final fundamentals of neuroscience

the primary somatosensory cortex corresponds to ______ located in the ______

brodmans area 3b, post central gyrus

postcentral gyrus has what somatic sensory areas

areas 1, 2, and 3a and 3b

somatic sensory area of posterior parietal cortex

areas 5, 7

why is area 3b the primary somatic sensory cortex?

• recieves dense inputs from the VP nucleus of the thalamus

• super responsive to somatosensory stimuli

• electrically stimulated

what happens if there’s a lesion on brodmans area 3b

impair of somatic sensation

primary somatosensory cortex 3b projections to area 1 and 2

area 1= texture information

area 2= size and shape

which layer do thalamus inputs to 3b terminate in?

IV

slow adapting and rapidly adapting neurons in S1’s area 3b

• stacked vertically into columns extending across cortical layers

what is somatotopy and how is it visually represented?

• mapping of the body surface sensations onto brain

• represented by the homunculus where cortical size is proportional to the density of sensory input

wilder penfield

• used electrical stimulation of the S1 surface to map localized somatic sensation

• showed which parts of the cortex correspond to specific body parts

barrel cortex

• found in rats/mice

• sensory signals from each whisker follicle go to one clearly defined cluster of s1 neurons

what are similar mapping concepts to somatotopy

• tonotopy

• retinotopy

example of multiple somatic sensory maps in primates?

areas 3b and 1 in own monkey show mirror image maps of the hand

how can somatotopic maps be studied experimentally

removing digits (fingers) or overstimulating and examining before vs after

are somatotopic maps fixed?

no they are dynamic

what happens with overstimulation of digits?

representation of stimulated digits expands on details of cortical map compared to adjacent unstimulated ones

what happens after digit amputation

cortex layer previously devoted to that digit responds to adjacent digits (example- d3 area would now be merged with d2 and d4)

phantom limb

• amputation phenomenon of perception of missing limb

• happens when other body parts are touched

characteristics of red muscle fibers

• many mitochondria and enzymes

• slow contraction

• sustained contraction

• found in antigravity muscles (leg, torso)

characteristics of white (fast) muscle fibers

• fewer mitochondria

• anaerobic metabolism

• contract and fatigue rapidly

• found in human arm muscles

types of fast / white muscle fibers

fatigue resistent and fast fatigable

describe fatigue resistant vs fast fatiguable white fibers

FR: moderate strength, fast contraction

FF: fastest, strongest and fatigue very quickly

do all three different muscle fiber types coexist in muscles?

yes

what type of fibers does one motor unit contain

one fiber type per motor unit

what are the types of motor units of muscles

• slow motor units

• fast motor units

describe a motor unit of a muscle

• the motor neuron and all the muscle fibers it innervates

what do repeated action potentials cause?

• different rates of fatigue

what does a single action potential do in a motor unit?

• triggers variable contractions

what happens in crossed-innervation experiments?

switching nerve input → switch in muscle phenotype

• changing muscle characteristics due to altered innervation.

what determines a muscle phenotype

activity of the motor neuron

how does activity level affect muscle?

hypertrophy or atrophy

how are muscle fibers formed?

a fusion of muscle precursor cells during fetal development

nucelus in muscle fibers (cells)

more than one nucleus per fiber

what surrounds muscle fibers?

• an excitable membrane called the sarcolemma

what cylindrical structure inside muscle fibers contracts

myofibrils

what neurotransmitter do alpha motor neurons release?

Acetylcholine (ACh)

what does Ach do in muscle fibers?

• produces a large EPSP

give overview of excitation -contraction coupling steps

alpha motor neurons release ACh→ Ach produces large EPSP in muscle fiber → ESPS causes action potential → action potential triggers Ca2+ release → fiber contracts → Ca2+ reuptake → fiber relies

z lines

division of myofibril into segments by disks

the sarcomere is composed of

two z lines and myofibril

thin filaments of myofibril

series of bristles anchored to Z lines

where are thick filaments located

between and among thin filaments in myofibril

how does muscle contraction occur structurally in myofibrils

• thin filaments slide along thick filaments

• pulls z lines closer together

sliding filament model

ca2+ binds → myosin (thick filament) binds to actin (thin filament) → myosin heads pivot, causing them to slide → atp releases the bind → repetition of process (myosin heads walk along thin filament)

what happens to muscles after death?

stiffening occurs

what specialized structures are found deep within skeletal muscles?

muscle spindles

what type of sensory feedback do muscle spindles provide?

stretch receptor feedback

what do muscle spindles and their associated IA axons detect?

