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Flashcards in Nervous System Deck (49):

Functions of the Nervous system

1. Integrate multiple inputs to sense changes in internal/external enviro
2. Initiate adaptive responses from body systems (voluntary/involuntary)
3. More sophisticated functions (memory, anticipation, learning and co-operation)


Cells of central nervous system (6)

-Nerve cells (neurons)
-Glial cells (glia)
-ependymal cells


Basic structure of neuron

-Dendrites, soma, axon hillock, axon, axon terminal

-Dendrites: receive input signals from other neurons
-also increases SA
-Soma (cell body): housekeeping functions (nucleus to store genetic material, ER, mitochondria - energy)
-uses 20-25% of body's energy
-Axon hillock: Synaptic integration, action potential triggering
-Axon: conveys electrical output signals (action potentials)
-Axon terminal: Specialised to release transmitter (chemical) to signal to next cell in pathway


Neuron definition

-Specialised to receive, integrate and transmit information to other cells
-info transmitted as a chemical and/or electrical signal


Classification of neurons by location

1. Central nervous system
-brain and spinal cord
2. Peripheral nervous system
-outside brain and spinal cord -> includes cranial and spinal nerves
*any neuron that sends a process out of CNS


Classification of neurons by function

*based on where neurons taking info to/from
-Interneuron: are connecting neurons whose processes are restricted to a particular CNS region -> allow increased complexity of network activity
-Afferent Neuron: From sense organ to integrating centre
-Efferent Neuron: from integrating center to effector organ


Classification of neurons by structure

-Fascicle and nerve definition

*based on number of processes arisng from soma
-Multipolar, bipolar and unipolar
-Fascicle: bundles of axons from many different neurons
-Nerves: are bundles of fascicles


Glial cells - functions/role

-Make up more than 90% of cells in CNS
-Physically support neurons - also have housekeeping functions
-Cannot fire action potentials
-Can influence synaptic transmission


5 Glial cells and their functions

-Myelin definition

-Schwann cells: form myelin (PNS)
-Oligodendrocytes: form myelin (CNS)
-Astrocytes: transport nutrients to neurons
-also form blood brain barrier
-Microglia: Remove debris/dead cells from CNS (derived from macrophages)
-Ependymal cells: line fluid filled cavities of CNS

Myelin: layers of lipid-rich glial cell plasma membrane, provides electrical insulation for nerve axons


Evolution of nervous system

-Radially symmetrical animals

-Bilaterally symmetrical animals

-Almost all multicellular organisms have nervous system
-Radially symmetrical animals (e.g. corals, jellyfish) have nerve nets - scattered neurons, diffuse connections
-no clear afferent/efferent division (no integrating centre
-Bilaterally symmetrical animals (like humans - and most other animals)
-emergence of ganglia (act as integration centres)
-cephalisation increases w/ increasing nervous system complexity (tendency for integration centers and sense organs to be clustered at anterior end)


Features of vertebrate nervous system (4)

-Highly cephalised
-Part of nervous system (CNS) encased w/in cartilage or bone
-Dorsal nerve cord
-Hollow nerve cord (Contains CSF)

*nerve cord = brain and spinal cord
-Brains have same basic parts - expanded or reduced depending on functional requirements


Seven regions of the vertebrate central nervous system

-Spinal cord
-Diencephalon (thalamus and hypothalamus)


Spinal cord


Spinal cord:
-somas in the center (grey matter), axons around the outside (white matter)
-main function is mediating reflex arcs
-sensory nerves in dorsal horn, motor in ventral horn
-integrates sensory, motor and vestibular (balance) inputs
-Functions include learning motor skills, co-ordination, eye movements and maintaining posture
-loads of neurons


Brainstem: Medulla, pons, midbrain

-Medulla Oblongata: regulation of blood pressure, digestion and breathing
-Pons: relays info between cerebellum and cortex
-regulates breathing, sleep
-Midbrain: controls sensory functions (visual, auditory, touch)
-controls reflex responses to sensory input


Diencephalon - 2 main divisions

-2 main divisions;
1. Thalamus: relay station, controls transfer of sensory info from periphery to cortex
2. Hypothalamus: regulates hormonal secretions of pituitary gland, regulation of circadian rhythms, important in motivation



-Corpus callosum

-cerebral cortex = info processing (cerebral hemispheres)
-deep structures
-basal ganglia: fine movement control
-amygdala: social behaviour and emotion
-hippocampus: memory

*Corpus callosum: larg tract of axons that links left and right hemispheres of brain


Cerebral cortex and 4 hemispheres

-Thin, outermost layer of cerebrum -> performs the highest lvl of info processing
-4 lobes: frontal, temporal, parietal and occipital
-neuron numbers and cortical infoldings (sulci) tend to increase through evolution




-Charge - and two types

-Potential differences

*when current will flow (2 requirements)

