A&P Ch. 11 The Nervous System

Functions of the Nervous System

1. Sensory input- monitors changes inside and outside the body

2. Integration- processing and interpreting information, decides what to do

3. Motor output (motor responses)- response is carried out, sent to effector organ

The Central Nervous System

composed of the brain and the spinal cord

function: responsible for interpreting sensory input and deciding motor output

The Peripheral Nervous System

composed of nerves that extend from the CNS to the rest of the body

function: allows information to be sent between the CNS and the rest of the body

creates sensory input + sends out motor response to effector organ

General cell types in the nervous system

1. Neurons- nerve cells that can respond to stimuli and transmit electrical signals (excitable)-chief cell type

2. Neuroglia- provide support and maintenance to neurons, smaller cell types, ensure neurons can perform function

Types of Neuroglia

1. Astrocytes (CNS)

2. Microglial cells (CNS)

3. Ependymal cells (CNS)

4. Satellite Cells (PNS)

5. Oligodendrocytes (CNS) and Schwaan Cells (PNS)

Astrocytes

CNS- projections from membrane surface connect to and wrap around neurons, nerve endings, and surrounding blood capillaries

main functions: provide nutrients for neuron cells (travels from capillary through astrocyte), allows migration of young neurons (in right direction), “clean up” area surrounding neurons (leaked K+ ions, leftover neurotransmitters)

Microglial Cells

CNS- contact nearby neuron cells to monitor neuron health

if there is a damaged neuron: migrate toward injured neurons, transform into macrophage, phagocytize the neuron

Ependymal Cells

CNS- most have cilia, lines central cavities of CNS to circulate cerebrospinal fluid (CSF) within cavities

only found in cavities of CNS called ventricles

fills ventricles-cushions + protects brain and spinal cord

Satellite Cells

PNS- support and protect neuron cell in PNS, like astrocytes in CNS

Oligodendrocytes and Schwaan Cells

Oligodendrocytes- CNS

Schwaan- PNS

wrap around nerve fibers in CNS and PNS- wraps around axon of neuron

function: creates an insulating covering called a Myelin Sheath for neurons- allows electrical messages to be sent much faster

Neurons

cells specialized to generate or transmit electrical signals- nerve impulses

other characteristics: longevity (can function for a lifetime), amitotic (cannot reproduce mitotically), metabolism (always doing something)

3 structures of neurons

1. Cell Body

2. Dendrites

3. Axon

Cell Body

portion of the cell containing the nucleus, synthesizes proteins and neurotransmitters

clusters of cell bodies in CNS- nuclei, in PNS- ganglia

Dendrites

the receptive region of the neuron, first part that gets the message, shorter, more numerous

function: provide increased surface area for incoming signals and convey incoming messages toward the cells body

Axon

single, long, nerve fiber extending from the cell body

the conducting region of the neuron, sends message to next neuron in line

function: generates and transmits nerve impulses away from the cell body

bundles of axons in CNS- tracts, in PNS- nerves

Axon terminals

end of the axon where it branches out and formed terminal branches, the secretory region of the neuron

function: neurotransmitter released at axon terminal to pass the impulse to the next neuron

Myelin Sheaths

protects and electrically insulates long and or large nerve fibers (axons) to increase speed which impulses are transmitted

found only on the axon

not all axons are myelinated

created by schwaan cells wrapped around multiple times

Myelination in the PNS

multiple schwaan cells on the axon form myelin sheath, but none of the schwaan cells contact each other

Myelin sheath gaps- region of axon that is “exposed” due to absence of schwaan cell covering- space in between

Afferent (sensory) neurons

neurons that transmit signals to the CNS

Efferent (motor) neurons

transmit motor signals away from the CNS

Interneuron

found in between sensory and motor neurons

function: pass signals through CNS pathways where integration occurs

can connect to other interneurons, can communicate with neighbors

Membrane potential

the difference in electrical charge across the plasma membrane

should be more negative inside the cell → -70 mv resting

How do neurons change their membrane potential

they change the permeability of the plasma membrane to one or more ions, with the use of ion channels in the membrane

Two Types of Potentials

1. Graded Potential

2. Action Potential

Ion Channels

proteins in plasma membrane that form “channels” to allow passage of ions into/out of cell

Leakage Channels

always open + allow free flow of ions across plasma membrane

Gated Channels

have a “gate” that must open before ions can move

Chemically gated ion channels

open when a certain chemical (neurotransmitter) binds to protein

Voltage gated ion channels

open and close in response to changing membrane potentials

Mechanically gated ion channels

open in response to physical deformation of receptor

Depolarization

when the inside of the cell becomes more positively charged (less negative) compared to the resting membrane potential

excitation (stimulates) of a neuron

Hyperpolarization

when the inside of the cell becomes more negatively charged compared to the resting membrane potential

inhibits a neuron, decrease chance electrical message is generated

Graded potentials

allow a neuron to receive a message from another neuron- neurons can’t generate a message before they receive a message

magnitude varies directly with stimulus strength

only occur over short distances- happens on the dendrites

can be depolarizing or hyper polarizing- depends on signal

necessary for initiating a nerve impulse

Action Potentials

necessary to send information from one neuron to another

only occur on the axon

specific to neurons and muscle cells

all have consistent strength- same every time no matter what

occur over long distances

are only depolarizing - -70mv → +30 mV, every single time

not possible without first receiving a graded potential

What type of ion channel is specific to generating action potentials

the voltage gated ion channel

Two gates of voltage gated Na+ ion channel

• Activation gate- voltage-sensitive and opens at depolarization

• Inactivation gate- blocks channel to prevent Na+ movement

Voltage gated K+ ion channel

only has only has one gate that slowly opens at repolarization

Step one of creating an AP

at resting membrane potential, neuron is not active

all voltage gated channels are closed, -70 mV, permeability of Na+ is slow

K+ leakage channels are open

Step 2 of creating an AP

Depolarization

resting neuron receives message through dendrites, Na+ enters through channels

If membrane reaches threshold voltage (-55mV) → AP will begin at “trigger point” of axon (where axon attaches to body)

activation gate of voltage gated Na+ channels open → Na+ freely enters the cell and membrane potential changes from -70mV to +30mV

