Neuronal Signaling and Nervous System - Final

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Neuron (definition and function)

-single nerve cell
-specialized for communication
-function: generate electrical impulses when can be carried to other excitable cells

Typical neuron consists of...?

1. dendrites - receive info

2. soma - cell body; carries out normal cellular activities

3. axon - nerve fiber; carries info toward other excitable cells

4. axon terminal - transmits message to other excitable cells

Conduction vs. transduction of messages

-conduction of messages = movement of impulse along same neuron, same cell

-transduction of messages = movement of impulse from one cell to the next

3 Types of Neurons based on structure

1. multipolar neuron (typical neuron)

2. bipolar neuron (cell body in middle)

3. unipolar neuron (cell body detached from axon)

3 Types of neurons based on function

1. afferent (sensory)
2. efferent (motor)
3. interneuron

Afferent Neuron

- accepts info and conveys impulses to CNS
- no dendrites, only bipolar axon
-soma lies outside CNS

Efferent Neuron

-conveys impulses toward peripheral nervous system
-has dendrites
-soma and dendrites lie in CNS

Interneurons

- connect afferent and efferent neurons together in CNS
-only found in CNS
-99% of neurons fall into this category

Synapse

-interface between 1 excitable cell and another

Glial Cells

-NOT neurons
-make up about 90% of cells in CNS
-different types: astrocytes, oligodendrocytes (Schwann cells), microglia, ependymal cells (blood/brain barrier)

Functions of Glial Cells

1. form myelin sheaths
2. provide metabolic substrate
3. remove metabolic wastes
4. provide structure/support for neurons
5. guide migration of neurons to target cells
6. involved in repair of damaged neurons
7. involved in intercellular communication

Resting Membrane Potential (define and explain what causes it)

-natural electrochemical potential across neuron's membrane is -70mV (membrane is "polarized" at rest...separated charge)

-caused by separation of charges across cell membrane
-inside of cell: nucleic acids, proteins, phosphates, sulfates, potassium (overall negative charge)
-outside of cell: cations like sodium and calcium (overall positive charge)

Magnitude of Resting Membrane Potential caused by:

1. difference between intra- vs. extracellular concentration of ions

2. permeability of membrane to specific ions

Equilibrium Potential (define and explain what it is determined by)

-"driving force" of each ion across membrane (unique for each ion at each cell type)
-different than overall resting potential of -70mV

Determined by:
1. concentration gradient
2. electrochemical nature of ions

If equilibrium potential for sodium is +60mV and -90mV for potassium then why isn't resting membrane potential -30mV instead of -70mV?

1. differences in membrane permeability (potassium more permeable than sodium at rest...more efflux of potassium than influx of sodium)

2. sodium/potassium pump is ELECTROGENIC (doesn't contribute equate amounts of each ion, contributes -4mV to overall resting membrane potential)

Electrochemical nature of ions

-positive charge attracts negative charge, vise versa, and like charges repel

-one of the factors that determine equilibrium potential

How do electrical signals (impulses) get generated?

-by changing membrane's permeability to ions (change in membrane permeability can result in graded potentials/action potentials)

Graded Potential

-initiated by opening of ligand gated channels (neurotransmitter bound to its receptor)
-slow change in membrane potential (low conductance but open for long periods of time)
-change in potential (current) restricted to small area (as current diffuse away from site of initiation, intensity decreases)
-intensity of graded potential determined by intensity of stimulation (stimulus = response; amount of neurotransmitters will determine intensity of response/change in potential)

Action Potential

-initiated by voltage gated ion channels
-changes in membrane potential are fast and brief (open wide, very fast, then close quickly)
-follows "all-or-none" principle
-self-propagating but can't start itself (needs graded potential)
-this is typically how electrical impulses are conducted over long distances

Ionic Basis of Action Potential (mechanism)

-depolarization of membrane to -55mV results in opening of voltage gated sodium channel
-rapid influx of + charge, membrane potential changes to +30mV
-voltage gated sodium channels close
-voltage gated potassium channels open (efflux of + charge) resulting in repolarization (slow to open and close)
-after brief period of hyperpolarization, membrane potential reestablishes at -70mV

Refractory Period (define and list the 2 different types)

-during hyperpolarization, there is a refractory period when neuron is resistant to subsequent action potentials

1. absolute refractory period (no matter what, channels won't open even if another action potential comes along)

2. relative refractory period (greater than normal stimulus can stimulate a second response)

Driving force of sodium and driving force of potassium

Na+ = influx

K+ = efflux

How and where will action potential be generated?

