Module 24 - Neuroanatomy of Sensory Systems Flashcards
What are the four functional divisions of the sensory system?
- Somatosensation
- Proprioception
- Visceral Sensation
- Special Sensation
What is somatosensation?
Sensation received from the skin and mucous membranes, as well from as the limbs and joints (GSA)
Somatosensation includes the conscious sensation of pain, temperature, and touch but such stimuli can also produce subconscious reflex responses.
What are the different types of touch somatosensation?
- Discriminative, providing specific information about location, shape, size, or texture.
- Crude touch, which simply reports that something has come in contact with the skin or mucosal surface.
What are the 4 discriminative (fine) touch mechanoreceptors?
Meissner
Pacinian corpuscles
Ruffini organs
Merkel disks
What kind of receptors mediates crude touch, pain and temperature sensation?
Free nerve endings → also found in joints
What is proprioception?
A sensation of body position and movement associated with muscles, tendons, and joints (GSA)
Proprioception is involved with conscious awareness of body position but is also a part of various subconscious pathways.
Which pathway do discriminative touch and conscious proprioception take?
The posterior column/medial lemniscus pathway
Which pathway does pain, temperature, and crude touch take?
Spinothalamic pathway
Which pathway does unconscious proprioception take?
Spinocerebellar pathways
Explain the posterior column/medial lemniscus pathway.
- The starting point is touch mechanoreceptors and proprioceptors in skin, muscle, and joints.
- Fibers from the upper limb (green) travel to C8 spinal level + Fibers from lower limb (blue), traveling L4 spinal levels, respectively, but understand that a complete representation would involve all spinal nerves.
- The somata (cell bodies) of the primary sensory neurons associated with these receptors are located in the posterior (dorsal) root ganglia. These ganglia are associated with the posterior (sensory) root of each and every spinal nerve and are commonly referred to as DRG.
- The axons from the primary neurons join white matter in the posterior (dorsal) funiculus (DF) of the spinal cord (see spinal cord inset, stained blue with yellow lettering). Those from the lower part of the body in the fasciculus gracilis (FG); those from the upper part of the body in the fasciculus cuneatus (FC). Note that when both are present, the FG is always medial relative to the FC. These are collectively called the “posterior columns.
- The FG, or the FG and FC, are evident at every level of the spinal cord.
- The axons in the FG and FC ascend through the rest of the spinal cord and pass into the medulla where they terminate (form
How is the thalamus laid out? What are certain characteristics of the thalamus?
It is composed of two hemispheres sometimes connected (by the interthalamic adhesion). Each hemisphere contains nuclei that are often named by relative location (e.g. ventral posterior). I certainly do not expect you to learn the anatomy of the thalamus in detail – but see if you can locate the VPL nucleus!
Explain the spinothalamic pathway.
- The starting point here is free nerve endings that are sensitive to pain, temperature, and crude touch.
- Primary sensory neurons located in the DRG.
- The diagram illustrates fibers carried by a spinal nerve from the upper limb (green) and lower limb (blue) but such fibers would in fact be a component of each and every spinal nerve.
- Axons from the primary sensory neurons project to Lissauer’s tract (white matter) before entering the posterior horn (gray matter) and contacting the dendrites of second-order neurons in the substantia gelatinosa or (deeper) the nucleus proprius.
- In Lissauer’s tract the axons can ascend or descend 1-3 spinal segments. For example, as shown in the diagram, an afferent that entered Lissauer’s tract at about the L4 spinal level (red arrow), is capable of ascending (as illustrated) to L2 or descend to S1. What this means is that the mapping of spinothalamic sensory information (pain, temperature, and crude touch) is spread out over several spinal cord levels.
- Fibres in the spinothalamic tract ascend through the spinal cord white matter (lateral funiculus) and then through the brainstem, reaching the VPL nucleus of the thalamus where they synapse (onto 3rd order neurons). VPL neurons project to the primary somatosensory cortex.
The projections to the primary somatosensory cortex represent pathways that lead to conscious sensations. But the same kinds of stimuli can generate other responses. What type of responses are possible?
