What is the lemniscal system? What is the anterolateral system?
What kind of information do they each convey?
Where do they travel relative to one another?
The term often used for the combination of the dorsal columns, medial lemniscus and the receptor input is lemniscal system. The anterolateral system is often referred to as the lateral spinothalamic tract (or simply the spinothalamic tract). Note their different positions within the spinal cord and brainstem and the site of synaptic relays. The two systems are further distinguished by the different sensory information they each carry. The anterolateral system carries primarily information about pain and temperature, while the lemniscal system carries information regarding touch, vibration and proprioception from the skin, muscles and joints. Separation of these somatosensory modalities is maintained within and between these systems as they ascend the neuraxis, i.e. see labeled lines. Somatotopic organization is maintained within these systems as they ascend the neuraxis.
What are the receptor/tansducer types within the sensory system? What kind of information do they convey?
TRANSDUCERS/RECEPTORS - begin to answer the question: what is it?
– touch, vibration, position, stretch
● CHEMO (temp and chemoreceptors also convey pain)
– smell, taste
Approximate the length of axons that convey the following modalities:
mechanoreception, pain, temperature, proprioception in limbs and trunk
Keep in mind that metabolic and other factors/processes occuring in axons comes from the cell bodies. In neurons with axons such as those that go from the limbs and trunk to the spinal cord this is a long distance. For this reason, they may require different metabolic processes than shorter axons and may cover different vascular territories.
Explain the difference btwn the activation of mechanoreceptors and chemoreceptors.
See attached figure
Mechanoreceptors: anchored to the cell membrane. Distorion of the membrane leads to activation of the receptor
Chemoreceptors: activated by a ligand. Often have second messenger systems that are activated upon ligand binding (although some are linked to ionophores)
What is a receptive field?
What is a receptive field determined by in the periphery?
In the CNS?
The region of the sensory surface within which an appropriate stimulus can elicit a response from a sensory neuron or receptor is the receptive field of that cell: stimulus quality and spatial location/extent
– in the periphery it is determined by the distribution of the receptor endings
– in the central nervous system it is determined by selective convergence and divergence of excitatory and inhibitory synapses
How does the size of a receptive field determine sensivity? Compare the size of receptive fields in human fingers vs forearm.
The larger the receptive field, the less sensitive. Note in the attached diagram that if there is a stimulus that activates the larger receptive field in the forearm/wrist area, the different parts of the same neuron are activated and the same information will be conveyed regardless of the part that is stimulated. If there is a similarly sized stimulus in the fingertip, multiple neurons will be activated because of the smaller receptive field-increased sensitivity.
Note that if one holds a quarter to their fingertips, they can tell what kind of coin and what side of that coin they are holding. If a coin is placed on the back, they may not even be able to tell what kind of coin they are holding let alone what side of the coin is on their back. This is due to the difference in size of receptive fields.
What is lateral inhibition? What does it do as it pertains to receptive fields of central neurons?
How is information from receptive fields made meaningful in the CNS? (Is information from every receptive field simply conveyed to the brain??)
Inhibitory interneurons, by lateral inhibition, produce the antagonistic surround of the receptive fields of higher-order sensory neurons. As a result each central neuron may have a very well localized and distinctive receptive field.
The nervous system uses convergence and divergence as well as inhibition and excitation to create meaningful form from information conveyed by peripheral receptors (see attached pic). If all of the information from each receptor was simply conveyed to the brain as is, the brain would have a series of pixels and no form/shape will have been created.
Note in attached pic that when more receptive fields are stimulated (picture B), inhibitory interneurons synapse on the second order neuron such that when a line of receptors longer than a certain length are stimulated the second order neuron is no longer "interested" (no longer processes that information). This is how shapes are built and conveyed to the brain.
Cortical neurons may respond to a specific stimulus orientation (edges), size and texture. They can also code velocity and direction of moving stimuli.
Explain how some sensation in a patch of skin may be preserved if there is injury to a single spinal root.
