Neurophysiolgy Notes PDF

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Lecture 1: Neurophysiology 1Learning Objectives:  Describe the organization of the brain in terms of hemispheres and functional areas.  Outline how the nervous system can store a memory.  Describe the role of glial cells in supporting brain function and why the brain is vulnerable to stroke.Descr...

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Lecture 1: Neurophysiology 1 Learning Objectives:  Describe the organization of the brain in terms of hemispheres and functional areas.  Outline how the nervous system can store a memory.  Describe the role of glial cells in supporting brain function and why the brain is vulnerable to stroke. Describe the organization of the brain in terms of hemispheres and functional areas:  The brain is bilaterally symmetrical  The hemispheres are connected by only a few bands of axons  Most of us use both hemispheres when performing tasks  Sensory and motor functions are lateralized by hemisphere  There is no compelling evidence that some people favor one hemisphere over another

Functionally defined areas of cortex:

Two broad classes: extero-and intero- receptors:

Outline how the nervous system can store a memory.  We are all capable of forming memories from a single experience, but the detail is highly selective.  Memories are formed by making synaptic connections between neurons and by changing their strength. Neural plasticity refers to changes in synaptic connections and their strength:  Synapse strength depends on factors such as the amount of neurotransmitter released and number of post-synaptic ligand-gated channels — modifiable.

Does our brain switch off when we sleep? NO!  Electroencephalogram (EEG) recordings from the human brain show activity in REM sleep is much like walking activity  We are paralyzed when in REM sleep, presumably to stop us falling out of bed. Sleep is critical for memory formation:  Experiences are transiently stored in the hippocampus by rapid synaptic plasticity.  During sleep, synaptic plasticity occurs in cortex to lay down long-term memories.

Describe the role of glial cells in supporting brain function and why the brain is vulnerable to stroke.

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Will 10s without blood flow to the brain will make you lose conscious? YES! (Why I the brain vulnerable to stroke?  The brain has no energy stores, so when the supply of oxygen and glucose stops, brain function ceases.  Permanent damage starts after about 3 mins due to excitotoxicity as neurons become depolarized and then depolarize others. Blood-brain barrier — selectively leaky:  The blood-brain barrier restricts the permeability of brain capillaries.  Astrocytes work with the capillary cells to make the tight junctions less leaky.  The main purpose is to protect the brain from toxins.

Glial cells are of five main types and outnumber neurons by 10:1: Glial cells Ependymal cell Astrocyte Microglial cell Oligodendrocyte Schwann cell

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Functions Make CSF Blood-brain barrier traffic with neurons Immune response (protect CNS from foreign matter) Make myelin Make myeline

Lecture 2: Neurophysiology 2 Learning Objectives:  Explain sensory transduction and its importance in brain function. Describe how afferent type, location and activity encodes the properties of a skin stimulus.  Provide an example of each of the receptors and afferents used to signal touch, pain and temperature.  Outline the dorsal column/medial-lemniscal and spinothalamic pathways to somatosensory cortex. Explain sensory transduction and its importance in brain function. Transduction and encoding:  Receptors transduce a stimulus to a change in membrane potential: 1. Stimulus 2. Change in ionic permeability of receptor cell or afferent nerve ending 3. Change in membrane potential: the receptor potential 4. Generation of action potentials in afferent nerve terminal if potential cross threshold 5. Propagation of action potentials to CNS

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Job description of sensory system: o Afferent type: modality (type of stimulus) o Afferent location: somatotopy of receptive fields (where the stimulus is occurring) o Afferent activity: rate code (intensity and duration)  Extension over time and space (population code) provides a complete description of the stimulus

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Afferent activity: Rate coding of stimulus intensity: o The rate of generation of action potentials depends on the amount of depolarization o Stronger depolarization causes a new action potential earlier in the relative refractory period of the previous AP o Adaption — the gradual reduction in the response of a neuron to a sustained stimulus o Adaption usually has a physical basis in the receptor such as photoreceptor bleaching in the retina, or fluid displacement inside touch receptor capsules. o The rate of adaption is usually determined by receptor properties:  Rapidly adapting — receptors are good for