The projections to the primary somatosensory cortex represent pathways that lead to conscious sensations. But the same kinds of stimuli can generate subconscious reflex responses generated within the spinal cord alone (and not involving projections to thalamus or cerebrum).
What are the three subconscious proprioception pathways carried by the spinal cord to the cerebellum?
- Posterior spinocerebellar tract
- Anterior spinocerebellar tract
- Cuneocerebellar tract
Explain the posterior spinocerebellar tract
- The posterior spinocerebellar tract pathway (shown in blue in the diagram) begins with proprioceptors (muscle spindles, Golgi tendon organs) from the lower body (leg).
- Primary sensory neurons are located in the DRG.
- Axons from the primary neurons ascend in fasciculus gracilis and then synapse with dendrites of Clarke’s nucleus that is found only at spinal levels T1-L2/L3.
- Axons from neurons in Clarke’s nucleus ascend in the posterior spinocerebellar tract (PSCT) to the caudal pons where they enter the cerebellum via the inferior cerebellar peduncle
Explain the anterior spinocerebellar tract
- The anterior spinocerebellar tract pathways begins with input from lower limb proprioceptors (Golgi tendon organs), but there is also input from skin mechanoreceptors.
- Primary sensory neurons are in the DRG.
- The primary neuron axons synapse with dendrites of “spinal border cells” located at T12-L5 spinal levels.
- Axons of the spinal border cells cross via the anterior white commissure…
- …and enters and then ascends in the anterior spinocerebellar tract (ASCT) to the rostral pons where the fibers enter the cerebellum via the superior cerebellar peduncle. Within the cerebellum, the fibers then cross back within the cerebellum. This pathway is sometimes said to be “double-crossed.”
- It is said to be double-crossed because…
- It crosses once at the level of the anterior white commissure in the spinal cord
- And then crosses again within the cerebellum itself
Why two spinocerebellar pathways from the lower limb?
It has been suggested that the additional sensory information (from cutaneous mechanoreceptors) in the anterior spinocerebellar pathway could allow for “more complex” processing by the cerebellum.
There may also be additional input to “spinal border cells,” from spinal interneurons or even from descending (motor) tracts.
Explain the cuneocerebellar tract.
- Starts with proprioceptors (muscle spindles, Golgi tendon organs) in the upper body (e.g. arm) with primary sensory neurons in the DRG.
- Axons from the primary neurons ascend in fasciculus cuneatus and then synapse with dendrites of neurons in the lateral cuneate nucleus (in the medulla).
- Axons from neurons in the lateral cuneate nucleus ascend in the cuneocerebellar tract (CCT) to the caudal pons and enter the cerebellum via the inferior cerebellar peduncle.
These pathways carry sensory information delivered by spinal nerves from the body to the cerebrum (via the thalamus) or cerebellum. What about similar sensory information from the head?
THE TRIGEMINAL SYSTEM
The trigeminal nerve carries GSA from the skin, joints, and muscles in the head carried by three peripheral branches: ophthalmic (V1), maxillary (V2), and mandibular (V3).
Where are primary sensory neurons for discriminative touch, conscious proprioception, pain & temperature and crude touch located?
The trigeminal ganglion
Axons from the primary neurons synapse with 2nd order neurons in EITHER the ___________________ OR ____________________________________.
Axons from the primary neurons synapse with 2nd order neurons in EITHER the spinal nucleus of CN V (pain & temperature, crude touch) OR the Chief sensory nucleus of CN V (discriminative touch, conscious proprioception).
What is the location of the spinal nucleus? What is the location of the Chief sensory nucleus?
- The spinal nucleus is found throughout the medulla and extends up into the pons;
- The Chief sensory nucleus is in the pons at the level where the trigeminal nerve is attached to the brainstem (mid-pons).
All the fibres of CN V enter the brainstem at the mid-pons level. Therefore, for fibres to reach all the different levels of the spinal trigeminal nucleus, they first travel in the __________________.
All the fibres of CN V enter the brainstem at the mid-pons level. Therefore, for fibres to reach all the different levels of the spinal trigeminal nucleus, they first travel in the spinal tract of CN V.
Axons from the 2nd order neurons in the spinal nucleus of CN V cross and join the _____________ tract (overlaps and medial to the spinothalamic tract).
