Final Exam Questions

Chapter 4

1.      Describe the four phases of the action potential and the order in which they occur?

Rising Phase (depolarisation): Sodium channels open and sodium enters the cell, depolarising it.

Overshoot Phase: The peak of the action potential where membrane potential is positive after potassium channels start to open and sodium channels start to deactivate.

Repolarisation: Membrane begins to repolarise as potassium channels are activated and potassium leaves the cell to restore membrane potential. Sodium channels are inactivated by a flap.

Undershoot Phase (Hyperpolarisation): Too much potassium is let out the cell and it hyperpolarises (more negative than original potential) before potassium gates close, and it slowly returns to the resting membrane potential (-70 mV). Potassium and sodium gates eventually close, and the membrane potential slowly returns to the resting level of -70 mV. During this phase, it is more difficult to initiate another action potential, as the membrane potential is more negative, but it can be done because sodium channels are closed not inactivated.

2.      First define the concept of ion channel selectivity in neuronal membranes. Then briefly explain how the selectivity filter in K⁺ channels work. (Hint: You will want to explain how the selectivity filter restricts the passage of some ion species that are larger than K⁺ and some ion species that are smaller than K⁺.)

Because sodium is smaller than Potassium the ions in cushion of water around sodium must be removed to pass through the filter. K has 8 water molecules which are removed when it passes through but because Na is smaller it is too small to interact with oxygen lining the pore wall ions that remove the water and the water is not able to be removed and it is therefore unable to pass through because Na more stable water shell outside channel than inside.

Because chlorine & calcium are larger than potassium, they cannot fit through the 4 subunits are assembled to form a k channel and 4 poor loops form a poor that serves as a narrow tunnel allowing k to move across but blocking larger ions.

3.      Describe two functional differences between ion channels and active transporters.

Ion channels: Allow the passive flow of ions across their electrochemical gradient and does not require energy.

Active transport: Use energy in the form of ATP to transport ions against their electrochemical gradient.

Chapters 11 and 12

1.      Describe the five basic classes of neurons in the retina.

Photoreceptors (Rods & Cones): House disks and photopigment which is responsible for initial light detection (outer segment) while the inner segment has nucleus and contacts other cells.

Horizontal: Lateral connections for contrast and luminance modulate responses of photoreceptors and bipolar cells

Bipolar: Receive inputs from photoreceptors and convert the graded potentials into action potentials to be transmitter to ganglion cells. They also have parallel processing of visual information by different bipolar cell types allows the retina to extract and encode various features of the visual scene, such as colour, contrast, and motion and contribute to the centre-surround receptive field organization of ganglion cells, enhancing contrast and edge detection.

Amacrine: Lateral connections many subclasses which are responsible for direction selective responses in the retina and contribute to the processing and integration of visual information.

Ganglion: Send visual information to the brain via the optic nerve and encode features of the visual scene such as contrast edges and motion.

2.      Describe the two types of photoreceptors and how they function differently.

Rods: No colour, night and low light vision as they are more sensitive to light, low acuity less in fovea and more in peripheral regions. Less spatial acuity due to multiple rods converging on a bipolar cell.

Cones: Colour vision during the day high acuity in fovea but as you move away from the fovea concentration of cones decreases and concentration of rods increases decreasing acuity in peripheral vision. More spatial acuity due to one-on-one connections with bipolar cells. There are three types of cones for different wave lengths of light (short, medium and long). Essential for tasks that require detail such as reading, object and colour recognition.

3.      Explain why visual acuity is so poor in the periphery?

Visual acuity is reduced in peripheral vision as there are less cones outside the fovea which is the region for central vision as cones contain colour and detail for increased visual acuity as cones have 1 on 1 connections with bipolar cells. The rods for your peripheral vision have lower acuity because they do not have 1 on 1 connections or perceive colour.

4.      In the context of phototransduction, what changes the membrane polarisation?

In light photons are absorbed by retinal this initiates the phototransduction process by changing 11-cis retinal to all-trans retinal. This causes opsin to change an activate transducin which activates PDE which hydrolyses cGMP. This reduces cGMP concentration so cGMP cannot bind to cGMP gated channels causing this channel

to close blocking sodium and chlorine from entering the cell, hyperpolarising the cell. In the dark this does not happen and there is an abundance of cGMP which binds to cGMP gated channels and lets in sodium, chlorine and other anions depolarising the membrane in the dark.

5.      Why are photoreceptors in the deepest layer?

So tips can touch pigment epithelium to regenerate disks as the move to tip where they are pinched of and phagocytosed or recycled by pigment epithelium which also provides nutrients. Phagocytosed approximately every 12 days.

