Sensory and Motor Physiology

Question 1

Outline the relationship between the location of lesions at various points on the visual pathway and resulting visual field defects.

Answer 1

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.

Question 2

Outline the relationship between the location of lesions at various points on the visual pathway and resulting visual field defects.

Answer 2

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.

Question 3

Describe Age-related macular degeneration (AMD)

Answer 3

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)

Question 4

describe how transduction occurs in photoreceptors

Answer 4

Phototransduction involves three main biochemical events:

1. 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.

Question 5

define absolute scotoma

Answer 5

Absolute scotoma - nothing can be seen at all within that area.

Question 6

describe Amplification in the phototransduction cascade

Answer 6

• 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.

Question 7

what nerves are associated with taste and where do they go

Answer 7

Glossopharyngeal N. (IX)

Chorda tympani N. (VII)

Project to NTS – Post Central Gyrus

Question 8

describe length motitoring in muscles

Answer 8

Afferent fibres from spindle, Type Ia (large myelinated, fast conducting, type II, myelinated, but slower conducting) - sense the change
Efferents too (γ efferents)- ?

Question 9

describe the nuclear bag and chain fibres

Answer 9

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

Question 10

what is the function of y efferents in muscles

Answer 10

• 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

Question 11

describe the muscle stretch reflex

Answer 11

• Monosynaptic reflex
• Muscle contracts against stretch
• Reciprocal innervation
• Smooths and damps muscle movement
• Signals length and velocity of muscles centrally (spinocerebellar and dorsal tracts)

Question 12

what is the function of Golgi tendon organ in muscles

Answer 12

• 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.

Question 13

briefly describe the crossed extensor reflex

Answer 13

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

Question 14

briefly describe the positive supporting reflex

Answer 14

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.

Question 15

describe the rubrospinal tracts in the subcortical pathways

Answer 15

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

Question 16

within the subcortical pathways describe the pontine (excitatory) and medullary (inhibitory) reticulospinal tracts

Answer 16

originating in the reticular nucleii of the medulla

These nucleii receive connections from the motor cortex and cerebellum and innervate anti-gravity muscles via the reticulospinal tracts, to help maintain an upright posture in standing.
- Reticular nucleii
> Pontine (ex)
> Medullary (inh)

The vestibulospinal tracts originate in the vestibular nucleii of the medulla and travel ventrally in the spinal cord.
They innervate neck and antigravity muscles too.

The tectospinal tract originates in the superior colliculus of the midbrain and controls eye movements.

Question 17

what is the primary motor cortex

Answer 17

a map of the movement areas of the brain, what we currently term the primary motor cortex (area M1).

Question 18

describe the Premotor cortex and Supplementary Motor Area of the brain

Answer 18

premotor cortex is thought to set a posture appropriate for movement, aids in selecting movement and the supplementary motor area is involved in integrating complex, planned patterns of movement.

Question 19

describe the Corticospinal tracts

Answer 19

The main descending tracts responsible for motor control are the corticospinal tracts.

80% of these travel from the cortex to the pyramids in the medulla where they decussate, hence the name often given to them (pyramidal tracts).

They then travel down via the lateral corticospinal (pyramidal) tracts to innervate distal muscles responsible for fine control (remember new V old phylogeny).

The other 20% (ventral corticospinal tracts) don’t decussate until they reach the level at which they leave the spinal cord and innervate ________________ muscles responsible for posture (remember new V old phylogeny).

Motor neurons innervating the face are travel via the corticobulbar tracts.
30% of pyramidal neurons come from the primary motor cortex
another 30% from the premotor and supplementary motor areas
the final 40% originating in the parietal association area, and the primary somatosensory area.

Question 20

describe the final common path

Answer 20

the neuron which eventually innervates a motor unit recieves input from many other neurons, either from posture maintenance tracts, or from movement controlling tracts, often via interneurons. This is why the lower motor neuron is often referred to as the final common path from upper motor neurons.

Question 21

describe the basal ganglia

Answer 21

Not real ganglia, but nuclei (groups of neurones)

Virtually all motor fibres pass
through internal capsule, the
space between caudate
nucleus and putamen

Release of some movements and suppression of others

Complex anatomic and pharmacologic connections

2 circuits, putamen and caudate

Question 22

describe the putamen circuit

Answer 22

Loop from cerebral cortex and back via putamen-GP-subthalamic Nucleii-SN, thalamus

Problems in it can cause:
Athetosis - abnormal contractions - writhing movements
Ballismus - wild movements
Chorea - jerky movements

Pre programmed mvmt patterns

Question 23

describe the Caudate circuit

Answer 23

Cognitive control of movement - cognitive control of motor pattern sequences, ie the subconscious determination of what pattern of movements will achieve a desired goal

Common to both – excitatory dopaminergic pathway from SN to Striatum

Question 24

describe the cerebellum and its function

Answer 24

Anatomic c’ffn into folia (leaves) lobes

More appropriate today to use a functional, Physiological c’ffn:
-Vestibulocerebellum
> Flocculonodular lobe
- Spinocerebellum
>Vermis
>Intermediate zone
- Cerebrocerebellum
>Lateral

FUNCTION:
Movement co-processor, “repair shop”, error correcter

Vestibulocerebellum:

• Equilibrium
• eye mvmts

Spinocerebellum:

• Recieves efference copy (what the brain wants the body to do)
• Proprioception (what the body is actually doing)
• Makes the 2 the same

Cerebrocerebellum:

• Planning movements
• Movement accuracy
• Learned skills