Unit 2: Function and Components of the Nervous System

two key control structures

enfocrine system and nervous system

3 main roles of the nervous system

receive, integrate and transduce info

3 main roles of the nervous system: receiving information

uses receptors to receive from external environment

3 main roles of the nervous system: integrating information

organizes and combines received information with already stored information

3 main roles of the nervous system: transducing information

sends signals to targets (mostly muscle and glands)

general pathway of the nervoys system

stimulus → sensory/receptor → afferent (sensory) pathway → integrating center → efferent (motor) pathway → effector cells → response

2 main parts of the nervous system

central nervous system and peripheral nervous system

central nervous system (CNS)

made of the brain and the spinal cord

peripheral nervous system (PNS)

nervous system outside of the brain and spinal cord); made of afferent (sensory) and efferent (motor) neurons

2 kinds of nervous system cells

neurons and glial cells

neuron

generate and transmit electrical impulses over long distances

4 main parts of the neuron

soma (cell body), dendrites, axon, axon terminals

4 main parts of the neuron: soma

contains the nucleus and all biosynthetic machinery; center of chemical processes

cluster of soma (cell bodies) in the CNS

known as nuclei

cluster of soma (cell bodies) in the PNS

ganglia (plural) or ganglion (singular

4 main parts of the neuron: dendrites

slender tree-like structures that receive information; transmits signals toward the soma

4 main parts of the neuron: axon

cytoplasmic extension that sends out information away from the soma

bundles of axon in the CNS forming a pathway

known as tracts

bundles of axon in the PNS forming a pathway

nerves

4 main parts of the neuron: axon terminals

ends of the axon; connect neurons to other cells. apart if the synapse → presynaptic

3 general structures of axon

pseudounipolar, bipolar, multipolar

pseudounipolar neuron

• somatic sensory neurons

• axon and dendrites fuse into 1 structure

bipolar neuron

• smell/vision sensory neurons

• single axon, single dendrite

multipolar neuron

• CNS and efferent (motor) neurons

• have 1 axon but 2 or more dendrites

afferent neurons

sensory; considered PNS. receive info from receptor cells and transmit it to the CNS interneurons

interneurons

located in the CNS (96% of all neurons). transmit signals within the CNS (laterally in the spinal cord or vertically in the brain). receive from AFFERENT transmit to EFFERENT

efferent neurons

motor; receive info from interneurons and transmit to effectors. cell bodies are in the CNS however they are still considered a part of the PNS

glial cells

associated with neurons; they don’t carry signals over long distances. communicate with each other and with nearby neurons via electric and chemical signals

how do glial cells help neurons

aid in conduction and maintain the microenvironment around neurons

PNS glial cells

• Schwann cells → wrap around the axon to form myelin in one spot

• satellite cells → non-myelinating Schwann cells that support the soma

myelin

layers of membrane; acts as an electrical insulator for neurons

4 CNS glial cells

oligodendrocytes, astrocytes, microglia, and ependymal cells

oligodendrocytes (oligodendria)

glial cell in the CNS; CNS version of Schwann cell (form myelin around the axon for insulation) BUT can myelinate many parts of the same axon and act on different axon

astrocytes (astroglia)

glial cell in the CNS; star shaped cells connecting blood vessels and neurons, maintain homeostasis in the ECM of neurons

microglia

glial cell in the CNS; small specialized immune cells that remove damaged or foreign cells (macrophage-like)

ependymal cells

glial cell in the CNS; epithelial cells that make cerebral spinal fluid (CSF) and form semi permeable compartment barrier of the brain

how do neurons transmit electrical impulses

by energy stored in electrochemical gradients

charge of the human body

electrically neutral

role of the cell membrane in relation to charges and electrochemical gradients

seperate the charges; acts as an electrical insulator

charge of ICF

negativec

charge of ECF

positive

how to charges move across the membrane

ion channels; excludes the anions because of their size

membrane potential

difference in electrical potential inside and out the cell in millivolts (mV); all living cells have membrane potential

what ion(s) have a higher concentration outside the cell

sodium (Na+), chlorine (Cl-) and calcium (Ca 2+)

what ion(s) have a higher concentration inside the cell

potassium (K+) and anions (large, negative proteins)

