Nerve tissue consists of neurons, which are excitable cells that transmit information as electrical signals, and glial cells (e.g., oligodendrocytes, Schwann cells, astrocytes, microglial cells), which perform a variety of nonsignaling functions such as forming myelin to provide support and insulation between neurons, phagocytosing and removing cellular debris, removing excess neurotransmitters, and forming the blood-brain barrier. Oligodendrocytes myelinate neurons in the central nervous system (CNS), while Schwann cells myelinate neurons in the peripheral nervous system (PNS). Myelin sheaths increase the conduction velocity of signals across axons. Inflammation and loss of the myelin sheath are the underlying pathologic processes in multiple sclerosis (CNS) and Guillain barre syndrome (PNS). Neurons are composed of dendrites, cell bodies, axons, and axon terminals. Based on their conduction velocity, diameter, and myelination, nerve fibers (axons) are classified into large, myelinated fibers with fast conduction velocity (group A); small, myelinated fibers with slow conduction velocity (group B); and small, unmyelinated fibers with slow conduction velocity (group C). Neurons communicate through the transmission of action potentials across junctions between them called synapses. Synaptic transmission can be chemical or electrical. Chemical synaptic transmission is the transfer of signals through the release of neurotransmitters (e.g. acetylcholine, dopamine, norepinephrine) from presynaptic terminals to postsynaptic receptors. Electrical synaptic transmission is the transfer of electrical signals through gap junctions. Alterations in neurotransmitter levels have been observed in various neurological diseases, including Parkinson disease (decreased dopamine), schizophrenia (increased dopamine), depression (decreased dopamine, norepinephrine, and serotonin), and Alzheimer disease (decreased acetylcholine).
- 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|| ||-|| |