of analgesia 3. epidural can be performed at any level, spinals only in lumbar region 4. epidurals can be used segmentally whereas spinals are more generalized
104
Blood supply of spinal cord and innervation of surrounding structures
Arterial supply of the spinal cord, venous drainage, innervation of vertebral column contents
105
3 main arteries of spinal cord
they are branches of the vertebral arteries within the cranium
a. Single anterior spinal artery
b. paired posterior spinal arteries
106
single anterior spinal artery
located in the anterior median fissure, supplies twice as much as the smaller posterior spinal arteries
107
paired posterior spinal arteries
passing longitudinally near posterolateral sulci
108
secondary suppliers of the spinal cord
they enter laterally - the segmental spinal arteries branch from various vessels ( deep cervical, ascending cervical, and vertebral arteries in the neck; posterior intercostal arteries in the thorax; and lumbar arteries in the abdomen) and enter through each intervertebral foramin and give rise to
1. anterior and posterior radicular arteries
2. anterior and posterior segmental medullary arteries
109
anterior and posterior radicular arteries
course along the anterior and posterior spinal root/rootlets to help supply these structures
110
radicular refers to
roots
111
anterior and posterior segmental medullary arteries
periodically reinforce the main spinal arties (8-10 in number). usually, one is significantly larger than the others, the great anterior segmental medullary artery.
112
where does the great anterior segmental medullary artery typically arise
on the left side from 1 of the lower intercostal arteries. This is our first looks into arterial anastomosis
113
arterial anastomosis
where arteries communicate from 2 different sources for purposes of re routing blood to regulate or maintain blood flow.
114
Venous drainage
1. dorsal and central spinal veins, which directly drain the spinal cord, generally distribute similar to that of the spinal arteries
2. internal vertebral plexus
115
dorsal and ventral spinal veins
1. vary in number but generally 3-5 anterior and 3-5 posterior, one at each primary sulcus/fissure
2. found on surface of spinal cord, enveloped within the spinal pia mater
3. drain into the internal vertebral plexus which lies in the epidural space
116
internal vertebral plexus
a valveless system of venous channels in the epidural space, which connects with the dural sinuses in the cranium, the external vertebral plexus surrounding the vertebral column and with major veins of the thorax, abdomen, and pelvis
117
internal vertebral plexus clinically
the valveless system and its high degree of interconnection allows for the spread of infection and neoplastic cells (precursors of tumors, cancer, etc)
118
innervation of vertebral column contents
1. each spinal nerve provides 2-4 small recurrent meningeal branches (aka sinuvertebral nerves, recurrent nerves of Luschka, meningeal branch of spinal nerves)
2. They return to the vertebral canal via the intervertebral foramina to supply fibers
119
recurrent meningeal branches return to the vertebral canal via the intervertebral foramina to supply....
1. sensory fibers for the periosteum of the vertebrae, the posterior longitudinal ligament, the outer portion of the annulus fibrosus, and the meninges
2. autonomic fibers to the vessels to aid in regulation of blood flow, sensory and sympathetic fibers to meninges, ligaments, and IV discs
120
clinical observation of innervation of vert. column contents
intervertebral disc rupture and meningitis MAY irritate these recurrent branches, leading to increased reflexive tone and referred pain.
121
what is a spinal reflex
a fast, automatic, hard wired muscular reaction to a stimulus. Initially, the neural activity reaches only as high as the spinal cord, thus does not directly incorporate voluntary, brain activity. These reflexes are important for protecting the body
122
reflex arc
the neural pathway from the stimulus to the effector. 5 components
123
5 components in a typical spinal reflex
1. receptors (muscle spindle)
2. sensory neurons (Ia)
3. synapsing (integration, communication, processing)
4. motor neurons (alpha)
5. effectors (muscles, glands)
124
receptors
range from a simple dendrite to a complex sense organ, specialized to sense or receive a specific change or status in the environment, termed a stimulus.
125
2 kind of receptor
exteroceptive and proprioceptive
126
exteroceptive receptors
are aroused by changes inn the external environment and involve such organs as 1. the skin for touch, vibration, pain or temp 2. the eyes for vision
3. the vestibular apparatus for acceleration of the head, pull of gravity
127
proprioceptive receptors
excited by changes in skeletal position, thus important for movement control.
