Synapses: Electrical and Chemical

### Synapses
- A synapse is a connection between a nerve cell and another cell.
- Synapses are specialized junctions where a nerve cell, or neuron, communicates with a target cell.
- Synapses target cell types include nerves, muscles, and glands.

### Electrical Synapses
- Electrical synapses are relatively rare in vertebrates.
- Membranes of two cells are in tight contact, producing electrical coupling.
- Electrical coupling enables a nerve impulse or action potential to pass swiftly and reliably to the next cell.
- Electrical synapses are much faster than chemical synapses.

### Chemical Synapses
- Chemical synapses are more complex than electrical synapses.
- Presynaptic and postsynaptic cells are physically separated by a gap, or synaptic cleft, roughly 10-15 times larger than electrical synapses.
- Chemical synapses prevent simple electrical transmission of the action potential to the postsynaptic cell.
- Transmission is accomplished by the release of a chemical neurotransmitter from the presynaptic cell. There is more control with chemical synapses than in electrical.
- The cytoplasm of the presynaptic nerve terminal in a chemical synapse is packed full of small vesicles, each containing a few thousand molecules of neurotransmitter. 
- When an action potential arrives in the terminal, it stimulates the opening of voltage-gated calcium channels in the terminal membrane.
- Consequently, calcium ions flood into the cell and trigger the synaptic vesicles to release their contents into the synaptic cleft.
- The neurotransmitter molecules that are liberated diffuse across the cleft and interact with specialized protein receptor molecules in the postsynaptic cell membrane.

### Neurotransmitters
- The molecular structure of a neurotransmitter and its receptor are matched.
- The neurotransmitter and receptor fit one another like a lock and key.
- At nerve-muscle synapses, and in many nerve-nerve synapses, the receptors have a double function, also serving as ion channels.
- Binding of a neurotransmitter produces a change in the three-dimensional shape of the receptor that opens a tiny intrinsic pore in the protein.
- For neurotransmitters that excite the postsynaptic membrane, the pore permits positively-charged sodium ions to move into the cell, making the potential across its membrane less negative.
- This local depolarization is known as an excitatory post-synaptic potential or EPSP.
- Its amplitude is roughly determined by the number of vesicles released from the presynaptic cell.
- If sufficiently large, the synaptic potential reaches threshold and initiates an action potential in the cell.
- If the target cell is a neuron, the action potential sweeps along its fiber.
- If it is a muscle, it propagates over the surface of the muscle cell and 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 and thus less likely to generate an action potential.
- Inhibitory transmitters often act on receptors that act as channels for chloride or potassium ions, and generally make the interior of the postsynaptic cell even more negative, or hyperpolarization.
- Acetylcholine is an excitatory transmitter at nerve-skeletal muscle synapses, whereas glutamate is the main excitatory transmitter in the CNS.
- Inhibitory neurotransmitters include glycine and gamma aminobutyric acid (GABA).

### Neurotransmitter Action Steps in Synaptic Transmission
- 1. Action potential reaches an axon bulb and causes calcium ion gates to open, and calcium ions move into the axon bulb. 
- 2. The rise in calcium ions in the axon bulb causes synaptic vesicles containing neurotransmitter to move towards the presynaptic membrane. 
- 3. Synaptic vesicles merge with the presynaptic membrane and exocytosis of neurotransmitters into the synaptic cleft occurs. 
- 4. Neurotransmitters diffuse across the synaptic cleft and bind to receptor proteins on the postsynaptic membrane. 
- 5. If sufficient excitatory neurotransmitter binds to receptors, an action potential is produced in the postsynaptic membrane and travels along the length of the second neuron.
- 6. Neurotransmitters are broken down to prevent continuous stimulation or inhibition of the postsynaptic membrane.

### The Neurotransmitter Serotonin
- A presynaptic cell makes serotonin from the amino acid tryptophan, then packages it in vesicles in the end terminals. 
- An action potential passes down the presynaptic cell into its end terminals. 
- This stimulates the vesicles containing serotonin to fuse with the cell membrane and exocytose serotonin into the synaptic cleft. 
- Serotonin passes across the synaptic cleft, and binds with specialized protein receptors and depolarizes the postsynaptic cell where a new action potential will be propagated.
- Remaining serotonin molecules in the cleft as well as those released by the receptors after use get degraded by the enzyme monoamine oxidase, or MAO.
- In the presynaptic cell, MAO destroys reabsorbed serotonin molecules, enabling the nerve signal to be turned "off" and the synapse readied to receive another action potential. 
- Catechol-o-methyltransferase, or COMT, is a similar enzyme that targets catecholamines but not serotonin.

