Sensory, Motor, and Integrative Systems: Synapses

• Chapter 16 focuses on sensory, motor, and Integrative Systems

Synapses

• Synapses are connections between a nerve cell and another cell
• A synapse is a specialized junction where a nerve cell (neuron) communicates with a target cell
• Target cells include: nerves, muscles, and glands
• Electrical synapses are rare in vertebrates and produce electrical coupling to enable a nerve impulse (action potential) to pass swiftly and reliably to the next cell
• Chemical synapses are complex; presynaptic and postsynaptic cells are separated by a gap, the synaptic cleft which is 10-15x larger than electrical synapses
• The action potential to the postsynaptic cell is transmitted through the release of a chemical neurotransmitter from the presynaptic cell
• The increased control of chemical synapses vs electrical synapses, is important

Synapse Details

• The cytoplasm of the presynaptic nerve terminal in a chemical synapse contains small vesicles that contain neurotransmitter molecules
• When an action potential reaches the terminal, it stimulates the opening of voltage-gated calcium channels in the terminal membrane
• Calcium ions then flood into the cell which triggers the synaptic vesicles to release their contents into the synaptic cleft
• The neurotransmitter diffuses across the cleft and interacts with protein receptor molecules in the postsynaptic cell membrane

Neurotransmitters

• Transmitters' and receptors' molecular structures match like a lock and key.
• At nerve-muscle and nerve-nerve synapses, receptors have a double function and serve as ion channels
• Neurotransmitter binding causes a change in the receptor, opening a tiny intrinsic pore
• In synapses that excite the postsynaptic membrane the pore lets positively charged sodium ions into the cell, making the membrane potential less negative
• Local depolarization of the membrane is known as an excitatory post-synaptic potential, or EPSP; its amplitude is determined by the number of vesicles released from the presynaptic cell
• If the synaptic potential is large enough, threshold is reached and an action potential is initiated in the cell
• An action potential sweeps along the fibre if the target cell is a neuron
• If the target cell is a muscle, it also causes it to contract
• Not all synaptic transmission is excitatory, they can be inhibitory neurotransmitters too
• Inhibitory transmitters render the post-synaptic cell less excitable
• Inhibitory transmitters act on receptors that act as channels for chloride(common) or potassium ions to make the interior of the post synaptic cell more negative, or hyperpolarization
• Acetylcholine is an excitatory transmitter at nerve-skeletal muscle synapses, and glutamate is the main excitatory transmitter in the CNS
• Glycine and gamma aminobutyric acid (GABA) are examples of inhibitory neurotransmitters

Synaptic Transmission

• Step 1: The reaching of an axon bulb by an action potential causes calcium ion gates to open and calcium ions to move into the axon bulb.
• Step 2: The rise in calcium ions in the axon bulb causes synaptic vesicles containing neurotransmitter to move towards the presynaptic membrane.
• Step 3: Synaptic vesicles merge with the presynaptic membrane and neurotransmitters are liberated. Exocytosis of neurotransmitters into the synaptic cleft occurs; this step requires ATP energy.
• Step 4: Neurotransmitters diffuse across the synaptic cleft and bind to receptor proteins on the postsynaptic membrane. Excitatory neurotransmitters cause Na+ ions to move through those receptor proteins, depolarizing the membrane. Inhibitory neurotransmitters do not depolarize the postsynaptic membrane
• Step 5: If sufficient excitatory neurotransmitter binds to receptors, an action potential is produced in the postsynaptic membrane and travels across the neuron.
• Step 6: To prevent continuous stimulation or inhibition of the postsynaptic membrane, neurotransmitters are broken down by enzymes or are reabsorbed through the presynaptic membrane by endocytosis. This also requires ATP energy

Key Steps in Neurotransmitter Action

• Neurotransmitter molecules are synthesized
• Neurotransmitter molecules are stored in vesicles.
• Neurotransmitter molecules that leak from their vesicles are destroyed by enzymes.
• Action potentials cause vesicles to fuse with the presynaptic membrane and release neurotransmitter molecules into the synapse.
• Released neurotransmitter molecules bind with autoreceptors and inhibit subsequent neurotransmitter release.
• Released neurotransmitter molecules bind to postsynaptic receptors.
• Released neurotransmitter molecules are deactivated either by reuptake or enzymatic degradation

Serotonin Actions

• The presynaptic cell makes serotonin (5-hydroxytryptamine, 5HT) from the amino acid tryptophan (found in Turkey), packages it in vesicles in the end terminals, and uses vitamin B6 to help it
• An action potential travels down the presynaptic cell
• That action potential stimulates the vesicles containing serotonin for exocytosis into the synaptic cleft
• Serotonin then passes across the synaptic cleft and depolarizes the postsynaptic cell if the threshold level is reached; if the threshold is reached, a new action potential is propagated

