Nervous System 1 - Part 2 - Lecture 11
Dendrites are short, often highly branched cytoplasmic extensions that are tapered from their bases at the neuron cell body to their tips. Many dendrite surfaces have small extensions, called dendritic spines, where axons of other neurons form synapses with the dendrites. When stimulated, dendrites generate a small electrical currents, which conducted toward the neuron cell body.
Dendrites Dendrites are extensions of the cell body and are the receiving portion of the neuron. They receive input from other neurons' axons and from the environment
Astrocytes also release chemicals that promote the development of synapses and help regulate synaptic activity by synthesizing, absorbing, and recycling neurotransmitters.
Astrocyte regulation of Synaptic Activity
The endothelial cells with their tight junctions form the blood-brain barrier (BBB), which determines what substances can pass from the blood into the nervous tissue of the brain and spinal cord. The BBB protects neurons from toxic substances in the blood, allows for the exchange of nutrients and waste products between neurons and the blood, and prevents fluctuations in blood composition from affecting the brain.
Astrocytes Involvement in Blood Brain Barrier (BBB)
2. These extensions widen and spread out to form foot processes, which cover the surfaces of blood vessels, neurons, and the Pia mater. They can form a supporting framework for blood vessels and neurons. Astrocytes help regulate the composition of extracellular brain fluid by releasing chemicals that promote the formation of tight junctions between the endothelial cells of capillaries. 3. There are two types of astrocytes: Protoplasmic and Fibrous -. Protoplasmic Astrocytes are found within gray matter. -. Fibrous Astrocytes are found within white matter and surrounding blood vessels.
Astrocytes are glial cells that are star-shaped because their cytoplasmic processes extend from the cell body
A single axon (most neurons) arises from a cone shaped area of the neuron cell body called the axon hillock. The beginning of the axon is called the initial segment. Action potentials are generated at the trigger zone, which consists of the axon hillock and the part of the axon nearest the cell body. An axon can remain as a single structure or can branch to form collateral axons, or sides branches. The cytoplasm of an axon is sometimes referred to as the axoplasm and the membrane is also called the axolemma. Axons terminate by branching to form small extensions with enlarged ends called presynaptic terminals. Action potentials conducted along the axon to the presynaptic terminal stimulate exocytosis of neurotransmitters from their vesicles into a synapse. Then neurotransmitters cross the synaptic cleft to stimulate or inhibit the postsynaptic cell.
Axon
1. Movement away from the cell body (within the axon) is called Anterograde. 2. Movement toward the cell body (within the axon) is called Retrograde. Damaged organelles, recycled plasma membrane, and substances taken in by endocytosis can be transport up the axon toward the neuron cell body. 3. Movement of materials within the axon is necessary for its normal function, but it also provides a way for infectious agents and harmful substances to be transported from the periphery to the CNS. For example, Rabies and Herpes Viruses can enter damaged axons in the skin and be transported within the axons to the CNS
Axon transport mechanisms move cytoskeletal proteins, organelles, and vesicles containing neurotransmitters down the axon through the axoplasm toward the presynaptic terminal.
1. Ependymal Cells line the ventricles (cavities) of the brain and the central canal of the spinal cord. 2. Specialized ependymal cells and blood vessels form structures called choroid plexuses, which are located within certain regions of the ventricles. 3. The choroid plexuses secrete the cerebrospinal fluid (CSF) that flows through the ventricles of the brain. 4. The ependymal cells frequently have patches of cilia that help circulate CSF through the brain cavities. Ependymal cells also have long processes at their basal surfaces that extend deep into the brain and the spinal cord and seem, in some cases, to have astrocyte-like functions.
Ependymal Cells
There is very little myelin, and these areas appear darker in appearance. In CNS 1. The cortex consists of gray matter on the surface of the brain. 2. Nuclei are clusters of gray matter located deeper within the brain. In PNS 1. Gray matter consists of clusters of neuron cell bodies called ganglion.
Gray Matter Gray matter consists of groups of neuron cell bodies and their dendrites.
They phagocytize: 1. Necrotic Tissue 2. Microorganisms 3. Foreign substances that invade the CNS Areas of the brain or spinal cord that have been damaged by infection, trauma, or stroke have more microglia than healthy areas. A pathologist can identify these damaged areas in the CNS during an autopsy because large numbers of microglia are found there.
Microglia are CNS-specific immune cells (macrophage). These cells become mobile and phagocytic in response to inflammation.
1. Cytoplasmic extensions (of Schwann Cells or Oligodendrocytes) that wrap many times around axons to form an insulating material called myelin sheaths. 2. They protect and electrically insulate axons. Myelinated axons conduct action potentials more rapidly than along unmyelinated axons. 3. They are rich in phospholipids, with little cytoplasm sandwiched between the membrane layers. Their white appearance comes from the high lipid concentration. 4. Schwann cells provide myelin in the PNS. 5. Oligodendrocytes provide myelin in the CNS. 6. Nodes of Ranvier: Regions of the axon not covered by myelin. Action potentials will jump from these regions.
Myelin Sheaths
1. Sensory Neurons (Afferent Neurons) conduct action potentials toward the CNS. 2. Motor Neurons (Efferent Neurons) conduct action potentials away from the CNS toward muscles or glands. 3. Interneurons conduct action potentials within the CNS from one neuron to another.
Neurons are classified functionally and structurally. Functional classification is based on the direction in which action potentials are conducted.
1. Oligodendrocytes have cytoplasmic extensions that can surround axons. 2. If the cytoplasmic extensions wrap many times around the axons, they form an insulating material called a myelin sheath. 3. 1 Oligodendrocyte can form myelin sheaths around axons of multiple neurons.
