Physiology Flashcards
What is the somatosensory pathways?
The somatosensory pathway is the neural pathway that transmits sensory information from the skin, muscles, joints, and viscera to the central nervous system. It is responsible for processing touch, pressure, vibration, proprioception, pain, and temperature. The pathway consists of three-order neurons:
- First-order neurons: Detect stimuli and transmit signals from peripheral receptors to the spinal cord or brainstem.
- Second-order neurons: Relay signals from the spinal cord or brainstem to the thalamus, where they cross the midline (decussation).
- Third-order neurons: Project from the thalamus to the somatosensory cortex (postcentral gyrus) for perception.
The receptors, which are specialized for each sensory function, connect to ganglion cells responsible for transducing the stimuli into electrical signals.
What are ganglion cells? Why are some myelinated and others aren’t?
In the nervous system, ganglion cells are neurons located in ganglia, which are clusters of nerve cell bodies found outside the central nervous system—typically in the peripheral nervous system.
There are different types based on location and function : sensory ganglion cells, autonomic ganglion cells, retinal ganglion cells.
Some fibers are highly myelinated like the A-alpha and A-beta others aren’t myelinated like C fibers.
The high velocity of the conduction depends on the myelin sheath : more myelin means faster the conduction.
What are some different receptors involved in the sensory system?
Mechanoreceptors—> mechanical stress causes the opening of sodium channels which depolarized the cells. The change in the membrane potential is proportional to intensity of the stimulus.
Chemoreceptors—> selective binding of a specific molecule with a portion of the receptor causes sodium channels to open and triggering of an action potential.
Photoreceptors—> photons will trigger downstream signaling pathway that ultimately result in an action potential.
During a neurological evaluation, how can you evaluate the somatosensory pathways of the patient?
- Cutaneous :
- mechanoreception—> non painful mechanical stimuli
- thermoreception—> cold induces an increase firing rate while warmth decreases. But as temperature drops below 0 or over 45/50 a painful stimulus is conveyed.
- Deep
- position and movement of muscles, tendons and joints—> the patient can be actively or passively moved in order to test if he or she can receive information.
How do we asses proprioception of a patient?
To assess the ability of the patient to identify the position of a limb, a wrist is passively moved and he/she, with eyes closed, is asked to describe its spatial orientation.
To assess the ability of the patient to comprehend movement of joints, muscles and tendons, a finger (or a wrist, if testing with the finger fails) is moved and the patient is asked whether he can perceive the movement or not. Patient answer yes and often lie therefore sometimes the question is asked when the physician isn’t moving anything or the physician can ask a different question, like is the limb moving up or down.
Moreover, a test for proprioception is also the finger-to-nose, performed with eyes closed
What are some superficial and deep receptors?
Superficial :
Meissner’s corpuscles—> rapid, found in superficial layers of skin and most abundant in non hairy areas like palms, maximal response to local skin pressure.
Merkel disks—> slow, located in superficial layers of skin and most abundant in non hairy areas like fingertips, involved shape and material perception.
Deep :
Pacini corpuscles—> rapid, deep in the skin, maximal response to vibration, especially at 200 Hz.
Ruffini receptors—> slow, deep in the skin, unidirectional skin tension and unconscious hand and finger movement.
Muscle spindle fibers 1A —> proprioception.
Golgi tendon organs 1B —> tendon stretch.
How is vibration perception assessed during neurological examination?
It is assessed using a tuning fork, specifically a Rydel-Seiffer tuning fork. It uses an arbitrary scale of 0 to 8 which indicates the vibration amplitude, 0 being very fast vibration.
What is a long refractory period of the visual system responsible for?
It causes afterimages, where retinal photochemical activity continues briefly even after the stimulus ends.
What is the two point discrimination test?
It is a test used to discriminate the minimal distance at which two stimuli are perceived as separate is measured. Patients with a loss of peripheral nerve receptors will have a reduced 2 point discrimination and will perceive 2 close points as one single one.
