Week 6 Neuro 1: Neurotransmission and Sensory Transduction

action potential

result of changes in Vm leading to openin go fvoltage gated ion channels, but one does not for much unless can be transmitted to an effector organ/other neuron (where they can process info, amplify or block signals, or synthesize multiple outputs)

divergence

example of principle transmission, characterized by AP branching out and diverging from a single neuron initially

convergence

example of principle transmission, characterized by many AP from many neurons tunneling down into a global control center

limits of signal transmission 

• each AP will look like the preceding, IF that preceding is able to reach threshold 

• the applied voltage change will spread along the membrane, but only so far before it disperses and dies 

space constant

factor that determines the distance over which the applied voltage is able to spread out, distance at which the voltage has decayed by 63% from where the source was produced, typical values are 0.1-1.0 mm, is proportional to the membrane resistance and inversely proportional to the internal resistance

importance of myelinated sheaths

• constant generation of new APs is energetically expensive and slow, if one fails, the signals dies

• the sheaths allow for greater space constant, so have to generate AP

membrane resistance

how well the membrane is able to hold the charge, a low Rmembrane means that the membrane is “leaky” and will lose a lot of the current, decreasing its space constant

membrane internal resistance

how much current resistance there is inside of an “tube”, the less internal resistance, the more the current will be able to travel, which will increase the space constant, a big axon will have a low Ri

structure of more efficient axons “cables”

larger diameters (which decreases the Ri), insulation from myelination (increase Rmembrane)

cells that make up myelinated sheaths 

schwann cells and oligodendrocytes(in the CNS instead of schwann cells bc they branch and encompasses a higher density of neurons that we have in CNS)

saltatory conduction 

term that defines the movemnt of the AP along an axon, in a myelinated axon, the AP spreads more rapidly as they move through the myelinated portions and reach the nodes of ranvier, which contain high densities of voltage gated ions channels, allowing for the generation of more APs 

Aalpha Group I peripheral nerves

largest, diameter of 13-20, fast speed of 80-120. and serve as proprioceptors and in skeletal muscle

Abeta Group II peripheral nerves

medium sized 6-12, medium speed 35-75, serve as mechanoreceptors

Agamma Group III peripheral nerves

smaller size 1-5, slower speed 5-30, involved in pain and temperature reception

C Group IV peripheral nerves

smallest size 0.2-1.5, slowest 0.5-2, involved in pain, temperature, itch (non-myelinated)

transduction

changing different forms of energy into nerve pulses to be sent to the CNS, example of one not in the nervous system is a dynamometer

external environmental sensations

exteroceptors- things like chemicals, temperature, light, position in space, sounds, applied loads

internal environmental sensations

interoceptors- things like termperature, hydration, movement, chemicals, mechanical forces (stretch, pressure) and since our system is homeostatic, it doesn’t like changes to these internal baselines

unipolar 

bipolar

pseudounipolar 

lots of sensory nerves fall into this morphology, more so than the multipolar that we see as a classic model 

multipolar 

classic model, lots of dendrites, terminal branches  

types of receptors

chemical, thermal, mechanical, photon, important bc we process so much information, which are SPECIFIC to the modality

determining factor of signal transduction

the receptor! since they are modality specific, so sensory neurons may contain multiple receptor types, whcih will be able to affect local Vm by either depolarizing or hyperpolarizing the membrane

sensory receptors

can be categorized by structure or function, encode the modality of the stimulus (location, intensity, and duration), each will have an adqueate stimulus that allows them to encode the nature of the stimulus

meissner’s corpusles

superficial cutaneous receptors, small receptive fields which means good localization, numerous in skin in fingertips, have a role in fine touch and are rapidly adapting 

merkel’s disks

superficial cutaneous receptor, disc shaped receptors, small receptive field, terminal extension of sensory fibers, found in hair and glabrous skins, involved in fine touch sensation and encode pressure, slowly adapting

types of superficial cutaneous receptors

meissners corpusles, merkel’s disk, hair receptors, free nerve endings

hair receptors

superficial cutaneous receptor, wrapped around a hair follicle, sensitive to displacement of hair, direction of motion

types of deep cutanous receptors

pacinian corpusles, ruffini’s corpusles, larger receptive fields and less localization

pacinian corpusles 

encapsuled nerve ending similar to that of an onion, touch and vibration, rapidly adapting 

free nerve endings

superficial cutaneous receptors, pain and temperature sensation, both slow adapting and rapid adapting

ruffini’s corpusles 

deep cutaneous receptors, pressure and stretch of skin, detect tension on joint capsule, slowly adapting, an example are mechanoreceptors 

