What is the structure and function of the retina? 

The retina is located at the back of the eye, composed of two types of photoreceptors; rods and cones. 

• Rods: Peripheral, monochromatic, low light. 
• Cones: Abundant in the fovea, color vision, high light. 
• Visual Pigments: light-sensitive material present in disks of the outer segment
• interactions between light energy and visual pigments changes the configuration of the pigment - changing the cell membrane potential. 
• All the rods/cones in a receptive field converge to one bipolar neuron, which then converge into a ganglion cell to send signals to the optic nerve. 
• Function of the retina is to transmit light energy into electrical impulses to send to the brain to interpret visual stimulus.  

• Axons of the ganglion cells form the optic nerve
• ganglion cells from the nasal hemiretina cross at the optic chiasm, ganglion cells from the temporal hemiretina stay on the same side. 
• Synapse on neurons in the lateral geniculate nucleus in the thalamus project on to the visual cortex 
• [image]

• Ventral Pathway: What an object is. (Inferior temporal) 
• Ventral stream is activated in a face matching task: determining whether a target face is the same as a previously presented face. 
• Damage can cause deficits in percieving colour/shape
• Certain areas of the ventral stream specifically process face stimuli - a stroke affecting these areas can lead to prosopagnosia (difficulty recognizing faces) 
• Dorsal Pathway: Where an object is, and how to move to it. (posterior parietal)
• Dorsal stream is activated in a location matching task: determines whether a target object is at the same location as a previously presented target 
• Damage can cause deficits in percieving portions of space or visual motion
• A stroke in the right parietal cortex would result in visual neglect 

• Visual input is related to the ventral pathway (what) and dorsal pathway (where) 
• Vental and dorsal pathways project to the prefrontal cortex - decision making centre of the brain 
• Ventral pathway determines what the object is, dorsal pathway determines where the object is. 
• Must perceive what and where the object is to plan the motor response 

In relation to perception vs. action: 

• Damage to the ventral stream results in visual form agnosia; cannot percieve the objects physical properties, but can grasp them without trouble.  
• Basically: Ventral stream processing is the influence of illusion on perception, dorsal stream processing is the lack of influence of illusin on action. 

• Feedforward control: CNS is able to predict the sensory consequences of actions. 
• prediction of what action is appropriate based on experiences we have with movement 
• Efference Copy: copy of the motor command that is sent through a predictor which predicts the sensory consequences, if the desired command isn't completed the motor command will be adjusted accordingly. 
• Cerebellar activity increases the longer the delay between the predicted and actual sensory consequences (tickling) 
• The cerebellar predictions also control the interjoint coordination during reaching; proven by the difficulty shown by patients with cerebellar damage. 

The cerebellum is the main brain structure associated with predicted sensory consequences of actions. 

• Posterior Parietal Cortex: Sensorimotor integration, spatial processing
• Premotor Cortex: Visually guided movements and planning 
• Supplementary Motor Area: Complex, bilateral movments, and performed/imagined sequences of movements 
• Forward models allow us to change our motor behaviour via efference copies. 

• Muscles: Extraocular Muscles 
• Nerves: 1) oculomotor nerves - medial, inferior, superior recti, and inferior oblique 2) Trochlear Nerve - superior oblique 3) Abducens Nerve - Lateral rectus.
• Brainstem Structures: 1) Vertical Saccades - mesencephalic reticular formatoin 2) Horizontal Saccades - paramedian pontine reticular formation 3) Superior Colliculus 
• [image]

1. Saccades: Very rapid eye movements that occur constantly and that change the focus on the retina from one point to another..
2. Smooth Pursuit: Slow, tracking movements of the eyes designed to keep a moving object aligned with the fovea..
3. Vergence: movements of the eyes in opposite directions
4. Vestibular Ocular Reflex: a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement
5. Optokinetic Nystagmus: allows the eye to follow objects in motion when the head remains stationary

• Omnipause Neurons: release the eye from fixation 
• Pulse Neurons: Burst's of fixation 
• Tonic Neurons: Constant on/off fixations 
• [image]

The superior colliculus projects to the mesencephalic and paramedian pontine reticular formations and recieves from the retina dorsal stream visual areas, frontal eye fields, and basal ganglia. 

