Neuroanatomy Quick Review Flashcards
Associations for Brain and Behavior
1
Q
Cerebellar peduncles
A
(Set of three) axon bundles that connect the brainstem with the cerebellum
2
Q
Regions of the brainstem
A
Medulla
Pons
Midbran
3
Q
4-4-4 Rule
A
Of the 12 cranial nerves, 4 emerge from each of the three regions of the brainstem: 4 from the medulla 4 from the pons 4 from the midbrain **or higher**
4
Q
4 CNs emerging from the medulla
A
IX, X, XI, XII (Glossopharyngeal, Vagus, Accessory, Hypoglossal)
5
Q
4 CNs emerging from the pons
A
V, VI, VII, VIII (Trigeminal, Abducens, Facial, Vestibulocochlear)
6
Q
Three components of the cerebellum
A
Cerebellar cortex
Deep cerebellar nuclei (one on each side)
3 cerebellar peduncles (on each side)
7
Q
Functional units within the cerebellar cortex (3)
A
Corticocerebellum
Sprinocerebellum
Vestibulocerebellum
8
Q
What is contained in the subarachnoid space?
A
Contains the CSF and the large cerebral blood vessels and their branches that are too large to be contained in the anastemosing network within the thin pia.
9
Q
Choroid plexus
A
Specialized secretory epithelia found in long, highly convoluted, vascularized grapevine like-strands within each ventricle that continuously produce about a half liter of CSF every day
10
Q
Ventricles
A
Four fluid-filled spaces in the brain, including two lateral ventricles (think alien), and a third and a fourth ventricle.
Another space that is often included is the CEREBRAL ACQUEDUCT.
11
Q
Hydrocephalus
A
When there is an obstruction to CSF flow (such as in the cerebral aqueduct) such that the actively-secreted CSF accumulates in the ventricles, which can expand dramatically and cause significant damage to the brain tissue (which is held in the fixed-volume-cranium)
12
Q
Hematomas- name three major types
A
A medical emergency involving bleeding outside the brain and around the meninges.
The three most common types are:
- epidural (ie lens shaped)
- subdural
- subarachnoid
13
Q
Parts of the diencephalon
A
The two largest are the thalamus and hypothalamus.
Also the subthalamus and the epithalamus.
14
Q
Mylencephalon
A
Medulla
15
Q
Metencephalon
A
Pons
Cerebellum
16
Q
Mesencephalon
A
Midbrain
17
Q
Telencephalon
A
Cerebral hemispheres - only part that is visible without cutting
Basal forebrain
Basal ganglia
Amygdala
Hippocampus
18
Q
What is the central sulcus?
A
A sulcus that coronally divides the frontal lobe from the two parietal lobes.
19
Q
Sylvian fissure/large lateral sulcus
A
Runs rostrally to caudally, separating the temporal lobes from the frontal and parietal lobes on either hemisphere of the brain.
20
Q
Functions of the frontal lobe
A
Executive function
Working memory
Motor control (think premotor and primary motor cortices located anterior to the central sulcus)
21
Q
Function of the occipital lobe
A
Vision (both the primary/striate visual cortex V1, and the extrastriate cortices)
22
Q
Function of the temporal lobe
A
Audition (primary auditory cortex on transverse gyri of Heschl, in the superior temporal lobule deep within the Sylvian fissure)
Vision (“what” pathway for identifying and naming objects).
Memory ( hippocampus in the medial temporal lobe)
23
Q
Function of the parietal lobe
A
Language processing
Bodily sensation (primary and secondary somatosensory cortices)
Association cortex
24
Q
What are the main functions of the cerebellum?
A
Control and coordination center for movement by modulating the force, speed, and pattern of muscle contraction.
Also, it is involved in learning motor skills, and detecting errors in movement when the movement produces is not what was intended.
25
Which motor axons synapse directly onto muscle?
Lower motor neurons.
26
What signals course through the ICP?
Input from the spinal cord and CNVIII to the spino- and vestibulocerebellum
(proprioceptive input?)
27
Uncus
A small cortical structure that is the most medial part of the temporal lobe. (Latin=hook)
Often the target of the olfactory tract.
Often (damaged) because it herniated, when there is an increase in intracranial pressure. Uncal herniation can be accompanied by a CN III palsy due to oculomotor nerve being compressed against the uncus.
28
Veins in brain drain mainly into...
Dural sinuses. Eventually, they are returned to the heart via the jugular veins.
29
The spinal cord extends from...
The foramen magnum to approximately the level of the L2 vertebra in adults.
The end of the spinal cord is called the CONUS MEDULLARIS, and it from it emerges the CAUDA EQUINA.
Note that the dura extends until approximately the S2 level.
30
What does the intermediate zone of spinal cord grey matter contain?
