a&p 1 - nervous system

CNS

• integrating, processing, and coordination

• higher cognitive functions

• brain and spinal cord

PNS

• connects CNS to rest of body

• delivers sensory info to CNS

• carries motor commands to the body

• cranial nerves, spinal nerves, and ganglia

• sensory and motor divisions

nervous system functions

• sensation - monitor external (room temp) and internal (core temp) changes

• integration - process sensory input and determine response

• response - by activating effector organs

sensory nervous system - in PNS

• also called afferent system

• Transmits information from receptors to the CNS

• divides into somatic and visceral subcategories

somatic sensory

• division of sensory nervous system in PNS

• sensation from skn, muscles, bones, and joints

  • receptors for:

    • touch, pressure, vibration

    • pain, temperature

    • proprioception

• special senses - hearing, vision, taste

visceral sensory

• division of sensory nervous system in PNS

• sensation from organs

  • mainly stretch, temperature, and pain

    • stomach stretching when you eat

somatic motor

• a motor division in PNS

• contraction of skeletal muscles

• voluntary movement

• somatic reflexes - involuntary contractions of skeletal muscle

visceral motor / autonomic nervous system

• a motor division in PNS

• regulates smooth and cardiac muscles, also glands

• involuntary

• parasympathetic and sympathetic division

what makes up nervous tissue?

neurons and glia

glia

support cells in nervous tissue

neurons roles

• excitability - respond to stimuli

• conductivity - produce and transmit electrical signals

• secretion - release neurotransmitters

neuron features

cells in nervous tissue that have high metabolic rates (need constant oxygen and glucose) and live for a long time that do not go through mitosis

• parts:

  • soma (body)

  • dendrites (receive signal)

  • axon (transmit signal)

  • axon terminal (communicate with target cell)

types of neurons

• sensory (afferent)

• interneurons

• motor (efferent)

sensory (afferent) neurons in PNS

neurons that conduct signals from receptors and send to CNS

interneurons in CNS

neurons that integrate incoming signals and mediate responses

motor (efferent) neurons in PNS

conduct signals from CNS to effector

• brain telling muscle to do something

neuron categories

• multipolar -one axon and multiple dendrites extending from the cell body

• pseudounipolar -a single axon that splits into two branches, bypassing the cell body

• bipolar - cell body in middle with a long dendrite and then axon and axon terminals at ends

multipolar neurons examples

motor neurons and interneurons

pseudounipolar neuron example

sensory neurons

bipolar neuron example

special sensory neurons

dendrites

part of neuron that provide surface area for receiving signals from other neurons and conveys incoming signals towards the cell body

astrocytes - type of glial cells of the CNS

• provides structural support

• regulate extracellular environment

• part of blood-brain barrier - projections from them help mediate what can leave blood and go into CNS

• divide to replace damaged tissue since the neurons do not divide

  • formation of scar tissue

types of glia in the CNS

• astrocytes

• oligodendrocytes

• microglia

• ependymal cells

oligodendrocytes - glial cells of the CNS

wrap around axons to form myelin sheath

microglia - glial cells of the CNS

phagocytotic defense cells

ependymal cells - glial cells of the CNS

form and help circulate CSF

• ciliated cells that go around the central canal

glial cells of the PNS

• satellite cells

  • provide structural support

  • regulate extracellular environment

• schwann cells

  • form myelin sheath - myelinate the CNS

  • help repair damaged axons

anatomy of a nerve

• axon - surrounded by myelin sheath

• fascicle - group of axons

• nerve - group of fascicles with blood vessels

myelin sheath

• electrical insulation around axons

  • plasma mebrane of schwann cells or oligodendrocytes

• facilitates signal transmission

• segmented

what are the segmented parts of a myelin sheath?

unmyelinated nodes or myelinated internodes

myelination in the PNS

• begins prenatally

• schwann cells spiral around a single axon

myelination in the CNS

• oligodendrocyte processes wrap around multiple axons

• begins postnatally

• need enough lipids during childhood for these to be made

grey matter

somas, dendrites, and unmyelinated axons

white matter

myelinated axons

synapses

• point of communication between neuron and adjacent cell (postsynaptic cell)

• electrical

• chemical

electrical role of synapses

gap junction joins adjacent cells and allows for rapid signal transmission

chemical role of synapses

neurotransmitters released from synaptic vesicles and are received by receptors on postsynaptic cell membrane

NTs

chemical signals that alter postsynaptic cell physiology

• ACh, NE, GABA, serotonin

• excitatory like ACh and NE often

• GABA often inhibitory

• can vary based on receptor type

membrane potential

• differences between intra and extracellular charge caused by unequal electrolyte distribution between ICF and ECF

  • K+ is higher in ICF

  • Na+ is higher in ECF

• resting membrane potential

what is the resting membrane potential at rest?

more polar → more negative internally

ligand-gated ion channels

• ligand = chemical messenger

• only open when specific ligand is bound (normally closed)

  • receptors on protein channel

  • chemical messengers must bind with their receptor for the channel to open

where are the receptors for a ligand-gated ion channel?

on a protein channel

Immediately after an action potential has reached its depolarization peak, what opens?

voltage-gated potassium channels

what are the kinds of ion channels?

