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Neurons and action potential (Glial cells (Types of glial cells…
Neurons and action potential
Neurons
Cells of nervous system --> responsible for reception --> conduction --> transmission of electrochemical signals
Electrical signals --> within axon Chemical signals --> in synapses
Classes of neurons
Multipolar neurons --> most common --> 1 axon --> many dendrites
Unipolar neurons --> 1 process exceeding from cell bodies --> known as sensory neurons --> or afferent nerves in PNS --> pass signals to CNS
Bipolar neurons --> two processes exceeding from cell bodies --> pass signals within CNS
Interneurons --> short or no axons --> responsible for conducting signals within a single brain structure
Anatomy of neurons
External anatomy of the neuron
Cell body --> a.k.a soma --> contains cell nucleus --> all metabolic reactions occur there
Cell membrane --> semipermeable --> controls movement of substances in and out of neuron
Dendrites --> short extensions connected to soma --> receive information
Axon hillock --> junction between soma and axon --> plasma membrane composition changes here --> voltage-gated Na and K channels appear --> from here on --> action potential possible
Axon --> long "tail" of cell --> responsible for carrying electrical signals
Myelin sheath --> fatty white substance that surrounds axon --> insulates axon --> facilitates transmission of nervous signals
Nodes of Ranvier --> tight gaps between myelin --> facilitate action potential process --> only place where ions can pass through membrane--> (in myelinated axons)
Buttons --> endings of axon branches --> release neurotransmitters into synapses
Synapses --> gaps between neurons --> neurotransmitters travel through them
Internal anatomy of the neuron
Endoplasmic reticulum --> network of flat folded membranes --> rough ER covered with bound ribosomes --> rough ER --> site of protein production --> ones for secretion produced here --> smooth ER --> for production of fats
Nucleus --> stores DNA --> responsible for neuron replication and transcription --> in soma
Cytoplasm --> clear fluid inside cell --> promotes cellular activities
Mitochondrion --> double-membraned elongated organelle --> contains own DNA --> site of ATP production --> in aerobic respiration
Ribosomes --> round structures --> responsible for protein synthesis --> two types -->
bound ribosomes
--> located in rough ER --> produce protein for secretion -->
free ribosomes
--> in cytoplasm --> responsible for protein synthesis --> don't leave cell
Golgi complex --> network of flat membranes --> responsible for processing and packing proteins for secretion
Microtubes --> thin tubes in cytoplasm --> responsible for transporting material through neuron quickly
Synaptic vesicles --> single-membraned structures --> storage of neurotransmitters
Neurotransmitters --> chemical substances--> released when electrical signal from axons needs to travel to others --> quantity and type influences activity of other cells
Glial cells
Somethings --> different names --> vary on location
Cluster of cell bodies --> CNS --> nuclei --> PNS --> ganglia
Bundles of axons --> CNS --> tracts --> PNS --> nerves
Number of glial cells similar to number of neurons --> found throughout nervous system
Types of glial cells
Oligodendrocytes --> composed of extensions rich in myelin --> surround axon of neurons within CNS --> presence facilitates signal condition within neurons
1 oligodendrocyte= several myelin segments
Schwann cells --> presence facilitates signal condition within neurons too --> performed PNS--> the only cells capable of axonal regeneration
1 Schwann cell --> one myelin segment
Microglia --> small --> responsible for triggering inflammatory responses --> perform phagocytosis --> engulf and digest damaged --> or dead --> cellular bodies
Astrocytes --> largest --> provide coverage of blood vessels in brain --> allow or block exchange of chemicals between blood and neurons of CNS --> responsible for establishment of synapses --> modulation of neural activity
Action Potential
Resting membrane potential
Membrane potential
--> difference in electrical charges outside and inside of cell
Resting membrane potential
--> when the charge across membrane -->-70mV --> neuron polarised
During resting membrane potential --> ratio of negative to positive charges is
greater
inside
the neuron than outside
Ie. K+ ions --> mostly concentrated inside --> Na+ --> Cl- ions --> more outside
Types of transport
Passive transport
Diffusion
--> when particles randomly move from high to low concentrated areas --> down the concentration gradient --> why ions constantly distributed across plasma membrane --> movement is random --> no energy required --> = passive
Electrostatic pressure
--> moves molecules from high to low concentration by
activating molecules
with
opposite charges
and repulsing those with same charges
Ion channels --> example of facilitated diffusion --> ions move through them with concentration gradient
Active transport
Requires energy from action potential --> within neurons --> active transport --> performed by sodium-potassium pump --> uses ATP --> why glucose is needed constantly --> for pump to work
Also for nervous system to function properly --> transports ions against concentration gradient -->each turn of pump uses 1 ATP --> pumps 3 NA+ ions out for two K+ ions in --> work constantly to maintain the gradient --> ion leakage --> means ions flow through channels even when closed --> so pump needs to work all the time -->
facilitated dffusion
Action Potentials
Membrane potential --> temporarily reversed from -70 to +50mV --> action potential occurs --> referred to as
all-or-none responses
--> has to reach the threshold to work or won't at all
Rate law
--> strong stimulus will increase axon's rate of firing --> production of action potentials --> so high rate of firing --> stronger muscle contraction
Refractory periods
Absolute refractory period
--> when impossible to elicit two action potentials one after another --> lasts about 1-2 ms --> afterward is the
relative refractory period
--> where firing of the neuron is possible only by receiving abnormally high levels of stimulation
During refractory periods --> electrical signal cannot travel backwards --> explains the direction of the signal conduction within neurons
Conduction
Antidromic conduction
--> when action potential travels towards cell body --> resulting from very intense electrical stimulation to the axon terminals
Orthodromic conduction
--> normal condition --> electrical signal from cell body --> to end of axon
Voltage-gated channels
Sodium Na+ channels
Sodium --> accumulates at axon hillock --> reaches threshold potential --> channels open
Lots of sodium ions
inside
cell
When action potential is reached --> sodium channels
close
Potassium K+ channels
Potassium channels
open
at action potential
Potassium ions --> flow
out
of cell
Potassium channels -->
close
--> after reaching resting potential again