Midterm 2 - PSL300

Energy Conservation in the CNS

The central nervous system's method of conserving energy by limiting communication between neurons.

Dorsal Root

Carry afferent (incoming, sensory) signals to the spinal cord.

Dorsal Root Ganglion

Contains the cell bodies of neurons carrying sensory signals.

Ventral Root

Carries efferent (outgoing) signals from the CNS to the body, including motor signals.

Gray Matter

Area in the middle of the spinal cord consisting mainly of the cell bodies of neurons.

Dorsal Horn

Posterior portion of the gray matter in the spinal cord that receives sensory information.

Ventral Horn

Anterior portion of the gray matter in the spinal cord that sends out motor signals.

Somatic Sensory Nuclei

in dorsal horn. Receive signals from the skin.

Visceral Sensory Nuclei

In dorsal horn. Receive signals from the viscera (internal organs).

Autonomic Efferent Nuclei

Found in ventral Horn. Send commands to glands and smooth muscle.

Somatic Motor Nuclei

In ventral horn. Send commands to skeletal muscle.

Ascending Tracts

Carry sensory signals to the brain, located dorsally.

Descending Tracts

Carry signals from the brain, located ventrally.

Propriospinal Tracts

Stay within the spinal cord.

Spinal Reflex

Response to stimuli without consulting the brain. Enters dorsal root ganglion, to dorsal horn then emits an efferent signal via the interneuron.

Brain Stem

Medulla +Pons+midbrain. Main control center for autonomic functions and reflexes, such as breathing, swallowing, vomiting, and regulating blood pressure.

Thalamus

Processes information going to and from the cerebral cortex.

Hypothalamus

Regulates behavioural drives and endocrine and autonomic homeostasis.

Diencephalon

Thalamus, hypothalamus, pituitary and pineal

6 major divisions of the brain

medulla,Pons, midbrain cerebellum, midbrain, , Diencephalon, cerebrum

Cranial nerves

Connect directly to the brain. 12 pairs, one on each side.

Forebrain

formed from Prosenphalon. Contains diencephalon and cerebrum. Involved with higher brain function.

Midbrain

Formed from the Mensencephalon. Processes visual and auditory stimuli.

Hindbrain

Formed from Rhombencephalon. Contains the medulla, Pons and Cerebellum.

Medulla

continuation of the spinal chord, relays signals and regulates autonomic functions. In hindbrain.

corpus callosum

mylenatied axons between left and right hemisphere.

Cingulate gyrus

Emotional responses

4 lobes of hemisphere

Frontal, Parental, occupational and temporal

limbic sytem

cingulate gyrus (emotional response) + Hippocampus (memories) + Amygdala (fear)

5 special senses

Vision, hearing, equilibrium, taste and smell

4 somatic senses

touch, temperature, proprioception and nociception (pain and itch)

Receptor cells

A receptor cell converts stimulus energy into a graded change in
membrane potential called a receptor potential. The receptor may
then release neurotransmitter to affect a downstream neuron or if the receptor is itself a neuron, it may fire action potentials.

Adequate stimulus

The form of energy to which a specific receptor cell is most responsive to. Receptors can respond to other forms of energy (i.e thermoreceptors can be triggered by chemicals). Muller’s Law (Law of specific nerve energies)

5 groups of sensory stimuli

• Chemoreceptors pH, O2, organic molecules
  ■ Mechanoreceptors vibration, acceleration, sound
  ■ Photoreceptors light
  ■ Thermoreceptors temperature
  ■ Nociceptors tissue damage (pain)

Receptor threshold vs perceptual threshold

Receptor threshold- The weakest stimulus that will cause a response in the receptor

Perceptual threshold: Weakest stimulus that will cause a conscious perception in the organism

Four stimulus properties to create an accurate sensory stimuli representation:

1. Stimulus modality

2. Stimulus intensity

3. Stimulus duration

4. Stimulus location

Stimulus modality

Sensory systems in ideate modality by labeled lines. Ex. if neutrons from visual pathway are activated, brain perceives it as light.