3 receptors are needed to discriminate two close points: two firing and one non firing in the middle. The neurons compete with one another sending inhibitory signals through interneurons. Eventually the higher firing neurons will suppress the lower firing neuron, enhancing spatial resolution. This is called center surrounding inhibition.
What is somatotopy?
Somatotopy refers to the organization of the body’s sensory and motor functions in specific areas of the brain or spinal cord, corresponding to specific parts of the body.
From an upright image it may seem that the lumbar roots innervate structures located lower than those from the sacral roots. If you change the perspective as if we were a quadruple animal, the sacral roots actually innervate the furthers part of the body.
It is important to remember the somatotopic arrangement of the ascending tract. The sensory signals arriving from the lower portion of the body are more medial while the more superior parts are more lateral.
What are afferent fibers and how are they classified?
They are the nerve fibers that send the information from the PNS to the CNS. They vary based on axon diameter and myelinization, which both affect conduction velocity of the signal.
They can be classified based on the Erlanger and Gasser classification which divides them in three classes, A being largest and fastest, B and C being smallest and slowest.
They can also be classified based on specific sensation conveyed of fibers : Ia, Ib, II, III, IV.
What is the lemniscal pathway?
Also known as the dorsal column pathway, is responsible for transmitting tactile (2 point), proprioceptive, pressure and vibratory sensory information from the body to the brain.
Pathway—> sensory information is received by specialized receptors, signals transmitted via afferent nerve fibers to the dorsal root ganglion where the sensory neurons are located. From the dorsal root ganglion (1st order neuron) the signal travels along the dorsal columns of the spinal cord composed of the fasciculus gracilis (belowT6) and cuneatus (above T6), they then ascend through the spinal cord and synapse in the medulla oblongata at specific nuclei called nucleus cuneatus and gracilis (2nd order neuron) depending on whether the information is coming from the upper or lower body respectively. Then the information crosses over through the internal arcuate fibers and becomes contralateral to the original stimulus. It then ascends the medial leminiscus tract through the brain stem, finally it reaches the thalamus, specifically the ventral posterior nucleus (3rd order neuron). They then pass through the posterior 1/3 of the internal capsule, and than relayed to the primary and secondary somatosensory cortex in the parietal lobe through the corona radiata.
Which nerves does diabetic neuropathy affect first and why?
They first lose the ability to coordinate lower limbs. This is because the pathway is longer for the legs. Therefore a longer cell has more chance to undergo a degenerative process.
How can we test coordination in patient with diabetic neuropathy?
Heel to knee, which is performed first with eyes open and then with eyes closed.
Romberg test, asking the patient to stand up with heels tight together, performed first eyes open then eyes closed.
Moreover patients will also have visual deficits, and will have problems orienting themselves in the dark.
How can you distinguish the lack of coordination because of afferent deficits from cerebellar lesion?
If you have a cerebellar involvement, the performance will be bad equally with eyes closed or open, while if the problem is in the afferent, the performance will get worst.
Also patients with central conditions affecting the long pathway will have more problems in the lower limbs that the upper limbs.
What are some deficits caused by afferent fibers lesions of the dorsal column of the spinal cord?
- 2-point discrimination comes when patients have to recognize very small objects, like buttons, that they do not feel properly.
- Vibration is lost as well. This can be tested by the tuning fork.
- Conscious proprioception (limb position, passive movement) is tested by asking the patient to tell whether the examiner is moving the finger up and down or not.
- Astereognosia (object recognition).
- Agraphestesia (impossibility to recognize “words written on the skin”).
- Ataxia, loss of coordination worsening with eyes closed.
- Parasthesia, patient feels something that is not there aka Lheermitte’s sign.
What is Lhermitte’s sign? Why is it important?
It refers to demyelinating lesions in the cervical dorsal columns where the axons are still capable of conducting but they are more sensitive to mechanical stress.
E.g if the patient is flexing the neck anteriorly and leads to a sort of electrical current running through arms or legs.
It is important because it could be the first sing of someone having a demyelinating disease without actually having any symptoms.
