physiology exam 1

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nervous system functions

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203 Terms

1

nervous system functions

regulate and control other systems of the body by communicating through electrochemical impulses

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Neuron

nerve cell that responds to stimuli, conducts electrical activity and releases chemical regulators

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what are the structural classes of neurons based on?

the number of processes.

<p>the number of processes.</p>
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Parts of a neuron

cell body (soma)

dendrites

axon

axon hillock/initial segment

axon terminal

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what are the functional classes of neurons based on?

the direction of impulse

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Interneurons (association neurons)

located completely within the CNS and integrates functions of the nervous system (mulitpolar)

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motor neurons

efferent neurons; conduct impulses from the CNS (brain & spinal cord) to target organs (muscles or glands)

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sensory neurons

afferent neurons; conduct impulses from sensory receptors to the CNS

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Glial cells in CNS

astrocytes, oligodendrocytes, microglia, ependymal cells

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glial cells

constitute about half of the cells in the CNS, can divide by mitosis (unlike neurons), and provide physical and metabolic support

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glial cells of PNS

Schwann cells and satellite cells

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Oligodendrocytes

CNS; forms myelin sheaths which insulates and covers axons and speeds up the conduction of electrical signals along axon

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Schwann cells

PNS; forms myelin sheath in PNS, same function as oligodendrocytes

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membrane potential of neurons

resting potential = -70mV

established by large negative molecules inside the cell, Na+/K+ pumps, and permeability of the membrane

ions are constantly moving to maintain concentration gradients

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channels in the membrane of a neuron

ligand-gated

voltage-gated

mechanical gated

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ligand-gated channel

specified by ion, opening in response to the binding of a chemical ligand to its receptors

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voltage-gated channels

protein channel that when stimulated, depolarizes the membrane to a threshold, specific to the ion

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mechanical gated channels

open when physical deformation to membrane occurs (like stretching)

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where are ion gated channels located on neuron?

on the receptive segment, dendrites & cell body

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Where are voltage gated channels located on neuron?

axon hillock/initial segment

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Where are mechanically gated channels found?

Found in sensory receptors (touch, pressure, vibration)

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threshold

an approximate value needed for an action potential to occur. there is enough positive ions flowing in to move the membrane potential from -70mV to -55mV

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does the strength of stimulus have an affect on action potential?

no, the stimulus strength doesn't matter after passing the approximate value. the strength of the stimulus affects the FREQUENCY of AP and may recruit more neurons to have an AP

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why does the AP peak stop at +30mV?

this is where the K+ gates open and the Na+ gates close

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changes in membrane potential are controlled by

changes in the flows of ions through channels such as Voltage gated K+ channels and voltage gated Na+ channels

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action potential

all or nothing electrical event in a single cell where the membrane potential quickly becomes positive and returns to resting potential after

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compound action potential

the sum of all the action potentials occurring in the individual neurons of the whole nerve

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absolute refractory period

the period where a second stimulus will not produce an action potential

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why does the absolute refractory period occur?

during this period, the neurons have to wait for the first AP to complete because since all Na+ channels are opening up another action potential cant start

- Na+ channels are inactivated

- As soon as inactivation is removed and Na+ are closed, the channel can reopen to the second stimulus

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relative refractory period

a second action potential could happen only if the stimulus strength is greater than usual

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Why can relative refractory period occur?

This occurs in a period of hyperpolarization (even more negative), have to bring it from a further negative point to +55

- This wavelength of action potential will have a lower amplitude

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Why does the AP only travel in one direction down the axon?

because behind it is the refractory zone which is essentially a block that stops any other channels in that zone from opening. this forces channels in front of it to open ensuring the neuron operates forward

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action potential conduction

- The depolarization of the first AP is the stimulus for the new action potential in the region just ahead of it and so on

- each AP is its own separate event and is said to be regenerated

-However, positive feedback of Na+ allows the action potential to travel. without decrement (decrease) thus reaching the end with the same amplitude.

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saltatory conduction

Rapid transmission of a nerve impulse along an axon due to myelinated neurons.

myelin prevents Na+ and K+ from moving through the membrane causing the AP to move faster since its "leaping from node to node" compared to ion channels located ALL along the axon

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Synapes

the functional junction between two neurons at which point the impulse is transmitted. use both chemical and electrical stimuli to pass info

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synapses can either be _________ or _________ depending on the neurotransmitter (chemical signal) being transmitted

inhibitory or excitatory

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presynaptic neuron

conducts impulses toward the synapse

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postsynaptic neuron

conducts signals away from synapse

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electrical synapses

Pre- and post-synaptic cells are connected by gap junctions. found in cardiac and smooth muscle to allow contraction as a single unit to occur.

