Sensory and Motor Physiology Flashcards
Outline the relationship between the location of lesions at various points on the visual pathway and resulting visual field defects.
see slide 36-38 of lecture 16
Lesions before the chiasm:
- One eye only.
- These will produce a field deficit in the ipsilateral eye.
- Lesions just before the chiasm can also produce a small defect in the upper temporal field of the other eye as the anterior looping nasal fibres of the contralateral eye may become affected.
Lesions at the chiasm:
- Bitemporal hemianopia (blindness in over half the field of vision).
- If they spread up from below, for example, pituitary tumours, the defect is worse in the upper field.
- If the tumour spreads down from above , e.g. craniopharyngioma, the lesion is worse in the lower quadrants.
- May be asymmetrical
Lesions after the chiasm:
- Homonymous (same side both eyes) field defects.
- A lesion in the right optic tract produces left visual field defect.
- Midline respected
- Congruity increases the further back the lesion occurs.
- Lesions in the main optic radiation cause complete homonymous hemianopia without macular sparing.
- Lesions in the temporal radiation cause congruous upper quadrantic homonymous hemianopia commonly with macular sparing.
Outline the relationship between the location of lesions at various points on the visual pathway and resulting visual field defects.
see slide 36-38 of lecture 16
Lesions before the chiasm:
- One eye only.
- These will produce a field deficit in the ipsilateral eye.
- Lesions just before the chiasm can also produce a small defect in the upper temporal field of the other eye as the anterior looping nasal fibres of the contralateral eye may become affected.
Lesions at the chiasm:
- Bitemporal hemianopia.
If they spread up from below, for example, pituitary tumours, the defect is worse in the upper field.
If the tumour spreads down from above , e.g. craniopharyngioma, the lesion is worse in the lower quadrants.
May be asymmetrical
Lesions after the chiasm:
- Homonymous (same side both eyes) field defects.
A lesion in the right optic tract produces left visual field defect.
Midline respected
Congruity increases the further back the lesion occurs.
Lesions in the main optic radiation cause complete homonymous hemianopia without macular sparing.
Lesions in the temporal radiation cause congruous upper quadrantic homonymous hemianopia commonly with macular sparing.
Describe Age-related macular degeneration (AMD)
Accumulation of waste material known as drusen in Bruch’s membrane and RPE leads to impaired dark adaptation
Strongly age-related therefore set to become a major public health concern
In the early stages rods affected more than cones therefore dark adaptation and dark adapted sensitivity measurements are able to pick up deficits much earlier than standard measures such as visual acuity (its mainly just cone based so doesn’t pick up early changes)
describe how transduction occurs in photoreceptors
Phototransduction involves three main biochemical events:
- Light entering the eye activates the opsin molecules in the photoreceptors:
- Upon photon absorption, 11-cis-retinal undergoes an isomerization to the all-trans form, causing a conformational change in the rhodopsin.
- The activated rhodopsin is called metarhodopsin II.
- The precursor for 11-cis-retinal is all-trans-retinol (vitamin A).
- A diet rich in vitamin A is crucial for vision, since vitamin A cannot be synthesized by humans.
Activated rhodopsin causes a reduction in the cGMP intracellular concentration:
- The cytoplasmic cGMP levels are controlled by cGMP phosphodiesterase, an enzyme that breaks down cGMP.
- In the dark, the activity of this enzyme is relatively weak.
- When the photoreceptor is exposed to light, metarhodopsin II stimulates the activity of cGMP phosphodiesterase via transducin, a G protein.
- GDP-bound inactive transducin will exchange GDP for GTP following interaction with activated rhodopsin.
- GTP-bound active transducin will increase the activity of cGMP phosphodiesterase.
- The result is decreased levels of cGMP in the cytoplasm.
The photoreceptor is hyperpolarized following exposure to light:
- Decreased levels of cGMP cause the closing of cGMP-gated ion channels which will
lead to membrane hyperpolarization.
define absolute scotoma
Absolute scotoma - nothing can be seen at all within that area.
describe Amplification in the phototransduction cascade
- The activation of a single rhodopsin by a single photon is sufficient to cause a significant change in the membrane conductance.
- This is possible due to amplification steps present in the transduction cascade.
- A single photoactivated rhodopsin catalyses the activation of 500 transducin molecules.
- Each transducing can stimulate one cGMP phosphodiesterase molecule and each cGMP phosphodiesterase molecule can break down 10^3 molecules of cGMP per second.
- Therefore, a single activated rhodopsin can cause the hydrolysis of more than 10^5 molecules of cGMP per second.
what nerves are associated with taste and where do they go
Glossopharyngeal N. (IX)
Chorda tympani N. (VII)
Project to NTS – Post Central Gyrus
describe length motitoring in muscles
Afferent fibres from spindle, Type Ia (large myelinated, fast conducting, type II, myelinated, but slower conducting) - sense the change Efferents too (γ efferents)- ?
describe the nuclear bag and chain fibres
Within spindle different kinds of fibres
Sensing velocity:
- Dynamic nuclear bag (bag1 fibre)
- Innervated by Ia fast conducting nerves
- Fast adapting
- Phasic response
- As the name suggests, their function is to signal rapid changes in muscle length (velocity), firing maximally when the muscle is being rapidly stretched (dynamic response)
Sensing length:
- Static nuclear bag fibre (bag2 fibre)
- Nuclear chain
- Innervated mainly by type II, slower conducting than Ia
- Slowly adapting
- Static response
what is the function of y efferents in muscles
- The fusimotor system
- Efferents to:
> Dynamic nuclear bag (bag1) fibres (dynamic efferents)
> Static nuclear bag (bag2) fibres
> Nuclear chain fibres (both static efferents) - Sets sensitivity of spindles (spinal tap stylee, up to 11)
- Prevents spindle unloading because unloading would render the muscle neurally silent
- Via stretch reflex sets baseline levels of α discharge and therefore muscle tone
describe the muscle stretch reflex
- Monosynaptic reflex
- Muscle contracts against stretch
- Reciprocal innervation
- Smooths and damps muscle movement
- Signals length and velocity of muscles centrally (spinocerebellar and dorsal tracts)
what is the function of Golgi tendon organ in muscles
- Force of the muscle is monitored by the golgi tendon organ consisting of a net of knobbly nerve endings embedded within the fascicles of a tendon.
- located in indistensible tendon
- in series with the muscle
- Fire in response to muscle contractile force, not passive stretch.
- Synapse via inhibitory neuron on α fibre
- thought to be the signal feeding back force information to the supraspinal systems controlling movement, enabling us to finely modulate not just length of a muscle but also force exerted.
briefly describe the crossed extensor reflex
In the opposing limb, the muscles reflexively act the opposite way, with extensors contracting and flexors being inhibited - if you touch something hot and pull hand away quickly
briefly describe the positive supporting reflex
Another reflex integrated at a spinal level is the positive supporting reflex where the leg extends to push down on a finger touching the sole of the foot. The foot can actually “follow” the finger, even in the absence of supraspinal influences
Supraspinal reflexes normally override it in conscious humans though, but, as with many symptoms of Upper Motor Neuron lesions, the absence of cortical drive “unmasks” all of these more primitive survival reflexes.
describe the rubrospinal tracts in the subcortical pathways
originate in the Red nucleus (nucleus ruber) of the midbrain.
They cross over the midline and innervate distal musculature
Its thought that these tracts are responsible for control and modulation of the walking
Theyre rather unique in that they are midbrain nucleii that innervate distal muscles
