Neurobiology Final

Know the four major features of sound

waveform, phase, amplitude and frequency

Can most natural sounds be modeled as sums of sinusoidal waves of varying amplitudes, frequencies and phases?

No, because they have little or no periodicity

Know the human’s audible spectrum

20 Hz to 20 kHz

Why smaller mammals are more sensitive to sounds of higher frequencies than humans?

smaller auditory structures resonate at higher frequencies

What is tonotopy?

systematic representation of sound frequency along an auditory structure

Know the major components of the external ear and their respective functions

pinna, concha (selectively filter different sound frequencies for elevation detection),

auditory meatus (selective boosts of sound pressure 30-100 folds for frequencies around 3 kHz)

tympanic membrane (transmission of sound from the air to the ossicles)

pinna, concha

selectively filter different sound frequencies for elevation detection

auditory meatus

selective boosts of sound pressure 30-100 folds for frequencies around 3 kHz

tympanic membrane

transmission of sound from the air to the ossicles

What is the major function of the middle ear?

amplify sound pressure by almost 200 fold

By what mechanisms it accomplishes the amplification?

the size difference between the tympanic membrane and the oval window; the lever action of the three middle ear bones

What would flaccid paralysis of either of the tensor tympani or stapedius cause?

hyperacusis—painful sensitivity to moderate or even low-intensity sounds

Know the major components of the cochlea

oval window, round window, basilar membrane, tectorial membrane, inner hair cells and outer hair cells

Know the organization and function of the inner ear

The cochlea is tonotopically organized, with the basal region sensitive to high frequency stimuli, and the apical region sensitive to low frequency sounds.

Know how mechanoelectrical transduction is accomplished by the hair cells

Traveling waves of sound causing vertical movement of the basilar membrane, leading to shearing force that bend the stereocilia of the hair cells, resulting in opening of K+ channels, K ions influx to depolarize the hair cells

What is the another name for these K ion channels located on tips of the stereocilia?

mechanoelectrical transduction channels

What are the tip links

mechanical linkages, consists of cadherin-23 homodimers and protocadherin-15 homodimers, that open and close the K+ channels

Why K ions serve in both depolarizing and repolarizing the hair cells?

the hair bundles are bathed in endolymph which is K ion rich, and Na ion poor, with a large electrical potential of +80 mV, so when the K ion channels in the tips of stereocilia open, the strong electrochemical driving force pushes K ions into the hair cell to depolarize the cell.

The soma of the hair cell is bathed in perilymph that is similar in composition as typical extracellular fluid (Na ion rich and K ion poor).

There are two types of K channels in the soma membrane: voltage- sensitive and Ca ion-sensitive. Both channels are open by depolarization of the hair cell (Ca ion influx through voltage-sensitive Ca channels in the soma membrane).

K ion efflux in the soma causes repolarization of the hair cell

Major similarities of outer/inner hair cells

both types are similarly organized with similar cell soma and ion channels, hair bundles with K+ channels linked by tip links, and similar mechanoelectrical transduction mechanism

Major differences between the inner/outer hair cells

there are far more outer hair cells (OHC) than inner hair cells (IHC), OHC receive efferent innervation, while IHC send afferent projection, IHC are the major sensory cells, and OHC are amplifiers

What are the major causes of conductive hearing losses

due to damages to the external and/or middle ear components

What are the major causes of sensorineural hearing losses?

due to hair cells or auditory nerve damages

What is a tuning curve?

a plot of amplitude of sounds at various frequencies necessary to elicit a response from a central auditory neuron

Where are endolymph and perilymph found?

scala media for endolymph, scala vestibuli and scala tympani for perilymph

What is the characteristic frequency?

the lowest threshold of the tuning curve

Know the major central auditory pathways.

auditory nerve—cochlear nuclei—superior olive complex—inferior colliculi—MGC—auditory cortex

Some auditory axons project to the lateral lemniscus nucleus, then to the inferior colliculi, while others directly from the cochlea nuclei to the inferior colliculus

What are the two strategies used to localize the horizontal position of sound sources?

nteraural time differences (ITD) and interaural intensity differences (IID)

Know how MSO (low frequency sounds) and LSO (high frequency sounds) compute horizontal positions using ITD and IID, respectively

coincidence detecting by MSO neurons, differential excitation/inhibition by LSO and MNTB circuit

Where in the auditory pathways a computed topographical representation of auditory space is constructed?

inferior colliculus

Where are the auditory cortex located?

superior temporal gyrus of the temporal lobe

Know the major anatomical and functional features of the auditory cortex.