2 types of ion channels

passive (leak) or gated

equilibrium potential

E_ion; membrane potential exactly opposing the concentration gradient of an ion. chemical and electrical forces are equal and opposite → this is the membrane potential where there wouldn’t be any net movement of said ion

nernst equation

E_ion = (61/z) log( [ion] out/ [ion] in)

• E_ion is the equilibrium potential of one specific ion (mV)

• z is the electrical charge of the ion

  • 61 is related to the gas constant, temperatur and faraday constant (61 = 2303 RT/F at 37C)

equilibrium potential for K+ in a typical neuron

-90 mV

equilibrium potential of Na+ in a typical neuron

+60 mV

key of the nernst equation

looks at what membrane potential would be if the membrane was permeable to only ONE ion

resting membrane potential

charge difference between the outside and inside of the cell at rest; -70mV for a typical neuron. can vary between cells but all will be negative

what sets resting membrane potential

concentration of each of the ions and their relative permeability

• K moves more easily than Na

• Ca2+ doesn’t cross the membrane → doesn’t contribute to resting potential

• the more permeable an ion is, the more it contributes to resting membrane potential.

Goldman-Hodgkin-Katz (GHK) equation: what does it tell us

predicts membrane potential using multiple ions → considers membrane permeability of each ion EXCEPT ca2+

Goldman-Hodgkin-Katz (GHK) equation: what is it

V_m= 61 log ( (P_k[K+]_out + P_Na[Na+]_out + P_cl[Cl−]_in) / (P_k[K+]_in + P_Na[Na+]_in + P_cl[Cl−]_out) )

• V_m → membrane potential, mV

• 61 → 2.303 RT/F at 37C

• P → permeability of ion to the membrane

  • if the membrane isn’t permeable to an ion, it isn’t considered

how does membrane potential change

• change in permeability to an ion → ion moves down electrochemical gradient → only take a few ions to change potential away from RMP (-70 mV)

• changes in membrane potential DOESNT CAUSE concentration gradient changes

depolarization

a decrease in the membrane potential (by magnitude; becomes more positive)

hyperpolarization

an increase in membrane potential (by magnitude, becomes more negative)

mechanically gated ion channels in neurons

found in sensory neurons, open in response to physical forces (eg stretch)

ligand gated ion channels in neurons

respond to ligands, such as neurotransmitters

voltage gated ion channels in neurons

respond to changes of voltage; important in initiation and conduction of electrical signals along the axon

4 types of selective ion channels in neurons

Na, K, Ca, Cl

2 divisions of signals generated by neurons

graded potentials and action potentials

graded potentials

• can be depolarization or hyperpolarization

• occur in dendrites or soma

• triggered by opening/closing of ion channels

• short distance

• amplitude of the potential is proportional to the strength of the event

why can graded potentials only travel short distances

they lose strength from current leak (positive charges leak back in depolarization wave) and cytoplasmic resistance

graded potentials: what causes ions to enter the cell

initiated by neurotransmitters binding to membrane receptors and opening ion channels → channels open → ions move in/out the cell → depolarization or hyperpolarization wave spreads

graded potential: signal strength

proportional to the number of ions in/out, strength diminishes as distances increases

depolarizing graded potentials

bring membrane potential closer to threshold potential (-55 mV); called excitatory post synaptic potentials (EPSPs) because they increase the chance if exciting the axon to fire

hyperpolarization graded potentials

bring membrane potential farther from threshold potential (-55 mV); called inhibitory post synaptic potentials (IPSPs) bcz they decrease the change of exciting the axon to fire

how is graded potential affected by the duration of stimulus

a longer duration of stimulus means a longer lasting graded potential but not a strong graded potential

action potential

• all identical — either on or off with no strength control

• don’t diminish in strength with distance→ long distance signaling

• start at the trigger zone (integrating center) of the neuron

action potential: trigger zone

different in different kinds of neurons

• in sensory (afferent) neurons it is adjacent to the receptor

• in motor (efferent) neurons and interneurons, it is the axon hillock and initial segment of the axon