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signaling change such as movement (semi-circular canals, many touch receptors)  Slowly adapting — receptors are good for signaling intensity of a steady stimulus (joint receptors, otolith organs) Receptive field: region of the stimulus space that activates a neuron

Afferent location: Receptive field — region of the stimulus space that activates a neuron

Provide an example of each of the receptors and afferents used to signal touch, pain and temperature. Somatosensory Modalities:  The 4 somatosensory modalities use distributed receptors rather than specialized sense organs:  A(beta) fibres (large myelinated): o Tactile: sub-modalities include hair afferents, Pacinian corpuscles, Merkel discs o Proprioception: limb and joint position (or movement which is kinanesthesia).  Receptors include spindle afferents, Golgi tendon organs, joint receptors.  A(delta) fibres (small myelinated) /C fibres (unmyelinated): o Thermal: sub-modalities — hot and cold receptors o Pain: sub-modalities are a nociceptor responding to mechanical, chemical or thermal stimuli

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Responses of thermoreceptor fibres: There are cold receptors, with A(delta) axons and warm receptors with C axons

Pain is a distinct sense:  Somatic/skin pain:

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o Fast pricking pain — readily localized A(delta) fibres o Slow burning pain/itch — poorly localized C fibres Deep/visceral pain: o Pain with a dull or diffuse character — mainly C fibre Nociceptors (pain receptors) are free nerve endings: o Classified by stimulus sensitivity as mechanosensitive, thermosensitive, chemosensitive and polymodal Projected pain: o The site at which the noxious stimulus acts is not that at which the pain is sensed (e.g, phantom limbs) Referred pain: o Nociceptive stimulation of the viscera often produces sensation of pain not in the affected organ but rather in distant superficial structure (e.g., heart attack)

Outline the dorsal column/medial-lemniscal and spinothalamic pathways to somatosensory cortex. Tactile system:  Tactile and proprioceptive afferents travel in the ipsilateral dorsal column.  Decussate at medulla and form medial lemniscus. Pain and temperature:  Pain, temperature and crude touch synapse in the spinal cord.  Decussate and travel in the contralateral spinothalamic tract. Spinal hemi-lesions may affect contralateral and ipsilateral differently:  A lesion affecting the left half of the spinal cord at around level T9 will cause: o No change for upper limb sensation o Loss of touch and proprioceptive sensation from the left leg o Loss of pain and temperature sensation from the right leg

Lecture 3: Neurophysiology 3 Learning Objectives:    

Describes the structure and function of the outer, middle, and inner ear Explain how mechanical energy is transduced into a neural signal by cochlear hair cells Explain the cochlear place code for pitch Explain how the semi-circular canals and the otolith organs both use hair cells to transduce different mechanical signals.

Describes the structure and function of the outer, middle, and inner ear. Properties of sound:    

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Sound is a mechanical wave and formed of regions where we compress the air molecules — called the regional compression. The region where the air is less stance is — region of rarefaction. Sound had several qualities that relate to physical characteristics of the wave. Three main properties:

o Pitch (tone) depends on frequency — same loudness o Intensity (loudness) depends on amplitude — same note o Timbre (quality) depends on overtones — same loudness, same notes

Structure of the ear: 

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Pinna — modifies incoming sounds, by creating reflections of different intensities and delays, which helps in localizing the sound source. Auditory canal — allows sound waves to reach the tympanic membrane. Middle ear is for impedance matching — allows sound waves in air to move water molecules o Allows 75% of energy to be transmitted o Due to piston effect (x17) from tympanic membrane to oval window o Lever effect (x1.3) from ossicles Labyrinth (inner ear) contains 3 types of hair cell, each transducing a different stimulus. o Linear acceleration o Angular acceleration o Sound waves

Explain how mechanical energy is transduced into a neural signal by cochlear hair cells. Transduction of sound:   