voltage, mechanically, and ligand gated

voltage gated ion channel

channels that open or close in response to membrane potential changes

• stimulated by action potentials

• more voltage outside cell, but it increases (-70 to -50) when gates are open

local anesthetics

• work by blocking voltage-gated Na+ channels to prevent depolarization

• blocks afferent neurons from carrying sensory info to the brain

• work non selectively and can impact efferent motor neurons leading to numbness and muscle weakness

• examples include novocaine and lidocaine

mechanically gated ion channels

• open in response to mechanical stimulation

  • pressure and stretch

• close when stimulus is removed

• important for things like sense of touch

sequence of membrane potential changes

• resting potential

• depolarization

• repolarization

• hyperpolarization

what happens when ions move in the electrochemical gradient?

it alters the membrane potential

depolarization

increase in internal cell charge

repolarization

return to RMP

hyperpolarization

decrease in internal charge below RMP - linked with refractory period by making it longer

local potentials

• small and local changes in membrane potentials

• response varies with stimulus strength

  • length of stimulus, number/type of ion channels that open

• reversible - neurons turn to RMP if stimulus is too small/short

• effects get weaker with distance

where do local potentials take place?

at dendrites or somas but they do not reach axons

signal integration

summation - combined effect of multiple local potentials trigger action potential

• temporal summation

• spatial summation

temporal summation

• frequent stimulation of the same location

• multiple APs from a single presynaptic cell

spatial summation

• simultaneous stimulation of many locations

• APs from multiple presynaptic cells

action potentials

• all or none

• irreversible

• maintained along length of axon

• self propogating

all or none - APs

proceeds only if threshold is reached

irreversibility of APs

continues to completion once threshold is reached

APs across distance

they are maintained along the length of an axon to allow signals to be sent over long distances

self propagating - trait of APs

meaning that each one triggers the next

AP generation

• NTs open ligand-gated Na+ channels

  • local depolarization of cell membrane → local potential

• summation opens voltage-gated Na+ channels

• full AP triggered if depolarization reaches axon

• rapid depolarization of axon

• Na+ channels close, K+ channels open

rapid depolarization of axon in AP generation

regional depolarization triggers adjacent Na+ channels - positive feedback loop

during AP generation Na+ channels close and K+ channels open at one point, what does this lead to?

the outflow of K+ repolarizes the cell

what happens at the axon terminal during the end of the AP?

voltage gated Ca2+ channels open

what happens that opens voltage gated Ca2+ channels?

depolarization of axon terminals

what happens after Ca2+ flows in at a chemical synapse (axon terminal)?

it triggers exocytosis of neurotransmitters which then bind to receptors on postsynaptic cell

• finally is signal cessation

signal cessation at axon terminal during AP

• presynaptic cells stop NT release

• NT cleared away from postsynaptic receptors

refractory period

• resistance to new stimulation

• either absolute or relative

• local effect - other parts of neuron can be stimulated

• maintains unidirectional movement of AP toward axon terminals

absolute refractory period

time in which no stimulus can trigger another AP, comes before other period - red here

relative refractory period

time in which only a very strong stimulus can trigger another AP and this period lasts until restoration of RMP - comes after other period - yellow here

Na+/K+ antiport

• antiport is a membrane protein

• exchanges K+ ions for Na+ ions

  • K+ moves into cell and Na+ moves out

• important for maintaining electrochemical gradients

  • but, requires ATP

continuous signal conduction

• unmyelinated fibers

• voltage-gated channels along entire length of axon

• opening of Na+ channels causes local depolarization → excites adjacent Na+ channels

  • initiates new AP distally

• wave of successive APs continues down length of axon

• ensures unidirectional spread of the AP

what kind of channels are in continuous signal conduction?

voltage gated channels which are along the entire length of axon

what are the two types of signal conduction?

continuous and saltatory

saltatory signal conduction

• myelinated fibers

• myelin insulates axons - APs only generate at nodes

• signal seems to “jump” from node to node

• faster than the other kind of conduction

is saltatory signal conduction myelinated or unmyelinated?

myelinated - segmented myelin sheath causes signals to have to “jump” from node to node - APs only need to be generated at unmyelinated segments

is saltatory or continuous signal conduction faster?

saltatory

signal conduction speed

• myelination increases speed of conduction

• faster in larger fibers

• small unmyelinated fibers < small and myelinated < large and myelinated

multiple sclerosis

• autoimmune destruction of oligodendrocytes → demyelination of CNS neurons

• interrupts saltatory conduction

• common symptoms: muscle weakness, numbness, visual impairments, speech deficits

• no cure but anti-inflammatories and pt can help

nerve regeneration

• only possible in PNS + if soma is intact

1. fiber distal to injury degenerates because macrophages clear away debris

2. axon stump sprouts multiple growth processes

3. schwann cells form regeneration tube

4. single growth process continues to growh

   1. schwann cells release gH

   2. regeneration tube guides growth

5. synaptic contact re-establishes

   1. re-myelination over time

6. can take up to 2 years

axon stump

forms earliest formation of newly growing axon, called growth processes

what do schwann cells do in nerve regeneration?

it forms the regeneration tube and dominant growth process continues to grow because this cell releases growth factors and the tube guides it toward the target cell