transmission of course touch, temperature and nociception

sensed through free nerve endiings, information then sent to either A-delta (myelinated) or C fibers (unmyelinated), sensory testing for sharp/dull, hot/cold. some examples: numbness, tingling/prickling, anesthesia, hypesthesia (reduced sensibility)

nociception

a component of pain, sends signal of something bad happening in tissue, with chronic there is disfunction in this sensation

graphic of some receptors in free nerve endings

Bradykinin, Seratonin, TNF-alpha, protein degredation products, etc

intensity 

freqquency of AP generated at initial segment will encode proportionally for this 

phasic receptor behavior 

• bursts of action potentials when stimulus applied 

• if the stimulus is maintained the magnitude to generator potential dies quickly 

• might see another burst at the end of stimulus- “on-off” of the stimulus 

• fast adapting

tonic receptor behavior

• responds the entire time the stimulus is applied

• APs are fired at the same rate (given the intensity stays constant)

• “duration of stimulus”

• slow-adapting

rate of adaptation

• the neural response to stimulus 

• rate of firing is proportional to stimulus strength 

• can be slow (tonic) or fast (phasic)

• both contribute to perception and duration of stimulus 

receptor activation influence

receptors may be used to influence things that they don’t directly sense- such as thirst or urine production (baroreceptors and osmoreceptors in blood vessels)

transmission of nerve impulses

electrocal current runs down along dendrite and axons, when reach the terminal ends enter the synapses with other neurons

synapses

specialized region of communication between neurons, ruled by 2 main processes: chemical and electrical transmission, each neuron forms abotu 1000 connections, and recieves anothe 10,000, and human brain has 10^14 of these in one human brain!

features of a generalized synapse

terminal bouton, synaptic cleft, synaptic vesicle, presynaptic membrane, post synaptic membrane, postsynaptic web, effector cell

electrical synapses

• rare in the neural circuits

• direct connections between cells (connexins)

• gap junctions, span synaptic cleft 3.5 nm

• ionic currect transmits signal

• virtually no delay and bidirectional (though some may be rectifying-not bidirectional)

• examples are oculomotor nuclei and inferior olive

chemical synapses 

• more common in nervous system 

• no direct physical connection, synaptic cleft 20-40 nm 

• neutrotransmitters released from presynaptic neuron and diffuse across cleft, mostly unidirectional for a given signal 

• NT binds to ligand gated post synaptic receptor 

• considerable synaptic delay (0.3-5 ms, though mammalian skeletal is 0.3)

some common neurotransmitters

acetylcholine, dopamine, norepinephrine, epinephrine, serotonin, histamine, GABA, glycine, glutamate, NO, ATP, also many other peptide

advantages of chemical vs electrical synapses

• unidirectional

• inhibition/facilitation is easier

• can summate different inputs

• can delay in order to coordinate effects

• plasticity- change in efficacy of signal, can change and mold signal

transmitter release 

• triggered by Ca2+ release from the presynaptic membrane, starts a signaling cascade

• released in quanta (discrete increments) 

• vesicles bind to plasma membrane bc of calcium and release NT into the cleft by exocytosis (vesicle is recycled)

• NT removed from synapse either through enzyme degradation or reuptake (glial cells)

neurotransmission 

communication between neurons 

receptor potential

what sensory receptors tranduce physical stimulus into, also known as local membrane potential, temporal and spatial decrement, encode intensity and duration, a larger intensity will generate a larger potential and increase the rate of APs

post synaptic potentials

• also graded potentials

• can be (as a function of the receptor

  • excitatory (depolarizing)

  • inhibitory (hyperpolarizing)

• summation of PSPs similar to those of receptor potentials at axon hillock

pre-synaptic control

synaptic efficiency may be changed by other synapses (connected), either presynaptic inhibition or facilitation

presynaptic inhibition

• synapse at pre-synaptic terminal

• results in decrease of excitatory NT release

  • hyperolarize membrane or alter CA2+ conductance

• allows for selective elimination of specific inputs without altering effectiveness of others

• IPSPs would hyperpolarize post-synaptic membrane and reduce effects of ALL inputs

presynaptic facilitation 

• synpase at pre-synaptic terminal 

• less clear on mechanism to which it occurs 

• still allows selective modulation without altering effectiveness of others 

effector organ that allows perception

brain, no sensation without perception! signal has to get to brain for us to sense it

rapidly adapting cutaneous receptors

messieners corpusles, pacinian corpusles

slowly adapting cutaneous receptors

merkel’s discs, ruffini’s corpusles

both rapid and slowly adapting receptors

free nerve endings (hair receptors and cutaneous free nerve endings)