1. Pre-ganglionic SYMPATHETIC neuron cell bodies-\> intermediolateral cell column (found only in T1-L2 )
2. Neuronal cell bodies in Clark's nucleus -\> (also found only T1-L2)
3. Neurons that perform pattern generation for locomotion -\> (found only in the lumbosacral enlargement)
31
"Fight or Flight" response
Sympathetic division of the autonomic nervous system, segments T1-L2/3
Effects of sympathetic stimuli include:
1. Pupil dilation
2. Bronchodilaton
3. Cardiac acceleration
4. Stimulate sweat glands
5. Piloerection ("goose bumps")
6. Inhibition of digestion, salivation, and lacrimation
7. Stimulates of glucose production (gluconeogenesis, glycogenolysis) and release
8. Systemic vasoconstriction
9. Stimulate secretion of adrenaline and noradrenaline
10. Stimulates penile emission
11. Relaxes urinary bladder
32
Which are the two divisions of the dorsal root entry zone?
Medial or lateral divisions. All fibers have synapses at the spinal cord segment that they enter at.
Medial division fibers also send collateral branches that ascend the spinal cord in the dorsal column tracts.
33
Classification system for axons entering the dorsal root
Dorsal roots from **motor sensory neurons** are classified with Roman numerals in order of decreasing fiber diameter (Ia, Ib, II)
Dorsal roots from **cutaneous sensory nerves** are classified alphabetically in order of decreasing conduction velocity (A-beta, A-delta, C)
34
Colliculi
("little hills")
Two pairs of little bumps on the dorsal surface of the midrbain.
35
Tectum
A general term referring to the dorsal region of the brainstem (= "roof")
36
Superior collicus
Is involved in reflexive head and eye movements in response to salient stimuli of many modalities (ex. reflexively turning head and orienting eyes to focus on an object that appears in the peripheral field of vision)
Receives input from the ascending auditory and somatosensory pathways and a direct projection from the retina.
37
Three lobes of the cerebellar cortex...
- Anterior lobe (anterior to primary fissure)
- Posterior lobe (largest lobe, posterior to primary fissure)
- Flocculonodular lobe (or vestibulocerebellum)
38
Flocculonodular lobe
The smallest lobe of the cerebellar cortex, the flocculonodular lobe is subdivided into the flocculus (hemispheral component) and the nodulus (vermal component).
It is concerned with vestibular system and eye movements, and thus receives direct input from the vestibular nuclei (or nerve).
39
Each of the lobes of the cerebellum is subdivided into..
A vermal (midline) and a hemispheric (lateral) portion.
The hemispheres coordinate movement for the more peripheral/distal body, while the vermis coodinates movement for the more proximal/axial body.
40
Cerebellar tonsil
Inferior portion of the posterior lobe of the cerebellum.
It can herniate into the foramen magnum, thus creating a serious medical emergency.
41
The hills and valleys of the cerebrum are called:
Gyri and sulci, respectively
42
Folia
The gyri of the cerebellum. Run longitudinally from one side of the cerebellum to the other.
43
Functional subdivisions of the cerebellum:
1. Corticoocerebellum (neocerebellum)
2. Spinocerebellum (paleocerebellum)
3. Vestibulocerebellum (archicerebellum)
The names in parenthesis refer to the pylogenetic age of each respective division.
44
Cerebrocerebellum/Corticocerebellum
Consists of the hemispheres of the anterior and posterior lobes, but not the vermis or paravermis.
Dominated by inputs from the cerebral cortex -\> cerebral peduncle -\> synapse on neurons in the basis pontis -\> middle cerebellar peduncle -\> axons terminate on the mossy fibers in the granule cell layer of the cerebellar cortex.
45
Mossy fiber
Axon of a pontine neuron that innervates cerebellar granule cells.
46
Spinocerebellum
Consists of the vermis and paravermis of the anterior and posterior lobes.
Dominated by inputs from the spinal cord:
Pathways from Clark's nucleus (DSCT = dorsal spinocerebral tract) and the external cuneate nucleus (cuneocerebellar tract) carry information about:
* joint angles
* muscle length and tension
* cutaneous events
into the cerebelar cortex again as mossy fibers.
Enter through the inferior cerebellar peduncle and are UNCROSSED pathways.
47
Vestibulocerebellum
= flocculonodular lobe.
48
MCP
Middle cerebellar peduncle
Carries information from the cortex that was received in the (pons) pontine nuclei into the corticocerebellum.
49
ICP
INPUT: proprioceptive afferents from the spinal cord directly into the cerebellum (i.e. the spinocerebellar tract AND \_\_\_\_)
OUTPUT:
(a) two-way communication with the vestibular system (direct uncrossed projections from the vestibular portion of the 8th nerve) and uncrossed projectionf from the cerebellum to the vestibular nuclei and reticular formation
(b) crossed olivo-cerebellar tract, which originates in the inferior ollive and distributes to the entire cerebellar cortex as climbing fiber inputs to Purkinje cells
50
SCP
Exclusively OUTPUT.
