Cells of the Nervous System

Central Nervous System (CNS)

Brain and Spinal Cord

Peripheral Nervous System (PNS)

ANS, spinal nerves (peripheral nerves attached to spinal cord), cranial nerves (peripheral nerves attached directly to brianstem)

Trigeminal Nerve

3 branches that innervate the lower jaw, upper jaw, forehead, and teeth

Ophthalmic Branch

Forehead

Maxillary

Upper Jaw

Mandibular

Lower Jaw

Sensory Neurons of Trigeminal Nerve

Lie in the trigeminal ganglion adjacent to the brainstem

Afferent Neuron

Bring information into CNS (sensory axons)

Efferent Neuron

Information coming out of CNS (motor axons)

Interneurons

The majority of neurons; can be excitatory or inhibitory (intermediates). Connect sensory and motor neurons.

Sensory Neuron

Enter spinal cord

Motor Neuron

Leave spinal cord

Soma

Cell body. Integrator (summing excitatory and inhibition)

Dendrite

Receiving terminal (input)

Axon

Carries nerve impulse away from cell (output)

Axon Hillock

Start of an axon, final integration (adds all the signals in the dendritic field). Decides whether the cell will fire.

Secretory Cell

Produces abundant proteins. Looks similar due to NT release, acting as secretory products.

ER-Golgi Amount

Abundant with high rates of protein synthesis as it requires a lot to transfer and ship products to large axon.

Axoplasmic Transport

Anterograde and retrograde

Dynamic

Continual reorganizations of contacts

Signals

Passive transmission works for short distances

Action Potentials (APs)

Used to send long-range signals. Anesthetics block APs by blocking voltage gated sodium channels.

Anesthetics Effect

Anesthetics block APs by blocking voltage-gated sodium channels.

Dye Used for Neurons

Nissl Stain Dye

Nissle Stain Dye

Stains nucleic acids (DNA & RNA)

Nissle Bodies

Stacks of rER

Somatosensory Neurons (pseudounipolar)

Receptor endings in the periphery, then the cell body, then axon endings in the CNS. No synapse on the cell body

Trigeminal Nerve (Pseudounipolar)

Pain, pressure, vibration causes AP to fire across the axon through the trigeminal ganglion and then reaches the trigeminal sensory neurons synapse in the brainstem.

Schwann Cels

Produce myelin for axons in the PNS

Oligodendrocytes

Produce myelin for axons in the CNS (multiple axons at once)

Myelin

Type of insulation which increases speed of axon conduction (fast)

Free Nerve Endings

Sense pain & temperature. Thin axons without myelin or thin myelin (still have Schwann cells)

Encapsulated/Specialized Endings

Fine touch, vibration, pressure, texture. Thick myelinated axons.

Synapse

Neurons pass information from one cell to another chemically

Action Potentials

Na+ and K+ Channels, with their concentration gradients, explain membrane potentials and action potentials.

APs at Rest

Some K+ channels are partially open, Na+ channels are closed. K+ wants to leave cell while Na+ wants to go into the cell due to relative concentrations.

APs at Peak

Depolarization causes Na+ voltage-gated channels to open quickly, going into the cell. K+ channels open more slowly, Na+ channels close even more slowly.

Sodium Channels

Closed in resting state. Depolarization opens in a (+) feedback loop, leading to the rapid, explosive opening of all channels, causing further depolarization. Inactivation closes the channels, and the membrane repolarizes. With a delay (recovery from inactivation), channels are ready to open again for the next AP. Timing is critical, inactivation is slow, and activation is fast.

Myelin

Type of insulation preventing ion flux, and therefore APs, in parts of the axon covered by myelin. Faster conduction speed.

Nodes of Ranvier

Spaces without myelin in myelinated axons

Myelinated Axons

Rapid conduction of AP due to layers of insulation and localized concentration of Na+ channels.