-Electricity: Presence and flow of electrical charge
-Charge: a physical property of matter, which means the matter experiences a force if it encounters other charge material
-two types; positive and negative
-Difference in charge between two places = potential difference (measured in volts)
-net movement of charge = current (measured in amperes - amps)
*current will flow if two places with a potential differences are connected by a conductor


-Electrons and ions
-2 types of ions

-Electrons: what carries a current in a wire (negatively charged)
-Ions: what current is carried by in a solution
- 2 types;
-Cations: positive ions
-Anions: negative ions


Cell Membrane potential

-what it is

-how it occurs

-All cells are filled with and surrounded by aqueous solution of ions
-ICF and ECF have different solute conc
-unequal distribution of ion charges
-Potential differences across the cell membrane = membrane potential (Vm)
-Vm can change a lot without significantly changing the intracellular and extracellular ion conc


Forces driving ion currents in solution (2)

*movement of ions (and therefore current flow) in solution determined by 2 forces;
1. Chemical gradient: ions move from high conc. to low conc.
2. Electrical gradient: opposite charges attract, like charges repel

*only conc. of solute matters in chem gradients -> any other molecules don't matter


Electrochemical driving force

-electrochemical gradient

-Equilibrium potential

-Electrochemical gradient = overall force on an ion due to combination of chemical and electrical driving forces
-to determine net movement at a particular membrane potential - need to know equilibrium potential
-Equilibrium potential = value of Vm at which electrical gradient is EQUAl in magnitute and OPPOSITE in direction to the chemical gradient
-no net movement of the ion


Equilibrium potential: how to stop a chemical rush

-e.g. with K+ high inside of cell

*High K+ on inside of cell*
-K+ efflux due to conc. gradient (-5mV on inside)
-Negative membrane potential causes K+ influx down electrical gradient
-At one value, the electrical gradient is exactly same magnitude as, but in opposite direction to, the chemical gradient
-no net movement
-called the equilibrium potential


High concentration of Na+ on outside - what will equilibrium potential charge be?

-Chemical gradient into the cell
-electrical gradient must be going to outside
-What charge to move Na+ out down electrical gradient
-positive charge needed: would repel Na+ and move out


Ion movement if not at equilibrium?

-Cell with potassium equilibrium potential of -94mV
-at this charge, no net movement of K+
-If membrane potential is -100mV, net influx of K+
-will decrease membrane potential back to equilibrium

*same applies for lower membrane equilibrium -> will be efflux


Chemical gradient and membrane potential

-No matter what the membrane potential is, the chemical gradient stays constant
-but direction of electrical gradient flips around depending on the membrane potential


Resting Membrane Potential

-Equilibrium potential tells us about the movement of an individual ion
-in most cells, Vm stays constant for long periods
-Resting membrane potential: overall voltage across the cell membrane when the cell is not transmitting an electrical signal
-in all cells


Determinants of RMP (2)

-Concentration gradients of ALL ions across membrane
-Differing permeability of cell membrane to those ions


What solute affects RMP most?

-K+ ions
-proteins can't cross, Calcium conc gradient relatively weak and Cl- equilibrium potential is close to normal RMP
-25 x more K+ channels open than Na+ channels

-At RMP, membrane is stable but neither Na+ nor K+ is at equilibrium
-typical value for RMP = -70mV, Ek = -90 mV


Membrane pumps

-their effect on membrane potentials

-most important one

-At RMP, electrochemical forces lead a net efflux of K+ and a net influx of Na+
-Over time, can lead to a change in comp of ICF and ECF and run down of the cell membrane potenial

*need pumps to move ions against electrochemical gradient
-esp Na+/K+ ATPase pump
-moves Na+ and K+ against electrochemical gradients (moves Na+ out of cell, K+ into cell)


Electrical signalling
-what cells are excitable?

-How electrical signalling works (3 basic steps)

-All cells have RMP, but only neurons and muscle cells are excitable cells
-they transmit electrical signals rapidly over long distances
-do so by changing their membrane potential
*additional feature = can change the amount of signalling they can do
How it works:
-Open/closing of ion channels -> change in number or type of ions moving in or out -> change in membrane potential


Leak and Gated Ion channels

-3 types of gated ion channels

-Leak channels: always open
-Gated ion channels: only open in response to a stimulus
-are 3 types based on stimulus that opens them
1. Ligand-gated: ligand binding changes shape of pore and opens (w/out ATP input)
2. Voltage-gated: each has specific Vm to open
3. Mechanically-gated: connected to filaments of cytoskeleton -> physically yanks open when pressure applied


Describing changes in membrane potential

-Depolarisation, Rempolarisation and hyperpolarisation

*Changes in membrane potential described relative to resting membrane potential
-Depolarisation: Vm more positive than RMP
-Repolarisation: Vm returning to RMP after depolarization or hyperpolarization
-Hyperpolarisation: Vm less positive than RMP


Electrical signalling within a neuron

-2 types of potentials and what they are

-Neurons propagate two main types of electrical signals;
-Graded potentials: input signals
-Action potentials: output signals
*both types involve opening or closing of gated ion channels