All components are CROSSED.
Largest component is the axonal projections from the dentate nucleus to the thalamic nuclei VL and VA, which in turn, project to motor cortices. Smaller component: interposed nuclei to the red nucleus in the midbrain.
51
Red nucleus
Cell group in the midbrain involved in the control of movement.
The site of termination of the SCP, and the site of origin of uncrossed fibers to the inferior olivary nucleus and of the crossed rubrospinal tract.
52
Rubrospinal tract (RST)
A crossed pathway to the cervical spinal cord that is an agonist of the corticospinal tract (part of the lateral motor system)
Has very limited significance in humans.
53
(Intraventricular) Foramen of Monro
Narrow orifice between each lateral ventricle and the third ventricle.
54
Caudate nucleus
Most medial part of the striatum, bulging into the lateral ventricle with its large head in the wall of the anterior horn, tapering body immediately behind...
Principally connected with the prefrontal and other association areas of cortex and is involved more in cognitive functions, not so much in movement.
55
What two structures do the (2) setpum pellucidum separate...
They separate the two lateral ventricles, grading inferiorly into the septal nuclei. There are two, one on each side, but in post brains they are so closely apposed that they appear to be a single structure.
56
The thalamus is the floor of the body of the...
lateral ventricle.
57
Hippocampus
Specialized cortical area found in the **medial temporal lobe**.
Plays a critical role in the consolidation of new memories of facts and events (i.e. **episodic memories**) into long-term memories.
58
3 anatomical subdivisions of the hippocampus:
From most interior to most exterior:
1. Dentate gyrus
2. Hippocampus proper
3. Subiculum
59
Fornix
Major OUTPUT (efferent) pathway of the hippocampus, it loops back (in the same type of loop that the lateral ventricles take) and terminates principally in the mamillary bodies.
A prominent paired fiber bundle, mostly containing hippocampal efferents, that interconnects the hippocampus of each cerebral hemisphere and the ipsilateral septal nuclei and hypothalamus.
60
Mamillary bodies
Most posterior nucleus of the hypothalamus.
INPUT: revceives afferents from the hippocampus via the fornix.
61
Thalamocortical pathways: Medial Geniculate Nucleus (MGN) to....
Primary auditory cortex
62
Thalamocortical pathways: Lateral Geniculate Nucleus (LGN) to....
Primary Visual Cortex
63
Thalamocortical pathways: VB (ventral posterolateral and posteromedial (VPL and VPM) nuclei) to....
Primary somatosensory cortex
64
Where are the thalami located?
They form the floor of the body of the lateral ventricle on each side.
65
What are the main roles of the thalamus?
They serve as a relay center for primary inputs to all of the cerebral cortex.
They also regulate wakefullness and sleep cycles.
66
What inputs do the regions of the cortex receive?
Every region receives from one of the large collection of thalamic nuclei in addition to direct inputs from other cortical areas, as well as nonspecific inputs from nuclei such as the cholinergic fibers of the basal nucleus of Meynert.
67
Thalamocortical pathways: Vetrolateral/Ventroanterior Nucleus (VL/VA) to...
Motor cortex
68
The cerebrocerebellum modulates the activity of the motor cortex by...
acting on VL/VA via the superior cerebellar peduncle.
The collective action of the basal ganglia also operates through VL/VA to influence motor cortex output.
69
What does the internal medullary lamina divide?
The internal medullary lamina is the vertical band of white matter that divides the thalamus into MEDIAL and LATERAL nuclear groups.
It also bifurcates anteriorly to define the **anterior thalamic nucleus**.
70
Which nuclei are contained in the lateral nuclear group of the thalamus?
* MGN
* LGN
* VPL + VPM
* VL/VA
* pulvinar nucleus
71
Pulvinar nucleus
Innervates the vast expanse of extrastriate visual cortex in the occipital, parietal, and temporal lobes.
72
Which major nucleus is contained in the medial nuclear group of the thalamus?
Medialis dorsalis (MD) nucleus.
73
The anterior thalamic nucleus innervates:
The cingulate gyrus
74
Medialis dorsalis nucleus innervates...
much of the frontal lobe anterior to the motor cortices.
75
Papez Circuit
An important component of the **limbic system**.
It is a major player in the **consolidation of experience as memory**.
76
Which cerebral structures are contained in the complete loop of the Papez Circuit?
hippocampus -\> fornix -\> mamillary bodies of the hypothalamus -\> mamillothalamic tract -\> anterior thalamic nucleus -\> anterior limb of the internal capsule -\> cingulate gyrus -\> cyngulum -\> parahippocampal gyrus -\> hippocampus
77
Basal ganglia
Set of complex and highly interocnnected structures deep in the cerebrum.