Axons from the 2nd order neurons in the spinal nucleus of CN V cross and join the trigeminothalamic tract (overlaps and medial to the spinothalamic tract).
Axons from the 2nd order neurons in the Chief sensory nucleus of CN V cross and join the _________________ (more or less overlaps with medial lemniscus).
Axons from the 2nd order neurons in the Chief sensory nucleus of CN V cross and join the trigeminal lemniscus (more or less overlaps with medial lemniscus).
Both the trigeminothalamic tract and the trigeminal lemniscus ascend to the ____________________of the thalamus and synapse with 3rd order neurons. Thalamic neurons project to the primary somatosensory cortex in the ___________________.
Both the trigeminothalamic tract and the trigeminal lemniscus ascend to the ventral posteromedial (VPM) nucleus of the thalamus and synapse with 3rd order neurons. Thalamic neurons project to the primary somatosensory cortex in the post-central gyrus (parietal lobe).
With a pattern similar to that seen in the primary motor cortex, the _______________________
shows somatotopic organization.
With a pattern similar to that seen in the primary motor cortex, the primary somatosensory cortex shows somatotopic organization.
A third group of GSA fibres associated with CN V carry proprioceptive information from the teeth and gums. This pathway operates at a subconconscious level and involves the _______________________.
A third group of GSA fibres associated with CN V carry proprioceptive information from the teeth and gums. This pathway operates at a subconconscious level and involves the mesencephalic nucleus of CN V.
Primary sensory neurons for this modality are NOT in the trigeminal ganglia!!!!
Instead, they are located within the CNS, within the mesencephalic nucleus of CN V. GSA fibres enter at the level of the pons, reach the nucleus via the mesencephalic tract of CN V.
Projections from the mesencephalic nucleus (crossed and uncrossed) go to the ____________________ nucleus providing reflex proprioception of the muscles of mastication (e.g. controlling bite strength).
Not shown: there are also projections to the cerebellum, trigeminocerebellar fibres via the__________________________
- Projections from the mesencephalic nucleus (crossed and uncrossed) go to the trigeminal motor nucleus providing reflex proprioception of the muscles of mastication (e.g. controlling bite strength.
- Not shown: there are also projections to the cerebellum, trigeminocerebellar fibres via the inferior cerebellar peduncle.
What are the 5 different taste modalities?
- Bitter
- Sour
- Sweet/umami
- Salty
How are these modalities laid out on the tongue? Is it localized or spread out broadly over the tongue?
Sensitivity to each modality is not localized solely to any one area of the tongue but is distributed broadly over the tongue.
The tongue is covered with taste buds (~5000 in total) located in 3 kinds of papillae …???
- 250 Fungiform (3-5 taste buds per papillae)
- 20 Foliate (100-150 taste buds per papillae)
- 8-10 Circumvallate (250 taste buds per papillae, responsible for about half of the total number of taste buds on the tongue)
How do we perceive taste? How to make the distinction of our food?
- Taste cells express taste receptors. Each taste bud has multiple types of taste cells, but each taste cell has a single type of receptor (bitter, sour, sweet, umami, salty) located on apical microvilli
- When stimulated by the appropriate stimulus, taste cells release neurotransmitter onto dendrites of primary sensory neurons
Explain the taste pathway - FACIAL NERVE
- The central taste pathway associated with the facial nerve (anterior 2/3 of the tongue) involves primary sensory neurons in the geniculate ganglia.
- Axons (SVA of CN VII, facial nerve) project to the upper part of the solitary nucleus (gustatory nucleus) in the medulla and synapse with 2nd order neurons
- Second-order neurons project via the central tegmental tract and synapse with 3rd order neurons in the VPM nucleus of the thalamus.
- Third-order neurons project to taste cortex located in the insula and frontal operculum (the region of the frontal lobe that forms the margin of the lateral fissure).
What is the chorda tympani?
- IN FACT, taste cells (within buds of the anterior 2/3 of the tongue) synapse onto branches of the lingual nerve (a branch of CN V3) but the taste fibers exit the lingual nerve and join the facial nerve via the chorda tympani.
- Because the facial nerve carries the taste information to the brain, it is considered to be the source of this input.NOT the lingual or trigeminal nerve!!!