Two types of current flow in neurons


1. Flow across the plasma membrane: flow down electrochemical gradient
-K+ leak channels most important for RMP
-voltage-gated Na and K channels most import. during AP
-graded potentials involve curent flow through mechanically, voltage or ligand-gated channels
2. Flow through cytoplasm: down electrochem. gradient through cytoplasm
-current flow through ion channel
-nerve cells filled w/ ICF filled w/ ions that transmit signals
-two regions will have slightly different charge = current flow


Graded potentials in neurons


-Graded potentials: small, local potentials generated by a stimulus acting on a cell
-ions moving through gated ion channels at stumulus site cause a small change in Vm (graded potential)
*gated channels only opened at stimulus site
*amplitude can vary depending on stimulus amplitude and distance from site of stimulation


Decay of graded potential amplitude (why does it occur)

-Some passive current flows along axon via cytoplasm, but some leaks out through leak channels
-decays in amplitude - not useful for long-range signaling


Properties of graded potentials (4)

-amplitude depends on stimulus amplitude (more channels open = bigger graded potential)
-can be hyperpolarising or depolarising, depending on type of ion channels that are opened
-graded potentials can add together (if they occur in succession)


Action potentials in neurons


-how AP and GP relate

-Action potentials: large potentials generated by regenerative activation of voltage-gated ion channels
-involve a sequence of ion channel opening and ion movements
-Occur in exactly same shape and some amplitude (all or nothing)
-all involve period of depolarisation followed by a period of repolarisation
-*can be conducted over long distance

-Need a certain amount of deplarisation by graded potentials for action potential -> is a threshold that has to be reached.


Phases of nerve action potential

-4 stages and membrane potential changes

-At RMP: VGSC and VGKC both closed
-Depolarisation: VGSC open, Na+ influx (membrane potential becomes more positive)
-Repolarisation: VGSC close, Na+ influx stops. VGKC open, K+ efflux (membrane potential becomes more negative)
-Hyperpolarisation: VGKC stay open, continued K+ efflux (Membrane potential becomes more negative than RMP for a bit)



-what it is

-Threshold Voltage: change in cell membrane potential required to initiate an AP
-summed magnitude of graded potentials determines whether cell membrane potential reaches threshold and fires AP.
-graded potentials must produce enough depolarisation for VGSC
-Most overcome hyperpolarisation caused b K+ leak current


Transition from graded potential to action potential

-Stimulus opens ion channels, produces graded potential
-Depolarisation from graded potential opens voltage-gated sodium channels (VGSC)
-opening leads to sodium entry, more depolarisation
-Opening of more VGSCs -> leads to more depolarisation and opening of more VGSCs


Why are AP all or nothing?

-once AP fires, VGSCs opened by other VGSCs (not by original stimulus)
-soon so many BGSCs open that the amount of Na+ enetering the cell is limited by Na+ conc gradient, not by no. of open ion channels


Properties of AP (3)

1. Are large
2. Amplitude independent of stimulus (if > threshold)
3. Action potentials always start with a bid depolarisation


Conduction of action potential (6 steps)

-RMP, threshold and peak of AP

1. Stimulus triggers graded potential
2. Depolarisation spreads via cytoplasm to adjacent membrane
3. Adjacent membrane reaches threshold for regenerative opening of VGSCs
4. sodium ions enter through VGSCs producing AP
5. Depolarisation spreads via cytoplasm to adjacent membrane
6. Adjacent membrane reaches threshold for opening of VGSCs

RMP = -70mV
Threshold= -55mV
Peak of AP = +30mV


Two properties of axons that increase conduction velocity

1. Increased diameter of axon
2. Myelination of the axon


Fundamentals of current flows in axons

-what can make current flow faster

-2 things that affect current flow in axons

-APs involve current flow along cytoplasm and across membrane
-Faster current flows along an axon - faster the AP will travel
-passive current flow through cytoplams is quicker than opening new ion channels to top up AP
-further cytoplasmic current can get before decaying below threshold, faster AP will move

-Current flow affected by; membrane resistance and intracellular resistance


Axon Diameter and conduction velocity

-2 effects

-which one dominates

-why increasing axon diameter is not feasible

-Increasing axon diameter decreases membrane resistance and intracellular resistance (easier for it to flow out and move across)
-increasing diameter decreases intracellular resistance more than decreases membrane resistance
NET EFFECT: increase current flow along axon and increase AP conduction velocity

*not feasible to have large axons for everything


Myelin and conduction velocity

-How myelin affect conduction

-Saltatory conduction

-Myelin sheath acts as electrical insulator -> reduces current leakage across membrane
-myelin = lipid bilayer that doesnt let ions flow across
-Nodes of ranvier are sections of axon no covered by myelin

-In unmyelinated axons, VGSC must be opened all the way along axon (slow) to account for leakage
-In myelinated: voltage gated channels only open at notes -> AP jump from node to node (called saltatory conduction)