Include the:
* caudate
* putamen
* globus pallidus
* subthalamus
* (in the midbrain) the substantia nigra.
78
What is the function of the basal ganglia?
They ultimately modulate the activity of the MOTOR cortices by means of VL/VA in the thalamus.
They are important for the initiation and termination of movement.
79
Lentiform Nucleus
Collective name for the putamen + globus pallidus
80
Internal capsule
Is an area of white matter in the cerebrum that separates the caudate nucleus and the thalamus from the putamen and the globus pallidus. It contains both ascending and descending axons that run between the cerebral cortex and the pyramids of the medulla.
81
Corona radiata
Sheet of white matter that continues ventrally as the internal capsule and dorsally as the fanned-out centrum semiovale.
82
Amygdala/amygdaloid nucleus Location?
Hangs like an ornament at the **extreme rostral tail of the caudate nucleus** although there are no neuronal connections between the two at this point.
Situated far medial in the temporal lobe (near the uncus) and anterior to the temporal pole of the lateral ventricle.
83
Striatum
Region where the putamen and the caudate nucleus fuse (rostrally and ventrally).
It contains the **nucleus accumbens**, i.e. the "reward center" of the brain. Its cells contain many opioid and dopaminergic receptors.
84
When is the nucleus accumbens very active?
During the period of reward after and during behaviors such as eating, taking drugs, or getting into medical school.
85
How does the optic nerve arise?
As the axons of the **retinal ganglion cells** (RGCs) leave the eye.
86
Axons from the RGCs in the optic chiasm: Which cross and which don't?
CROSS: Axons from the RGCs in the nasal hemiretina. "When you make cross-eyes, you stare at the tip of your nose"
DON'T CROSS: Axons of RGCs in the temporal hemiretina.
87
Calcarine fissure/striate cortex?
Also known as the **primary visual corte, or V1. **
88
Meyer's loop
Lateral and rostral detour through the temporal lobe that the fibers from the inferior retinal quadrants take before turning posteriorly and terminating in the lower bank of the calcarine fissure.
Part of the **optic radiation**.
These axons carry information about the **superior portion of the contralateral visual field**.
LESION: results in upper quadrantanopsia ("pie in the sky scotoma")
89
Path taken by fibers from the INFERIOR retinal quadrants
They must pass through the temporal lobe by looping around the inferior horn of the lateral ventricle. Carry information from the SUPERIOR part of the visual field. A lesion in the temporal lobe that results in damage to Meyer's loop causes a characteristic loss of vision in a superior quadrant (quadrantanopia or "pie in the sky" defect.)
90
Upper (superior) homonomous quadrantanopsia
"Pie in the sky" scotoma. Results from damage to Meyer's loop.
91
Path taken by fibers from the SUPERIOR retinal quadrants...
Travel straight back through the parietal lobe to the occipital lobe in the retrolenticular limb of the internal capsule to the visual cortex. Carry information from the INFERIOR part of the visual field
92
How does the CSF escape the brain? Where does it go then?
It escapes the brain through the Foramen of Magendie and the foramen of Luschka, to then flow in the cisterna (magna, interpeduncular, and superios) and in the subarachnoid space.
93
Brain structures that have major impact on (i.e. are critical landmarks for) sectional anatomy are...
the ventricles, the basal ganglia, and the thalamus.
94
Where does the internal capsule pass?
Between the thalamus and caudate medially, and the putamen and globus pallidus (i.e. lentiform nucleus) laterally.
95
Where do the descending axons of the internal capsule go?
Various sites in the brainstem (i.e. pontine nuclei) and the spinal cord (e.g. motorneurons and interneurons in the ventral horn).
96
Coronal sections of the brain are...
... symmetric about the midline and roughly circular in shape.
97
Cingulate gyrus
Gyrus(i) located just above the lateral ventricle(s).
98
Claustrum
Thin but extensive layer of gray matter beneath the insula, separated from it and the underlying putamen by the extreme and external capsules, respectively. Has reciprocal connections with the cerebral cortex, yet is incompletely understood.
99
Insular cortex (aka insula)
fill in.
100
Operculum
The parts of the frontal, parietal, and temporal lobes bordering the lateral sulcus and overlying the insula, hiding it from view.
101
Sylvian fissure/lateral fissure
Cleft on the lateral surface of the brain that separates the temporal and frontal lobes.
102
Circular sulcus
Rings the entire insula, and is exposed after removing the overlying opercula.
103
Heschl's gyri
The (often two) gyri that run transversely across the lower banks of the lateral sulcus. These are the location of the primary auditory cortex.
104
Short association fiber (U-fibers)
U-shaped fibers that loop down into the cortical sulci and interconnect adjacent gyri and cortical areas. Like a hammock between two gyri.
