Introduction to the Nervous System and Neurocytology Flashcards
Central Nervous System (CNS)
Consists of the:
- Brain
- Spinal cord
Peripheral Nervous System (PNS)
Consists of:
- Cranial nerves and their derivatives from the brainstem
- Except for CN I (Olfactory) and CN II (Optic) which are parts of the CNS
- Spinal nerves and their derivatives from the spinal cord
What are the two divisionsof the peripheral nervous system?
- Afferent (sensory) division
- Efferent (motor+) division
Subdivisions of the afferent (sensory) division on the PNS
- Somatic: signals from special sensory organs (except olfactory and retina epithelium), skin, skeletal muscles, joints, and tendons
- Visceral: signals from internal organs and tissues
Afferent Neurons
- Part of the Peripheral Nervous System
- Are neurons that receive information from our sensory organs and transmit this input to the CNS
Efferent Neurons
- Part of the Peripheral Nervous System
- Are neurons that send impulses from the central nervous system to your limbs and organs
Subdivisions of the efferent (motor+) division on the PNS
- Somatic: voluntary, innervating skeletal muscle
- Visceral: involuntary, innervating smooth and cardiac muscles, glands, and adipose tissue. This subdivision is also known as the autonomic nervous system (ANS)
Neurons
- Electrogenic cells that allow for rabid signaling throughout the body
Glia (AKA neuroglia or glial cells)
- “Glue” of the nervous system
- Involved in structural support, nutrient and O2 delivery, insulation, immune function, synaptic signaling, formation, and plasticity
- Don’t fire action potentials
The full nervous system can be classified into which two groups?
- Grey matter
- White matter
Grey matter
- Neuronal and glial cell bodies (somata)
- Dendrites and unmyelinated axons
White matter
- Myelinated axons
Nuclei
Functionally homogeneous grey-matter densities of the CNS
Ganglia
Functionally homogeneous grey-matter densities of the PNS
Tracts
Bundles of axons (white matter) of the CNS
Nerves
Bundles of axons (white matter) of the PNS
Neuron Structure
Cell Body/Soma/Perikaryon
- Contains organelles
- Blue arrows in picture
Neuron Structure
Dendrites
- Shorter projections (yellow arrowheads in picture)
- Afferent: receive signals and sends them to soma
- Dendritic spines: increase surface area
Neuron Structure
Axon
- Much longer projection (green triangle in picture)
- Efferent: sends signals away from soma to synapse
Neuron Structure
Nucleus and Nucleolus
- Large central nucleus
- Strong nucleolus
Neuron Structure
Nissl Bodies/Nissle Substance
- Dark staining endoplasmic reticulum/ribosomes
- Soma makes tons of proteins
Neuron Structure
Axon Hillock
- Initiates action potential
- Lots of ion channels and microtubles
Neuron Structure
Myelin Sheath
- Fatty wrapping around axon
- Increases speed of impulse conduction
Neuron Synapse
Communication point between neuron and target
Multipolar Neuron
- Single axon and many dendrites
- Most common
Bipolar Neuron
- Single axon and single dendrite
- Seen in sensory organs
Unipolar/Pseudounipolar Neuron
- Single projection splits into axon and dendrite
- Common in the spinal ganglia
Neuroglia (glial cells) of the CNS
- Astrocytes
- Oligodendrocytes
- Microglia
- Ependymal cells
Neuroglia (glial cells) of the PNS
- Satellite cells
- Schwann cells
Astrocytes
- CNS Glial cells
- Large star-shaped cells
- Filled with intermediate GFAP filaments
- Large irregular neclei (arrows in picture)
- 2 subcategories:
- Protoplasmic (grey matter)
- Fibrous (white matter)
- Functions:
- Structural and metabolic support
- Scavenge NT and ions in synapse
- Form neural scar (glial scar)
- Form the blood brain barrier
Blood Brain Barrier
- Endothelial cells of capillaries have tight junctions
- Continous basal lamina/basement membrane
- Astrocyte foot processes form outer layer
- Small molecules can cross (gases, hormones, ethanol)
- Large molecules must be transported (glucose, amino acids)
- Functions:
- Protects the brain from compounds or pathogens in blood
- Restricts ion transport to protect neuronal activity
- Neutralizes drugs and toxins
Oligodendrocytes
- Smaller glial cells of the CNS
- Small, round dense nucleus with small halo (arrow in picture)
- Functions:
- Produce myelin sheath for brain and spinal cord
- Each oligodendrocyte wraps multiple axons
Microglia
- Very small glial cells of the CNS
- Derived from monocyte precursors in the bone marrow
- Small, cigar shaped nuclei (arrows in picture)
- Functions:
- Immune defense for CNS
- Antigen presenting cell
- Initiate inflammatory response
- Phagocytose cell debris after injury
- Glial scar formation
Ependymal Cells
- Glial cells of the CNS
- Line the ventricles and central canal (arrows in picture)
- Act as epithelium, lots of tight junctions
- Cilia and microvilli on apical ssurfaces
- Functions:
- Move and absorb cerebrospinal fluid (CSF)
