- Nerve tissue is the main tissue component of the nervous system and is primarily composed of neurons and supporting glial cells.
- The nervous system is divided into two main components:
- Polarized, signal-transmitting cells that comprise the central and peripheral nervous system
- Classified into unipolar, pseudounipolar, bipolar, and multipolar depending on the number of protoplasmic processes (neurites)
- Permanent cells
- Composed of soma (cell body), axon and dendrites
- Nissl staining positive in the cell body and dendrites, which have (aggregates of rough endoplasmic reticulum with bound polysomes)
- Soma: contains the cell organelles
The projection from a neuron's cell body along which action potentials travel to send intercellular signals.
- Ends in a synapse.
- Differentiated from the cell body by the presence of the axon hillock, an area that lacks Nissl substance.
- Initial segment: trigger zone for initiation of action potentials
- The projection from a neuron's cell body along which action potentials travel to send intercellular signals.
Supporting glial cells
|Astrocytes|| || |
|Tanycytes|| || |
- Insulating layer of modified plasma membrane that wraps around axons of nerve in a spiral fashion
- Increases the conduction velocity of signals traveling down axons
- Node of Ranvier
- Demyelination: A process in which myelin sheaths of nerves become damaged, which impairs electrical conduction.
- Responses to damage
Layers of peripheral nerves
- Layer of connective tissue around nerve fascicles
- Contains the blood-nerve barrier
- Clinical significance: important layer in microsurgery during limb salvage surgical procedures
- Outer layer of dense connective tissue around a nerve
- Contains nerve fascicles and blood vessels to the nerve
Nerve fibers are classified based on their conduction velocity, diameter, and axon characteristics.
|Nerve fibers||Myelinated||Characteristics||Conduction velocity||Size|
|A-alpha-fibers|| || || || |
|A-beta fibers|| || || |
| || || |
|A-delta fibers|| |
|B fibers|| || || || |
|C fibers|| || || |
C fibers have a slow conduction velocity due to their small diameter and lack of myelination.
Synapses are areas where signals or action potentials are transmitted from a presynaptic to a postsynaptic structure (e.g., neurons, muscle). There are different types of synapses according to the synaptic structures:
- Axodendritic synapses: signaling between axons and dendrites
- Axoaxonic synapses: signaling between axons
- Axosomatic synapses: signaling between axons and the cell body of neurons
- Dendrodendritic synapses: signaling between dendrites
- Excitatory postsynaptic potential (EPSP): Stimulates firing and propagation of the action potential as a result of increased Na+ influx into the cell. Examples include:
- Inhibitory postsynaptic potential (IPSP): Decreases firing and propagation of the action potential as a result of increased influx of Cl- into the cell. Examples include:
Neuromuscular junction (NMJ)
- Presynaptic neuron: Action potential → depolarization of the presynaptic membrane → opening of voltage-gated Ca2+ channels (in the presynaptic membrane) → influx of Ca2+ into the presynaptic terminal → release of presynaptic vesicles filled with acetylcholine (ACh) into the synaptic cleft
- Postsynaptic neuron: Binding of ACh to its receptor on the postsynaptic membrane of muscle (motor end plate) → depolarization of the postsynaptic membrane → end-plate potential (EPP) → stimulation of voltage-sensitive dihydropyridine receptors (DHPR) → coupling with ryanodine receptors (RR) → release of Ca2+ from the sarcoplasmic reticulum (SR) → tropomyosin release from myosin → binding of myosin and actin → muscle contraction
- Synaptic cleft: (AChE) breaks down ACh → acetate + choline → reuptake of choline into the presynaptic membrane → resynthesis of ACh
- Characterized by direct flow of current through cells via gap junctions
- Found in the heart and smooth muscle
- No chemical synapse is required → no delay during synapsis
Levels of neurotransmitter in disease processes
|Neurotransmitter||Location||Increased levels||Decreased levels|
|Norepinephrine|| || || |
|GABA|| ||-|| |