Stimulus intensity (2)

Population coding of intensity - increases number of activated neuron’s

Frequency coding: neutrons fire at a faster rate

Stimulus Duration

Reflected by changes in stimulus. Receptors and neurons have dynamics.

Stimulus Duration - Receptor dynamics - Phasic cells

Respond briefly to change then cease firing. Often retinal.

Stimulus Duration - Receptor dynamics - Tonic cells

Maintain their activity when stimulus is not changing, signals present level

stimulus Duration - Receptor dynamics - phasotonic cells

cells react to change but don’t return all the way to zero firing when the stimulus is constant, so they also carry information about its steady level.
Different cells have different dynamics

Temporal changes

Changes over time

Sensory processing

Primary sensory neurons (can be receptors) synapse onto secondary and tertiary neurons (integrates and processes information from multiple neuron’s). Convergence.

Convergence

Convergence of inputs onto a single sensory neuron enhances that neuron’s sensitivity, but reduces its spatial resolution

Contrast

spatial changes or differences between neighbouring regions in space. Locations with strong contrast have edges.

Lateral inhibition

Lateral inhibition means that cells inhibit their neighbors, or
they inhibit the cells their neighbours excite. Sharpens edged in contrast. Away from the edge the excitation and inhibition cancels out so cells are near baseline activity.

Higher processing

Most pathways run through thalamus, which is near the center of
the brain, out to the sensory cortices on the surface of the cerebrum.
■ Olfactory (smell) pathways are an exception: they don't project via
thalamus.

Equilibrium pathways project mainly to cerebellum

Why is sensory processing an “educated guess”?

Your brain uses inference, which is unconscious and fast. This is how optical illusions works.

Structure of the eye - Lens

The lens is a transparent disk that focuses light. It is suspended by ligaments called zonules which allow eye to focus and stay in place. Mesh of 12mm unnucleated cells. Packed with crystallins instead.

Eye - Aqueous humour

in front of lens is the anterior chamber, filled with aqueous humour. Helps maintain interopticular pressure. Produced by cilliary bodies in posterior chamber and flows into anterior chamber through the pupil.

Viterous chamber

behind lens.Filled with vitreous body/humor, a clear jelly that helps maintain the eyeball’s shape and supports retina.

Cilliary processes

Cilliary processes produces aqueous humour in posterior chamber and flows into anterior chamber through the pupil. Drains via trabecular meshwork (Regulates flow) into the canal of schlemm.

Pathway of light

Light enters through the cornea (transparent). Passes through cornea to lens via a hole in the iris known as the pupil. Cornea and lens bend light to focus on the retina.

Optic disk

Ganglian cells axons accumulate and exit the eye. Horizontal diameter 1.7mm. Vertical diameter is 1.9mm.

Eye- Fibrous layer

Cornea and sclera (maintain shapes and protects). Sclera connected to dura matter of brain.

Why are there no blood vessels running through sclera

Blood in the sclera could scatter light

What happens when pressure builds up in the brain?

In instances relating to high pressure in the brain, such as a brain tumour or head injury. Arachnoid space could transfer pressure to behind the eye. Eye function can thus be used to diagnose brain health.

Cornea layers (5)

1. Epithelium: The outermost layer, a single layer of epithelial cells that protects the cornea and absorbs nutrients from tears. 

2. Bowman's Layer: A tough, collagen-rich layer located under the epithelium. 

3. Stroma: The thickest layer, composed of collagen and water. 

4. Descemet's Membrane: A thin, strong layer that can repair itself, located between the stroma and the endothelium. 

5. Endothelium: The innermost layer. Filled with aqueous humour. Mono layer of specialised cells which help maintain corneal clarity. (non vascular)

Cornea- endothelium

The innermost layer, responsible for pumping fluid from the stroma to maintain its transparency. Filled with aqueous humour. Mono layer of specialised cells which help maintain corneal clarity. (non vascular). Sodium potassium pumps remove excess ions from stroma to aqueous humour

Fuchs endothelial corneal dystrophy (FECD)

Fluid accumulates in the cornea's stroma (the body of the cornea) due to the failure of the endothelial cells to properly pump out excess fluid. Treatment: Hypertonic saline solution

Choroid layer

Between scelera and retina. Rich in blood vessels and feeds retina.

