Physiology Flashcards
What are somatosensory pathways?
They consist of neural structures from receptor organs up to the cerebral cortex. They inform the brain, spina cord and brain stem about various stimuli including touch, pain, temperature, muscle contraction, etc.
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?
They are named like this because their cell bodies are located within a ganglia which is a group of neuron cell bodies in the PNS.
Ganglion cells are the fastest neurons in the somatosenosry system as the motor system relies on continuous information which needs to be conveyed as fast as possible.
The high velocity of the conduction depends on the myelin sheath : thicker myelin means longer the myelin segment can be, the faster the conduction.
Ganglion cells involved in other sensory pathway like pain have thinner or non existent myelin sheaths.
If all the nerve fibers where myelinated it would impair our flexibility, therefore in evolution only certain nerve fibers where selected to be myelinated.
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 from lost 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 lesions of the afferent 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 —> Dorsal root ganglion (1st order neuron), then it reaches Clark’s nucleus (2nd order nucleus) which is part of Clark’s column, located in laminate VII, that extend from C8 to L3. It then moves into the lateral white column and ascends the DSPT and finally entering the cerebellum (3rd order neuron) through the inferior cerebellar peduncle.
Ventral spinocerebellar tract —> Receives info from below L3 to C01. Dorsal root ganglion (1st order neuron), dorsal grey horn (2nd order neuron), then it crosses the mid line though the anterior white commisure and ascends. It enters the cerebellum through the superior cerebellar peduncle, it then crosses back into the ipsilateral cerebellum.
Cuneocerebellar tract —> Receives info from C8 to C1. Dorsal root ganglion (1st orders neuron), posterior grey horn and ascends ipsilaterally and reaches and synapses the lateral (accessory) cuneate nucleus in the medulla (2nd order neuron). It then moves through the external arcuate fibers through the inferior cerebellar peduncle.
Spino-olivary tract —> Dorsal root ganglion (1st order neuron), synapses in posterior grey horn (2nd order neuron), crosses to the contralateral side and ascends the spino olivary tract. It finally synapses on the inferior olivary nucleus and then cross the midline and enter the cerebellum through the inferior cerebellar peduncle.
What is the spinothalamic tract?
This pathway (antero-lateral) conveys temperature and pain information. In this pathway A delta are fast and receive mechanical and cold temperature stimuli, C fibers are slows pain fibers which mainly respond to chemical stimuli.
C fibers enter dorsally and synapses mainly in rexed lamina II and III, then ascends 2 levels, and then crosses contralateraly through the anterior commisure.
A delta mainly synapses in rexed lamina I and V, then ascends 2 levels, and then crosses contralateraly through the anterior commisure.
If you stab someone in the spinothalamic tract, you will lose ipsilateral proprioception and light touch, because these are axons running ipsilateral towards the brain, but contralateral pain and temperature from two levels below, because they come from the dorsal root on the other side.
A lesion of the spinothalamic tract will cause contralateral deficit in pain and temperature sensation starting from two dermatomes below the level of the lesion. Every neuron enters, ascends 2 levels, synapses and crosses the midline.
E.g if stabbed in T5 you lose light touch and proprioception ipsilaterally because of the axons running ipsilaterally towards the brain, but experience contralateral pain and temperature from two levels below the lesion (T7).
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 (tumor or enlargement of the canal), it will involve both sides of the fibers and result in bilateral damage, but the fibers crossing will only be those of the dermatomes of 2 levels below, indeed the deficit will be like a belt, involving a single dermatome.
How is pain and absence of pain classified?
- Negative somatosensory symptom : sensory reduction (deficit), absence of pain.
- Positive somatosensory symptoms : sensory increase/ alteration, presence in the absence of stimuli.
Positive symptoms include :
- Spontaneous—> no external stimulus but we feel like there is one.
- Paresthesia—> not painful, Lhermittes phenomenon. We feel stimulus which is not really there.
- Dysesthesia—> painful.
- Evoked—> Exaggerated pain sensation evoked by an actual stimulus.
- Hyperalgesia—> becomes severe from mild pain.
- Allodynia—> painful from non painful stimulus.
Touch sensory pathway for the face?
The pathway conveys touch information from the face. The pathway consists of—> sensory input which travels from the face via CN V, with the cell bodies located in the semilunar ganglion (Gasserian ganglion), then the afferent fibers enter the pons where they synapses with the main sensory nucleus of the trigeminal nerve. From there the neurons decussate and join the medial leminiscus which carries the fibers up to the thalamus, specifically the ventral posterior medial nucleus. From there it it relayed to the somatosensory cortex.
If lesion occurs at level of the nucleus—> loss of ipsilateral touch sensation of the ipsilateral face. If lesion occurs after the crossing (i.e. in the midbrain)—> loss of contralateral touch in the hemiface.
What is the corneal reflex pathway? Clinical relevance?
It is a protective reflex where touching the cornea of the eye triggers a blink response in both eyes. This reflex involves the trigeminal nerve as the afferent nerve and the facial nerve as the efferent nerve.
Pathway—>
Afferent pathway : the cornea sends sensory information through the ophthalmic branch of the trigeminal nerve, the signal travels to the main sensory nucleus and the spinal trigeminal nucleus in the pons.
Efferent pathway : the signal from the trigeminal nucleus is relaye to the facial nerve nuclei in the pons. Then they send motor signal via CN VII to the orbicularis oculi muscles, causing both eyes to blink.
Lesion in R main trigeminal nucleus :
- Touch right eye—> no eye movement bc loss of afferent ipsilateral.
- Touch left eye—> both eyes blink because left afferent and left efferent are not damaged.
Lesion in R facial nerve :
- Touch right eye—> only left closes, as right efferent is damaged.
- Touch left eye—> only left closes, same as before.
Pathway of pain and temperature sensation in the face?
Pain and temperature receptors (1st order neuron), afferent fibers then enter the pons and descend through the descending trigeminal tract to synapse the spinal nucleus in the medulla(2nd order neuron). After synapsing they decussate and ascend along the spinothalamic tract. They finally end up in the ventral posterior medial nucleus of the thalamus.