Serotonin Breakdown and other Actions

-The enzyme monoamine oxidase (MAO) degrades the remaining serotonin molecules in the cleft and those released by the receptors after their use

• In the presynaptic cell, the reabsorbed serotonin molecules are destroyed by MAO, which enables the nerve signal to be turned "off" and readies the synapse for another action potential
• Catechol-o-methyl transferase (COMT) is an enzyme that targets catecholamines(dopamine, epinephrine, norepinephrine), and catecholamines are targeted by monoamine oxidase(MAO)

Modulation Details

• 'Fast' neurotransmitters function briefly as they unbind quickly from receptors, break down and reuptake
• The receptor channels are in place for only as long as a receptor is open
• Binding is transient, the synaptic potential short and the resting level returns quickly
• Some other slow transmitters (neuromodulators) are serotonin, dopamine, noradrenaline
• 'Neuromodulators' have receptors that activate messenger molecules inside the cell
• It is known today that nerve terminals can contain two or more transmitters and/or modulators; it was once thought that they only released one neurotransmitter

Neurotransmitters - FYI

• Dopamine is an amine that is an excitatory neurotransmitter found in the brain and smooth muscle that controls arousal levels.
• Serotonin, an excitatory amine neurotransmitter, is found in the brain and smooth muscle; it affects mood, sleep, pain, and appetite
• Noradrenaline(Amine) is excitatory, and found in the Brain and Smooth muscle, inducing arousal and heightening mood,
• Brainstem, Role in memory/vasodilation, Acetylecholine- acetic acid with excitatory+inhibitory,
• GABA- Control of Anxiety lever- Amino Acid
• Brain+Spinal Cord- Redress stress, and promote calamine , natural pain killer- Neuropeptide

Transmission Nuances

• There are 100-300 vesicles discharged for each nerve impulse in a single fiber
• Each nerve cell may have tens, hundreds or thousands
• Nerve cell transmitter and postsynaptic variety may be have >100

Output

• Each input to the pre-synaptic cell releases few vesicles in response to the nerve signal, so that there may be less synaptic potential.
• The intergraded response neuron output will be a reflection of what is different within the different input lines

Synapse Abuse

• Most brain drugs act on synapses
• Nicotine activates nicotinic acetylcholine receptors, and the brain is triggered by the receptor, to release dopamine for euphoria.
• Curare, traditionally an arrow poison from South American natives use, paralyzes their victims through antagonization

Consequences

• In opiate receptors, the user acts in cannabinoid ways; similar to heroin, and morphine users
• On the other hand, if you block dopamine neurotransmitters in the synaptic left, you are essentially acting like a stimulant, an example is cocaine.
• Nerve Gas agents block acetylcholine from removing the signal of the transmitter, so you can kill with spasms, at the nerve muscle locations.
• Inherited forms of the disorders for neuromuscular
• Graves, and Myasthenia
• Epilepsy= GABA levels decreased of inhibitory functions
• Serotonin and wanting the serotonin to last longer) and dopamine.
• Parkinson's disease a result of lacking dopamine action. On the contrary you to have Tourettes through
• Neurons-

Neural Circuits

• Neurons do not function independently, and are combined in ensembles called neural circuits
• Arrangement is different depending on function but have several functions that they share,
• Circuits is defined by the axon terminals and glial processes(Neouropil)
• Not many cell bodies

Nuances

• Area where connectivity
• Occurs with connectivity
• Afferent bring towards spinal/brain cords and Efferrent away spinal to brain.
• Three classes affect afferent efferent to the three classes that influence it .

Spindle example of what influences circuit.

• anatomical entity is
• A knee jerk motion that controls myotonic reactions that happen that
• Limb reflex is sensory neurons that is
• spinal sensory

Neural Circuits Continued

• Dorsal area contains that are neurons, they are projected at the different flexions.
• Nerve fibres in all kinds of different output modes of neurons.

Neuronal Pools

• The body =10 millions sensory, 20 B InterNeourons and 1/2 million motor
• Organized into smaller Neurons for different functions
• Neuronal interconnected that pools many functions

Wiring Diagrams

• The pattern is where interaction among neurone provides the functional characters of the neurone pools.
• Neural circuits are named wiring diagrams as electrical circuits to electrical circuits wiring.
• Distinguishing 5 patterns that differ between these circuits.