Ciliary body

Changes shape of lens when eye focuses

Glaucoma

Drainage pathway of the aqueous humour is blocked, puts pressure on the optic nerve and leads to vision loss.

Pupil size

Pupil size is controlled by smooth muscles in the iris. In bright light, parasympathetic signals from the brain contract the ring-shaped pupillary constrictor muscle, shrinking the pupil. In the dark, sympathetic signals contract the radial pupillary dilator muscle of the iris, dilating the pupil.

What happens in bright light - pupil

parasympathetic signals from the brain contract the pupillary constrictor muscle, shrinking the pupil.

What happens in low light - pupil

sympathetic signals contract the radial pupillary dilator muscle of the iris, dilating the pupil

Depth of field - pupil

When the pupil is tightly constricted, we have full depth of field, i.e.
everything we see is equally in focus.

When the pupil is dilated, we have a shallow depth of field, i.e. only
objects near one specific distance are in focus (that distance de-
pends on the lens of the eye, as we will see). (zoning out)

Pupil- refraction

our corneas are made of clear collagen. They bend light strongly because there is a big difference between the refractive indices of air and collagen. The cornea is responsible for 2/3 of the eye’s refraction, and the lens for just 1/3

Why do objects appear blurry underwater?

the refractive indices of collagen and water are similar

Lens Accommodation

Parasympathetic nerve signals contract the ciliary muscle, reducing tension in the zonules, making the lens rounder. Light rays bend more and the focal point moves forward.
Sympathetic signals relax the ciliary muscle, making the lens
flatter for far vision

Round lenses

The rounder the lens, the more lights bends. Focal point is closer. “stronger focal power”.

How is lens accommodation stimulated?

The sympathetic system dilates he pupil when the retina is not receiving enough light, and the parasympathetic system constricts the pupil when too much light hits the retina.

Pupillary Light Reflex

1. Light is detected by photoreceptors in the retina.

2. travels via optic nerve (cranial nerve II), to reach the pretectal nucleus in the midbrain

3. From the pretectal nucleus, signals are sent bilaterally to the Edinger-Westphal nuclei

4. Preganglionic parasympathetic fibers from each Edinger-Westphal nucleus then travel with the oculomotor nerve to the ciliary ganglion

5. From there, postganglionic parasympathetic fibers continue via the short ciliary nerves to causing constriction of the pupil (miosis).

Response is bilateral even for unilateral stimuli. (not for accommodation!)

presbyopia

With advancing age the lens stiffens, near point of accommodation increases.

hyperopia

Far-sightedness. Focal point falls behind the retina. Lens and cornea may not have enough focus power. Solved by convex glasses.

myopia

Near-sightedness. Focal point falls in front of retina. Use concave lens/

Photoreceptors

Light sensitive neurons found in the retina that convert light energy into electrical energy, a process known as phototransduction. 2 main types of photo receptors are Cones and Rods. Retina contain more rods than cones. Graded membrane.

Cones and rods (similarities)

1. Structure: outer segment (house visual colours), Inner segment and basal membrane

Layers of retina (8). "Real Outstanding Organizations Often Inspire Intelligent Great Neuroscience"

Retinal Pigment Epithelium (RPE)
Supports, nourishes, and regenerates
photoreceptors.
Outer Segments (OS)
Contains stacks of membrane discs with visual
pigments — the site of phototransduction.
Outer Nuclear Layer (ONL)
Houses the cell bodies of rods and cones.
Outer Plexiform Layer (OPL)
Synapses between photoreceptors and bipolar
cells.
Inner Nuclear Layer (INL)
Contains the cell bodies of bipolar, horizontal,
amacrine, and Müller glial cells.
Inner Plexiform Layer (IPL)
Site of synapses between bipolar cells, amacrine cells, and ganglion cells.
Ganglion Cell Layer (GCL)
Contains the output neurons of the retina — the ganglion cells.
Nerve Fiber Layer (NFL)
Contains axons of the ganglion cells, heading
toward the optic nerve.

