CNS/PNS Flashcards
Like other organ-systems, the nervous system contains various cell types with neurons being the most important. Unlike other organ systems, the cell bodies of neurons have long processes called axons that allow one neuron to communicate with hundreds or thousands of other neurons.
These can synapse with nearby neurons, so-called interneurons, or project vast distances through the spinal cord and peripheral nerves. Most neurons have an inhibitory influence and suppress information so that only the important stuff gets the attention it deserves.
The nervous system can be divided into three major components:
the Central Nervous System or CNS,
the Peripheral Nervous System or PNS and
the Autonomic Nervous System or ANS.
What does the CNS include?
The CNS includes the brain, brainstem, cerebellum and spinal cord.
What does the PNS consist of?
The PNS consists of sensory nerves whose cell bodies originate in ganglia that lie outside of the CNS and motor nerves whose cell bodies lie in the anterior horn cell of the spinal cord and certain cranial motor nuclei in the brainstem.
Accurate diagnosis of nervous system disorders critically depends on determining the anatomy of the lesion. Does the lesion lie on the right or left side of the body? Is it rostral or caudal along the so-called “neuro-axis”. In other words, is the lesion located in the cerebral hemispheres, deep structures of the brain, the brainstem, the cerebellum, the spinal cord, nerve root, nerve plexus, peripheral nerve, neuromuscular junction, or in the muscle? An accurate anatomical diagnosis will limit the differential diagnosis to a manageable few disorders, and that will improve the efficiency and efficacy of the care you deliver to the patient.
The surface of the brain consists of creases and folds, called ____ and _____ that are unique to every individual, much like fingerprints, but common features permit identification of important brain topography.
gyri and sulci
What separates the frontal lobe from the parietal lobe?
The central sulcus
What divides the temporal lobe from the frontal and parietal lobes? Lobes are large areas of the brain that house specific functions
Sylvian fissure
The frontal lobe contains many executive functions such as what?
deciding what you want to do next, what thoughts you want to express in words or writing, where you want to move your eyes to view the world, where you want place your arm, manipulate an object or move your leg. Think of it as a “to do” or the motor part of the brain.
What does the parietal lobe do?
The parietal lobe is the area that receives and interprets sensory information from principal modalities such as fine touch, position sense, vibration, pain and temperature. It determines the texture, shape, temperature and other characteristics that allow you to identify an object held in your hand but also to know where your hand is in three dimensional space.
Think of it as an area of the brain that interprets the physical environment around you.
What does the temporal lobe do?
The temporal lobe has several important functions. In the superior temporal gyrus lies the primary auditory cortex (called Heschl’s gyrus) that processes sounds. It is shown by the asterisk. Just posterior to it lies “Wernicke’s area” that interprets the processed sounds as words and mediates the comprehension of language.
Another important function of the temporal lobe is memory (part of the Limbic system)
What are the main roles of the occipital lobe?
it also interprets the environment but is devoted to a special sense: vision.
The cerebellum consists of two lobes or hemispheres, right and left. What do they do?
Unlike the cerebral hemispheres that govern the contralateral side of the body and sensory world, the cerebellar hemispheres receive and organize motor information about the limbs on the same or ipsilateral side of the body. Their function is to smooth out anticipated movements so movements are coordinated with what the rest of the body is doing.
The medial view of the brain shows the four major subdivisions of the cerebral hemispheres, the frontal, parietal, temporal, and occipital lobes, plus the limbic lobe that contains elements of the limbic system. What does the limbic system do?
It is involved in emotion, learning and in the formation of new memories. (older memories are ok)
There is no obvious boundary between the parietal and occipital lobes on the lateral view of the hemisphere but in the medial view, the ______ clearly demarcates the two lobes.
parieto-occipital sulcus
This slide shows the major lobes of the brain from a lateral, medial, superior and inferior (base of the brain) views. Note the interhemispheric fissure. This is the other major fissure in addition to the lateral or Sylvian fissure.
What is the concept of hemispheric dominance?
The left hemisphere is devoted to language and calculation, which defines hemispheric dominance, and the right hemisphere is devoted to visual-spatial processing. Most individuals are left hemispheric dominant and right handed but about two-thirds of left handed individuals are also left hemispheric dominant.
The remainder exhibit language function in both hemispheres, and only a small minority actually house language in the right hemisphere.