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Cochlea is a long coil made of three internal chambers. The cochlea is the hearing part of the inner ear, the other segments relate to balance: the vestibular sense. The stapes pushes on Scala vestibuli. o The pushing of the stapes creates mechanical pressure pulses inside the cochlea. Stapes sets up fluid standing waves, that ultimately cause the round window to bulge. o Standing waves are mechanical disturbances that propagate along the basilar membrane. o The basilar membrane stiffness (resonance) varies along its length. o Then, the energy passes through the membrane to scala tympani and finally causes deflection of the round window. Cross-section of cochlea:

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o Scala vestibuli and scala tympani are filled with perilymph. o Scala media is filled with endolymph, which gives rise to a potential of +80mV o The membrane potential is then about -150mV, and this big driving force assists ion movement during the brief channel openings The Organ of Corti: o Has 3 outer hair cells (OHC) per inner hair cell (IHC). o OHC amplify the signal o Tip links connect stereocilia, and attach to the mechano-gated channels  By attaching to the channels, the tip links serve to focus the forces from the tiny movements of the stereocilia on to the mechano-gates channels — why ears are so sensitive  Bending of stereocilia, gates mechanoceptor ion channels to increase or decrease firing rate

Explain the code for pitch.

cochlear place

Coding of pitch: 

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Pitch coding results from localized hair cell activity due to the varying resonant frequency along the basilar membrane. o The point where the basilar membrane resonance matches the travelling wave frequency = the membrane movement is maximal and so strongly activates the hair cells. Relationship of frequency and apparent loudness:

Explain how the semi-circular canals and the otolith organs both use hair cells to transduce different mechanical signals. Vestibular system:  

Labyrinth is filled with endolymph shared between cochlea and vestibular organs. Head rotation and endolymph movement:

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Operation of hair cells in semi-circular canals due to inertial flow of endolymph: o In the semi-circular canals, the hair cells are activated by rotation, which causes deflection of stereocilia due to the inertia of the endolymph. Eye movements are driven by the semi-circular canals and show adaptation over 30s: o The vertical axis plots horizontal eye position during sustained rotation in the dark. o The sharp initial (tooth waveforms) are eye movements called — rotatory nystagmus o The slower phase is matching the velocity of head rotation.

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Otolith organs: utricle and saccule: o The otoliths are calcium chloride crystals and denser than the endolymph.

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o The inertia they confer makes these hair cells respond to linear acceleration from movement or from gravity. o The utricle and saccule can each signal two dimensions, and together have coverage of all three dimensions Brainstem vestibular pathways: o The vestibular system has a dedicated nucleus in the brainstem. o From here, the signals are used for control of posture and eye movements.

Lecture 4: Neurophysiology 4 Learning Objectives:    

Describe the contribution of the various parts of the eye to image formation on the retina. Describe myopia, hyperopia and presbyopia and the lenses used to correct them. Describe the retinal distribution of rods and cones and how this distribution affects visual perception. Explain the basis of color vision, and of anomalous trichromacy (red/green color blindness).

Describe the contribution of the various parts of the eye to image formation on the retina. Job description for the eye: 

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Image formation: o Objects in the environment emit or reflect light. o The eye uses refraction to focus these light sources into an image on the back of the eye (retina). Transduction:

o The image on the retina must be converted from electromagnetic radiation into membrane depolarization. o Properties such as frequency (color), amplitude (brightness) and location must be encoded. Refraction: 

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Refractive Index: o The ration of the speed of light in a vacuum to the speed of light in the medium o The refractive index of air is 1.00 o The refractive index of water is 1.33 (at 25 degrees) Refraction (not reflection): o The bending of light rays when they pass from one transparent medium to another. o The amount of bending increases with:  A larger difference in refractive index  An angle of incidence further from perpendicular A convex and concave lens: o Convex lenses: coverage rays emitted from a point, back to a point, to form a real image.  Converges light rays to a focal point  Focal length: distance between lens centre and focal point for imcoming parallel light rays  The reciprocal of the focal length measures thr power of the lens (in diopeters/D) o The power of a lens depends upon its:  Radius of curvature  The refractive index of the medium o Concave lenses: diverge rays, and so are considered to form a virtual image.  Can extrapolate back to get a negative focal length.  Make parallel light rays diverge, and so the power of concave lenses is expressed in negative diopter Focal length of the eye: Structure of the eye:

