PHYSIOLOGY OF SENSORY AND MOTOR SYSTEMS OF THE SPINAL CORD (Part 1)

Site where 2 neurons (or neuron-skeletal muscle or gland) come into close contact and functional, unidirectional interneuronal communication occurs

Also the region where axons of one neuron (______) terminates on dendrites, soma, or axon of another neuron (_______)

SYNAPSE

Presynaptic

Postsynaptic

TRUE or FALSE:

Presynaptic neuron axon can synapse on any part of the postsynaptic cell

TRUE

Parts of Synapse
Presynaptic terminal, Synaptic cleft, Postsynaptic terminal

___________-enlarged ends of presynaptic fibers

_________-Extensions from the surface of a neuron that form receptive sites for synaptic contact with boutons; Also helps with memory

Terminal Boutons / Synaptic knobs

Dendritic Spins / Synaptic spines

FUNCTIONAL ANATOMY OF SYNAPSES:

Classification According to Structural Association:

• Axodendritic - most common

• Axoaxonal

• Axosomatic

• Neuromuscular Junction (NMJ)

FUNCTIONAL ANATOMY OF SYNAPSES:

Classification According to Type of Communication

Chemical - neurotransmitters

Electrical - gap junctions; low-resistance bridges which ions can pass with relative ease

• not common in mammals
  

PRESYNAPTIC TERMINAL:

Contain many mitochondria, membrane-enclosed vesicles 

• which contain neurotransmitters called ____________

Synaptic Vesicles

3 KINDS OF SYNAPTIC VESICLES

1. Small, Clear

2. Small, Dense-core

3. Large, Dense-core

NT of Small Clear Vesicles

Acetylcholine, Glycine, GABA, Glutamate

NT of Small Dense-core

Catecholamines

NT of Large Dense-core

Neuropeptides

Vesicles and most proteins within are synthesized in neuronal cell body and transported via which type of transport?

Fast Axonal Transport

This NT directly acts on the postsynaptic membrane; only one can be active

Principal Activator NT

NTs that do not have a direct effect on the postsynaptic membrane but as modulators and modify the activity of the principal NT

• enhance, prolong, inhibit or limit the principal NT’s effect on the postsynaptic membrane 

Neuromodulators

TRUE or FALSE:
In chemical synapses, several principal activator neurotransmitters may be present

FALSE:

In chemical synapses, several neurotransmitters may be present

but only 1 can be the principal activator, others are neuromodulators.

How a NT gets transported outside the cell, cytoplasmic sides of the two membranes fuse (synaptic vesicle and phospholipid bilayer)

Exocytosis

How a NT gets transported into the cell, extracellular sides of the two membranes fuse (synaptic vesicle and phospholipid bilayer)

Endocytosis

Found throughout presynaptic nerve terminal

Releases via exocytosis from all the parts of the terminal

Large Dense Core Vesicle

are recycled in nerve ending 

Via exocytosis then endocytosis and refilling 

Enter endosomes and are budded off and refilled

Small Clear and Small Dense-Core Vesicle

 synaptic vesicle discharges its contents through a small hole in cell membrane and the opening reseals rapidly and the main vesicles stays inside 

 Kiss and Run Discharge

SMALL SYNAPTIC VESICLE CYCLE:

What do vesicles bud off of?

EARLY ENDOSOME

after they get filled with NTs, Vesicles translocate to the release sites of the plasma membrane where they dock. in this stage the vesicles are ___________

Primed

Once the vesicle is primed, what triggers exocytosis of the NTs into the synaptic cleft?

Ca²⁺ influx

After exocytosis the vesicle wall is covered in a substance which promotes endocytosis, what is this substance?

Clathrin

After the vesicle is back inside the cytoplasm its uncoated by clathrin and fuses with the _________ restarting the cycle

Early endosome

When an action potential moves down an axon and enters the axon terminal on the presynaptic side of the synapse, the opening of voltage dependent Ca⁺² channel allows influx of Ca⁺², initiating NT release

• Inc ________ Ca⁺² = enhances NT release

• Dec _________ Ca⁺² = dec NT release 

• Inc ____ = blocks release

Extracellular

Mg (Magnesium)

SYNAPTIC CLEFT AND POSTSYNAPTIC TERMINAL:

Synaptic Cleft - ________ wide

20-30 nm

____________ - complexes of specific receptors, binding proteins and enzymes induced by postsynaptic effects

Postsynaptic density

True or False:

Action of chemical mediator on its target is more dependent on the properties of the mediators than the type of receptor which acts on.

False

type of receptor which acts on is more important

True or False:

Each chemical ligand has potential to act on many subtypes of receptors

True

True or False:

Receptors for many NT are located on postsynaptic elements only.

False:

Receptors for many NT are located on both presynaptic and postsynaptic elements

These types of receptors inhibits further NT release.