The cortex can be divided into what?
primary and association cortex.
What are the four types of primary cortex?
one motor and three sensory: auditory, visual and somatosensory.
The primary sensory cortex receives the most direct connections from its sensory organs in the periphery. Each has a neighboring association area that processes the raw sensory data to give meaning to it.
For example, the primary visual cortex detects basic features of the visual world such as edges, light, dark, color, location, direction of movement, and so on. The nearby association cortex takes this information and recognizes faces, objects and makes sense of the visual world.
Most of the cerebral cortex consists of association cortex which can be unimodal or heteromodal. Unimodal means that only one sensory or motor modality is involved. Heteromodal association cortex manages information from multiple sensory modalities and involves higher levels of information processing. It is this heteromodal cortex that is most developed in humans when brains are compared among different animal species.
These two drawings nicely depict the flow of information from the primary sensory cortex to association cortex (left) and to higher order association cortex (right) that is heteromodal cortex. (However, notice for motor how the flow is reversed- info first in association cortex then to the primary motor cortex) How is sound processed?
Sound is detected in its basic qualities such as pitch, frequency, tone in primary cortex centers and that information is processed by neighboring association neurons into words, for example, and these are given meaning in Wernicke’s area nearby.
T or F. Comprehension of language in Wernicke’s area is heteromodal in so far that language can be heard (auditory), read (visual), and felt (Braille for the blind).
T.
Where does the primary motor cortex lie? How does it work?
in the precentral gyrus and it uses the corticospinal tract to carry information to the spinal cord to produce an intended movement.
Flow of information here is reversed when compared to sensory information flow but the general scheme applies. Complex planning begins in the region with “executive function”, the so-called prefrontal association cortex that receives input from multiple sensory and other areas of the brain. It sends a motor command to the premotor cortex, that receives input from the cerebellum and basal ganglia and then conveys the information to the primary motor cortex for execution. Some of this information, however, is also sent from association cortex to the spinal cord via the corticospinal tract.
What is visual agnosia?
A lesion to the association area produces problems with information processing. For example, visual agnosias involve the inability to recognize objects although its features can be described in detail.
If you take a photo of someone you recognize and look at it upside down, the recognition disappears although you can describe the details just as well. That is a visual agnosia.
What occurs with injury to the somatosensory association areas?
the patient has difficulty identifying items in his hand (astereognosis) and will often not be able to identify numbers written on the palm of his hand (agraphesthesia).
Some patients with injury to the right parietal lobe association areas develop what?
a geographic agnosia. They become lost in their own neighborhood, a common problem in patients with Alzheimer disease.
A lesion to the motor association areas will produce what?
an apraxia, which is a defect in producing a complex motor task such as dressing one self, striking a match, combing one’s hair or making a pot of coffee.
One recurrent theme in Neuroscience is that the body is topographically organized in its gray and white matter. For example, the primary motor cortex in the precentral gyrus controls the contralateral body as shown in this motor homunculus.
The corticobulbar and corticospinal tracts that arise in the primary motor cortex maintain their topography as they descend through the white matter, internal capsule, brainstem and even after the fibers cross in the cervical spinal cord. The head, arm and leg show a medial to lateral relationship respectively. How do we know this? In the 1930s, Wilder Penfield, a neurosurgeon in Montreal, used small electrodes to stimulate brain areas in awake individuals undergoing tumor surgery under local anesthesia. The aim was to identify normal brain from diseased tissue that needed resection. Diseased tissue or tumor produced no response from the patient and was presumed safe for removal. If the electric current caused a small twitch, a sensory sensation or called up a memory, then one left that area alone. In this way, he was able to map out a motor and sensory brain topography as represented by the homonculus.
There are three anatomical planes in which the brain and spinal cord may be sectioned:
the coronal plane, the sagittal plane and the horizontal plane, also called the axial plane.
These terms are important for communication about brain anatomy especially when describing CT or MRI images. MRI brain images that correspond to their respective tissue planes are shown below the brain figures. Notice how the outer cortical grey matter stands out from the deeper white matter in the MRI images.
The distinction between gray and white matter is evident in this gross horizontal section of the brain. What is ‘white matter’?
Because axons in the CNS are heavily myelinated to accelerate signal conduction, bundles of axons forming pathways and tracts appear white. Brain tissue containing these tracts is called white matter. The whiteness is due to the high fat content of myelin.