- if intensity of stimulus is great enough to reach soma (via passive diffusion), then action potential will be generated at axon hillock

-must be able to elevate change in membrane potential from -70mV to -55mV

What determines speed of impulse conduction (AKA propagation of AP)?

-speed of impulse conduction determined by thickness of axon and myelination
-think of resistance as deflection
-as input resistance increases, ions will be deflected more, not able to reach center of axon and will travel down axon much more quickly
-thicker myelin sheath = faster impulse

Nodes of Ranvier

-interruptions along myelin sheath where axonal membrane is exposed
-responsible for recharging AP
-at each node of ranvier, new AP generated and impulse continues down myelin sheath

Salutatory Conduction

-AP jumps from node of ranvier to node of ranvier
-carries impulse to nerve terminals

Types of Synapses

1. electrical synapse = usually found found in heart (not nervous system)...cell membranes of 2 excitable cells joined together by gap junctions

2. chemical synapse = substance (neurotransmitter) released from terminal of pre-synaptic neuron and binds to receptors on post-synaptic cell

Mechanism of Synaptic Transmission

-AP reaches axon terminal and opens voltage gated Calcium channels (no myelination at terminals)
-Calcium enters terminal
-neurotransmitter bound in vesicles released via exocytosis
-causes local graded potential of post-synaptic membrane (post-synaptic potential)
-either excitatory or inhibitory post-synaptic potential

Excitatory Post-Synaptic Potential vs. IPSP

EPSP = has depolarizing effect, makes cell more positively charged

IPSP = has hyperpolarizing effect, makes cell more negatively charged

Driving force of calcium?

influx

Temporal summation vs. Spatial Summation

*typically a single EPSP not strong enough to evoke AP (EXCEPTION: endplate potential of peripheral nervous system at NMJ)

Temporal Summation = several EPSP's delivered in quick succession from single pre-synaptic neuron

Spatial Summation = several EPSP's from several different pre-synaptic neurons arrive simultaneously (convergence)

Pacemaker Potential

-excitable cells that are able to spontaneously generate AP
-due to greater permeability (leakiness) of these membranes
-resting potential of these cells is less than that of other neurons (-55mV vs -70mV)
-regulated by voltage gated channels (open around -40mv)

Mechanism of Pacemaker Potentials in Smooth Muscle

-calcium enters, causes depolarization and opens voltage gated calcium channels
-potassium channels open and efflux of potassium causes repolarization

Mechanism of Pacemaker Potentials in Myocardium

-influx of sodium depolarizes membrane to -40mV
-voltage gated sodium channels open (fast channels)
-causes voltage gated Calcium channels to open (slow phase)
-potassium channels close, process repeats itself

same principle as smooth muscle except myocardium uses sodium channels instead of calcium channels to start things off

What causes repolarization of membrane?

always caused by efflux of K+

What are the 3 major levels of the Central Nervous System?

1. Spinal Cord Level (walking movements and reflexes)

2. Lower Brain Level (subconscious bodily functions and emotions)

3. Higher Brain Level (always functions in conjunction is lower centers of CNS...higher thought process...Limbic System)

Cerebellum

-fine motor movements that require significant degree of coordination
-regulation and coordination of movement, posture, and balance

Hypothalamus

-part of the Limbic System ("emotions")
-homeostasis, circadian rhythms, emotion, control of autonomic NS

Medulla

controls heart rate and breathing

Breakdown of the Peripheral Nervous System

1. Afferent Division
2. Efferent Division
- somatic
-autonomic

Somatic (voluntary)

CNS -> effector organ (skeletal muscle)

-neurons from CNS to skeletal muscle only carry excitatory signals (Ach)

Autonomic (involuntary)

CNS-> pre-ganglionic fiber (Ach) -> ganglion -> post-ganglionic fiber (Ach or Norepi) -> effector organ (smooth or cardiac muscle, glands, or GI neurons)

Sympathetic Branch

-fight or flight
-nerves leave spinal cord from thoracic and lumbar regions
-post-ganglionic neurotransmitter is norepinephrine
-adrenal medulla-specialized autonomic ganglion

Parasympathetic Branch

-rest and digest
-nerves leave spinal cord from cervical and sacral regions
-most of this neural info passed along to vagus nerve to thoracic and abdominal regions
-post-ganglionic neurotransmitter is Ach

Blood Brain Barrier

-capillaries of brain much less permeable than those to other organs
-endothelial cells joined together by tight junctions

Cerebrospinal Fluid

-produced within and fills ventricles of brain
-leaves ventricles and bathes exterior of brain and spinal cord (cushions and protects)

hydrocephalus = accumulation of CSF

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