action potential: how are they initiated

graded potentials sum together at trigger zone → threshold potential is reached (minimum depolarization required for action potential); -55mV

summation of graded potentials can be…

spacial summation or temporal summation

graded potentials: spacial summation

more than one location of graded potential fires at the same time → adds at the trigger zone

graded potentials: temporal summation

single location graded potential firing over and over until threshold potential is reached

what happens when a neuron reaches threshold potential

action potential is triggered → voltage gated Na and K ions begin opening → Na rushes in to max of 30mV →Na channels close and slower K channels open (peak of AP) → K exits the cell causing repolarization to RMP → hyperpolarization below -70mV → voltage gated K channels close → membrane returns to RMP from Na leak and K retention

voltage gated sodium channels

have 2 gates (regulate ion movement) → activation and inactivation gate regulate sodium movemtn across the membrane

voltage gated sodium channels: activation gate

little flap on the cytoplasmic domain of the channel. Closed at resting membrane potential, opens at -55mV (threshold potential)

voltage gated sodium channels: inactivation gate

ball and chain of amino acids on cytoplasmic domain of the channel. Open at resting membrane potential, triggered by reaching threshold potential (-55mV) but delay causes it to close at the peak of action potential (30 mV)

describe the sequence of events impacting the voltage gated Na channel

depolarizing stimulus to -55mV stimulates both gates → activation gate opens and Na rushes into the cell → Na entry causes further depolarization and more Na channels open (positive feedback) → inactivation gate closes at 30mV (peak of action potential) → positive feedback halts and Na influx stops

Na channel: closed conformation

activation gate is closed and inactivation gate is open

Na channel: open conformation

activation gate is open and inactivation gate is open

Na channel: inactivated conformation

activation gate is open and inactivation gate is closed

how does membrane potential return to resting level after an action potential is triggered

K ions leave the cell, causing the falling phase of action potential

voltage gated K+ channels

one gate, allows movement of K out of the cell. triggered at threshold potential (-55mV) BUT slower to open than Na → full opening is around 30 mV (peak of ap)

voltage gated K+ channel sequence of events

cell reaches threshold potential → K channels begin to open slowly → complete opening is at 30 mV (peak of curve) → K leaves the cell (repolarization) but overshoots resting potential (hyperpolarization) →

key about concentration gradients during an action potential

only a few ions move across the membrane during an action potential → concentration gradient essentially unchanged about one action potential

refractory period

occurs during the hyperpolarization period (undershot). made up of 2 parts, absolute refractory period and relative refractory period

absolute refractory period

no action potentials can be triggered because the Na channels are in inactive state → membrane must repolarize before channel can return to rest state

relative refractory period

large (suprathreshold) stimulus required to start an action potential → k channels are still open thus more Na is needed to reach threshold stimulus

what is the significance of the refractory period

set the direction of current flow, prevents temporal summation and prevents action potentials from going backwards

how do action potentials travel long distances along neurons without decrease in strenght

because Na channel’s positive feedback loop → depolarization in one area depolarizes the region next to it, causing identical action potentials travelling down the axon in one direction (bcz refractory periods)

what determines how fast an action potential can travel along the neuron

diameter of the axon and resistance of the axon to ion leakage

what determines how fast an action potential can travel along the neuron: diameter of the axon

greater diameters lower resistance to ion flow, speeding conduction

• eg invertebrate (squid and earth worms) have giant axons, allows fast conduction

what determines how fast an action potential can travel along the neuron: resistance of the axon membrane to ion leakage out of the cell

a more insulated (myelinated) axon leads to less ion leakage, speeding conduction

• glial cells create the myelin sheath → schwann cells in the PNS, oligodendrocytes in the CNS

node of ranvier

unmyelinated spaces along the axon. they are concentrated with voltage gated Na channels thus action potentials jump from one node of Ranvier to the next

saltatory conduction

action potentials jump from one node of Ranvier to the next in myelinated axons, causing fast conduction down the axon compared to unmyelinated axon

3 components of the synapse

presynaptic cell (axon terminal), synaptic cleft (small space between cells), postsynaptic cell (membrane; not always another neuron)

2 types of synapses

electrical synapse and chemical synapse