Brain areas with dense populations of neurons have much less myelin, appear darker and are called ‘gray matter’.
The grey vs. white difference is also detected by CT scanning but unlike the MRI picture, the white matter appears darker and the grey matter appears brighter. Why?
The dark to light scale on CT is based on tissue density with low density fat of myelin appearing dark and higher density gray matter and blood appearing brighter.
T or F. The grey matter is metabolically three times more active than white matter because of its ongoing membrane depolarizations and repolarizations.
T. Hence blood flow is three times greater in grey matter than in white matter so as to meet the increased metabolic demands. The presence of the blood and hemoglobin add to the density of the grey tissue which shows up as brighter than the white matter.
This slide presents three sections through the central nervous system.
Compare the coronal and horizontal sections of the cerebrum with the axial section of the spinal cord on the lower left.
Describe the gray/white matter distribution in the forebrain
In the forebrain much of the gray matter lies on the outer surface of the brain with the noted exceptions of several large gray matter nuclei that lie deep in the brain while all the white matter is deep.
Describe the grey/white matter distribution in the spinal cord
Note that this pattern is reversed in the spinal cord where the gray matter is central in the cord and the white matter is placed nearer the surface.
This has interesting implications for traumatic injury to the spinal cord. The interior grey matter is more susceptible to trauma than the outer tracts that are insulated with myelin fat. The result is a so-called central cord syndrome about which you will hear more later.
Deep in the cerebral hemispheres lie two major collections of gray matter, _____ and __________
the basal ganglia and the thalamus.
What does the basal ganglia do?
help to regulate/initiate/smooth out movement in coordination with the cerebellum. Sends their info to the thalamus (like the cerebellum) before they go to the motor cortex
What two diseases are primarily related to dysfunction of the basal ganglia?
When its function is reduced, movement is reduced as occurs in Parkinson Disease. When its function is abnormally increased, involuntary movements result such as chorea in Huntington disease.
The caudate and putamen are part of a single nuclear structure called the _____, shown in purple, artificially separated by fibers of the internal capsule better seen in the upper left picture under the asterisk.
striatum
In axial cuts, the globus pallidus nuclei and putamen have the shape of a lens and are sometimes called what?
the lenticular nucleus.
Other nuclei that have a close relationship with the basal ganglia, but are not part of the basal ganglia, include the substantia nigra and the subthalamic nucleus. As in the case of the cerebellum, the basal ganglia channel their motor input to the prefrontal motor cortex via the thalamus shown in red
What is this?
A coronal section of the brain at the level of the basal ganglia. The Weigert stain stains myelin black and nuclei stand out as lighter areas.
ID the parts of this image
The C identifies the caudate nucleus, P = putamen and the asterisk shows the internal capsule artificially separating these two otherwise identical nuclei.
You can see the GP globus pallidus and the most medial portion, GP internal, is just barely visualized.
CM= centromedial nuclei is part of the reticular activating system (aka the ascending arousal system), which is responsible for consciousness (and you have to be awake to use your cortex)
What are the main components of the brainstem?
The main components of the brainstem include the aqueduct (drains CSF from the ventricles) midbrain, colored blue, the pons (P), and the medulla (M) that connects the brain to the spinal cord. (both the pons and medullar house the CN nuclei)
Spinal cord composition
There are 8 cervical cord segments shown in green but only 7 cervical vertebrae.
The thoracic spinal cord contains 12 segments shown in blue as does the spinal column vertebrae.
There are 5 lumbar spinal nerves, shown in purple and 5 lumbar vertebrae.
The 5 sacral spinal nerves shown in red are paired with the fused sacral vertebra forming the sacral bone.
Note that as a child grows into adulthood, the spinal cord is pulled rostrally such that the lower thoracic, lumbar, and sacral nerves exit the spinal canal several segments below their exit from the spinal cord. Note also that the adult spinal cord ends at approximately the T12 – L1 vertebral level.
This slide presents an axial section through the spinal cord. Note once again the central location of the gray matter and the outer location of the white matter. The gray matter in the spinal cord is organized into what?
bilateral dorsal horns and bilateral ventral horns.