The vagus nerve (CN X) carries taste information from epiglottis with 1st order neurons in the ____________________________(IG X; also called the nodose ganglion)
The vagus nerve (CN X) carries taste information from epiglottis with 1st order neurons in the inferior ganglion of CN X (IG X; also called the nodose ganglion)
In each case, the axons of the primary sensory neurons terminate in the upper part of the ________________ (and travel in the adjacent solitary tract to reach the nucleus).
In each case, the axons of the primary sensory neurons terminate in the upper part of the solitary nucleus (and travel in the adjacent solitary tract to reach the nucleus).
Multiple elements of CN I (SVA) pass from the olfactory epithelium through the skull to reach the ________________ where they synapse. Note that some authors (and textbooks) classify CN I as SSA – grouping it with vision (CN I) and hearing & balance (CN VIII). I prefer classifying it as SVA because it is a ________________.
Multiple elements of CN I (SVA) pass from the olfactory epithelium through the skull to reach the olfactory bulb where they synapse. Note that some authors (and textbooks) classify CN I as SSA – grouping it with vision (CN I) and hearing & balance (CN VIII). I prefer classifying it as SVA because it is a chemical sense
Numerous areas of cerebral cortex receive input from neurons in the olfactory bulb. Collectively can be regarded as the “____________________” and is associated with the inferior, medial temporal lobe.
Numerous areas of cerebral cortex receive input from neurons in the olfactory bulb. Collectively can be regarded as the “primary olfactory cortex” and is associated with the inferior, medial temporal lobe.
Is visceral sensation just taste and smell?
The visceral sensation is not just taste and smell!
What are some examples of general visceral afferent systems that are sensory receptors and are part of the visceral sensation?
- An example of a distinct general visceral afferent (GVA) system are sensory receptors located in the walls of the carotid artery and aorta:
- Carotid body: chemoreceptors monitoring blood O2, CO2 and pH.
- Carotid sinus: baroreceptors monitoring blood pressure.
- Receptors in carotid body/sinus are innervated by CN IX with the primary sensory neurons in the inferior (petrosal) ganglia of CN IX.
- Aortic bodies: chemoreceptors and baroreceptors. Receptors innervated by CN X fibres with the primary sensory neurons in the inferior (nodose) ganglia of CN X.
- Neurons in these ganglia project to lower part of solitary nucleus.
- From the solitary nucleus, connections are made (via various tracts) with several brain areas, including the hypothalamus, to mediate cardiovascular and respiratory (autonomic) reflex responses to changes in blood pressure, and blood levels of CO2 and O2.
What are the different GVA that travel from the viscera (especially the gut)?
- as a component of the vagus nerve (CN X; via solitary nucleus).
- carried with parasympathetic branches of sacral spinal nerves (pelvic nerves).
- carried with sympathetic branches of spinal nerves.
The visceral afferents carried with sympathetics often signal ____________(mechanoreceptors, e.g. excessive distention, blockage; chemoreceptors, e.g. acid reflux, ischemia) and can be ___________.
The visceral afferents carried with sympathetics often signal pain (mechanoreceptors, e.g. excessive distention, blockage; chemoreceptors, e.g. acid reflux, ischemia) and can be conscious.
Give an example of referred pain.
Here an inflamed appendix stimulates GVA that travel back to primary sensory neurons located in the DRG. These primary sensory neurons then travel with GSA, that receive sensory information from the T10 dermatome on the skin, back to the brain (spinothalamic pathway) so that the pain is perceived as originating from the surface of the body, in the region of the T10 dermatome.
This way, the two overlapping portions of each visual hemifield (nasal: peripheral, temporal: central) end up together. After the chiasma, the right hemifield is represented on the left side (shown in green); the left hemifield is represented on the right side (shown in purple).
After the chiasma the fibres (still RGC axons) are contained within the optic tract. Most of the fibres travel to the lateral geniculate nucleus (LGN) of the thalamus where they synapse. Note that the LGN protrudes from the posterior portion of the thalamus forming the lateral geniculate body.
look at image
Neurons in the LGN then project to the primary visual cortex of the cerebrum (occipital lobe) via the _______________.