105
Superior longitudinal bundle
Thick band of long association fibers interconnecting the occipital, parietal, and frontal lobes, located just above the circular sulcus.
106
What two important areas does the superior longitudinal bundle connect?
Broca's area in the front with the association cortex in the angular gyrus.
107
Angular gyrus
Cortial region in the parietal lobe. Location of the association cortex, which is involved in many higher functions such as language, number processing and spatial cognition, memory retrieval, attention, and theory of mind
108
What does the arcuate fasciculus connect?
This long association fiber bundle connects the association cortex in the angular gyrus with Wernicke's area near the auditory cortex in the temporal lobe.
109
Uncinate fasciculus
Fiber bundle interconnecting the frontal and temporal lobes, located under the inferior surface of the frontal lobe (orbial gyri) and the tip of the temporal lobe. Beneath this structure is the ventral surface of the striatum.
110
Cerebral peduncle
Massive sheaf of tightly packed corticospinal, corticobulbar, and corticopontine fibers traveling along the base of the midbrain.
111
Optic radiation (geniculo-calcarine pathway)
A conspicuous, sharply defined, and heavily myelinated bundle of visual fibers originating in the lateral geniculate nucleus, departing the thalamus throught the retrolenticular and sublenticular parts of the internal capsule, curving in a broad fan around the atrium and the posterior and inferior horns of the lateral ventricle, and terminating in the primary visual cortex in the upper and lower banks of the calcarine sulcus.
112
Axial sections of the brain
Midline symmetry Oval shape
113
Axial sections of the brain
Midline symmetry Oval shape
114
Amygdala
Medial temporal lobe structure..
115
Hippocampus
If you are far back enough (in coronal sections) to see the pons, the structure in the medial temporal lobe will be the hippocampus.
116
What are the four(\*) commissures of the brain?
Commissures are places where the two halves of the brain interact. 1. Anterior commissure 2. Corpus callosum 3. Optic chiasm 4. Posterior commissure
117
The thalamus has a very heterogeneous appearance.
NULL
118
Interpeduncular cistern
Between the two cerebelar peduncle.
119
Pontine Tegmentum
In the posterior aspect of the pons. Responsible for REM sleep.
120
Crus cerebri
Anterior white matter portion of the cerebellar peduncle. Think "Mickey Mouse" ears, or penile crura.
121
Pineal gland
FILL IN
122
Why does the substantia nigra appear black?
Some of the dopamine oxidizes to form quinones that polymerize into a pigmented deposit called neuromelanin, which makes the cells appear black.
123
Principal neurotransmitters of the basal ganglia
Dopamine GABA (gamma-aminobutyric acid) Glutamate Acetylcholine (ACh)
124
What is the ultimate effect of the basal ganglia direct pathway?
Initiation of movement is permitted by decreasing GPi/SNr's inhibition of the thalamus, thereby increasing cortical activity.
125
What is the ultimate effect of the basal ganglia INdirect pathway?
Termination of movement by increasing GPi/SNr's inhibition of the thalamus, thereby decreasing cortical activity.
125
What signals course through the MCP?
Input from the cerebral cortex VIA the pons TO the corticocerebellum
126
What signals course through the SCP?
OUTPUT to the midbrain and to the portion of the thalamus that projects onto the motor cortices. ONLY cerebellar peduncle that contains mainly output/efferent fibers.
127
Schematically, what are the parts of the "butterfly" of gray matter in the spinal cord seen in transverse sections?
The two dorsal horns and the two ventral horns make up the "wings," while the intermediate zone makes up the "body."
128
Dorsal horn
THINK SENSORY. Contains cell bodies that receive input from sensory neurons that have THEIR cell bodies in dorsal root ganglia.
129
Ventral horn
THINK MOTOR Contain the cell bodies of lower motor neurons that synapse directly on and innervate skeletal musle
130
Where are Clarke's nuclei found? What do they contain?
T1-L2 Cell bodies that send axons that ascend the spinal cord in the DORSAL SPINOCEREBELLAR TRACT. These axons provide proprioceptive input to the cerebellum from the lower limbs.
131
What information does the spinocerebellar tract convey?
It conveys proprioceptive information about joint and limb position to the IPSILATERAL cerebellum.
132
How does the cerebellum get proprioceptive input from the lower limbs (i.e. legs)?
From the axons in the spinocerebellar tract, that originate in Clarke's nuclei between the levels of T1-L2, and course through the spinal cord until they enter the cerebellum through the ICP.
133
What is a fasciculus?
A tract of axons.
134
Where are preganglionic sympathetic cell bodies found?
In the intermediate zone of spinal cord segments T1-L2.
135
Neurotransmitter of PREganglionic sympathetic neurons...
Acetylcholine (ACh)
136
Neurontransmitter of POSTganglionic sympathetic neurons?