128
proprioception
provides knowledge of body position (static) and body movement (dynamic). Arguably the most dominant proprioceptive receptor is the muscle spindle. Golgi tendon organs also provide important information about the strain/tension of the muscle and its associated tendon
129
Afferent (sensory) neuron
1. A pseudounipolar (t shaped) whos cell body is located in the dorsal root ganglia
2. the axon is divided into a portion peripheral to the cell body (peripheral process) and a portion central to the cell body (central process)
3. the receptor stimulates the peripheral process, the signal propagates along the axon, continuing through the dorsal root and rootlet on the central process to enter the spinal cord
130
synapsing/integration - simple
synapsing may be simple, a sensory neuron synapsed directly with a motor neurons, called a monosynaptic reflex
131
synapsing/integration - complex
incorporation of interneurons increases the number of synaptic points in a reflex arc to 2 (disynaptic) or more (polysynaptic). The interneuron is usually a multipolar neuron whos cell body and dendrites are located in the dorsal gray horn and the very short axon terminates on the efferent neuron, also within the gray matter.
These more complex circuits result in various muscle activation patters.
132
Interneurons will most typically have
inhibitory influence on its target, but may be excitatory
133
efferent (motor) neuron
multipolar neurons, whos cell bodies and dendrites are located in the gray matter and their axons pass out the ventral rootlets to the ventral root to spinal nerves, to rami and finally to an effector (muscles, glands)
134
2 general types of motor neurons
somatic and visceral
135
describe somatic motor neurons
1. cell bodies of GSE neurons in ventral gray horn
2. GSE neurons are LMN, which are stimulated by UMN from the motor cortices of the brain for voluntary contractions and or stimulated by GSA and interneurons (IN) for reflexes
136
what are alpha motor neurons
GSE/LMN neurons which stimulate skeletal muscle (extrafusal fibers)
137
motor unit
an alpha motor nueron and all the muscle fibers it innervates
138
size of motor unit
can vary in size from ~5 muscle fibers (slow or fast twitch) to ~2000 muscle fibers (ex: gastrocnemius). The degree of fine tune control v power needed from that muscle dictates this variation
139
details of the motor unit
contains only 1 type of muscle fiber (slow or fast twitch), however, the entire muscle has a unique mixture of various fiber types, thus various types/sizes of motor units that the CNS can influence to control strength and endurance outputs
140
gamma motor neurons
GSE/LM neurons which stimulate muscle spindle fibers (intrafusal)
141
describe visceral motor neurons
1. cell bodies of GVE neurons in lateral gray horn found at levels T1-L2
2. GVE neurons originating inside the spinal cord are preganglionic neurons, these fibers course through ventral root, spinal nerve, white ramus communicans to the sympathetic trunk
3. eventually it stimulates a postganglionic neuron that acts as the effector (motor) neuron to stimulate an effector (smooth muscle, gland)
142
effector
the structure acted on by the efferent nueron
143
for a GSE neuron, what are the effectors
the target organ is skeletal muscle OR intrafusal muscle spindle fibers
144
for a GVE neuron, what are the effectors
smooth muscle, cardiac muscle, arrector pili muscle, or glands
145
Myotatic or stretch reflex
the most fundamental spinal reflect. It is a monosynaptic reflex that is elicited by tapping a tendon
146
what is the result of the tendon tap (stretching of the tendon)
subsequent stretch of the skeletal muscle and its associated muscle spindle
147
what happens to quickly stretch muscle spindles
stimulate their Ia afferent neurons
148
what do Ia afferent neurons do during a muscle stretch
propagate their action potentials through the dorsal spinal nerve system to directly terminate on the efferent neurons (GSE alpha motor neurons) within the ventral gray horn of the spinal cord
149
efferent neurons after a muscle stretch
(alpha mn) travel through the ventral spinal nerve system to terminate at the neuromuscular (myoneural) junction
150
what happens to the extrafusal muscle fibers after a tendon tap
they are stimulated leading to muscle contraction. This is called autogenic excitation because the muscle spindle stimulated its own skeletal muscle.
