Immune System Exam 1

Sensory neurons are known as

afferent neurons

Motor neurons are colloquially known as

Efferent neurons

What is the role of neurons?

recieve, conduct, and transmit information

What is the role of glia?

Support neuron function and environment

What are the regions of the neuron?

Dendrites - receptive region
Soma - body of neuron, contains metabolic and synthetic sensors
Axon hillock - trigger region
Axon - conducts impulse along “tail”
Axon terminals - signals travel to next dendrite

Function of dendrites

receive synaptic input, contains organelles near soma

Function of soma

holds nucleus and organelles, including the Nissl substance and intense protein synthesis, receives and integrates information

Function of axon

begins with hillock, moves to synaptic terminals to send signals using microtubules and neurofilaments

What are the four kinds of neuron structures? What do they look like?

Multipolar - one axon and many dendrites
Bipolar - one dendrite and one axon, soma sits inbetween
(Pseudo)Unipolar - one body that bifurcates off of rest of nerve
Anaxonic - No apparent axon, used as interneurons

What kind of neuron is most common in the retina?

Bipolar

What are the three sensory neuron classifications?

Sensory/afferent - receptors to CNS
Motor/efferent - CNS to receptors in muscles or glands
Interneurons - connects CNS to local neuron circuits

What is myelin?

Sheathes axons, allows for faster movement along nodes of Ranvier and induces saltatory conduction

What two structures produce myelin?

Oligodendrocytes in CNS, Schwann cells in PNS

How do Schwann cells and oligodendrocytes differ?

Oligodendrocytes produce multiple segments of myelin, Schwann cells are one per segment

What is the optic nerve pathway?

retinal ganglion cells leave eye through optic disc
axons form optic nerve that leave through optic chiasm

turns into optic tract

When does the optic nerve become myelinated?

myelinates after the lamina cribrosa towards brain

How to tell where myelin is in histo slide

Looks like large white sections with small dyed pink sections between (nodes of Ranvier)

PNS histo organization

neurons are grouped in ganglia, axons are grouped into nerves, satellite cells surround somata, Schwann cells appear, connective tissue organizes around nerves

How to recognize peripheral ganglia

LARGE neuronal cell bodies surrounded by small satellite cells, Nissl substance in neuronal cytoplasm, loose CT between bodies

How to recognize PNS nerves

Longitudinal - fibers are wavy, Schwann cells are elongated
Cross section - individual round fibers

surrounded by pale myelin
Axons grouped in fascicles

What are the three layers of the PNS CT?

Endoneurium - closest to nerve fibers
Perineurium - groups individual fibers together
Epineurium - dense CT around entire nerve, provides blood and nutrients

Receptor types

Thermoreceptors - sense temperature
Nociceptors - respond to pain
Photoreceptors - respond to light
Mechanoreceptors - respond to mechanical distortion

CNS histologic organization

Neurons, neuroglia, and blood vessels surrounded by neuropil
groups of neurons are called nuclei
Groups of axons are called tracts
covered by meninges

What is neuropil?

DENSE background of neuronal and glial processes in gray matter

Grey matter

non-myelinated, rich in neuronal cells, dendrites, synapses, and glia

White matter

Myelinated, dominated by axons and glia

What are the two types of glia?

CNS glia - astrocytes, oligodendrocytes, microglia, and ependymal cells
PNS glia - Schwann cells and satellite cells

Astrocytes

Star shaped and dark, provide blood-brain barrier, metabolic and immune support, and regulate extracellular ions and neurotransmitters

Microglia

Immune cells, “surveillance” cells, phagocytosis - release nitric oxide and free radicals to kill bad neurons, antigen presenting

Ependymal cells

lines ventricles of brain, often ciliated, specialized ones participate in CSF and choroid plexus

How do nerves regenerate?

Not a true regeneration - only found in myelinated neurons in PNS, Schwann cells are cut and help guide the nerve back into place

How effective is neuron regeneration?

Depends on type of injury, location, scarring, and inflammation

Components of the CNS

Cortex - thinking, learning, memory, consciousness - motor control
Cerebellum - receives input of body position and helps guide movement
Diencephalon - 2 components
- thalamus - integrates sensory information and motor control
- hypothalamus - affects ANS and endocrine system - homeostatis
Brainstem - integrates ANS function - CNIII - XII
Spinal cord - ascending and descending tracts

Dorsal column tract

Sensory afferent neurons
Synapse in brainstem (medulla oblongata)
decussate in medulla oblongata
A alpha and A beta afferent fibers
proprioception and mechanoreceptors

Spinothalamic tract

Sensory afferent neurons
Cross midline BEFORE cortex

A delta and C afferent fibers

Pain and temperature

Somatosensory axis

Dorsal column and spinothalamic tract

Primary sensory neurons synapse with 2nd order neurons in spinal cord or brainstem

Project to thalamus and cortex

Skeletal motor axis

Projection from motor cortex to motor neurons

Receives extensive input from thalamus, brainstem, and cerebellum

A alpha and A gamma efferent fibers

Nerve fiber C differs from the rest of the nerve fibers because

it is unmyelinated

What structures protect the brain?