Norepinephrine/noradrenaline
137
How many spinal cord segments/spinal nerves are there?
31. 8 cervical 12 thoracic 5 lumbar 5 sacral 1 coccygeal
138
Where do you see a lateral horn and what does it contain?
Only seen in transverse sections of levels T1-L2. It is made up of the intermediolateral cell column containing preganglionic sympathetic neurons.
139
What is the function of interneurons?
Contribute to sensory processing or to voluntary and REFLEX contractions of skeletal muscles.
140
Ascending spinal cord tracts/fasciculi carry...
...information up to higher levels of the CNS.
141
Descending spinal cord tracts/fasciculi influence...
1. The voluntary OR reflex contraction of skeletal muscle 2. Preganglionic autonomic neurons
142
Intersegmetal propriospinal axons?
?
143
Medial division dorsal root fibers
Large-diameter, heavily myelinated axons classified as Ia, Ib, II, and A-beta fibers. Fastest-conducting, largest-diameter afferent fibers. Most sensitive to anoxia but most resistant to anesthesia.
144
Class Ia (medial division dorsal root fibers)
Muscle spindle afferent sensory fibers.
RESPONDS: to **change in length or rate of change in length** of intrafusal fiber in response to changes in stretch of extrafusal fibers in skeletal muscle. They are sensitive to BOTH change and rate of change of stretch because they associate with both dynamic and static intrafusal fibers.
EXCITE: Monosynaptically excite alpha-LMNs, which cause a contraction of the **muscle being stretched** and its agonists.
Also, its collaterals provide reciprocal inhibition by synapsing with interneurons that **inhibit** alpha-LMNs to the ANTAGONIST muscles.
145
Class II fibers (medial division dorsal root fibers)
Afferents from flower spray endings and joint receptors ( also innervate muscle spindles)
RESPOND: to events of muscle strength and change in joint angle ( **while** the muscle is being strretched but NOT to the **rate** of stretch)
\*\*\*\*They don't monitor the rate of change of muscle stretch because they only associate with and receive input from the STATIC intrafusal fibers, which are sensitive to maintained length changes.
146
Class Ib (medial division dorsal root fibers)
Golgi tendon organ afferent.
RESPOND: to changes in muscle force or tension (i.e. contraction) sensed by the GTOs. Their firing rate is a linear representation or function of the muscle force sensed by the GTO.
SYNAPSE: with a variety of interneurons that regulate the activity of alpha-LMNs by distributing the force generated by the contracting muscle by inhibiting or activating alpha motor neurons to that muscle (an autogenic inhibition). Ul;timate effect is to inhibit the contraction of its respective muscle if it senses excessive force, thereby allowing that muscle to relax.
147
Lateral division dorsal root fibers
Slowest-conducting, smallest-diameter afferent fibers. More resistant than medial division fibers to anoxia, but most sensitive to anesthesia. Can be thinly myelinated (A-delta fibers) or unmyelinated (C fibers)
148
A-beta fibers (medial division dorsal root fibers)
Innervate mechanoreceptors in skin. These are FINE tactile (epicritic) sensory fibers, in that they sense light touch, vibration, and proprioception Therefore, recall that they send their information up to the higher CNS via the dorsal column/medial lemniscus system.
149
Merkel cells
Sense pressure, texture
150
Meissener's corpuscle
Sense touch velocity and flutter (i.e. low frequency vibration)
151
Ruffini corpuscle
Sense skin stretch
152
Pacinian corpucle
sense vibration
153
Hair receptors
Sense hair stroking
154
Joint receptors
Sense joint angle
155
A-delta (lateral division dorsal horn fibers)
Bare nerve endings that sense sharp pain, cold, and visceral receptors (pain)
156
C-fibers
Bare nerve endings that sense burning pain and warmth.
157
Pain, temperature (protopathic), and visceral sensory fibers
A-delta and C-fibers
158
Three types of motor axons that exis the spinal cord in a ventral root:
1. Axons of alpha and gamma motor neurons 2. Preganglionic autonomic axons (between T1-L2 segments, and from S2-S4 segments)
159
Alpha motor neurons
Exit the spinal cord from ventral horn in all segments. INNERVATE: extrafusal muscle fibers at neuromuscular junctions (NMJs) FUNCTION: contract a motor unit
160
Gamma motor neurons
Exit the spinal cord from ventral horn in all segments.
INNERVATE: Provide efferent innervation to either dynamic or static intrafusal fibers within muscle spindles, but NOT GTOs.
FUNCTION: Increase the sensitivity of spindles to rate of stretch and change in muscle length BY causing the polar (end) regions of the muscle spinde to contract.
In other words, they extend the dynamic range of the spindle response by "taking in the slack" during extrafusal contractions, thereby adjusting the sensitivity of the spindles.