Local Anesthetics

Block voltage-gated Na+ channels from the inside, stopping APs (Lidocaine). Requires time to diffuse through all layers.

pH and Anesthetic Effects

Lidocaines uncharged form penetrates tissue and into cell, then dissociated into charged (active form)

Inflammation and Anesthetics

Patients with infection, have inflammation sites that are acidic, meaning less uncharged form, taking longer to numb.

Peripheral Nerves

Contain both sensory and motor axons

Chemical Synapse

Axon terminus (pre-synaptic terminal) exocytosis vesicle with NTs, then go into the synaptic cleft, which bind receptors in the postsynaptic cell.

Synapse

Electrical synapse depolarization reaches axon terminal, causing Ca2+ to enter cell, causing exocytosis of NT, binding receptor post-synaptic cell, leading to depolarization of next neuron.

Most Common NT in Brain

Glutamate

Presynaptic Terminal

Synaptic vesicles hold NT, voltage-gated Ca2+ channels increase intracellular Ca2+ when AP arrives, MITO for energy. Each synaptic vesicle contains thousands of molecules of an NT.

Postsynaptic Cell

NT receptors excite (depolarize) or inhibit (hyperpolarize) dendrite.

Synaptic Cleft

At the neuromuscular junction, an enzyme breaks down and gets rid of NT. In CNS, NTs are removed through diffusion and transported in glia.

GABA (Gamma amino butyric acid)

Inhibitory (mainly brain)

Glycine

Inhibitory (spinal cord)

Acetylcholine

Excitatory (NM Junction) or inhibitory (cardiac to slow HR)

NT Receptor

Some NTs bind to different receptors; for example, glutamate and acetylcholine can be excitatory or inhibitory.

Glutamate

Excitatory in CNS

Oral Cavity Sensory Axons

Pseudo-unipolar cell bodies of sensory neurons lie in trigeminal (Gasserian) ganglion

Nocireceptors

Pain receptors from free nerve endings in tooth pulp (unmyelinated)

Ruffini Endings

Periodontal ligaments respond to pressure and stretch of the periodontal ligament (myelinated). Missing when impact is done, and the ability to precisely control of holding and crushing food by the teeth is impaired.

Epineurium

Around the nerve (adipocytes, fibroblasts, CT fibers, mast cell, small blood and lymph)

Perineurium

Around fascicles of axons (concentric layers of flattened cells + collage)

Endoneurium

Around single axons (Collagen fibers, fibroblasts, capillaries)

Glia

Cells in the brain without neurons, wrapping both myelinated and unmyelinated axons in the CNS and PNS. Insulation for axons, response to chemicals, releases trophic factors, and removes NTs after release.

Types of Glia

Schwann cell, oligodendrocytes, astrocyte, microglia

Astrocytes

Signal to blood vessels to increase blood flow or active brain areas, remove NTs, signal for BBB to form, surrond each synapse

Microglia

Respond to injury and act like macrophages by removing dead cells (phagocytosis, come from bone marrow)

Blood Brain Barrier (BBB)

Formed by tight junctions between endothelial cells of blood vessels.

White Matter

Myelinated axons

Grey Matter

Unmyelinated Axons

Wrapping of Schwann Cells

Myelinated (one Schwann cell per axon), unmyelinated (multiple axons per schwann cell)

Glia Maintenance

All neurons in the brain and spinal cord are covered by glia, which maintain homeostasis.

Axon Damage in PNS

The distal axon segment degenerates, while the proximal segment, which is connected to the cell body, begins to grow out, resulting in better recovery from a crush injury compared to a cut. Axons will find and grow down any basal lamina back to the target. With a crush, the basal lamina is intact, but with a cut, the growing axon can find the basal lamina of any distal axon and grow down the wrong basal lamina tube.

Axon Damage in CNS

Same as PNS, but axons will regrow. CNS axons recognize molecules on glia and tell them to stop growing. Axon regeneration is poor.