• Organized topographically - fovea represented rostrallly, periphery represented caudally 
• Rostral cells fire during fixations; caudal cells fire during saccades. 
• Rostral cells inhibit caudal saccade cells from firing
• Caudal cells inhibited by input from substantia nigra pars reticulata in basal ganglia. 
• the SNr is itself inhibited by the caudate nucleus within the basal ganglia. 
• Both the caudate and caudal Sc cells recieve excitatiory input from frontal eye fields - decision to saccade - this causes an increase in caudal SC activity (direct and indirect) 
• [image]
• When the caudal SC cells fire, they inhibit the rostral SC cells and a saccade is released; caudal SC cells project to burst neurons and rostral SC cells project to omnipause neurons. 

• Frontal Eye Fields: send excitatory input to the SC caudal cells and caudate to determine the decision to saccade. 
• Caudate Nucleus: influenced by the Substantia Nigra Pars Reticulata (inhibited) 
• Substantia Nigra Pars Reticulata: influenced by the caudate nucleus within the basal ganglia (inhibited)

[image]

• outflow theory: "efference copy" of oculomotor command is also sent to the visual areas of the brain - cancels out visual stimulation arising from eye motion. 
• Inflow theory: proprioceptive feedback from eye muscles sent to visual areas of the brain - cancels out visual stimulation arising from eye motion. 
• passive eye rotation biases pointing movements in the direction of the rotation - suggests that proprioception must play some role in monitoring eye position. 

• generated to follow a moving target - after target starts to move, generate initial pursuit, catch-up saccade, then maintenance pursuit. 
• Pursuit breaksdown when target velocity is above 40'/sec - unless using a hand to follow target. 
• arm motor signal must be interacting with pursuit signals. 

• use of hand in addition to smooth pursuit eye movements to increase efficiency. 
• Arm motor signal must be interacting with pursuit signals 
• Deafferented patients with no proprioception still show the effect. 

Opposite excitation and inhibition on the sides of the head, eyes move in opposite direction of the head (counterclockwise vs. clockwise) 

[image]

Damage to the vestibular system would render the patient unable to detect head position, which in turn would make the VOR impossible to perform correctly. 

alternating pursuit and saccadic phases to follow target objects flowing past the observer - such as watching water run. 

[image]

Inward/outward rotations of eyes in response to blurry vision or retinal disparity - very short latency. 

• retinal disparity: what you are focusing on is falling on two different parts of your retina
• Vergence and accomodation of the lens go hand in hand. 
• Blurry vision is a result, accomodation occurs in the lens as well. 

• Postural control: controlling body position n space to maintain stability and orientation - controlling the COP with respect to the COM 
• Postural Orientation: ability to maintain an appropriate relationship between body segments, body, and environment. 
• Centre of Mass: centre of total mass of body - centre of gravity is to vertical projection of centre of mass
• Centre of pressure: cetnre of distribution of the total force applied to the support surface
• Ground reaction force: force exerted by the ground on the body 

Alignment: how closely to body is aligned to the line of gravity - when perfectly aligned, energy requirements are minimized, and stability is maximized. 

• Specific set of muscles that are tonically active during quiet stance - function to keep body within alignment of line of gravity. 
• Stretch reflex alerts the body of changes in muscle tone - whether force generated during stretch reflex is sufficient to maitnain balance is controversial. 