### Modulation
- Action of ‘fast’ neurotransmitters is brief, because they unbind quickly from their receptors, and rapidly cleared from the synaptic cleft by breakdown or reuptake.
- The synaptic potential is brief and the membrane potential returns rapidly to its resting level because receptor channels remain open only as long as a neurotransmitter is bound, and their binding is transient only.
- Other transmitters, or modulators, such as serotonin, dopamine, noradrenaline, and small peptide molecules, act more slowly and for longer periods of time.

### FYI: Names, Types, Postsynaptic Effects, Locations and Functions of Neurotransmitters
- Dopamine is an excitatory amine, which controls arousal levels within the brain and smooth muscle.
- Serotonin is an excitatory amine, which affects mood, sleep, pain, & appetite within the brain and smooth muscle.
- Noradrenaline is an excitatory amine, which induces arousal, and heightens mood in the brain and smooth muscle.
- Acetylecholine is an excitatory and inhibitory acetic acid component used in the parasympathetic nervous system, brainstem and acts to promote memory and vasodilation.
- GABA is an inhibitory amino acid component, which primarily controls anxiety levels of the brain. 
- Enkephalin is an inhibitory neuropeptide component within brain and spinal chord which promotes stress release, calm, and acts as a natural painkiller.

### General Properties of Synapses
- Each skeletal muscle fibre is innervated by a single excitatory nerve fibre, which discharges 100-300 vesicles.
- This is enough to produce an action potential in the muscle cell.
- A single nerve cell may have tens, hundreds, or thousands of synapses that are not only inhibitory or excitatory.
- Synapses may involve many different type of transmitters and post-synaptic receptors.
- It is thought there may be more than 100 different neurotransmitters.
- Each pre-synaptic input may release just a few vesicles in response to a nerve impulse.
- Synaptic potential may be far smaller than that of a muscle fibre.
- Multiple simultaneous or closely-successive inputs are needed to elicit one action potential.
- The output of the post-synaptic neuron is an integrated response to all of its many different inputs.

### Synapse Abuse
- Most drugs that work on the brain, as well as drugs of abuse, act on synapses.
- Nicotine activates nicotinic acetylcholine receptors, especially in the brain, and causes dopamine release, resulting in euphoria.
- As opposed to nicotine, caffeine is of worse quality.
- Curare, traditionally used by South American natives as an arrow poison, paralyzes prey because it acts antagonistically to the acetylcholine receptor, blocking neuromuscular transmission.
- Morphine and heroin act on opiate receptors, and cannabis acts on cannabinoid receptors.
- Cocaine blocks reuptake system, clearing the neurotransmitter dopamine from the synaptic cleft, which increases dopamine concentration for longer, acting as a stimulant. 
- Some nerve gas or sarin agents block removal of acetylcholine at nerve-muscle synapses, causing death most likely from spasms.
- Inherited neuromuscular disorder, Myasthenia Gravis, occurs when the body produces antibodies that act antagonistically to the acetylcholine receptors on muscle fibres.
- This causes them to be bound to the cell, reducing the number of receptors, thus compromising neurotransmission.
- Affected patients are easily fatigued.
- Graves disease is an example of disease related synapse dysfunction where Auto-antibodies are agonistically bound to the thyroid gland, causing an increase in thyroid hormone.
- Epilepsy is sometimes linked to a decrease in the efficiency of inhibitory, (GABA), transmission in the brain, leading to over-excitability of networks of neurons.
- There is some evidence that major psychiatric conditions such as depression and schizophrenia, involve disorders of synapses where serotonin and dopamine neurotransmitters act respectively.
- A lack of dopamine is connected to Parkinson's and too much dopamine is connected to Tourette's syndrome.