Oligodendrocytes
In both the CNS and the PNS, nervous tissue is organized such that axons are grouped together, forming bundles, while neuron cell bodies and dendrites are grouped together. These groupings give nervous tissue distinctive areas: Gray Matter and White Matter
Organization of Nervous Tissue
Human peripheral nerves regenerate about 1mm per day for small nerves and 5 mm per day for larger nerves. On average, human peripheral nerves will regenerate at a rate of about 1 inch per month.
Peripheral Nerve Regeneration Rate
When the extracellular K+ concentration increases as a result of local neural activity, K+ enters Astrocytes via membrane channels. The extensive network of astrocytic processes helps dissipate the K+ over a large area.
Potassium Spatial Buffering by Astrocytes
Action potentials conducted along the axon to the presynaptic terminal stimulate the release (by exocytosis) of the neurotransmitters that cross the synapse to stimulate or inhibit the postsynaptic membrane of the postsynaptic cell.
Presynaptic Terminals Axons terminate by branching to form small extensions with enlarged ends called presynaptic terminals (terminal buttons). Within the presynaptic terminals are numerous small vesicles that contain chemicals called neurotransmitters
Astrocytes aid both beneficial and detrimental responses to tissue damage in the CNS. Almost all injuries to CNS tissue induce reactive astrocytosis, in which astrocytes wall off the injury site and help limit the spread of inflammation to the surrounding healthy tissue. Reactive scar-forming (glial scarring) astrocytes also limit the regeneration of injured axons.
Reactive Astrocytosis
3-5 Days: The axons in the part of the nerve distal to the cut break into irregular segments and degenerate. This occurs because the neuron cell body produces substances essential to maintain the axon, but these substances have no way of reaching parts of the axon distal to the point of damage. Eventually the distal part of the axon completely degenerates. As the axons degenerate, the myelin part of the Schwann cells around them also degenerates, and macrophages invade the area to phagocytize the myelin. The Schwann cells enlarge, undergo mitosis, and finally form a column of cells (regeneration tube) along the regions once occupied by axons. 2 Weeks: Axonal sprouts enter the regeneration tube. Regeneration of damage nerve tracts within the CNS is very limited compared to the PNS due to the Oligodendrocytes' short distance from the axons.
Schwann Cell and Nervous Tissue Respone to Injury When a nerve is cut, either it eventually heals or it is permanently interrupted. The final outcome depends on the severity.
Multipolar Neurons Multipolar neurons have many dendrites and a single axon. The dendrites vary in number and in their degree of branching. Most of the neurons within the CNS (most neurons and interneurons) and motor neurons are multipolar Bipolar Neurons Bipolar neurons have two processes: one dendrite and one axon. The dendrite is often specialized to receive the stimulus, and the axon conducts action potentials to the CNS. Bipolar neurons are located in sensory organs, such as the nasal mucosa (smell), retina (sight), tongue (taste), Vestibulocochlear nerve (hearing and balance) Pseudo-Unipolar (or Unipolar) Pseudo-unipolar neurons have a single process extending from the cell body, which divides into two branches a short distance from the cell body. The two branches function as a single axon. One branch extends to the CNS, and the other extends to the periphery and has dendrite-like sensory receptors. The sensory receptors respond to stimuli, producing action potentials that are transmitted to the CNS. Most sensory neurons are pseudo-unipolar. These neurons can be found within the skin, joints, and muscles.
Structural classification is based on the number of dendrites
Injury & Disease 1. In cases of injury or disease, neuroglia multiply to fill the spaces formerly occupied by neurons. 2. Brain tumors derived from glia, called gliomas, tend to be highly malignant and grow rapidly Types (6) CNS: 1. Astrocytes 2. Ependymal Cells 3. Microglia 4. Oligodendrocytes PNS: 5. Schwann cells 6. Satellite
The supporting cells of neurons are referred to as Neuroglia. "Glial" = Glue. 1. Generally , neuroglia are smaller than neurons, and they are 5-25 times more numerous (typically 10: 1 ratio to neurons). 2. Glial cells do not generate action potentials. 3. Glial cells can multiply and divide in the mature nervous system
Schwann Cells 1. Schwann cells form myelin sheaths. 2. Unlike oligodendrocytes, each Schwann cell forms a portion of the myelin sheath around only one axon. Satellite Cells Satellite cells surround neuron cell bodies in sensory and autonomic ganglia. Besides providing support and nutrition to the neuron cell bodies, satellite cells protect neurons from heavy metal poisons, such as lead and mercury, by absorbing them and reducing their access to the neuron cell bodies.
There are two types of glial cells in the PNS: Schwann Cells and Satellite cells
1. Regardless, the muscle will initially atrophy (shrink in size). 2. Without stimulation from the nerve, the muscle is paralyzed and atrophies. 3. When the 2 ends of an injured axon are aligned in close proximity, healing and regeneration of the axon are likely to occur. 4. After re-innervation, the muscle can become functional and hypertrophy (increase in size). 5. When the 2 ends of an injured axon are not aligned in close proximity, regeneration is unlikely to occur. Without innervation from the nerve, muscle function is completely lost, and the muscles remains atrophied.
When a nerve is injured, there are 2 possible outcomes (based on severity of injury).
In CNS White matter of the CNS forms nerve tracts, or conduction pathways, which propagate action potentials from one area of the CNS to another. In PNS Bundles of axons and their connective tissue sheaths are called nerves.
White Matter White Matter consists of bundles of parallel myelinated axons.
In addition to the structural classification described earlier, some neurons are named for the histologist who described them or for an aspect of their shape or appearance. Examples include the Purkinji Cells in the cerebellum and pyramidal cells found in the cerebral cortex of the brain, which have pyramid shaped cell bodies.
structural classification based on the organization of their dendrites or cell shape.