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chemical synapses

(the majority)

axon terminals hold synaptic vesicles. pre-synaptic neurons release neurotransmitters

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Neurotransmitters

chemical messenger that travels across the synaptic cleft and binds to receptors on post-synaptic neurons

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at the post-synapse

neurotransmitters come in large amounts across the synapses to ensure binding to a post synaptic receptor. unused neurotransmitters are transported away from the site

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SNARE complex

set of proteins that allow the vesicle to attach to the plasma membrane, they loosely dock the vesicles

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why are calcium ions important for the functioning of the SNARE complex?

Ca+2 ions trigger a change in the SNARE proteins that leads to the fusion and release of the neurotransmitter

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Exitatory Postsynaptic Potentials (EPSPs)

opening Na+ or Ca2+ channels results in a graded depolarization. this brings the postsynaptic membrane closer to the threshold (more positive) and is a graded potential

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inhibitory postsynaptic potential (IPSP)

opening K+ or Cl- channels results in a graded hyper-polarization. this brings the postsynaptic membrane further from the threshold (more negative). this decreases the likelihood of an action potential

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ESPS and IPSPS are GRADED POTENTIALS:

amplitude decreases as the signal moves toward axon hillock. graded potentials MAY lead to action potentials

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action potentials can begin at the hillock due to

a high amount of Na+ and K+ channels

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characteristics of graded potentials

summation and lack of a refractory period

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example of neurotransmitter at the synapse

acetylcholine

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Nicotinic ACh receptors

Ach binds at post synaptic cell, ex. skeletal muscle cells (how muscles contract)

- agonist = nicotine

- antagonist = curare

binding of 2 acetylcholine molecules opens a channel

- due to electrochemical gradient, more Na+ flows in than K+ out, ESPS begins

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Muscarine ACh receptors

Ach binds at post synaptic cell

ex: digestive cells or cardio cells

- agonist = muscarine

- antagonist = atropine (from plants)

binding at the receptor open sion channels indirectly using a G-protein

- dopamine and norepinephrine receptors do this too

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Monoamines

synthesized from amino acids

ex: dopamine, norepinephrine, epinephrine (all 3 called catecholamines, and all use a second messenger system)

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after activating their receptors, monoamines are _________________________

transported back into the axon terminal or are broken down by enzymes

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neurons releasing catecholamines located in the CNS:

regulate mood, attention, hormone release, states of consciousness and more

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Monoamine Oxidase Inhibitors (MAOIs)

enzyme breaking down monoamines

used to treat depression & other diseases/disorders. it inhibits the process of breaking down the catecholamine by enzymes, therefore increasing concentration of dopamine and norepinephrine at synapse

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57

sensory neuron structure

sensory neurons have an end to receive sensory stimuli and produce the nerve impulse and the other end delivers impulses to synapse in the CNS. cell body located in the middle

<p>sensory neurons have an end to receive sensory stimuli and produce the nerve impulse and the other end delivers impulses to synapse in the CNS. cell body located in the middle</p>
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Plasticity

the brain's ability to change, rewire/ build connections between neurons. associated with muscle memory, normal brain function during stress, during depression this is limited

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sensory receptors

specialized cells that generate graded potentials called receptor potentials in response to a stimulus

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receptor potentials

A slow, graded electrical potential produced by a receptor cell in response to a physical stimulus

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Stimulus

energy or chemical activating a sensory receptor

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Modalities

types of senses: arise from different receptors. every sensory neuron is specific to a sensation

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major categories of sensory receptors:

mechanoreceptors = mechanical deformation (touch, pressure)

thermoreceptors = heat/cold

photoreceptors = light

chemoreceptors = chemical composition

nociceptors = pain

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neural pathways in sensory systems

eyes: visual cortex

ears: auditory cortex

information (the coded action potential) then moves to the association area of the cerebral cortex.