### Neural Circuits
- Neurons never function in isolation.
- Neurons are organized into ensembles or circuits that process specific kinds of information.
- The arrangement of neural circuits varies greatly according to the intended function.
- Synaptic connections that define a circuit are typically made in a neuropil.
- A neuropil typically has a dense tangle of dendrites, axons terminals, and glial cell processes, while cell bodies are few.
-  A neuropil between nerve cell bodies is the region where most synaptic connectivity occurs.
- Essential to function is the direction of information flow in any particular circuit.
- Nerves that carry information toward the central nervous system, or toward the spinal cord or brain centrally, are called afferent neurons.
- Nerves that carry information away from the brain or spinal cord, are called efferent neurons.
- Nerve cells that only participate in the local aspects of a circuit, are called interneurons or local circuit neurons.
- These three classes of afferent neurons, efferent neurons, and interneurons, are the basic constituents of all neural circuits.
- Neural circuits are both anatomical and functional entities.
- A simple example is the circuit that controls the myotatic spinal reflex, or "knee-jerk".
- The afferent limb of the reflex is sensory neurons of the dorsal root ganglion in the periphery.
- Afferents target neurons in the spinal cord, while the efferent limb comprises motor neurons in the ventral horn of the spinal cord.
- One efferent group projects to flexor muscles in the limb, and the other to extensor muscles.
- The third element is interneurons in the ventral horn of the spinal cord. The sensory afferent neurons contact with interneurons and make synapses on the efferent motor neurons that project to the flexor muscles.
- Synaptic connections between sensory afferents and the extensor efferents are excitatory, causing the extensor muscles to contract.
- Interneurons activated by the afferents are inhibitory and their activation therefore diminishes electrical activity in motor neurons and causes the flexor muscles to become less active. 
- The result is a complementary activation and inactivation of the synergist and antagonist muscles that control the position of the leg.
- More complex circuits involve processes such as addiction.

### Neuronal Pools
- The human body contains approximately 10 million sensory neurons, 20 billion interneurons, and one-half
million motor neurons.
- Interneurons are organized into a smaller number of neuronal pools.
- A neuronal pool is a group of interconnected neurons with specific functions, defined on the basis of function.
- A pool may be diffuse, and involve neurons in several regions of the brain, or localized and restricted, with neurons restricted to one
specific location in the brain or spinal cord.
- Estimates of actual pool number range between a few hundred and a few thousand.
- Each pool has a limited number of input sources and output destinations, and may contain both excitatory and inhibitory neurons.
- The output of an entire neuronal pool may stimulate or depress the activity of other pools, or it may exert direct control over
motor neurons or peripheral effectors.
- Interaction pattern among neurons provides clues to a neuronal pool's functional characteristics.
- Functionality is classified by wiring diagram such as neural or electrical circuit.

### Divergence (in Neural Circuits)
- Divergence is the spread of information from one neuron to several neurons or from one pool to multiple pools.
- Permits broad distribution of specific inputs.
- Considerable divergence when sensory neurons bring data to the CNS.
- Spinal chord and brain are sent information distributed to neuronal pools.
- Visual information from eyes reaches consciousness at the same time it distributes to the areas that control balance and posture.

### Convergence (in Neural Circuits)
- Occurs when several neurons synapse on the same post synaptic neuron.
- Patterns of activity in presynaptic neurons can therefore have a similar affect on the postsynaptic neuron.
- Convergence assists same motor neuron action under conscious and Subconscious control.
- The movements of your diaphragm and ribs shift from conscious to subconscious.

### Serial Processing (in Neural Circuits)
- Stepwise fashion information relays from one neuron to another. This is called serial processing.
- Serial processing relays brain information.
- Information moves from one hemisphere to another in this way.

### Parallel Processing (in Neural Circuits)
- Several neurons or neuronal pools process the same information at one time.
- Must involve divergence before parallel processing occurs.
- Several responses occur simultaneously under parallel procession such as moving you, weigh etc and even feel the pain, from stepping on a sharp object.

### Reverberation (in Neural Circuits)
- Occurs in some neural circuits due to use of feedback.
- Arrangement via collateral branches somewhere leads back toward the sequence for the source of an impulse, and has affects further on the pre-synaptic neurons.
- Stimulates itself, or shuts off, depending. Complicated examples exist across neuronal pools, aiding consciousness, muscular coordination breathing.