- Help form the blood–CSF barrier
- Specialized ependymal cells form the choroid plexus in third ventricle of brain
Satellite Cells
- Small glial cell of the PNS
- Surrounds neuron somata in spinal ganglia (arrows in picture)
- Functions:
- Insulate neurons from electrical impulses
- Metabolic support
Schwann Cells
- Small glial cell of the PNS
- Extends cytoplasm around developing axon (each schwann cell wraps a single axon)
- Multiple layers of cell membrane form the myelin sheath
- Node of Ranvier: point between two Schwann cells
- Function:
- Form myelin sheath for peripheral nerves
- Insulate axons from each other
Synaptic Vesicle
- Filled with neurotransmitters
- Fuse with membrane to release NTs after calcium influx via opening of voltage gated Ca2+ channels
Pre-Synaptic Membrane
- Releases neurotransmitters
- Recycles membrane for new vesicles
Synaptic Cleft
- Space between pre-synaptic membrane and post-synaptic membrane
- 20-30 nm gap
Post-Synaptic Membrane
- Where neurotransmitter binds receptors
- This causes ion channels to change membrane potential
- Causing new action potential in target neuron
Axodendritic Synaptic Connections
Axon synapses on dendrite
Axosomatic Synaptic Connections
Axon synapses on soma
Axoaxonic Synaptic Connections
Axon synapses on axon
Excitatory Signals
- Open Na+ channels to depolarize neuron
- Makes it easier to reach threshold and fire action potential
Inhibitory Signals
- Open Cl- channels to hyperpolarize neuron
- Makes it harder to reach threshold and fire action potential
Electrical Synapses
- Contains gap junctions linking cytoplasm together for direct cell to cell contact
- Instant transmission; no neurotransmitters
- Examples: smooth muscle and cardiac muscle
Chemical Synapses
- Cells do not physically touch
- Requires neurotransmitters
- Delayed transmission (~0.5 ms)
- Examplse: neuromuscular junction, most neurons
Peripheral Nerve Structure
Epineurium
- Dense outer connective tissue covering (arrow in picture)
- Can contain blood vessels or adipose
Peripheral Nerve Structure
Perineurium
- Wraps of bundle (fascicle) of axons together
- (Golden arrowhead in picture)
Peripheral Nerve Structure
Endoneurium
- Innermost layer of reticular fibers
- Surrounds individual axon; red substance between axons
- Secreted by fibroblasts and Schwann cells
- (Green triangle in picture)
Axon Histology
- Round, central region
- (Blue arrow in picture)
Myelin Shealth Histology
- Dense lines surrounding axons
- (Yellow arrowhead in picture)
Schwannn Cell Nucleus Histology
- Folden around the myelin sheath
- (Green triangle in picture)
Endoneurium Histology
- Innermost layer of reticular fibers
- Surrounds individual axon; red substance between axons
- Secreted by fibroblasts and Schwann cells
Nodes of Ranvier
- Unmyelinated spaces between adjecent glia on axons
- Lots of ion channels
- Location where action potentials are propagated down myelinated nerves
How does myelin increase speed of nerve impulse
- Saltatory Conduction: action potential “jumps” to noes of ranvier
- Only small areas of axon membrane depolarize
- Unmyelinated Rate: 0.5-10 m/sec
- Myelinated Rate: up to 150 m/sec
What myelinatess the CNS?
- Oligodendrocytes
- A single oligodendrocyte wraps around multiple axons
What myelinates the PNS?
- Schwan cells
- A single schwann celll wraps one axon
Chemical Synapse Steps
- Action potential arrives
- Voltage gated calcium channels open; calcium influx
- synaptic vesicles fuse to pre-synaptic membrane; NT is released and diffuses across synapse
- Neurotransmitters bind receptors on post synaptic membrane
- Target cell membrane potential changes
Nerve Injury Process (Basics)
- Damage to axons can heal; damage to cell bodies is irreversible (neurons are post-mitotic/cannot replicate)
- Axonal damage triggers rapid electrical injury signal; can kill the neuron if strong enough
- Injury signal proteins slowly move via retrograde transport to soma via dynein
- Wallerian degeneration and chromatolysis occur
- Soma produces proteins and lipids, ships them down axon via anterograde transport (kinesin)
Wallerian (anterograde) degeneration
- Axon segment distal to injury degrades
- A short piece of the proximal segment also degrades (retrograde degeneration)
Chromatolysis
- Nucleus and Nissle bodies move to periphery, soma swells
Peripheral Nerve Regeneration
- High regeneration potential
- Macrophages eliminate degraded axon/myelin
- Schwann cells proliferate to form a channel
- Axon growth cone sprouts from soma and travels down channel to re-establish synapse with target
CNS Nerve Regeneration
- Low regeneration portential
- Oligodendrocytes undergo apoptosis
- Blood brain barrier blocks macrophages
- Microglia can’t clear debris (inflamation increases)
- Left over myelin blocks regrowth
- Gliosis: Hypertrophy of astrocytes
- Glial scar forms and blocks growth cone