Patterns

• Divergence- many neurons /pool so it it will permit the
• Broad distribution, and in spinal cord/brain and for
• Conscious vs Subconscious
• Convergence- Patterns of activity and can have the same effect on
• Convergence motor neurons can control Sub ,-Diapraghm and Ribs controlled by the level above,
• Serial processing can happen neuron to neuron with
• Relayed information

Other Processes

• Parrell Processing : Several Pool/Neureon process same info at the same time , many responses must take place,
• Reverberation- Use a feedback, and Positive in the
• Collateral
• Continous activation
• Help conciness

Chapter 16 continued

• Chapter focuses on levels of components, the motor and integrative actions

sensation

Brain must do integrate, store, and transmit the responses from all the sensory input

• Neural Pathway transmits, that directives,
• It has different neural pathways from those

Conscionos Sensation

• Sensation- Internal and External stimuli and
• Impulses are delivered to to the spinal cord vs relaid.
• Some reactions have some reactions the sensory input is sent through the brain stem

Sensation versus presecption influence it

• It is the stimuli that stimulate or sense to
• Some have some may not have different sensory

Sensation Details continued.

• Sensation can is unique, this is also sensory modular
• Touch /Temp is an example
• These is general plus special senses
• Conditions also include both conditions to inner internal to organs,
• Vibration to touch

Transduction

• Is sensory that changes from stimulus to energy that electrical.
• Amplitude determines propagation

Generation.

• There a integration to the
• Neurons carry from Periperihal(First Order)
• 4 steps that are needed and to reach sensation

Receptors

• The General sensory that no structures or and some with structure

Classification are based on:

• Structures, Stumuli,
• Response Origin

Structural

• Nerves /dendritcs and not myelinated are spread through tissues.

Separate Classifcation

• They have specialized cell synapses at the receptors with special

Stiumli

• Mechano- Touch Proprioception is mechanical stimulation
• Thermo - Increases and decreases in temp

Potential Damage

• Nocioceptor- intense deformation to change signals
• These stimulation cause in hurt
• Hyperalgesia stimulation.

other factors in Stiumli

• Convergace affect interuneron action,
• Referred pain is result

Otehr Stiumli

• Chemo receptors- Chemical detect
• Photo receptors- light and they are sensitive

Detection

• Ampullae of lorerzin--Electromagnetic receptors respond to the
• response in those

Response to stimmuli

• Potentials, and action potential,
• Amplitud is the is related
• Type of receptor

Location.

• Exteroceptors- stimuli of visions
• Interoceptors - monitors of what concision is
• Proprioception - what tendons are

Adapatation

• is most in in response with
• The sensitivity will decline for
• Some have long lasting actions with

Receptors Types and Adaptation Times

• Tonic Receptors - Sensory slow response
• Indicate levels that stimmuli are at
• Nociceptors to Proprioceptors.

Fast Acting.

• Sensory that have rapid resoponce are fast responding

• It measures amount of force exerted.

Tactile Sensaion.

• They may or the same
• They sense similarily
• Sensations like touch, etc

Toch

Discriminative

• ability
• Is on spinal pathways

touch 2

• Corpuscles is the receptors that sense
• located
• Hairy skin, movement to skin surface

type 1 cutaneuous receptors.

• flat make connection
• and pressure in Merkel.
• More in hand

Touch type II

• Riffuni located deep in the
• in legs and tendon.
• Skin stretching when you move
• Also Pressure

Vibration is what senses:

• vibration tactile

Vribraiton cont.

• Lamellar - quickly and through sub mucoas
• in mucous + and membrane
• joints tendon muscles
• Means pressure

Itch

• a mystery
• Bradykinin

Ticle

• Stimulation one can provide
• can not be done to one self
• pacinicioles

Phantom Limb

• that in limb still if there
• with expiirence

Sensation for phantom limb

• the cause of sensation
• reocrgaized from

Thermal Sensation.

• field that have endings at the skin.
• Straum basale, that are mediam diamet

Pain Sensation.

• Not in the body
• chemical

Pain

• slow or increase
• Pain and pain killer

Speed

• 0.1 to have stimuli in section

Pain fast slow

• and
• Fast
• to is in
• 0.1 to have stimuli in section

Pain Fast Slow.

• to hurt and slower at

P Threshold Vs Tol

• P threshold before it
• is stimulus and tolerance is take is
• is of how

P threshold +DRogs

• Anti prostaglandin's stimulation
• Inhibit for cyclooxgenase
• Sodium

Drugs

Can change how they feel them like euphoria, and the chronic

Proprioception.

• That allow head and are or with
• what we can
• is of

Proprioceptos

• Propionceptortes : are
• That contraction
• Balance,joints,

Prop

• weight and adjustmnets to coordinations,.
• assess wieght effort

Prorio.

• is cortex and Cerber
• muscles joints.

Muscle

• what muscles
• is muscle.
• that sensory

Muscle Cont.

• motor and
• end intrafusal fibers. contract fibers keeps that region. more

Golgi tendon Organs

• tendons
• tension . that impulses generate Muscle that and tension . reflexes
• Muscle that causing inhibits .

Golgi Tenon -1

• muscle tendosn is and bundle and signal