Neurons in the LGN then project to the primary visual cortex of the cerebrum (occipital lobe) via the optic radiations.
The optic radiations must pass around lateral to the lateral ventricle. Fibers carrying the representation of the superior visual field (shown in green) pass most laterally, forming_____________, relative to fibres carrying the representation of the inferior visual field (shown in blue).
The optic radiations must pass around lateral to the lateral ventricle. Fibres carrying the representation of the superior visual field (shown in green) pass most laterally, forming Meyer’s loop, relative to fibres carrying the representation of the inferior visual field (shown in blue).
The primary visual cortex is associated with the calcarine sulcus on the medial surface of the occipital lobe. The superior visual field is _________(lingual gyrus) and the inferior visual field is __________ (cuneus).
***The foveal representation is most posterior. Note: the representations are not just on the medial surface but extend into the depths of the calcarine sulcus.
image
Broadly, there are two streams of visual processing: a superior (or dorsal) ___________________in the parietal lobe (“where” pathway) and an inferior (or ventral) __________________in the temporal lobe (“what” pathway).
Broadly, there are two streams of visual processing: a superior (or dorsal) motion processing stream in the parietal lobe (“where” pathway) and an inferior (or ventral) object recognition stream in the temporal lobe (“what” pathway).
Not all RGC projections go to the LGN! What are the two major targets?
- Superior colliculus – involved with visually guided eye and head movements (recall from Module 23 the extrapyramidal tectospinal tract).
- Pretectal area – involved with the pupillary light reflex.
Explain the pathway of the pupillary light reflex.
- The pupillary light reflex involves RGCs that project via the optic tract to the pretectal nucleus and synapse.
- Neurons in the pretectal nucleus then project to and synapse on pre-ganglionic parasympathetic neurons of the Nucleus of Edinger Westphal (NEW).
- The pre-ganglionic parasympathetics in the Nucleus of Edinger-Westphal then project via the oculomotor nerve (CN III) to the ciliary ganglion and …
- …synapse onto post-ganglionic parasympathetics that then project to pupillary constrictor muscle (via ciliary nerves, not labelled).
- Therefore, when RGC are excited by light, this results in constriction of the pupil.
The _______________ is the sensory structure sensitive to sound. The auditory sensory information is carried by the ____________ nerve. Primary sensory neurons are in the _______________(not shown in the diagram).
The Cochlea is the sensory structure sensitive to sound. The auditory sensory information is carried by the cochlear nerve. Primary sensory neurons are in the cochlear (spiral) ganglion (not shown in the diagram).
Explain the auditory pathway
- The auditory pathway involves neurons in a cochlear (spiral) ganglion that then projects to cochlear nuclei in the medulla and synapse.
- The neurons in cochlear nuclei make crossed (shown) and uncrossed (not shown) projections and synapse with neurons in the superior olivary nucleus (SON) in the medulla.
- SON neurons then project to and make synapses on neurons in the inferior colliculus in the midbrain.
- Neurons in the inferior colliculus project to the medial geniculate nucleus (MGN) of the thalamus.
- Neurons in the MGN project to the primary auditory area in the superior temporal gyrus.
Explain the vestibular pathway
- In the case of the vestibular pathways, vestibular (Scarpa’s) ganglion neurons project to the vestibular nuclei (located in the upper medulla and lower pons). There are four different nuclei, but we shall consider them collectively.
- Vestibular nuclei project to:
- Nuclei of abducens, trochlear and oculomotor nerves for the coordination of eye movements via the medial longitudinal fasciculus (MLF).
- Spinal motor neurons for control of body posture and balance via the extrapyramidal vestibulospinal tracts (mentioned in Module 23).
- Vestibular nuclei project to cerebellum (details considered in Module 18).
- For conscious awareness of body position in space, other vestibular nucleus fibres project via the MLF and synapse onto neurons in various ventral posterior (VP) nuclei of the thalamus, that then project to the cerebral cortex.
- The cortical region for conscious awareness of vestibular sensation is somewhat uncertain.
- usually thought to be near the lowest part of post-central gyrus
- possibly also the posterior insula (PET scan image to the right shows increased blood flow during horizontal head movement)