6.      How do centre cones, bipolar cells and ganglion cells respond to a light spot in the centre?

The cone will hyperpolarise due to the light and the on centre bipolar cells will be depolarised and the off-centre bipolar cells will hyperpolarise due to the glutamine released by the cone. The off centre bipolar will deactivate off centre ganglion and the on centre bipolar cell will activate on centre ganglion cells.

7.      How do centre cones, bipolar cells and ganglion cells respond to a dark spot in the centre?

The cone will depolarise due to the dark and the off-centre bipolar cells will be depolarised, and the one centre bipolar cells will hyperpolarise due to the glutamine released by the cone. The on centre bipolar will deactivate on centre ganglion and the off-centre bipolar cell will activate off centre ganglion cells.

8.      Explain topographic organisation and provide two examples.

Topographic organization (somatotopic or retinotopic organization) refers to the spatial arrangement of sensory or motor representations in the brain, where neighbouring regions of the body or visual field are represented by neighboring neurons or brain regions. The topographic organization of sensory and motor representations in the brain is believed to facilitate efficient processing and integration of information, as well as enable the brain to maintain the spatial relationships and somatotopic mapping of the body and visual field.

Somatosensory cortex: The primary somatosensory cortex (S1) located in the parietal lobe of the brain has a topographic organization of the body. Different regions of the somatosensory cortex are dedicated to processing and representing specific body parts. The relative size of the representation for each body part is proportional to the density of sensory receptors (homunculus representation). Neighbouring body parts are represented by neighbouring regions of the somatosensory cortex, preserving the spatial arrangement of the body.

Visual cortex: The primary visual cortex (V1), also known as the striate cortex, located in the occipital lobe, has a retinotopic organization. The visual field is mapped onto the surface of the visual cortex, such that neighbouring regions of the visual field are represented by neighbouring neurons or cortical regions. The central (foveal) region of the visual field is represented by a larger area in the visual cortex, reflecting the higher visual acuity in the central vision. Closer to the back of the brain (occipital lobe) is the region for central vision, while the peripheral visual field is represented more anteriorly.

9.      Describe the major targets of the retinal ganglion cells and their main roles.

Pretectum: The pretectum is involved in the pupillary light reflex, which controls the constriction of the pupils in response to light and ensures both eyes constrict together. This reflex helps protect the retina from excessive light and aids in adjusting vision to different lighting conditions

Thalamus (Lateral geniculate nucleus (LGN)): The LGN in the thalamus is the primary relay centre for visual information it processes and organizes visual information before transmitting it to the primary visual cortex (V1) for further processing. The LGN helps in forming a coherent visual representation by maintaining the segregation of various types of visual information, such as colour, brightness, and motion.

Hypothalamus: The superchiasmatic nucleus in the hypothalamus regulates circadian rhythms by receiving light information from melanopsin-containing retinal ganglion cells to synch the bodies internal clock with the external light dark cycle.

Superior Colliculus: Coordinates eye and head movements to direct visual attention visual attention, reflexive eye movements, head movements in response to visual stimuli, directing gaze shifts and integrating visual and motor information for quick orientation towards stimuli.

10.  Describe an orientation tuning curve and explain how neurons in V1 are organised by their receptive fields and selectivity.

Vertical columns of neurons respond to similar areas of space and orientations whereas adjacent columns respond to slightly shifted receptive field and orientations to enable precision and help the information be organised and sorted easily.

Chapter 16

1.      Define upper motor neurons, lower motor neurons, and local circuit neurons.

Upper motor neurons originate in cerebral cortex and travel down brainstem or spinal cord. Lower motor neurons begin in the spine and innervate muscles throughout the body. Local circuit neurons (interneurons) communicate between lower and upper motor neurons receiving sensory input and descending projection from higher centres for coordinated movement. 

2.      What are the two types of lower motor neurons and how do they differ?

Alpha: Innervate extrafusal fibres and motor units. When the alpha motor neuron is activated, it caused the extrafusal muscle fibres to contract generating force and movement. They are larger than gamma motor neurons and also have a higher conduction velocity.

Gamma: Innervate intrafusal fibres. Smaller than alpha motor neurons. Form part sensory receptors called muscle spindles which provide information about body position and are specialised to detect muscle stretch. They regulate the sensitivity of the muscle spindle by controlling tension and stretch of intrafusal muscle fibres..

3.      What is a motor unit?

A motor unit is an alpha motor neuron and all the muscle fibres it innervates. It is the smallest unit that can produce a measurable contraction of the muscle, and the central nervous system can precisely control muscle force and movement by varying the number of active motor units.

4.      What are the different types of motor units and how do they differ?

Slow motor units: Alpha motor neurons that innervate slow fatigue resistant muscle fibres. Recruited first only generate a small amount of force but can sustain the force for a longer period of time. Involved in things like posture.