### Chapter 16 Topics: Somatic, Sensory and Integrative Systems
- Levels and components of sensation
- Pathways for sensations from body to brain
- Pathways for motor signals from brain to body
- Integration Process
Wakefulness and sleep
Learning and memory

### Sensation
- Brain components interact to receive sensory input, integrate and store information, transmit motor responses.
- Nervous system uses neural pathways to transmit impulses from receptors to the circuitry of the brain, manipulates circuitry, and produces directives transmitted in neural pathways to as a response.
- Sensation is conscious, or unconscious awareness of external and or internal stimuli.
- Reaction nature varies according to impulse destinations and receptor type.
- Spinal chord sensory impulses serve as spinal reflex input such as for the stretch reflex.
- Impulses elicits more brainstem reflexes including heart rate to breathing rate.
- The brain can precisely identify sensation using the cerebral cortex including location, such as with touch, pain, hearing, or taste.
- Stimuli that the body is aware of is sensation.
- If receptors are in place, then brain is aware stimulus.
- Types of stimuli that we are aware of: Heat pain touch smells. We have stimuli for these.
- Stimuli we aren't aware of: X-rays ultra high frequency UV light. Due to not having sensory receptor.
- Perception is conscious awareness of a sensation/ interpretation.
- It also deals with location memory and identification related to the perception, which is stored in the cortex.
- Blood sugar can be sensed and not perceived in the context of "can you feel your blood sugar".
- Modalities are distinguished from one another, i.e. the different senses.
- Sensations are of different types like touch, pain, temperature, vibration, hearing, vision, and receptors most often only detect one type, however there are exceptions.
- Sensory modalities are of two classes: general and special senses.

### General &  Special Senses
- Somatic and visual senses that include information of internal organs, which are general senses.
- Sensory touch, pressure vibration including itch. In addition, temperature thermal and pain sensations as well as perceptive sensations, are included.
- Sensations are either conscious or unconscious relative to static positions i.e. of joint and muscle. Visual senses provide information for conditions by internal organs.
- Special senses are smell, taste, hearing and balance.

### Actions for Sensation
- Process for sensation in specialized cells or sensory neuron dendrites.
- 4 Events for Sensation to Arise.
- 1. Sensory receptor stimulation needs stimuli.
- 2. Transduction or transfer of stimulus by sensory receptor to create graded potential.
- 3. Impulses are formed as Grade potentials reach threshold
- 4. The CNS sensory input is integrated where sensory impulse is integrate with consciousness in the core. Should any step be missing Sensation does not occur.

### Structural Classification
- Consists of free nerve endings as bare dendrites that are specialized from temperature that have unmyelinated terminal branches that spread.
- Encapsulate ending are the connective kind that relate the pressure to vibration in deeper touch sensations.
- Sensory receptors from special senses that synapse with cells.

### Stimuli Detected
- Mechanoreceptors detect physical/mechanical stress such as touch, pressure, vibration including the concepts of hearing as well as receptive function with the body.
- Thermoreceptors respond to temperatures when the are decrease or increase.
- Nociceptors deal with tissue damage via responses to intense mechanical deformation.

### Pain & Related Pain Signals
- Initial is a sense of high sensitivity.
- Descending inhibits transference to provide analgesia.
- Pain results from fibers becoming activated and causing TENS signals to the brain.

### Other Types of Receptors
- Chemoreceptors detect chemical signals in blood, with the help of arterial oxygen.
- Photoreceptors detect light via color sensitive pigment.

### Responses to Stimuli
- Free nerve endings encapsulated from receptors create generate potentials from order neurons
- Vision, hearing, or cell related synapse may trigger sensory nerve impulses with similar stimulus intensity in the inner portion
With itches in the body.

### Receptor Location
- Surface receptors receive external stimuli for vision taste touch or pressure.
- Internally it will monitor blood pressure.
- It's with receptors & originate the stimuli to joint capsule..

### Adaptation
- Most Sensory receptors adapt which means Generator potential decreases.
- Sensitivity decrease due to responsiveness or disappearance.
- Slow tonic are permanently active indicating  that sensory receptors continue a constant stimuli over duration including of pain receptors.
- Response decreases quickly over for stimuli providing on rapid changes like in muscles fibers to the stimulus.

### Tactile Sensations
- Touch, pressure, and vibration are tactile sensations, along with itch and tickle, perceived based on activation of the same types of receptors.
- Touch ability of perception location to sense size and texture from stimulation. It is carried on separate pathways for discrminiation.

### Rapid Adaptation
- Rapid adapting to touch the cornea located are used mostly with hair follicle to the surface.
- Detect free and are a common adaptation to touch.

### Slowly Adapting
- Slow adapting is is contact with cell stratum that are related to external part of genitalia. They contact and are related or movement.

