Nervous System Flashcards
Functions of the Nervous system
- Integrate multiple inputs to sense changes in internal/external enviro
- Initiate adaptive responses from body systems (voluntary/involuntary)
- More sophisticated functions (memory, anticipation, learning and co-operation)
Cells of central nervous system (6)
- Nerve cells (neurons)
- Glial cells (glia)
- Oligodendrocytes
- astrocytes
- microglia
- 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
- Central nervous system
- brain and spinal cord
- 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
- Cerebellum
- Medulla
- Pons
- Midbrain
- Diencephalon (thalamus and hypothalamus)
- Cerebrum
Spinal cord
Cerebellum
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
Cerebellum:
-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
Cerebrum
-Corpus callosum
- cerebral cortex = info processing (cerebral hemispheres)
- deep structures
- basal ganglia: fine movement control
- amygdala: social behaviour and emotion
- hippocampus: memory
- basal ganglia: fine movement control
*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
Definitions;
- Electricity
- 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
- 2 types;
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
- ICF and ECF have different solute conc
- 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)
- esp Na+/K+ ATPase pump
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
- definitions
- 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
-features
- 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
- 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
-features
- 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
- ions moving through gated ion channels at stumulus site cause a small change in Vm (graded potential)
- 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)
- Small
- 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
- definition
- features
-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)
Threshold
-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)
- Are large
- Amplitude independent of stimulus (if > threshold)
- Action potentials always start with a bid depolarisation
Conduction of action potential (6 steps)
-RMP, threshold and peak of AP
- Stimulus triggers graded potential
- Depolarisation spreads via cytoplasm to adjacent membrane
- Adjacent membrane reaches threshold for regenerative opening of VGSCs
- sodium ions enter through VGSCs producing AP
- Depolarisation spreads via cytoplasm to adjacent membrane
- Adjacent membrane reaches threshold for opening of VGSCs
RMP = -70mV
Threshold= -55mV
Peak of AP = +30mV
Two properties of axons that increase conduction velocity
- 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)