Fatigue resistant motor units: Alpha motor neurons that innervate fatigue resistant muscle fibres. Generate more force than slow motor units but less than fast fatigable motor units and can sustain for around 20 min. Involved in activities such as walking. Have lots of myoglobin and mitochondria for energy and oxygen to allow stronger forces to be sustained.

Fast fatigable motor units: Alpha motor neurons that innervate fast fatigable muscle fibres, have less mitochondria and therefore fatigue faster but can produce larger amounts of force.

5.      What is Henneman's size principle?

Recruited in order of size; slow, fatigue resistant, fast fatigable.

Chapter 17

1.      Fill in the boxes in the following figure. Then explain the functional relationship between upper motor neurons and lower motor neurons. Finally, explain why the cerebellum and basal ganglia are best described as playing a "modulatory" role in motor control.

Upper motor neurons provide higher-level control and the lower motor neurons executing the final command to the muscles to ensure communication between the muscles and the brain.

The cerebellum receives input from various sources (motor cortex, sensory systems, muscles and joints) to adjust motor commands to optimise timing, accuracy, force and coordination of different movements but does not initiate.

Basal ganglia also modulate activity of the motor cortex and other motor areas, to select appropriate movements and suppress unwanted or competing movements for smooth voluntary movements.

Therefore, both the cerebellum and basial ganglia have a role in ensuring movements are accurate, coordinated and smooth by modulating other systems, which is why they are said to be modulatory.

2.      Describe the two tracts for descending motor control including their different motor control functions.

Corticospinal Tract: Originates in the cerebral cortex of the primary motor cortex and is responsible for the voluntary control of fine movements such as writing or typing. It terminates in the spine to connect the spine an lower extremities to the motor cortex. 90% of fibres descending from the brain cross the body to form the lateral corticospinal tract (contralateral), while the rest form the anterior corticospinal tract (ipsilateral). Topographical organisation.

Corticobulbar Tract: Originates in the motor cortex but terminates in the brainstem. Controls involuntary movements such as the muscles in the neck, breathing, heart rate, swallowing, muscle tone and posture. Modulates activity of motor neurons in the spinal cord. Basil ganglia, cerebellum and brainstem

3.      What is the difference between voluntary and involuntary movements? List at least 2 examples of involuntary movements.

Knee Jerk Reflex: When the patellar tendon is tapped sensory information from muscle spindles travels up afferents to the spine. Sensory neurons make a direct monosynaptic connection with motor neurons that innervate the muscle. The motor neurons are excited, and the muscle contracts and the leg extends. It is a

monosynaptic stretch reflex that causes the lower leg to extend (kick out) when the patellar tendon is tapped or stretched. - For tripping

Muscle stretch reflex: Maintains the position of a body part such as the arm as weight is added. When you are holding the object your arm flexor muscles are passively stretched. When more weight is added passive stretch increases and spindles detect the change in length. Muscle spindles send signals to the spinal cord where they make motor connections with the motor neurons innervating the same muscle. This reflex arc causes the stretched muscle to contract, counteracting the stretch and helping to maintain the muscle's original length. It is essential for counteracting sudden muscle stretch.

Flexion extension reflex: When pain receptors are stimulated the signal the spinal cord which connect with interneurons in the spine which activate motor neurons of the flexor muscles of the same leg causing it to withdraw. Spinal neurons also inhibit the motor neurons of the extensor muscles of the opposite leg to extend it and provide stabilisation as the opposite leg is withdrawn.

Chapter 20

1.      List some reasons we need eye movements and explain.

Foveal vision and attention: The fovea is the region of the retina with the highest concentration of photoreceptors (cones) and provides the sharpest, most detailed vision. Eye movements allow us to focus moving objects or areas of interest onto the fovea, where we can perceive them with the highest visual acuity and colour sensitivity. Eye movements, such as fixations and micro saccades, allow us to direct our visual attention and focus on specific areas or objects of interest. This selective attention is important for tasks that require detailed visual processing, like reading, object recognition, or scanning the environment.

Tracking moving objects: Eye movements, particularly smooth pursuit movements, enable us to track and maintain focus on moving objects in our visual field.

Image stabilization: Vestibulo-ocular and optokinetic reflexes help stabilize the image on the retina during head and body movements, preventing blurred vision. This image stabilization is crucial for maintaining a clear and stable visual perception of the world around us, even when we are in motion.

2.      What is the latency of saccades and why are saccades described as ballistic?

Saccades have a latency of 200ms, and they are described as ballistic because once initiated they cannot be stopped, changed or corrected.