The strip of skin that is innervated by the peripheral cutaneous branches of a given spinal nerve is called a dermatome. Adjacent dermatomes partially overlap. This overlap reduces the localized effect of injury to a single spinal root.
How can different classes of mechanoreceptors be distinguished?
What is transduction? What is it due to?
What is the typical result of transduction?
Somatosensory receptors are classified primarily as mechanoreceptors, but include chemoreceptors and thermal receptors (the latter will be discussed in the context of PAIN).
A. Different classes of mechanoreceptors can be distinguished by their peripheral specialization, distribution of endings, axon diameter, myelination and quality of the sensation (submodality).
B. The transduction (conversion of stimulus energy to change in electrical potential) of a mechanoreceptor is due to opening of transmembrane channels by the physical deformation of the membrane.
1. These channels are primarily permeable to sodium, and thus the receptor potential (a graded potential) is generally depolarizing (which may trigger action potentials).
Note that there are multiple receptor types. Some cutaneous and some in muscles. When talking about touch (vibration, position, etc.) a blend of receptors are activated. Each of these receptors, because of their specializations, carry different information. Note that there are multiple fiber types with free endings.
In top table of attached figure, note they are all cutaneous receptors. Also note that Meissner corpuscles and Merkel cells have the smallest receptive fields.
In bottom table, note the fiber types are larger than the cutaenous receptors.
What is the relationship btwn diameter and conduction velocity?
What fiber type is not myelinated?
Note the relationship between diameter and conduction velocity. Axons with largest diameter are the most heavily myelinated and have the highest conduction velocity. All except C fibers (group IV) are myelinated.
C-fibers are the thinnest fibers. They are unmyelinated and have the slowest conduction velocity.
All of these receptor types and fiber types contribute to our normal full appreciation of somatosensory stimuli. Various insults (e.g. trauma, compression, vascular or metabolic deficiency) along the peripheral nerve may result in a neuropathy with functional impairment affecting all or some subset of the somatosensory modalities (and motor and autonomic fibers).
Explain the difference btwn/what it means for a receptor to be slowly adapting vs rapidly adapting.
Are Pacinian corpuscles slowly or rapidly adapting? What part of the receptor plays a role in its adaptation?
Receptors are also classified physiologically as slowly adapting (tonic) or rapidly adapting (phasic). Adaptation is the reduction of response during a maintained stimulus. It is mostly due to characteristics of the specialized receptor endings. In the case of the Pacinian corpuscle much of the adaptation is due to the connective tissue laminae surrounding the nerve ending. Stripping off the laminae reduces the adaptation (see attached figure).
How is the strength of a stimulus coded?
The strength of the stimulus is coded by the amplitude of the receptor (or generator) potentials, and by the frequency of the resulting action potentials.
T or F: Action potentials attenuate when going from receptors to the spinal cord/brainstem.
False. Action potentials travel reliably, without attenuation, from their origin near the receptor to the spinal cord (or brainstem).
Picture: A dorsal root ganglion cell showing a graded depolarizing generator (receptor) potential large enough to produce several action potentials near the beginning of myelin, which travel to the spinal cord. The receptor shows adaptation to a constant stimulus resulting in decreased firing of action potentials.
T or F: Collaterals from sensory fibers may participate in reflexes at the spinal and brainstem levels.
What is the name for diseases of the spinal cord? What deficits/symptoms may present? Describe the changes to reflexes (if any) with damage to the spinal cord.
What is the name for diseases of spinal nerve roots? What deficits/symptoms may present? Describe the changes to reflexes (if any) with damage to spinal nerve roots.
What is the name for diseases of spinal nerves? What deficits may present? Describe the changes to reflexes (if any) with damage to spinal nerves.
Diseases of the spinal cord are referred to as a myelopathy, these may include a sensory and/or motor deficit, with deceased reflexes at that level and increased reflexes at spinal levels below the injury.