161
Topographic organization of alpha and gamma motor neurons in the ventral horn: The cell bodies of neurons that innervate flexors are more _____ and the ones that innervate extensors are more \_\_\_\_\_.
Dorsal, ventral.
162
Topographic organization of alpha and gamma motor neurons in the ventral horn: The cell bodies of neurons that innervate proximal structures are more _____ and the ones that innervate distal are more \_\_\_\_\_.
Medial, lateral. THINK: the more lateral cell bodies are the furthest from the middle of the spinal cord, just as the more distal structures are furthest from the trunk.
163
Corticospinal tract
Largest and most important UMN tract for VOLUNTARY control of flexor muscles. 60% of its axons arise in the primary motor cortex and premotor area (located just anterior to the precentral cyrus of the frontal lobe). The other 40% of its axons arise from the 1ary and 2ary somatosensory cortices.
164
Rubrospinal tract
fds
165
Reticulospinal tract
fds
166
Vestibulospinal tract
fsd
167
What are Renshaw cells? What is their function?
They are inhibitory interneurons found in the ventral horn.
Recurrent collaterals of axons of alpha-LMNs innervate Renshaw cells (through ACh binding muscarininc receptors), and the Renshaw cells in turn provide a recurrent, or feedback, inhibition by synapsing with the same alpha motor neuron.
FUNCTION/PURPOSE: They provide a mechanism to adjust the sensitivity of pools of alpha motor neurons and inhibit the firing of alpha motor neurons after a brief period of excitation.
168
Where are cell bodies of UMNs?
In the cerebral cortex and brainstem Most cases, are CONTRALATERAL to the LMNs that they innervate.
169
Where are the cell bodies of LMNs?
In the ventral horn, on the side ipsilateral to the muscles/structures that they innervate.
170
What do UMNs synapse with?
LMNs or interneurons that then synapse on LMNs.
171
Which structures in the brainstem contain UMN cell bodies?
-red nucleus - pontine and medullary reticular formation - laterall and medial vestibular nucleus
172
Path taken by the UMNs in the corticospinal tract
1. Axons arise in the primary and premotor cortices (60%) and in the primary and secondary somatosensory cortices 2. Axons exit the cortex in the internal capsule 3. Then descend through the ventral brainstem 4. In the lower medulla, 80-90% of corticospinal fibers cross at the DECUSSATION of the PYRAMIDS. 5. Crossed axons descend the full length of the cord in the lateral funiculus of the white matter.
173
Where are the cervical and lumbar enlargements? What do they contain?
Cervical enlargement: C5-T1 (i.e. brachial plexus) Lumbosacral enlargement: L2-S3 (Lumbosacral plexus) These enlargements contain the spinal cord segments that innervate the upper and lower limbs, respectively. In both enlargements, the ventral horns are large.
174
What is the topographic organization of LMNs in the ventral horns?
Proximal to distal (in the medial to lateral axis) arrangement.
175
Renshaw cells
hj
176
Which UMNs are the ones that do NOT cross at the decussation of the pyramids?
**Extensor-biased** UMN tracts that course through the anterior funiculus and perform **bilateral** innervation.
177
What are muscle spindles?
Intrafusal fibers in skeletal muscle that lie functionally in parallel with the extrafusal fibers.
Each muscle spindle is an encapsulated collection of ~10 specialized (intrafusal) muscle fibers. They are functionally divided into two types, called dynamic fibers and static fibers. The dynamic fibers are sensitive to ongoing length changes, while static fibers are sensitive to maintained length changes.
RESPOND TO:stretch or lengthening of the surrounding skeletal (extrafusal) muscle fibers.
STIMULATE: the class Ia and II afferent fibers (medial division).
178
How do Ia and II sensory fibers convey the information about the amount or degree of muscle stretch?
The firing rate of these fibers increases in a manner that is **directly proportional** to the degree of strech in the muscle spindle that they innervate.
179
Which spinal cord segments do the commonly-tested muscle strech reflexes rely on?
Gastrocnemious/soleus (achilles tendon/ankle jerk) reflex: S1-2
Quadriceps femoris (patellar tendon/knee jerk): L3-4
Biceps brachii: C5-6
Triceps brachii: C7-8
\*\*\* Mnemonic: follow the numbers 1-8. Remember Dr. Ellman's video.
180
How are muscle stretch reflexes graded?
From 0 (absence of muscle stretch reflex)
2+ (normal, moderately brisk reflexes)
To 4 (hyperactive stretch relexes with clonus)
181
Clinically, how do we grade muscle strength?
From 0/5 (patient unable to generate contractions that produce movement)...
....to 5/5 (normal strength, patient generates movement against full resistance).
182
Which elements are involved in the muscle stretch reflex? What does it sense?