Meninges
CSF
Blood brain barrier
Blood CSF barrier

CSF in the brain

Around 150mL in brain at any time
500mL a day produced

CSF circulation

secreted by choroid plexus

flows through subarachnoid space and absorbed by arachnoid granules

CSF flow

One direction only

absorption goes up due to pressure gradient

How is CSF different from plasma?

Lower K+ and AA and no proteins

CSF secretion

Na-K pump on apical membrane
Net transfer of NaCl and NaHCO3

CSF and brain extracellular fluid dynamics

readily exchange with each other
similar composition
CSF removes waste from brain extracellular fluid
no tight epithelium

Blood brain barrier

Vessels of brain are not leaky
tight junctions between capillary endothelial cells
astrocytes help transfer ions and sugars across membranes

Circumventricular organs

Lack a blood brain barrier
Hypothalamus (median eminence and pituitary)
Area postrema (vomiting center)

What do astrocytes provide for neurons?

Lactate, glucose (in the form of glutamine synthesis), exchange ions

Electrical synapse

Directly linked to postsynaptic cell, ions travel across gap junctions and move quickly, often bidirectional

Chemical synapse properties

Synaptic delay
Unidirectional
Ca ion dependent
Neurotransmitter release at terminal
Post synaptic membrane expresses receptors for neurotransmitters

Synaptic transmitter steps

Neurotransmitters synthesized and moved to terminal
Action potential arrives at terminal along with vesicle
Ca2+ voltage gated channels open and Ca2+ comes in
Synaptic vesicles bind to membrane of terminal and release neurotransmitters
neurotransmitters enter synaptic cleft, bind to receptors on other side, and any leftover diffuse away from synapse or into presynaptic terminal

How are vesicles moved to the synaptic cleft?

Synthesized with neurotransmitters in the ER/golgi in soma of nerve if peptide, moved down axon with microtubules, if not peptide are packaged in terminal

Presynaptic sequence of events

AP travels down axon to terminal
V gated Ca2+ channels are opened
Ca2+ bind to synaptotagmin on vesicles
Vesicle fuses with membrane and exocytoses
Neurotransmitter released

SNARE complex

Proteins that hold the vesicle and wind up to bring it in for membrane fusion
Synaptobrevin and SNAP-25 and syntaxin

Synaptotagmin

located on vesicle, Ca2+ binds to it and helps release the neurotransmitter

What toxins degrade SNARE proteins?

Botulinum toxin and tetanus toxin

Postsynaptic sequence of events

Neurotransmitter diffuses along synaptic cleft
NT binds to specific transmitter proteins on postsynaptic membrane
Ion channels open and close on postsynaptic membrane changing membrane potential
NT is removed from cleft by degredation with enzymes or reuptake into presynaptic cell

Postsynaptic potentials are what kind of potentials

graded potentials

What are excitatory postsynaptic potentials?

Depolarization (Na-K or Ca channel opens or K channel closes)

Inhibitory postsynaptic potentials

Hyperpolarization (K or Cl channel open)

What is convergence?

Multiple inputs to postsynaptic neuron - AP only fires if total EPSP - total IPSP is over the AP threshold

Temporal summation

Signals originate from same neuron at different times

Spatial summation

Signal originates from different neurons synapsing at different locations on same neuron

Groups of neurotransmitters

Small molecules (e.g. glutamate, serotonin)
Neuropeptides (e.g. endorphins, substance P)
Gaseous molecules (e.g. O2, CO2)
Endocannabinoids

Most neurotransmitters are

Small molecules

Some NTs derived from Amino acids are

Acetylcholine (Acetyl-CoA and choline)
Amino Acids (glutamate, aspartate, GABA)
Amines (serotonin, dopamine, histamine)
Nucleotides (ATP, adenosine)

Types of NT receptors

Ionotropic - binds to ion channel directly and alters permeability
Metabotropic - binds to G protein-coupled receptor that may indirectly affect permeability of ion channels

How does Acetylcholine bind to receptors?