Autoreceptors

2 types of NT receptors

Ligand-Gated (Ionotropic Receptors)

Metabotropic Receptors (G-Protein Coupled)

activation of this gate elicits brief inc in ionic conductance; fast synaptic transmission 

Ligand-Gated (Ionotropic Receptors)

utilizes secondary messenger system

Metabotropic Receptors (G-Protein Coupled) 

NT receptors are concentrated in clusters of __________

Postsynaptic membrane 

 loss of responsiveness of receptors due to prolonged exposure to ligands 

Desensitization

TYPES OF ACETYLCHOLINE RECEPTORS

Nicotinic Receptors

Muscarinic Receptors

Ligand-Gated Ion Channels

Nicotinic Receptors at neuromuscular junction

Nm receptors

Nicotinic Receptors in CNS and autonomic ganglia

Nn receptors

Receptors at smooth muscles and glands

Muscarinic

How many subunits form the central channel which permits passage of Na⁺ and other cations in Ligand-Gated Ion Channels

5 subunits

• NT are rapidly transported from synaptic cleft back into presynaptic terminals 

• Major factor in termination of NT action

REUPTAKE

Specialized area where a motor nerve terminates on a skeletal muscle fiber

NEUROMUSCULAR JUNCTION (NMJ) 

As the axons supplying a skeletal muscle fiber approaches its termination, it loses its _________ and divides into a number of __________

Myelin Sheath

Terminal boutons

Main NT of NMJ

Acetylcholine

depressions in  the motor endplate

Junctional fold

thickened portion of the postsynaptic muscle membrane at the NMJ

motor endplate

True or False:

Each endplate can receive input from multiple nerve fibers

False:

Each endplate receives input from a single nerve fiber

True or False:

 The space between the nerve and the thickened muscle membrane is much smaller to the synaptic cleft at a neuron-to-neuron synapse

False:

 The space between the nerve and the thickened muscle membrane is comparable to the synaptic cleft at a neuron-to-neuron synapse

Where does Ach synthesis take place?

Terminal Bouton

What enzyme facilitates the synthesis of Acetylcholine (Ach)

Choline acetyltransferase

2 main components to make Ach (Not the catalyst)

Acetyl CoA and Choline

How does choline get into the cytoplasm of the terminal bouton?

via Na⁺ depednent choline transporter (Hemicholinium)

How does Ach enter the synaptic vesicle?

Vesicle-associated transporter (VAT)

What triggers Ca⁺ influx in presynaptic terminal to release the vesicles?

Nerve signals / Action potential

After Ach is used it undergoes hydrolysis and splits into?

Choline and Acetate

What is the catalyst for hydrolysis of Ach?
Where is it concentrated?

Acetylcholinesterase

Postsynaptic membrane

NT are released from presynaptic terminal in small packets called?

Quanta

How many synaptic vesicles containing Ach can each nerve impulse release?

about 60

how many molecules of NT are in one vesicle

10,000

True or False:

The size of quanta released is directly proportional to Ca²⁺ concentration and inversely proportional to Mg²⁺ concentration

True

Stimulation of single presynaptic excitatory neuron evokes in the postsynaptic neuron an _____ which is a small, non propagated depolarization

EPSP (Excitatory Postsynaptic Potential)

 excitatory ligand-gated channels:

cholinergic or glutaminergic  channels

excitatory ligands:

Choline and Glutamate

True or False:

EPSP = AP

False:

EPSP a weak ass signal dat didnt reach AP level

Electrical stimulation of single inhibitory presynaptic neuron contacting a single postsynaptic neuron results in development of ______, which consists of transient _________ of the postsynaptic membrane

IPSP (Inhibitory Postsynaptic Potential)

Hyperpolarization

Inhibitory channels

transmitter-gated Cl^- channels or G protein associated K+ channels

Opening of Cl^- channels = Cl^- influx = more negative intracellularly = hyperpolarization

Opening of K⁺ channels = K⁺ efflux = less positive intracellularly = hyperpolarization

Inhibitory ligand

Glycine

Generation of AP in Postsynaptic Neuron:

• The ______of the neuron thus acts as an integrator of excitatory and inhibitory stimuli

• When the level of _________ reaches the threshold voltage, a propagated AP will occur

• ______________ - area with lowest threshold for AP generation

• The soma of the neuron thus acts as an integrator of excitatory and inhibitory stimuli

• When the level of depolarization reaches the threshold voltage, a propagated AP will occur

• Initial Segment of Axon - area with lowest threshold for AP generation

Temporal and Spatial Summation of Postsynaptic Potentials

________- determines the time course of the synaptic potential

________- degree to which  a depolarizing current is reduced as it spreads passively

Time Constant- determines the time course of the synaptic potential

Length Constant- degree to which  a depolarizing current is reduced as it spreads passively

Temporal Summation (Skibidi Explanation)