### Pressure
- Relates mostly to stimulation by sensory responses or stimuli with nerves and muscles.

### Vibration
- Result from repeat signal with stimulus. Detection can be sensed, related to intensity.

### Itch
- Mystery to human physiology since humans don't know the functionality.

### Tickle
- It's to do with nerve endings via laughter

###  Phantom Limb
- From when a amputated and causes pain. Cerebral cortex interprets from the damage.

### Thermal Sensation
- Free nerve endings with receptive skin surface are mostly Cold
- Most are connected by fibers activates the receptor, when not as are active this mean more active..

### Pain Sensation
- Free Endings or tissue not at brain by stimuli damage with irritation.
- Persist from pain are adaptive with stimuli or muscle that involve extension.

### Fast Pain
- Occurs rapidly from pain relating or sharpness.
- Nerve impulse propagates with diameter medium in fibers.

### Slow Pain
- Begins with slow excruciating burning pain from a stroke the teeth.
- Conductor is smaller and unmyelinate with fibers.

### Actions to Percieving Discomfort
Most people first stub and then have slowly have a sense and this sensation results with muscle joint with tendin
- A way receptors are mostly located with the body in many situations.

### Pain Threshold and Tolerance
- Pain threshold: How strong does the stimulation have
to be before it elicits a pain response?
Ie. Before the stimulus reaches receptor threshold
Involves only the PNS
Constant among population
- Pain tolerance: How much pain an individual can take?
Involves the CNS
varies widely among population
Not well understood

### Drugs for Pain
Blockers will act as a suppressant the reduce production.
- Sodium blockers produce short term and slow signals. The  other side will  inhibit  production  for   both.

### Proprioception
The brain helps and sends data help us know without the us to feel to how to type or talk
- Adaptor are slow.

### Motor and Sensory Neurons
Neuronal pool help control activities. The body is  consciouss  for it  and it acts to control.

### Muscle Spindle
- Proprioceptors in long length to skeletal participate with stretching. Muscle has fiber end.

### Main Function of Muscle
- Mostly to measure, when shotten is stimulate

### Additional
Limb have neuron gamma horn from the spinal cord they do not get too high.
During sensory the process is then related by activity.

### Overall
- Diagrammatic summarization of the various types of receptors throughout the limbs

### Somatic Motor Pathways
- Lower motor neurons extend from the CNS to skeletal muscles (PNS).
- Lower motor neurons are peripheral nerves.
- Lower motor neurons are also called the final common pathway due to regulatory mechanisms.
- LMNs as cranial nerves to extend skeletal mm of face / head to  spinal nn for limbs/trunk.

### Control of Body Movement
- Control from cerebral complex are to activate with the body control and assist normal functions overall
- Control mostly by neurons through inter communication to other sectors of the central nervous system

### Neural Circuits
- Neurons never function in isolation; they are organized into ensembles or circuits.
- Although arrangement of neural circuits varies greatly, there are shared structural features.
- Synaptic connections that define a signal are typically made in neuropil with few cell bodies. The direction of information flow within circuits are related to the following types of neurons:
   - Afferent Neuron carries electrical impulse from the spinal cord. This is located toward central region area of nervous system.
   - Efferent Neuron: Carry electrical pulses which away area of spinal cord.
   - Interneurons: Participate primarily local area circuits and all three types of class make area circuit.
- Spinal or knee jerk is anatomical with sensor.
- Sensor neurons target neurons related with other areas and form connections..
- Sensory and extensor are exciting causing motor to contract with other the fibers more action results if motor not the one with higher levels.

### Neuronal Pools
- With ten million censor and neuron help work motor to organise functions. Sensory pool helps defuse a few localized.
- Limited, the pool may control the stimulus.
- Functionality are similar by type.

### Sensation
- 4 events happen: when stimulating to transduction with receptors

### Stimulated Detected
- Mechanoreceptors, thermorecptors are located through out the body tissue:
- Analgesic is lead with higher stimuli

### Response The Stimuli
- There is type and graded potentials including from from synaptic with the PSP

### Location of recptor.
- It located close to area surface with stimuli that are high for temperature

### Adaption
- Most help maintain slow down rate to help signal

### Tickle
- Neuromuscular system in nerve ending.

### Slow Pain
- Has slow response mostly to tooth and other tissues that help the body with c type fibre activation

### Motor Pathways
- Neuron area motor and they send output for area and limb movement area

### Direct Motor pathways

### Motor Pyramidal pathway.
- help for for movement.
All that is activated works through all types fibres