Cellular Components of the Nervous System Flashcards
Neurons
- Dendrites relay impulses towards the cell body (soma)
- Axons vary in length from microns to meters
- Protein synthesis is mainly carried out in soma
- Anterograde axonal transport is kinesin-dependent and carries nutrients, enzymes, etc to axonal terminal (ex herpes simplex reactivation)
- Retrograde axonal transport is dynein returns materials for lysis or recycling. Tetanus toxin and viruses (rabies, herpes simplex, polio) are transported into CNS via this mechanism
Glial Cell Classes
Macroglia
Microglia
Ependymal
Glial Cells: Origin, types, location
Astrocytes
- Respond to injury (reactive gliosis); produce neurotrophic factors for neuronal survival
- Optimize intersitital space composition for synaptic transmission
- Remove some neurotransmitters from synaptic cleft (glutamate, GABA)
- Common source of tumors; histological marker for pathology is Glial Gibrillary Acidic Protein
- Vascular end-feed couple neuronal activity with blood flow and support endothelial cells forming blood brain barrier
Blood Brain Barrier
- The Blood Brain Barrier provides protective separation of the circulating blood from the CNS extracellular fluid, limiting penetration of agents (bacteria, drugs, toxins). Biochemical barrier for selective transport in/out of CNS
- Better BBB penetration: increase lipid solubility, decrease molecular weight, decrease charge (non-polar). Transporters can faciliate entrance of polar molecules (glucose and amino acids via facilitated transport that does not require energy; insulin and transferrin via active transport that requires energy)
- Mediate by tight junctions and astrocyte end-feet
- Circumventricular organs (and other parts) lack a BBB
- Area postrema (vomiting center)
- paraventricular nuclei
- organum vasculosum lamina terminalis (OVLT) (osmolarity regulation)
- neurohypophysis (antidiuretic hormone secretion)
- BBB can open in stroke, tumors, trauma, infections, epilepsy, MS, neurodegenerative diseases
Oligodendrocyte
- Formation and maintenance of CNS myelin (axonal insulation)
- There are interruptions in myelin, important for impulse conduction
- One oligodendrocyte can interact with ~50 axons
- Also produce neurotrophic factors (nerve growth factor or NGF)
- Damaged in MS
Microglia
- Bone marrow derived
- immune cells of the CNS
- phagocytosis of debris after injury
- may contribute to pathophysiology of neurodegenerative disorders
Neurons and Glia in the Peripheral Nervous System
- Schwann Cells (PNS counterpart of CNS oligodendrocytes)
- Capsular of Satellite Cells (surround sensory and autonomic ganglia)
- Macrophages are PNS counterpart of CNS microglia
- Synapse onto peripheral tissues in different organ systems
Schwann cells
- envelop part of an axon rather than multiple axons
- outer layer called neurolemma
- myelin sheets are located within lamellae
- interruptions are called nodes of ranvier
- important in regeneration of damaged axons
- injured in guillain-barre syndrome
Sequence of morphological changes after injury to a myelinated PNS axon
A. Normal condition
B. Axon reaction (chromatolysis, swelling, eccentric nucleus, synapses disconnect) and anterograde (wallerian) axonal and myelin degeneration
C. Growth sprout regenerating into schwann cell tube
D. Axon regenerating to effector site
E. return to normal function
CNS axon reaction and anterograde degeneration
CNS axon reaction and anterograde degeneration occur; however, regeneration does not take place.
- CNS axons have intrinsic capacity to regenerate, but extrinsic factors prevent
1. astrocyte glial scar creates physical and chemical barrier
2. gap formation
3. oligodendroglial proteins trigger collapse of axonal growth cone via Nogo receptors
4. lack of neurolemmal tube-like guidance channels
Physiological properties of neurons
- Resting membrane potential
- Action potential generation and propagation
Examples of excitatory inputs that trigger action potential firing
- somatosensory input
- light, sound
- excitatory synaptic transmission
Action Potential Firing
Nonsaltatory Conduction in Unmyelinated Axon
- Propagation occurs continuously in meighboring axonal membrane segments
- Slower impulse transmission
Normal Action Potential Propagation
- Conducatnce changes occur at gaps (nodes) between myelin sheets
- Na+ channels are pesent in high concentrations in nodes; K+ channels are spread over axonal membrane
- Propagation velocity is higher in myelinated fibers and large diameter fivers (80-100 m/s)
Synaptic transmission
- Action potentials activate voltage-gated Ca2+ channels in axonal terminals, triggering neurotransmitter release into synaptic cleft.
Tetanus and botulinum toxins affect on neurotransmitters
Neurotransmitters are on which two types of receptors ?
- Ligand gated ion channels
- G protein coupled receptors
Examples of Neurotransmitters (table)
Biosynthesis of Catecholamines
Epinephrine <–(phenylethanolamine-n-methyltransferase) Norepinephrine <– (hydroxylase) Octopamine <–(dopamine B-hydroxylase) Tyramine <– (L-amino acid decarboxylase) Tyrosine
(tyrosine hydroxylase) → Dopa (dopa decarboxylase) → Dopamine (dopamine B-hydroxylase) →Norepinephrine (phenylethanolamine -N-methyltransferase) → epinephrine
*start from tyrosine and go foward, alternative pathway is on top
Synthesis of serotonin and melatonin
Tryptophan (tryptophan hydroxylase) → 5-hydroxytryptophan (aromatic amino acid decarboxylase) → Serotonin (serotonin N-acetyl transferase SNAT) → N-acetylserotonin (hydroxyindole-0-methyltransferase HI0MT) → Melatonin
*Receptors are all G protein coupled receptors except for 5HT3, a ligand gated ion channel. Block by anti-emetic drug, ondansetron: “You will not vomit with Ondanesetron, so you can go On dancing”
GABA shunt pathway
GABA receptors
Glutamate release
Glutamate Receptors
Inhibitory synaptic potential