Ionotropic (nicotinic) and metabotropic (muscarinic) both!

What are the excitatory NTs in the CNS?

Glutamate and aspartate

Glutamate receptors

AMPA - fast acting (Na-K channel)
NMDA - slow acting (cation channel blocked by Mg2+)

What does the membrane potential need to be at to activate NMDA

-40 or higher

GABA

Major inhibitory NT in brain, pumped back into nerves and metabolized, Cl- ionotropic channel, enhanced by benzodiazepines, barbiturates, neurosteroids, and ethanol

Where is glycine found and what does it do?

resembles GABA in action, found in interneurons of spinal cord

Neuromodulators

released into diffuse areas of the brain, bind to metabotropic receptors and modulate APs or NR releases

Norepinephrine

Makes potentials last longer/spike more

Reticular activating system

controls cortical arousal (Level of consciousness)
Stages: Maximal alertness, wakefulness, sleep, coma
Acts via cholergenic and adregenic systems

Factors that affect the RAS

Stimulated by sensory signals, cortical signals for motor and limbic system (emotion), and sympathomimetic drugs and histamine
Inhibited by withdrawl of sensory signals, damage to reticular formation, stim of sleep centers, general anaesthetics

Major neuromodulatory systems

Cholergenic - motor control, arousal, memory, learning, reward
Adregenic - arousal and reward
Dopamine - reward, motor control, cognition
Serotonin - mood, satiety, body temperature

How do different charges read on an EKG?

EPSP - positive charge, influx of (-)
IPSP - negative charge, influx of (+)

EEGs are used to diagnose

seizures and infarctions

Seizure disorder

unregulated and abnormal discharge in gray matter that interrupts brain function, tx includes anticonvulsants

Disorders associated with NT issues

Alzheimer’s, anxiety, depression, seizures, mania, Parkinson disease

How does the skeletal motor axis work?

Projection from motor cortex to motor neuron
Receives extensive input from thalamus, brainstem, and cerebellum

Motor cortex divisions

Primary motor cortex - “sensory homonculus”, coordinates fine motor movements
Premotor area - Stimulation from primary motor cortex creates muscle movements in different muscle groups to perform specific task, works in concert with other motor areas
Supplemental motor area - functions in concert with premotor area to provide positional and fixation movements to body, provides background movement for arms and hands

Cortical motor signal pathways

Direct pathway - corticospinal tract (pyramidal), directs fine motor
Indirect pathway - signals through basal ganglia, cerebellum, and brainstem nuclei (extrapyramidal), directs trunk musculature, upright posture, locomotion, orienting responses

Explain the corticospinal tract

giant pyramidal cells (Betz cells) originate in primary motor cortex and gives rise to large fibers with fast transmission, cross in pyramid of medulla and descend to corticospinal tracts to synapse with interneurons and motor neurons

Extrapyramidal tracts

Modulate signals from corticospinal tract to motor neuron and integrate sensory and posture information, important for trunk musculature, locomotion, and smooth muscle movement

Extrapyramidal symptoms

Fine motor control issues, tone irregularity, coordination

What supports the body against gravity?

muscles of the spinal column and extensor leg muscles - both under influence of brainstem nuclei
Pontine reticular nuclei excite the muscles, medullary reticular nuclei inhibit

Cerebellum functions

coordinate and sequencing muscle activities - compares actual movements with intended movements
afferent from motor and cortex (intended motion) and muscle spindles and Golgi tendons (status of muscle at given time)
efferent involve equilibrium control and opposing influences on motor neurons

Basal ganglia functions

control complex motor activity
Receives input from primary, premotor, supplemental, and prefrontal cortex
Inhibitory to cortex, suppresses conflicting motor activity
Contributes to postural stability and converts overall goal of movement

Spinal cord functions

preprogrammed movement patterns in all muscles

excitatory

The motor neuron in spinal cord is always ____ to muscle fiber

Motor neurons are __ than other neurons

Much larger

Motor fibers in the spinal cord can synapse where?

At the level of nerve or move higher to the cortex

How do interneurons synapse?

Synapse with brain and terminate at interneurons, small and highly excitable, comprise neural circuitry

Propriospinal fibers

travel up and down spinal cord for a few levels, help with proprioception

Motor neurons

Alpha - Large type, controls large swaths of muscle fibers (motor unit)
Gamma - Small type, stimulate intrafusal fibers in muscle spindle

Extrafusal fibers

Provide force for muscle contraction, stimmed by alpha neurons, make up bulk of muscle

Intrafusal fibers

Smaller, encapsulated in sheaths to form muscle spindles, shorter and run in parallel