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Retina seen through the pupil: o The optic disk produces the functional blind spot. o The fovea is where the image is centered and there are few blood vessels in this area. The retinal image is inverted: o The inversion of the retinal image is of no significance to the brain, as it has never known any other orientation. o The brain learns the mapping between stimulus and receptor. Refractive indices of parts of the eye: o The lens has the higher refractive index but it its less powerful than the cornea because the change in refractive index is smaller o Change at cornea is 1.00 to 1.38 o Lens is 1.33 to 1.40 Accommodation for near vision: o Relaxed lens for distance vision o Accommodated lens for near vision

Describe myopia, hyperopia and presbyopia and the lenses used to correct them. 

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Accommodation of the lens allows us to vary the eye’s focal length: o The power of the eye focused at infinity is 59 D — which equates to a focal length of 17mm o 2/3 of the power is the air/cornea interface o Normal or emmetropic eye has objects at infinity in focus on the retina with the lens is relaxed o Accommodation increases the power of the eye to bring near objects into focus. Correcting for long and short sight: o Hyperopia — long sight  Not usually corrected unless severe, as the patient can use the accommodation of their lens to compensate o Presbyopia: will ultimately require these people to use reading glasses o Myopia — short sight  Due to the eye being too long, or a cornea that has too much power

Describe the retinal distribution of rods and cones and how this distribution affects visual perception. Photoreceptors:

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Retina uses several cell types to capture and process visual information:

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Opsins are membrane-spanning proteins that bind retinal: o The rods and three types of cones have different opsin molecules.  This alters the photon wavelength they can best capture. o Retinal (yellow) sits in a pocket inside the opsin molecule.  Capturing a photon makes it change shape and exit the opsin molecule.

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Transduction is a cascade beginning with opsin which then activates second messengers: o Transduction occurs in the stacked membrane discs of the rod or cone.  These membranes are packed with opsin and the associated machinery.

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In the dark, transmitter is released: o The sodium channel is gated by cGMP. o It remains open in the dark because the concentration of cGMP is high.

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Light reduces transmitter release: o One photon activates one opsin which activates many transducins, each activating many phosphodiesterase — each using up many molecules of

cGMP and so affecting a large number of cGMP-gates channels in the photoreceptor.

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Eye movements help build a sharp image: o Use short-term memory to hold a stitched together hi-res color image of the world around us

Explain the basis of color vision, and of anomalous trichromacy (red/green color blindness). Color Vision:  

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Spectral colors relate to wavelength. Each photoreceptor type preferentially absorbs particular wavelengths of light: o Blue = S (short) cone o Green = M (medium) cone o Red = L (long) cone o M and L cones both prefer shades of orange Each rod or cone can absorb photons of different wavelength, although each has a preferred wavelength. o The bell-shaped absorbance curve makes color ambiguous at night when we only use rods

o However, by day we use the ratio of activity in the three cones to determine colors.

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Anomalous trichromacy is the most common form of red-green color blindness: o Both red and green cones are present but the peak absorbance of one of the opsins has been shifted closer to the other. o Red and green cones are both found on the X chromosome:  Males — more likely to have the condition (6%) than females (~0.5%) o Red and greens seem more similar than to normal viewers, as do purple and greys — most colors are still distinguishable. o Protanopia and deuteranopia (absence of the cone type) — males (2%) and females (0.1%)

Lecture 5: Neurophysiology 5 Learning Objectives:  

Describe convergence, divergence, and spatial summation in the central neural pathways. Give an example of an emergent neural property. Explain topographic organization and somatotopy, tonotopy and visuotopy.

Describe convergence, divergence, and spatial summation in the central neural pathways. Give an example of an emergent neural property. Convergence and divergence: 

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Neural processing requires convergence and divergence: o Convergence: multiple neuron synapses onto one neuron o Divergence: one neuron is branched out to multiple neurons

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o On average, each neuron makes 1000 synapses with and receives 1000 synapses from, other neurons. Convergence allows for spatial summation which can support integration of information.

Orientation selective cells first appear in the visual system at the level of primary visual c...