Note the composite spinal nerve with the dorsal root serving afferent nerves, that is peripheral sensory nerve fibers traveling to the cord, and the ventral root serving efferent nerves, that is motor nerves traveling to muscles and other organs.
The motor system is a two neuron system consisting of lower and upper motor neurons. What is a lower motor neuron?
The lower motor neuron (LMN) is the neuron that synapses on skeletal muscle. Note that there is some skeletal like muscle in the head so some LMN are in the brainstem
How do UMNs work?
The upper motor neurons (UMN) “talk” to the lower motor neurons. The basal ganglia and the cerebellum influence motor function by sending input through the thalamus to the upper motor neurons.
The wavy line separates the Peripheral Nervous System from the Central Nervous System.
The others will be discussed later but here, you should know that there are multiple types of upper motor neurons that influence lower motor neurons, and that some originate in the motor cortex and others in the brainstem.
The descending corticospinal tract is by far the most important one for you to learn.
Note that the basal ganglia and cerebellum, which are very important in motor movement, do not project directly to the lower motor neuron.
Also, note that the upper motor neurons found in the brain stem do not target any lower motor neurons in the brain stem. They project exclusively to lower motor neurons in the spinal cord.
Describe the corticospinal (pyramidal) tract. Where does it begin?
This pathway originates in large so-called Betz cells, that is, neurons in the primary motor cortex of the forebrain which send axons through the posterior limb of the internal capsule, down through the brainstem (through the middle 3/5ths of the crus cerebri in the anterior mibrain to the pons, where the fibers are broken up into many bundles).
The tract then descend as ‘pyramids’ in the anterior part of the medulla where they then decussate (~90%) (aka switch to the opposite side) before descending through the lateral spinal cord as the lateral corticospinal tract. (The remaining ~10% of fibers that dont cross, descend as the anterior corticospinal tract- no clinical relevance)
These axons terminate at a synapse to LMNs in the anterior horn of the spinal cord. This lower motor neuron sends its axon out through the ventral spinal root to enter a peripheral nerve and thereby reach the end organ, muscle that it will control.
The general scheme of the sensory system is a three neuron system (recall: The motor system was a two neuron system. This is a three neuron system.)
The primary sensory neuron cell body lies where?
In the dorsal root ganglion or DRG and it has receptor specialization at its afferent (receiving) terminus R. Receptors include golgi tendon organs, muscle spindles, Pacinian corpuscles, Meisner’s corpuscles and so on.
The wavy line demarcates the peripheral nervous system from the central nervous system, basically where the DRG axon enters the spinal cord.
The axon synapses on a secondary neuron located in the spinal cord (dorsal column system) or in the lower part of the brainstem.
Describe the protopathic pathway
This neuron sends one axon to the periphery where its specialized nerve endings sense pain and temperature. There is a central extension of this same axon that enters the spinal cord.
Within the spinal cord this axon synapses to a second neuron located in the gray matter of the dorsal horn. This secondary neuron sends its axon across to the opposite side, that is it decussates, and travels rostrally through the spinal cord via the spinothalamic tract, then via the spinal lemniscus in the brainstem to synapse with a third neuron lying in the ventral posterior nucleus of the thalamus. This tertiary neuron sends its axon shown as a solid orange line, to neurons lying in the primary sensory cortex of the forebrain to complete the pathway
The third pathway is also a sensory pathway that informs the brain of the position of body parts in space, that is position sense. This pathway, called the dorsal or posterior column, also has its origin from pseudounipolar neurons lying in the dorsal root ganglia. Describe it
One portion of the axon ends in the periphery as a special sensory terminal that detects vibration and position changes and the other end of the axon enters the spinal cord and travels rostrally on the same side of the cord up to the medulla of the brainstem where it synapses to a second neuron in either the gracile or cuneate nuclei. This secondary neuron sends its axon across to the opposite side and up to the ventral posterior lateral or VPL nucleus of the thalamus where it synapses on a third neuron. The VPL neuron sends its axon up to the sensory cortex to synapse on a cortical neuron.
General outline of cortex processing: Outside info from the thalamus first goes to the primary cortex (e.g. somatosensory, visual, etc.), and then to the association cortex (either unimodal or heteromodal)
Notice how the motor fibers for the legs originate medially but they cross over in the spinal column to lie laterally, so that in pts with spinal stenosis, the legs are the first things affected- expect to see weak legs, very slowing progressing
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