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complex integration occurs at

cortical association areas. this is where perception occurs along with emotional/varying factors that will affect perception

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factors that affect perception

1. receptor adaptation and afferent processing

2. emotions and experiences

3. not all stimuli give rise to a conscious sensation

-ex: stretch receptors monitor blood pressure in several large arteries

4. lack of receptors for certain stimuli

-ex: radio waves

5. damaged neural pathways

-ex. phantom limbs

6. drugs- hallucinogens

7. mental illness

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mechanoreceptors in the skin respond to

touch and pressure

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phasic receptors

respond quickly but just as quickly adapt to stimulus

ex: vision, eye

fast-adapting, stimulus withdrawn quickly

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tonic receptors

maintain response to stimulus

ex: pain. you stab yourself - that pain from the knife is going to continue to deliver a response

slow adapting

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receptor potential (graded potential): coding potentials:

stimulus strength: increasing the frequency of AP

adaptation: a decrease in receptor frequency

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Thermoreceptors

respond to thermal cues, some chemicals can open channels as well.

more receptors for cold (located close to surface) than warm

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Nociceptors

respond to intense mechanical deformation, extreme temps & chemicals (PAIN)

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transient receptor potential channels

Painful heat or painful cold stimulates a whole different set of channels to open in the membrane called: _________________________

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How is pain different from other sensory information?

after initial AP, changes can occur that may increase/decrease sensitivity to pain

- pain can last after stimulus gone

- pain can be altered by pas experiences/emotions, and simultaneous activation of other senses (ex: phantom limb)

- pain reduction depends on endogenous opioids

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eye: the shape of lens and thus degree of refraction is controlled by

muscles

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as lens changes shape,

adjustments or accommodations to distance of objects occur

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Where are photoreceptors located?

retina (back of the eye)

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photoreceptors contain _________ for effective light trap called ______ and ______

pigment; rods, cones

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rods

Retinal receptors that detect black, white, and gray, low levels of light

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Cones

retinal receptor cells that function in daylight or in well-lit conditions. The cones detect fine detail and give rise to color sensations. (bright light signals = red, blue, green)

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81

photoreceptors and bipolar cells ONLY undergo _________

graded responses, they lack the voltage-gated channels that mediate AP's

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action potentials working with photoreceptors

start with photoreceptor, go through bipolar cell, and AP occurs at the ganglion cells (first cells in the pathway where AP can be initiated)

- glutamate is a neurotransmitter released here

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neural pathways in the brain

optic nerve from each eye meet at the optic chiasm and project to many areas (mainly thalamus).

other inputs come from brainstem and visual cortex

some vidual pathway neurons project to areas other than the visual cortex like the hypothalamus

info is coded in spatial / temporal electrical activity - we perceive it all as a visual image consisting of lines, colors, contrast and movement

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84

tympanic membrane (eardrum)

air molecules push against it at same frequency as sound wave. pressures and movement indicate pitch and volume

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3 bones in ear

malleus, incus, stapes:

transduce sound by amplifying it through middle ear to the oval window

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Organ of Corti

Center part of the cochlea, containing hair cells, canals, and membranes

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organ of corti receptor cells

receptor cells called hair cells (mechanoreceptors)

- the hairs on the cell are called stereocilia, are bent back and forth as sound waves vibrate

- K+ channels open, Ca+ channels open

- bursts of neurotransmitters are then releases onto afferent neurons

- GLUTAMATE binds and causes AP in neurons that make up the vestibulocochlear nerve

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88

fluid in the ear (endolymph) is

highly positive with more K+ ions, K+ moves down its concentration gradient

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neural pathways in hearing

cochlear nerve fibers synapse with interneurons in the brainstem

- different arrival times from each ear and intensity determine sound source as well as the shape of outer ear and head movements

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neural pathways in hearing travel path

action potentials carried by vestibular cochlear nerve ------> brainstem (medulla obloganta) -----> thalamus -----> auditory cortex

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organization of CNS

association of interneurons form a network of communication from one area of brain to another

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CNS is composed of

brain and spinal cord

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CNS functions

1. receives input from sensory neurons and directs activity of motor neurons

2. association neurons integrate sensory info and help direct appropriate response to maintain homeostasis and respond to environment

3. humans capable of learning & memory adding a layer of modification to our behaviors

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forebrain (cerebrum)

Higher mental functions - The largest portion of the brain

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forebrain (cerebrum) consists of

cerebral hemispheres each divided into lobes

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cerebral cortex

outer gray matter of cerebrum

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corpus callosum

bundle of nerves that connects the cortex layers of the right and left hemispheres

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limbic system

neural system associated with emotions. physically linked to thalamus, hypothalamus, and endocrine

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Emotions controlled by the limbic system

Aggression: areas in the amygdala and hypothalamus

Fear: amygdala and hypothalamus, hippocampus

Hunger/satiety: hypothalamus

Sex drive: the whole system

Goal-directed behaviors: hypothalamus and other regions, reward/punishment system

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100

there are few connections between the cerebral cortex and limbic system, which

limits conscious control over emotion

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