A1 tonotopically organized; belt areas; EE and EI neurons, and EE and EI stripes alternate; higher order of processing of natural sounds, especially those used for communication; modulating auditory signal processing by lower auditory structures

Auditory info is further processed in other cortical regions including the frontal cortex and parietal cortex

e.g. “what” (dorsal posterior) and “where” (anterior ventral) pathways

Know the basic organization of the eye

three layers of sclera, choroid and retina; cornea, pupil, ciliary body, iris, lens, retina, choroid, aqueous humor and vitreous humor

Know the major retinal cell types

photoreceptors, bipolar cells and retinal ganglion

cells, amacrine cells and horizontal cells) and their major functions

What is the main function of rods

night vision

What is the main function of cones

daytime vision

Rhodopsin

consists of a large protein called opsin and a small molecule called retinal

How many types of cones does a person with a normal vision have

three types: S-, M-, and L-types, i.e. blue-, green- and red-types

Know the major steps in phototransduction in rods

light—retinal change from 11-cis inactive form to all trans active form—conformational change in opsin—activation of transducin—activation of PDE—hydrolysis of cGMP—closure of cGMP-gated ion channels—hyperpolarization of the rods

In a typical human retina, which type of cone is the most numerous

L-type, i.e. red-type

What is the function of calcium in light adaptation?

Light adaptation is the adjustment of the eye when the light threshold is increased. Calcium has an inhibitory effect on guanylyl cyclase, rhodopsin kinase, and it decreases cGMP’s affinity to the cGMP- gated channel. When light intensity increases, less calcium enters the photoreceptor, calcium’s inhibitory effects is reduced, resulting in more cGMP available (through increased cGMP production and reduced cGMP hydrolysis), and higher affinity to the channel, therefore more cGMP-gated channels become open, making the eye sensitive to further changes in light intensity

What are the major functions of retinal pigmented epithelium (RPE)?

removal of expended photoreceptor membrane discs and retinoid cycle

Know the major anatomical features of the primate retina that make the fovea as the most important region for day light vision.

almost all cones densely packed in fovea, other retinal layers and blood vessels are displaced, 1:1 ratio of cones to bipolar cells to retinal ganglion cells

What does a typical retinal ganglion cell receptive field look like

circular, with a center and surround

RGCs are grouped into two types based on their responses to spot light illumination

on-center RGCs and off-center RGCs

Most RGCs are either M type or P type based on their sizes

the P type is smaller and more numerous, good at perception of form and color. The M type is much larger and more sensitive to movement

Understand why hyperpolarization in a photoreceptors results in depolarization in a bipolar cell receiving direct input from the photoreceptor

photoreceptors release glutamate as their neurotransmitter, with ionotropic receptors in off-center bipolar cells (excitation) and metabotropic receptors in on-center bipolar (inhibition).

Hyperpolarization of the photoreceptor reduces its inhibitory effect on the on-center bipolar cells, allowing the bipolar cells become excited. This on-center bipolar cell makes an excitatory synapse with an on-center RGC. Therefore hyperpolarization in the photoreceptor causes excitation in the on-center RGC)

What do the receptive fields of bipolar cells look like

the same as RGCs

From which retinal cell type do RGCs obtain their type of receptive field?

bipolar cells

Which type of retinal cells is most important for the center-surround characteristic of the receptive field of RGCs and bipolar cells?

horizontal cells

Know the primary visual pathway

retina—optic nerve—optic chiasm—optic tract—LGN—primary visual cortex

Know the concept of retinotopic representation of the visual field in the LGN and primary visual cortex.

the peripheral spatial relationship of retinal cells is preserved in LGN and primary visual cortex

What are visual fields?

the entire area that can be seen when the eye is directed forward

How is LGN organized?

six layers, each layer receiving input from only one eye; M and P cells project to different layers

What do receptive fields of LGN cells look like?

similar to the RGCs’ and bipolar cell’s: circular with a center and surround

Know the basic organization of the primary visual cortex

six layers, LGN projects to layer IV; ocular dominance and orientation columns

Are receptive fields of the simple cells in the primary visual cortex similar to those in the retina and LGN?

no, they are bar shaped

Know that:

Both the simple cells and complex cells (sensitive to edges) have preference for special orientation of the stimuli.

