physiology exam 1

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nervous system functions

nervous system functions

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

Neuron

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

what are the structural classes of neurons based on?

the number of processes.

Parts of a neuron

cell body (soma)

dendrites

axon

axon hillock/initial segment

axon terminal

what are the functional classes of neurons based on?

the direction of impulse

Interneurons (association neurons)

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

motor neurons

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

sensory neurons

afferent neurons; conduct impulses from sensory receptors to the CNS

Glial cells in CNS

astrocytes, oligodendrocytes, microglia, ependymal cells

glial cells

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

glial cells of PNS

Schwann cells and satellite cells

Oligodendrocytes

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

Schwann cells

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

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

channels in the membrane of a neuron

ligand-gated

voltage-gated

mechanical gated

ligand-gated channel

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

voltage-gated channels

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

mechanical gated channels

open when physical deformation to membrane occurs (like stretching)

where are ion gated channels located on neuron?

on the receptive segment, dendrites & cell body

Where are voltage gated channels located on neuron?

axon hillock/initial segment

Where are mechanically gated channels found?

Found in sensory receptors (touch, pressure, vibration)

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

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

why does the AP peak stop at +30mV?

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

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

action potential

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

compound action potential

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

absolute refractory period

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

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

relative refractory period

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

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

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

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.

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

Synapes

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

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

inhibitory or excitatory

presynaptic neuron

conducts impulses toward the synapse

postsynaptic neuron

conducts signals away from synapse

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.

chemical synapses

(the majority)

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

Neurotransmitters

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

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

SNARE complex

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

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

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

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

ESPS and IPSPS are GRADED POTENTIALS:

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

action potentials can begin at the hillock due to

a high amount of Na+ and K+ channels

characteristics of graded potentials

summation and lack of a refractory period

example of neurotransmitter at the synapse

acetylcholine

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

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

Monoamines

synthesized from amino acids

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

after activating their receptors, monoamines are _________________________

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

neurons releasing catecholamines located in the CNS:

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

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

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

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

sensory receptors

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

receptor potentials

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

Stimulus

energy or chemical activating a sensory receptor

Modalities

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

major categories of sensory receptors:

mechanoreceptors = mechanical deformation (touch, pressure)

thermoreceptors = heat/cold

photoreceptors = light

chemoreceptors = chemical composition

nociceptors = pain

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.

complex integration occurs at

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

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

mechanoreceptors in the skin respond to

touch and pressure

phasic receptors

respond quickly but just as quickly adapt to stimulus

ex: vision, eye

fast-adapting, stimulus withdrawn quickly

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

receptor potential (graded potential): coding potentials:

stimulus strength: increasing the frequency of AP

adaptation: a decrease in receptor frequency

Thermoreceptors

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

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

Nociceptors

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

transient receptor potential channels

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

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

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

muscles

as lens changes shape,

adjustments or accommodations to distance of objects occur

Where are photoreceptors located?

retina (back of the eye)

photoreceptors contain _________ for effective light trap called ______ and ______

pigment; rods, cones

rods

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

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)

photoreceptors and bipolar cells ONLY undergo _________

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

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

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

tympanic membrane (eardrum)

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