The muscle stretch reflex uses:
* muscle spindles
* class Ia and II sensory fibers (both are sensitive to change in muscle stretch, while Ia is also sensitive to the rate of change in muscle stretch)
* alpha-LMNs
* inhibitory interneurons
The muscle stretch reflex provides the basis for muscle tone, i.e. the slight resistance to stretch found in all normally innervated muscles.
This is a "resistance reflex," where the end result is to excite the alpha motor neurons that innervate the same muscle (as well as agonist muscles at the same joint). This is the “resistance reflex”.
183
Which elements are involved in the Golgi tendon reflex? What does it sense?
The golgi tendon reflex involves:
* Golgi tendon organs (GTOs)
* class Ib dorsal root sensory fibers
* Interneurons
* Alpha motor neurons
Also known as the inverse muscle stretch reflex, the golgi tendon reflex senses large changes in muscle force or tension (due to that muscle's **contraction**), and reacts by **inhibiting the affected muscle, as well as it agonists**.
This is a **negative feedback reflex that helps stabilize joint position**.
184
Golgi tendon organs (GTOs)
Encapsulated clusters of ~10 extrafusal muscle fiber endings, with collagen fiber bundles connecting these endings to a tendon.
Are clustered at the tips of muscle-tendon junctions of skeletal muscles, and are functionally in series with these muscles.
RESPOND: contraction of the skeletal muscle fibers that terminate within that particular GTO.
INNERVATED: by the dendrites of a single Ib afferent fibers. These dendrites weave between the collagen fiber bundles.
185
How is the signal of muscle stretch transduces by the muscle spindle and its associates Ia and II dorsal root neurons?
The sensory neuron terminals wrap around and are closely associated with the central, non-contractile region of the intrafusal fibers.
When the intrafusal fibers are stretched, the sensory terminals are also stretched.
This local membrane stretch activates stretch-sensitive ion channels in the sensory neurons, causing a depolarization.
Sufficiently strong stretch causes a large depolarization, eliciting action potentials that then propagate along the sensory axon to the spinal cord.
186
Which are the only contractile regions of intrafusal (i.e. muscle spindle) muscle fibers?
The ends.
187
What is the function of the gamma-LMN loop?
It facilitates the contraction of skeletal muscles.
This is achieved when gamma-LMNs excite the polar/end regions of muscle spindles, causing them to contract. This stretches the central regions of the cell, which results in the firing of Ia fibers.
Since Ia fibers are meant to respond to muscle stretch by causing the encapsulating muscle to contract, this results in the activation of alpha-LMNs that innervate the same muscle, thus facilitating the contraction of extrafusal fibers of that muscle.
188
Alpha-gamma coactivation
PURPOSE: to keep muscle spindles active during muscle contractions (by usign gamma-LMNs to take out the slack), thereby allowing muscle spindles to continue to give appropriate proprioceptive information to the spinal cord.
This is especially important when muscles are contracting against an unexpected load.
Spinal cord and brainstem circuits are informed about this coactivation, in order to be able to interpret the information provided by the sensory afferents (i.e. information about load).
189
Tetanus toxin
EFFECT: **Sustained contractions and spasms** of axial and limb muscles. If CNs are involved, the patient may have spasm of muscles of mastication ("lock-jaw") and of the muscles of facial expression innervated by CNVII.
## Footnote
MECHANISM OF ACTION: Tetanus toxin is taken up and **retrogradedly transported** by axons that innervate skeletal muscle. It inhibits the release of **glycine** (an inhibitory neurotransmitters) from the synaptic terminals of Renshaw cells, thus inactivating Renshaw's cells inhibition of alpha-LMNs.
190
Flexor withdrawal/Crossed extensor reflex
This is a protective reflex that is activated by a noxious (i.e stepping on nail) or excessive thermal stimulation (touching a burning stove) of the skin that excites **cutaneous A-delta and class C dorsal root fibers**.
The A-delta and C fibers enter the spinal cord and synapse with mutliple excitatory and inhibitory interneurons.
**IPSILATERAL FLEXION**: On the side of the stimulus, the flexors are excited and the extensors inhibited so that you withdraw the limb from the bad stimulus.
**CONTRALATERAL EXTENSION**: One the side contralateral to the stimulus, excitatory interneurons make the extensor muscles contract to provide postural support during limb withdrawal.
191
Spinal reflex
* **Relatively rapid, involuntary response to a discrete stimulus**.
* Designed for rapid correction.
* Graded in intensity: Increasing the stimulus strength leads to an increase in the reflex response.
* These reflexes are also subject to modification by descending systems and central spinal circuits (see below). This means that spinal reflexes are flexible constructs that result in **state (behavioral)-dependent responses**.
192
Which portion of the spinal cord does the anterior spinal artery supply?
The anterior 2/3.
193
Which portion of the spinal cord does the posteriorspinal artery supply?
The posterior 2/3.