Diseases of the spinal nerve roots are referred to as a radiculopathy. These often include pain and may include a sensory or motor deficit, with decreased reflexes at that level. Multiple root disease is called polyradiculopathy. Radiculopathy often causes pain due to the fact that spinal nerve roots have surrounding membranes that are often damaged with the spinal nerve root and are highly innervated with pain fibers.
Diseases of the spinal nerves are referred to as a neuropathy. If only one dorsal root is involved (mononeuropathy), the sensory or motor loss would be restricted to the peripheral distribution of that nerve: one dermatome/myotome. Neuropathy often results in decreased reflexes at that level.
Representative section of the cervical spinal cord showing the large diameter AD and AE fibers on the right and small diameter AG and C fibers on the left, to avoid clutter. The sensory pathways in the spinal cord shown here provide the input for conscious sensation. NOT SHOWN are the spinal-cerebellar pathways for unconscious sensory input.
What are the Brodmann numbers for the primary somatosensory cortex?
What do these numbers refer to?
What can damage to the posterior parietal lobe cause?
The primary somatosensory cortex of the postcentral gyrus is referred to as Brodmann areas 3, 1 and 2.
1. These numbers refer to different proportions of stellate and pyramidal cells found in these regions.
2. The different areas correspond to different submodalities: 3a - deep (muscle spindles); 3b - superficial slowly and rapidly adapting receptors; 1 - superficial rapidly adapting receptors; 2 - deep (pressure and joint position) receptors. There are thus several parallel representations of the body (homunculi) in primary somatosensory cortex.
3. Damage to any site in the sensory pathway up to the cortex may produce a loss of sensation (numbness) in the part of the body represented by that site.
4. Damage to the posterior parietal lobe may produce an agnosia: loss of the ability to recognize objects by touch, even though primary sensitivity (sensory threshold, two point discrimination) is reserved.
Explain what somtatotopic representation is.
Why is the map of the body on the cortex (homunculus) distorted?
A. The body is mapped within the central nervous system in an ordered somatotopic representation, i.e. the relative position of the points on the body surface is preserved within the neuroanatomic pathways as they traverse the spinal cord, brainstem, thalamus, internal capsule and cortex. As a result points that are close together on the skin are represented by neurons that are close together at each level of the sensory pathway. Conversely, points that are distant from each other on the skin are distant from each other at each level of the sensory pathway.
B. The map of the body on the cortex (homunculus) is distorted because the amount of cortical area for each body part is inversely related to the size of receptive fields of cortical neurons representing that body part...the smaller the receptive fields-the larger the cortical area for that body part. Fine two-point discrimination is dependent on small receptive fields of cortical neurons.
What is the vertical organization unit of the cortex?
Explain the similarities of neurons within the same unit of vertical organization.
Explain the difference btwn the complexity of receptive fields in the cortex vs those at lower levels of the sensory pathway.
In addition to the somatotopic organization, the vertical organizational unit of the cortex is the column. Neurons within a column have overlapping receptive fields and respond to similar sensory modalities.
Neurons in the cortex have more complicated receptive fields than those at lower levels of the sensory pathway. They may respond to specific stimulus orientation (edges) and textures. They may also code the velocity, speed and direction of moving stimuli.
Figure: geographic separation is common way CNS organizes info. Cortical Columns: Inputs of individual modalities to the somatosensory cortex are organized into columns of neurons extending from the pial surface to the white matter, as seen here for digits 2, 3 and 4. For each digit there are separate columns of rapidly and slowly adapting neurons (RA, SA)
What is stocking glove distribution of sensory loss suggestive of?
What are common cause of this type of sensory loss?
Stocking glove distribution suggests a peripheral neuropathy
Common causes: diabetes, alcoholism, HIV, toxin exposure, metabolic abnormalities, vitamin deficiency, or adverse effects of certain drugs
What is neglect syndrome?
What is the anatomic basis of neglect syndrome?
Neglect syndrome: neglect of contralaterl world (somatosensory, visual, auditory)
Damage of posterior parietal lobe, most commonly right parietal.