Nerve Fiber Layer (NFL)

(outermost layer of retina). Contains axons of the ganglion cells, heading
toward the optic nerve.

Ganglion Cell Layer (GCL)

Contains the output neurons of the retina — the ganglion cells.

Inner Plexiform Layer (IPL)

Site of synapses between bipolar cells, amacrine cells, and ganglion cells

Retinal Pigment Epithelium (RPE)

Supports, nourishes, and regenerates photoreceptors.

Outer Segments (OS)

Contains stacks of membrane discs with visual pigments — the site of phototransduction.

Outer Nuclear Layer (ONL)

Houses the cell bodies of rods and cones.

Outer Plexiform Layer (OPL)

Synapses between photoreceptors and bipolar
cells.

Inner Nuclear Layer (INL)

Contains the cell bodies of bipolar, horizontal,
amacrine, and Müller glial cells.

“Backwards design of retina?”

1. Phot receptor placed against RPE, which recycles pigments, disks and can absorb excess light. Also ensure delivery of nutrients from the blood

2. Muller Glial cells guide light through retina layers

3. Evolved from Vertebrae design

Vdiosual pigments

When light hits them, pigment molecules change shape, starting a chemical cascade that hyperpolarizes the cell, reducing its release of glutamate, i.e. photoreceptors are more active in darkness.

How many pigments do photoreceptors contain?

Each photoreceptor contains millions of molecules of its pigment,
but each type of photoreceptor has just one type of pigment:
rhodopsin in rods, 3 other pigments in 3 types of cone

Phototransduction - Dark

Rod cells; 11-cis retinal doesn’t detects light, Rhodopsin (inactive). CG produces cyclic GMP. This binds to CNG (cyclic nucleotide-gated channels), allowing calcium influx. Potassium-dependent sodium-calcium exchanger (NCKX) works to exchange calcium and sodium and potassium, maintaining depolarisation state.

Phototransduction - Light

Rod cells: 11-cis retinal to isomerise to All trans retinal. Activates Rhodopsin to metarhodopsin 2 (inactive). Sets of G protein cascade. GDP is replaced by GTP in transductin. Alpha sub unit dissociates and activates PDE6 (phosphodiesterase 6). PDE6 breaks down cGMP to GMP, so CNG remains closed. Ca2+ drops and Hyperpolarizes rods. Reduces glutamate release.

Photoreceptor resets

With CNG closed, low calcium., NO GC inhibitor, which is the activated, which makes cGMP from GTP. In RPE, All trans retinal is recyled into 11-cis retinal.

Why are carrots good for your eyes?

Because they contain vitamin A which is a precursor for 11-cis retinal.

Cones and rods differences

• Cones are for bright light, less sensitive than rods; they are responsible for vision in bright light and for distinguishing colors, but they don’t operate in dim conditions. The fovea contains almost exclusively cones
  ■ Rods can detect single photons. But they operate only in low light:
  in daylight they are “bleached out”, i.e. their rhodopsin is broken
  down so they can’t sense light More-peripheral
  retina contains mainly rods
  
  

Macula

Photoreceptors most densely packed. Central pit called fovea. Fovea used for detailed vision.

Blind spot

Ganglian cells that connect to brain (no photoreceptors). ~15 degrees nasal

3 layers of neurons in the retina

Photoreceptors synapse onto bipolar cells, which synapse onto ganglion cells.

Receptive field

Area where light can change activity of neuron. Divided into a centre region and surround region.