Know orientation columns

cortex cells within each orientation column—from pia surface to white matter

Orientations columns are especially sensitive to light of a particular axis of orientation and ocular dominance columns

Each eye project to different stripes of cortex cells in the visual cortex, and the stripes alternate regularly

What is the projection pattern of visual inputs in the primary visual cortex?

from layer 4 to layers 2/3, then to layer 5, and finally to layer 6. Neurons in layer 6 also send feedback projections to layer 4

Know the respective function of dorsal stream

motion and visual control of action, toward the dorsal parietal area

Know the respective function of ventral visual streams

object recognition, mainly toward the inferior (ventral) temporal lobe

Where are face recognition neurons predominantly located?

inferior temporal cortex

What are motor systems consist of?

all the muscles and the neurons control them

What are the three types of movements generated by the motor systems?

reflexes, rhythmic motor patterns and voluntary movements

What is flexion?

movement that makes joint angle smaller

What is extension?

movement that makes joint angle larger

What are synergist muscles?

muscles that work together to extend or flex joints

What are antagonistic muscles?

muscles that pull joint in opposite directions

What are motor neuron pools?

groups of motor neuron clusters in the spinal cord innervating individual muscles

What is proximal-distal rule?

it describes the organization of motor neurons in the spinal cord: those innervating more proximal muscles are located in more medial regions of the ventral horn, while those innervating the more distal muscles are located in more lateral regions of the ventral horn

What is flexor-extensor rule?

motor neurons innervating extensor muscles are located ventral to those innervating flexor muscles

What is a motor unit?

A motor neuron and all the muscle fibers it innervates)

How can the nervous system control the force of a muscle contraction?

(1) by changing the numbers of motor units activated,

(2) by changing the rate of action

potentials in a motor neuron

Motor unit classification

Motor units can be classified as fast fatigable (FF), slow fatigue-resistant (S) and fast fatigue-resistant (FR)

Which of the three motor units generates the largest force?

FF type

Which generates the smallest force?

S type

What is size the principle?

motor units are recruited in a fixed order from the weakest to the strongest

What are the two types of muscle receptors?

muscle spindles and Golgi tendon organs

What are muscle spindles good at detecting?

changes in the muscle fiber length

What do Golgi tendon organs mainly respond to?

changes in the tension of muscle fibers

Know the main function of the gamma motoneurons

keep the muscle spindles from going “off air”

What is a reflex?

A stereotyped (involuntary) motor response elicited by a defined

stimulus

What is a spinal reflex?

a reflex mediated through the spinal cord

What are the three main levels of control for spinal reflexes?

1) control of individual muscles

2). coordinate muscles actions around a single joint

3). coordinate muscles actions at several joints

Stretch and flexion reflexes

Stretch reflex (e.g. knee jerk reflex) is an example of control of individual muscles and coordinate muscle actions around a single joint

Flexion withdrawal reflex is an example of coordination of muscles at several joints

Understand how locomotion movements are controlled

initiated by the brain, but maintained by spinal circuits—central pattern generators

The central motor system is organized in a hierarchical way:

cerebral cortex—basal ganglia—cerebellum—brain stem—spinal cord.

What is the central pattern generator?

a neural network within the CNS that is capable of generating a rhythmic pattern of motor activity without phasic sensory input from peripheral receptors

What are the two major descending motor pathways?

the lateral pathways and the ventromedial pathways

What do the lateral pathways (corticospinal tract, corticobulbar tract and rubrospinal tract) mainly control?

distal muscles, particularly the flexors, facial and head muscles

What do the ventromedial pathways mainly control?

maintain balance and posture, orientate head toward visual stimuli, and liberate the extensor muscles from reflex control

Do all corticospinal axons cross to the contralateral side?

no, the ventral corticospinal tract are bilateral

Where does planning of movements occur?

mainly in the prefrontal cortex

Which cortex areas are involved in the execution of planned motor activities?

premotor cortex and primary motor cortex

What brain structures belong to basal ganglia?

caudate nucleus, putamen, globus pallidus; some textbooks also include subthalamic neucleus and substantia nigra as basal ganglia structures

Which part of the basal ganglia is the output region?

globus pallidus

Which part of the basal ganglia is the input region?

striatum

From what structures does the striatum receive inputs?

Mainly from association areas of the cortex including the frontal and parietal cortices. Some inputs also arrive from the temporal cortex, and substantia nigra pars compacta