Thalamus divisions?
The thalamus has many subdivisions but we are going to focus on the motor and somatosensory thalamus. The somatosensory thalamus is the ventral posterior portion, of which the medial portion receives and processes afferent from then head and face while the lateral from limbs and trunk.
Then 3 neurons, ganglion cell, medial longitudinal fasciculus and thalamocortical projection, send the signal to the contra lateral somatosensory cortex.
What is the somatosensory homunculus?
It is the visual representation of the body sensory cortex.
The lower limbs in the medial portion of the cortex and shoulder/trunk at the vertex. Laterally, there are different parts of upper limbs, with the most distal parts that are more lateral.
Somatotopy is important for recognizing lesions. If we have a lesion medially, somatosensory processing to the lower limb is lost.
What if a person has a loss of somatosensory processing of only one lower limb?
It could be caused by a lesion in the medial portion of the contralateral cortex or a lesion on the ipsilateral spine.
What is the top down mechanism?
The cortex is able to inhibit all central neurons that are participating to the afferent pathway, so that only the strongest signals will pass the inhibition. Works like a filter.
What is the spinocerebellar tract?
All of these tracts mainly respond unconscious proprioception.
Dorsal spinocerebellar tract —>
Origin: Clarke’s nucleus (T1–L2 spinal segments)
Pathway:
• Ascends ipsilaterally in the spinal cord.
• Enters the inferior cerebellar peduncle and projects to the vermis of the cerebellum.
Function:
• Carries unconscious proprioceptive information from lower limb muscles and joints.
Ventral spinocerebellar tract —>
Origin: Spinal border cells (lumbar and sacral spinal cord)
Pathway:
• Crosses to the contralateral side in the spinal cord.
• Ascends to the superior cerebellar peduncle.
• Crosses again within the cerebellum to terminate in the ipsilateral cerebellar cortex.
Function:
• Transmits whole limb movements and proprioception from lower limbs.
•Important for detecting motor efference copy (predicting movements).
Cuneocerebellar tract —>
Origin: DRG
Pathway:
• Carries upper limb proprioceptive input via the fasciculus cuneatus.
• Synapses in the external cuneate nucleus in the medulla.
• Ascends ipsilaterally through the inferior cerebellar peduncle to the cerebellum.
Function:
• Analogous to DSCT but for upper limbs.
Spino-olivary tract —>
Origin: Spinal cord neurons.
Pathway:
• Crosses in the spinal cord and ascends to the inferior olivary nucleus in the medulla.
• Then, projects to the contralateral cerebellum via the inferior cerebellar peduncle.
Function:
• Integrates proprioceptive information with motor learning and coordination.
• Important for error correction in movement.
What is the spinothalamic tract?
The spinothalamic tract is responsible for transmitting pain and temperature sensations to the brain. This pathway consists of two main types of fibers:
• A-Delta fibers: These are fast-conducting fibers that respond primarily to mechanical and cold temperature stimuli.
• C fibers: These are slow-conducting fibers that mainly transmit chemical pain stimuli, often associated with tissue damage or inflammation.
Both fiber types enter the spinal cord dorsally but synapse in different laminae of the dorsal horn before crossing to the contralateral side via the anterior commissure:
• C fibers synapse mainly in Rexed lamina II and III, ascend two spinal levels, and then cross contralaterally.
• Aδ fibers synapse primarily in Rexed lamina I and V, ascend two spinal levels, and then cross to the opposite side.
A lesion in the spinothalamic tract will result in loss of pain and temperature sensation on the contralateral side of the body, starting two dermatomes below the level of the lesion. This occurs because each neuron enters the spinal cord, ascends two levels before synapsing, and then crosses the midline.
For example, if a person is stabbed at the T5 level of the spinothalamic tract, they will experience loss of pain and temperature sensation on the contralateral side starting from T7 downward.
What happens if there is a lesion at the level of the crossing fibers in the spinothalamic tract?
If the lesion is at the level of the crossing of fibers, it will involve both sides of the fibers and result in bilateral damage.