Ankle: rotate about ankle only - body stays straight, generally used for small perturbations

• during forward sway - activate muscles on back of body distally to proximally (gastrocnemius, hamstrings, paraspinals) 
• During backward sway - activate muscles on front of body distally to proximally (tibialis anterior, quadriceps, abdominals) 

Hip: rotate about hip only - legs and torse stay straight, used furing larger perturbations 

• During forward sway - activates muscles on the front side of the body, proximal to distal, abdominals and quadriceps
• During backward sway - activate muscles on the back of the body, proximal to distal, paraspinals and hamstrings. 

Step: taking a step because perturbation was outside limits of stability. Used during the largest of of perturbations. 

Suspensory: crouch down - mostly observed in young children 

Modulated by higher cortical centres

• Anticipatory postural control: changes in arm movement can lead to a loss of balance
• Feedforward control: anticipating postural consequences of limb movement

• Feed forward for expected postural disturbance --> pre-emptive postural adjustment
• Unexpected postural disturbance --> postural adjustment 
• healthy subjects switch from ankle to hip strategy when standing on narrow beam - Parkinson's patients produce complex muscle activation patterns regardless of the context. 

Cerebellar patients have problems scaling response amplitudes (hypermetria) - response is too long/large, overshoot and must activate antagonist muscle as well. 

Cerebellum controls movement - acts on postural adjustments

Basal ganglia - controls muscle tone regulation, controls automatic postural responses, motor flexibility to adapt, and sutomatic execution of motor plans. 

Rhythmic alternating activity of the limbs on opposing sides of the body for purposes of progression

• locomotor and balance processes are similar, locomotion is a series of balance perturbations
• sensory contributions from visual, proprioceptive, vestibular, and cutaneous sources
• CNS processing and transformation into appropriate motor output (generally limited to the spinal cord) 

[image]

reciprocal right/left stance/swing phases, first and last 20% of stance phase is composed of double support. 

Right heel contact/left toe off, left heel contact/right toe off. 

Heel contact: eccentrically contract knee extensors to absorb impact, eccentrically contract ankle dorsiflexors to decelerate foot 

Toe off: concentric contraction of plantarflexors to propel forward, concentric contraction of quadriceps to accelerate  the thigh forward.

Swing: concentric contraction of dorsiflexors for toe clearance, eccentric contraction of hamstrings for forward deceleration 

Gait is still generated after severing the spinal cord when postural support is provided, therefore alternating control of muscle activity is a result of the spinal cord. 

Touch receptors signal contact, causing a reflexive modification of gait pattern

Mesencephalic locomotor region: modulates gait in the brainstem, increasing intensity causes increased gait speed (walking --> trotting --> galloping) 

Cerebellum: fine tunes gait cycle

Dorsal Spinocerebellar tract: sends efferent copy of motor command from central pattern generators in spinal cord to cerebellum

Basal Ganglia: provides dynamic stability by integrating posture into locomotion 

• visual information related to optic flow makes a major contribution to gait, manipulating optic flow changes it's influences on gait 
• when optic flow is slower than expected - gait speed increases
• when optic flow is faster than expected - gait speed decreases 
• when optic flow info is opposite to what is expected, gait velocity is systematically altered. 
• Middle Superior Temporal area processes optic flow, respons specifically to visual motion related to optic flow but not to translation. 
• Proprioceptive signals are used to time the different components of the gait cycle: stretching the hip flexor/inhibition of knee extensory, causes early onset of knee flexor
• Muscle receptors in hip flexor signal when swing phase should be initiated. 
• Proprioceptive signals from the leg extensor muscles controls timing of stance phase - stimulating extensor afferents during stance phase extends the stance phase
• end of extensor afferent activity signals the end of the stance phase 
• Cutaneous signals from the feet signal the timing of different events durign gait cycle
• During gait initiation, trajectory of the centre of pressure moves onto the support foot and forward towards the toes
• cutaneous receptors will sense this pressure trajectory and contribute to initiating difference muscle activations
• Vestibular signals serve to stabilize the head during the gait cycle
• allow visual system to pick up optic flow information 
• patients with vestibular damage have difficulty stabilizing the head and therefore have gait disturbances