• changes in muscle length - stretch

what type of receptors are muscle spindles?

proprioceptors (body sense)

what happens when a weight is placed on a muscle?

muscle spindles are stretched

what causes depolarization of la axon endings?

opening of mechxnosensitive ion channels during stretch

what is the result of increased Ia axon firing?

synaptic depolarization of motor neurons

what is the final outcome of the stretch reflex?

muscle contracts and shortens

what is the basic function of the stretch reflex?

a muscle is pulled → tendency to pull back

how is la sensory axon discharge related to muscle length?

as stretch increases, discharge rate increases

• shortening, discharge rate decreases

what type of loop is the stretch reflex?

feedback loop

is the stretch reflex monosynaptic or polysynaptic

monosynaptic

example of a stretch reflex

knee-jerk reflex

how are many inputs to motor neurons mediated?

through spinal interneurons

what is reciprocal inhibition?

contraction of one muscle set is accompanied by relaxation of the antagonist muscle

how does reciprocal inhibition occur?

• collaterals of Ia axons synapse on inhibitory spinal interneurons, which inhibit motor neurons of antagonist muscles

what is the purpose of the flexor withdrawal reflex?

withdraws a limb from an aversive stimulus

what type of input initiates the flexor reflex

excitatory input pain receptors

how is the flexor reflex coordinated in the spinal cord?

• interneurons in multiple spinal segments are activated

excitatory input activates interneurons in several diff spinal segments these cells eventually do what?

excite the motor neurons that control all the flexor muscles of the affected limb

what happens during crossed-extensor reflex?

• extensor muscles activated on opposite side

• flexors inhibited on opposite side

what is the purpose of the crossed-extensor reflex?

compensate for the extra load during limb withdrawal

what are central pattern generators?

circuits that produce rhythmic motor activity

example of central pattern generator function?

walking

where does walking circuitry reside?

spinal cord

what produces rhythmic activity in walking?

spinal interneurons

what does coordination of walking depend on?

multiple mechanisms

what initiates the rhythmic circuit for movement?

a steady input excites two interneurons

what regulates flexors and extensors motor neurons and how?

• two interneurons excited by steady input

• produces bursts of output

• alternate activity due to inhibition of each other

• flexion on one side is accompanied by extension on the other

what are examples of rhythmic brain activities?

• sleeping and walking

• hibernation

• breathing

• walking

• electrical rhythms of the cerebral cortex

what is characteristic of the cerebral cortex’s electrical activity?

• it shows a range of rapid electrical rhythms correlated with behaviors

what is an EEG?

electroencephalogram

• a classical method for recording brain rhythms

what are circadian rhythms?

• changes in physiological functions based on the brains internal clock

what does an EEG measure?

• generalized activity of the cerebral cortex

how is an EEG recorded?

from the surface of the scalp

the human EEG when discovered in 1929 showed that

walking and sleeping are distinctly different in the brains activity levels

an EEG is used to diagnose

neurological conditions such as epilepsy and sleep disorders

how are EEG signals collected

• electrodes on scalp with low resistance connections

• connected to amplifiers and recording devices

what do electrode pairs measure

activity from different brain regions

what type of signals are measured in an EEG?

voltage fluctuations in the tens of microvolts

what determines EEG amplitude?

the synchrony of underlying neurons

what generates EEG electrical fields?

currents from synaptic excitation of dendrites of pyramidal neurons

how many neurons contribute to EEG signals?

thousands of neurons activating together

what strongly influences EEG amplitude?

how synchronous the underlying neuronal activity is

how are rhythmic EEG signals often described?

by their relative amplitude, reflecting synchrony

• number of neurons also matters

how are EEG rhythms categorized?

frequency range, named after greek letters

alpha rhythm characteristics?

8-13 Hz , quiet walking state

beta wave characteristics

-15-30 hz

• activated or attentive cortex

gamma wave characteristics

• 30-90 Hz an activated or attentive cortex

delta waves

• less than 4 hz

• deep sleep

spindles waves

• brief 8-14 Hz waves associated with sleep

ripples (waves)

• brief bouts of 80-200 Hz oscillations in hippocampus, sleep

what can an EEG indicate about cognition?

helps determine if someone is thinking

what rhythms are associated with alertness or dreaming?

high frequency, low amplitude

what rhythms are associated with deep sleep or coma?

low frequency, high amplitude