Step 3 of creating an AP

Repolarization- ending the AP

once at +30mV- voltage gated Na+ inactivation gate closes

voltage gated K+ channels open, K+ leaves the cell, membrane potential becomes more negative

Step 4 of creating an AP

Hyperpolarization- inside of the cell becomes more negative than resting membrane potential

excess K+ leaves the cell

Na+-K+ pump works to re-establish resting Na+ and K+ concentrations outside and inside the cell→ slowly climb back up until at RMP

Action potential and stimulus strength relationship

independent- strength does not affect AP

Strong vs Weak Stimulus

• Strong stimuli- impulses are sent in large amounts- more APs are sent to the brain

• Weak stimuli- impulses are sent in less amounts- less APs are sent to the brain

Refractor period

time in which a second AP cannot be generated at axon, or is more difficult to generate an AP

Absolute refractory period

during this time, another AP absolutely cannot be generated in the area, no matter how strong the stimulus is

all activation gates are open or closed- can’t possible get another AP

this ensures each AP is a separate, all or non event and enforces one-way transmission of AP (away from body)

Relative refractory period

occurs immediately after the absolute refractory period

stimuli that are relatively weak cannot generate an AP, but an exceptionally strong stimulus can generate an AP (severe pain)

Speed of conduction is dependent on two factors

1. axon diameter- larger axon = faster conduction

2. degree of myelination- more myelination= faster induction

slower- less important info, ex: how digestive system is doing

faster- more important info, ex: damaged (pain) or about to be damaged (about to trip, catch self)

Continuous conduction

propagation of unmyelinated fibers→ slow

voltage gated ion channels are adjacent for the entire length of the axon

stop+go process, generate a new AP at every ion channel

Saltatory conduction

propagation in myelinated fibers

voltage gated ion channels only found in myelin sheath gaps

AP jumping huge distances- quicker

Synapse

space where neurons are nearly touching each other

where signals between neurons are transmitted

presynaptic neurons

conduct impulses toward the synapse- the neuron that is sending the message

postsynaptic neurons

conduct impulses away from the synapse

the neuron that is “receiving” the message

Synaptic cleft

fluid filled space that neurons are separated by

First part of transmission of APs

1. action potential arrives at axon terminal of presynaptic neuron

2. voltage gated Ca2+ channels in terminal open → Ca2+ enters the axon terminal of presynaptic neuron (higher in ECF than ICF

Second part of transmission of APs

3. synaptic vesicles (filled with neurotransmitter) in axon terminal fuse in membrane in response to Ca2+ influx- neurotransmitter enters the synaptic cleft

4. neurotransmitter crosses cleft

Third part of transmission of APs

5. neurotransmitter binds to receptors (dendrites) on the postsynaptic neuron membrane - causes ion channels to open, ion flow generates Graded Potential

6. neurotransmitter in synaptic cleft is disposed of

Three ways neurotransmitters can be disposed of

• Reuptake- neurotransmitter is shipped back to neuron by an astrocyte or the presynaptic neuron

• Degration- neurotransmitter is degraded by an enzyme- specific, one enzyme, one neurotransmitter

• Diffusion- neurotransmitter diffuses out of the synapse- floats away, least amount of work, won’t bind to postsynaptic membrane

Postsynaptic potenitals

a graded potential created by the postsynaptic neuron after binding to neurotransmitter

Graded potentials vary in strength according to

• amount of neurotransmitter released → stronger

• how long neurotransmitter stays in synaptic cleft - stays longer, bind to gated ion channels repeatedly → more flow of ions → stronger

Excitatory postsynaptic potential (EPSP)

binding to neurotransmitter causes the membrane to depolarize

BUT- a single EPSP cannot induce an AP alone, need several to generate AP

Inhibitory postsynaptic potential (IPSP)

binding of neurotransmitter causes the membrane to hyperpolarize

K+ channels or Cl- channels open, making the inside of cell more negative

pushes membrane potential farther from the threshold, less likely to generate AP

Neurotransmitters

chemical signals produced in the cell body and are transported to the axon terminal

most neurons produce at least two types

can be excitatory, inhibitory, or either depending on the receptor type they bind to

Acetylcholine

neurotransmitter, released at neuromuscular junctions to stimulate muscle contraction and is released by many neurons in the automatic nervous system

Norepinephrine and epinephrine

neurotransmitters, stimulates the “fight or flight” response automatic nervous system, usually released at the same time- adrenaline

Dopamine

neurotransmitter, learning (via reward), motor control(inhibitory of muscle contraction), cognition (attention + focus)

happy, calming, executive functions- decision making

Endorphins

neurotransmitter, natural opiates→ pain relief + mood booster

released during times of stress, prevent AP, someone can’t feel sever body damage

Gamma-aminobutyric acid (GABA)

neurotransmitter, reduces neuron activity by causing hyper polarization of neurons

The primary inhibitory neurotransmitter in CNS, regulates sleep - don’t wan’t neurons firing all of the time (overstimulated)