Longer time constant= longer time for potential to decay = more time for other potential to add up to da other potential = bigger chance of AP

Spatial Summation (Skibidi explanation)

Longer length constant = more distance covered before potential decays = more chance for other potential to find and add to da 1st potential to make big AP = win win win

OR

Longer length constant = more distance covered before potential decays = can reach initial segment before dying = initial segment has low threshold = win win win AP

INHIBITION AND FACILITATION AT SYNAPSES:

Via release of inhibitory NT (glycine or GABA) from presynaptic terminal to induce IPSP on the postsynaptic neuron

Postsynaptic Inhibition

Reciprocal Inhibition

U shud know ts alr

Presynaptic Inhibition:

Is a process mediated by neurons whose terminals are one excitatory endings, forming ________ synapses. 

s a process mediated by neurons whose terminals are one excitatory endings, forming axoaxonal synapses. 

NT of presynaptic inhibition?

GABA

Activation of the presynaptic receptor which inhibits the excitatory axon increases conductance of what ion?

Cl^-

Is produced when the AP is prolonged and the Ca⁺² channels are open for a long period

Presynaptic Facilitation

Organization of Inhibitory Systems

Neurons may also inhibit themselves in a negative feedback manner which is called?

negative feedback inhibition

For instance, a spinal motor neuron emits a recurrent collateral synapse on an inhibitory interneuron (Renshaw cell) that terminates on the cell body of spinal motor neuron 

The renshaw cell releases ______ to reduced the activity of the motor neuron

Glycine

5 basic types of sensory receptors

Mechanoreceptors

Thermoreceptors

Nociceptors

Electromagnetic Receptors

Chemoreceptors

Names are self explanatory

Classification of receptors based on structure

encapsulated vs nonencapsulated

NONENCAPSULATED RECEPTORS: (FREE NERVE ENDINGS, MERKEL DISCS, HAIR FOLLICLE RECEPTORS)

_____________- Found between epithelium of skin, cornea, alimentary tract, connective tissues and muscle

Free nerve endings

True or False:

Free nerve endings afferent fibers and terminal ends maybe myelinated or nonmyelinated.

FALSE:

Their afferent fibers maybe myelinated or nonmyelinated but terminal ends are devoid of myelin and schwann cells

What sensations can free nerve endings detect?

Detect pain, crude touch, pressure, tickle sensations and possible cold and heat 

Found on hairless skin and in hair follicles

Passes into the epidermis and terminates as a disc-shaped epithelial cell in the deeper part of the dermis, called ________

Merkel Discs

Merkel cells

Function of merkel discs?

Slowly adapting touch receptors 

Degree of pressure exerted on skin

Nerve fibers that wind around follicle in its outer connective tissue sheath below the _________

Hair Follicle Receptors

Nerve fibers wind around follicle in its outer connective tissue sheath below the sebaceous gland

Function of Hair Follicle Receptors

Detects bending of hair

If hair remains bent, receptor remains silent, once it is release, a burst of impulse is initiated

ENCAPSULATED RECEPTORS:

MEISSNER CORPUSCLES, PACINIAN CORPUSCLES, RUFFINI CORPUSCLES, NEUROMUSCULAR SPINDLES

Located in dermal papillae especially on palms and soles

_______ shape

Consists of stack of modified flattened _______ cells arranged transversely across the long axis of the corpuscles

Enclosed by connective tissue that is continuous with _________

Meissner Corpuscles

Ovoid shaped 

Consists of stack of modified flattened schwann cells arranged transversely across the long axis of the corpuscles 

Enclosed by connective tissue that is continuous with endoneurium

Speed of adaptation of Meissner Corpuscles

Rapidly adapting

Function of Meissner Corpuscles

Two-point discrimination

Abundant in dermis, subcutaneous tissue, ligaments, joint capsules, pleura, peritoneum, nipples, external genitalia

Also Ovoid

Capsule consists of numerous ___________ of flattened cells

Pacinian Corpuscles

Capsule consists of numerous concentric lamellae of flattened cells

Speed of adaptation of Pacinian Corpuscles

Rapidly adapting mechanoreceptor

Function Pacinian Corpuscles

Detects Vibration

Dermis of hairy skin

Consists of several large unmyelinated nerve fibers ending with a bundle of ____________ and surrounded by cellular capsule

Ruffini Corpuscles

Consists of several large unmyelinated nerve fibers ending with a bundle of collagen fibers and surrounded by cellular capsule

Speed of adaptation of Ruffini Corpuscles

Slowly adapting mechanoreceptor

Neuromuscular Spindles

Found in skeletal muscles; more towards tendinous attachments

Changes in muscle length and rate of change 

Surrounded by fusiform capsule

How many sender intrafusal muscle fibers are in a muscle spindle?

6-14

Extrafusal muscle fiber - ordinary muscle fiber outside the muscle spindles