151
path of tendon tap
tendon tap -> muscle spindle -> Ia -> alpha mn -> skeletal muscle contracts
OR
tendon tap -> muscle spindle -> Ia -> Ia-IN -> inhibits alpha mn of antagonist m
152
what else does the monosynaptic stretch reflect have
a disynaptic reflex response. Thi is called reciprocal inhibition. The same Ia afferent stimulates a Ia-interneuron (Ia-IN), which in turn inhibits the alpha mn of the antagonistic muscle. this activity may/should result in relaxation of the antagonist muscle (reduced tone or a reduction in the likelihood of contraction)
153
1a neuron
| from, to, characteristics, and yields
from: muscle spindle
to: spinal cord
characteristics: sensitive to length change and velocity of length change of the muscle
yields: proprioception; T reflex; reciprocal inhibition
154
II from, to, characteristics, and yields
from: muscle spindle
to: spinal cord
characteristics: sensitive to length changes only
yields: proprioception
155
1b from, to, characteristics, and yields
from: GTO
to: spinal cord
characteristics: stimulated by stress/strain on the GTO at muscle/tendon junction
yields: Proprioception, autogenic inhibition
156
alpha mn from, to, characteristics, and yields
from: spinal cord
to: muscle
characteristics: is a LMN, affected by sensory fibers, interneurons, and UMN
yields: contraction of the muscle
157
gamma nm from, to, characteristics, and yields
from: spinal cord
to: muscle spindle
characteristics: is a LMN, affected by interneurons and UPM
yields: resets the spindle; regulates muscle tone
158
what are muscle spindles
capsules scattered throughout skeletal muscles, aligned parallel to these muscle fibers
159
describe muscle spindles
sensitive to muscle length and the velocity of changes in muscle length. They provide a tonic signal via the Ia and II sensory fibers. They contain their own intrafusal muscle fibers within the capsule.
160
what stimulates the intrafusal muscle fibers of muscle spindles
the gamma motor neurons, yielding 2 functions: sensor reset and muscle tone
161
what happens with increased output of the muscle spindles
generally causes autogenic excitation and reciprocal inhibition
162
tonic means
being constantly active, never going to zero
163
Golgi Tendon Organs (GTO)
a proprioceptor that is sensitive to strain/tension on a muscle/tendon. It is embedded within the muscle/tendon junction in series with muscle fibers. It provides a phasic signal via Ib sensory fibers. It leads to inhibition of the stimulation of the sameP muscle it is found in (autogenic inhibition)
164
Phasic means
being active in bursts, particularly when stimulated, returning to 0 at some point
165
purposes of the gamma loop
1. reset (shorten) muscle spindle following skeletal muscle contraction/shortening
2. regulate muscle tone
166
how are gamma motor neurons stimulated
by higher brain centers and interneurons
167
what do gamma mn do
innervate muscle spindles, causing the intrafusal muscle fibers to contract, leading to 2 outcomes
168
2 outcomes from gamma nm causing muscle fibers to contract
1. the muscle spindle will shorten to match the shortened skeletal muscle, "resetting" the muscle spindle so it can pick up a new stretch within the skeletal muscle (maintain continuous monitoring of the muscle length changes)
2. the sensory components of the spindle will be stimulated, increasing the Hz of the signal on the Ia sensory neuron, leading to stimulation of the alpha motor neurons back to the associated skeletal muscle -> regulating muscle tone.
3. Thus, muscle length and tone can be regulated through a circuit (loop) within the lower centers
4. it adjusts the gain of the stretch reflex, which adjusts the activity level of the alpha mn, thus regulating muscle length and tone
169
muscle tone is the balance of
many excitatory and inhibitory influences acting on the alpha motor neurons of a given muscle.
170
Much inhibitory influence on muscle comes from
higher brain centers.
171
what does a major loss of inhibitory signal from the brain (like a spinal cord lesion) result in
hypertonicity (rigidity) or spasticity
172
damage to UMN usually results in
spastic paralysis (because reflexes still work)
173
damage to LMN usually results in
flaccid paralysis (no signal - voluntary or reflexive - can get to the muscle because the alphas are out
174
what can UMN do
excite Gamma mn, alpha mn (yielding more muscle activity), or have an inhibitory effect on the motor neuronal pool (yielding less muscle tone)
175
If UMN are damaged details
no voluntary control nor inhibitory signals can get to a particular level of the spinal cord, thus paralysis. however, the reflexes still work thus spastic paralysis is found.
176
example causes of spastic paralysis
stroke, MS, CP, ALS
177
symptoms of spastic paralysis
person involuntarily jerks, tremors, twitching, hypertonia, stiff muscles, etc
178
if LMN are damaged details
No voluntary nor reflexes can get tot he muscles. No activity on the alpha motor neuron, thus cannot contract the muscle at all. the voluntary pathway and the reflexive arc are broken, thus absolutely no muscle activity = flaccid paralysis
179
example causes of flaccid paralysis
myasthenia gravis, ALS