3.      Name the different extraocular muscles and describe/draw how each is responsible for different eye movements.

Lateral and medial rectus: Horizontal movements

Superior and inferior rectus: Vertical movements

Inferior and superior oblique muscles: Torsional movements

Vertical depression: Superior oblique and inferior rectus

Vertical elevation: Superior rectus and inferior oblique

To nose: Adduction or intorsion

Away from nose: Abduction or extorsion

4.      Explain the separate neural control of saccade amplitude and direction.

Saccade amplitude is controlled by the amount of time lower motor neurons in the brainstem fire for while saccade direction is controlled by which neurons are activated. The medial and lateral muscles control the direction, and the dorsal and ventral muscles control the amplitude of the eye movement. This is to allow separate control of amplitude and direction.

5.      How do the VOR (vestibulo-ocular reflex) and OKR (optokinetic response) relate to one another?

The VOR maintains a stable image during head movements by detecting head movement and producing rapid corrective eye movements. The OKR compensates for slow movements of the visual field through compensatory eye movements to stabilise images and can compensate for slow head movements to compliment the VOR.

6.      How does the alignment of visual and motor maps in the superior colliculus support sensorimotor integration?

Aligned visual and motor maps in the superior colliculus facilitate the direct translation of sensory information into motor commands, supporting the integration of visual perception and eye movement control for effective interaction with the visual environment.

Retinotopic visual representation: Contains a topographic map of the visual field representing the spatial arrangement of the visual field, with nearby regions of the visual space being represented by nearby neurons in the superior colliculus.

Saccadic motor representation: Contains a motor map that represents the generation of saccadic eye movements which is aligned with the visual map.

Sensorimotor integration: The alignment of the visual and motor maps in the superior colliculus allows for direct and efficient coupling between sensory information (visual input) and motor commands (saccadic eye movements). When a visual stimulus is detected in the visual field, the corresponding region of the visual map in the superior colliculus is activated. This, in turn, activates the aligned region of the motor map, resulting in the generation of a saccadic eye movement that shifts the gaze to the location of the visual stimulus.

Chapters 27 and 33

1.      Describe the 4 main similarities in structure across association areas. Then describe a major difference between the association cortex and the visual cortex.

Similarities in structure across association areas:

·         Highly interconnected with other brain regions

·         Receive inputs from multiple sensory modalities.

·         Lack clear topographical organization (unlike primary sensory cortices)

·         Involved in higher-order cognitive functions.

Major difference between association cortex and visual cortex:
The visual cortex has a clear topographical organization, with different regions representing specific parts of the visual field, whereas the association cortex lacks such a clear spatial organization.

2.      What brain areas are not typically characterised as association cortices? What are some of the distinctive features of association areas?

Brain areas not typically characterized as association cortices include the primary sensory and motor cortices.

Distinctive features of association areas:

·         Involved in complex cognitive functions like attention, memory, language, decision-making.

·         Receive inputs from multiple sensory modalities.

·         Lack clear topographical organization.

3.      What is contralateral neglect syndrome and where are you likely to find the damage responsible for it? What does it tell us about the neuroanatomical basis of attention?

Contralateral neglect syndrome is when attention on one side of space is not processed. Damage responsible for this is typically found in the right parietal lobe. This tells us that attention is lateralized in the brain, with the right hemisphere playing a dominant role in directing attention to both sides of space, but especially the left side. While the left parietal lobe directs attention for the right side of space and damage to this area will cause deficits of attention on the right side but less serve as the right side of the brain is able to accommodate for some of this.

4.      What is prosopagnosia and where are you likely to find the damage responsible for it?

Prosopagnosia is the inability to recognize people's faces due to damage in the fusiform gyrus of the temporal lobe on the left hemisphere.

5.      What is the Wisconsin Card Sorting Task and where are you likely to find the damage in a patient who struggles with this task?

The Wisconsin Card Sorting Task requires participants to shift between different sorting rules for example sorting by colour, then number, then shape. The damage responsible for struggles with this task is most likely to be found in the frontal lobe, as this region is involved in regulating personality, goals, and selecting appropriate actions.

6.      Explain what is meant by the statement "Language is localised and lateralised in the human brain".

This statement means that different language functions are localized to specific brain regions, and that language processing is typically lateralized to the left hemisphere in most people.

7.      Where is Broca's area? Where is Wernicke's area? Compare Wernicke's aphasia and Broca's aphasia.

Broca's area is located in the inferior frontal gyrus of the left hemisphere. Wernicke's area is located in the posterior superior temporal gyrus of the left hemisphere.

Wernicke's aphasia is characterized by fluent but nonsensical speech, with difficulties in comprehension. Broca's aphasia is characterized by non-fluent, effortful speech, with relatively preserved comprehension.