Nervous Tissue - High School Biology

As the action potential depolarizes the pre-synaptic terminal button, calcium enters the region and causes the vesicles (full of neurotransmitter) to fuse their membranes with the membrane of the neuron, leading to rapid release of their chemical content outside of the cell. The neurotransmitters must diffuse across the synaptic cleft in order to cause a post-synaptic effect. Typically, many, many excitatory inputs must summate to cause depolarization of the post-synaptic neuron. Each individual stimulus is generally well below threshold, and the post-synaptic neuron will only generate an action potential with several stimuli at once.

Neurotransmitters are released from the axon terminal into the synaptic cleft.

Neurons are essential for transmitting signals, but do so without actually touching one another. The space between neurons is known as the synaptic cleft, or synapse. When a signal reaches the end of one neuron at the axon terminal, it causes neurotransmitters to be released from vesicles. The neurotransmitter molecules travel to the dendrites of the next neuron, which receives the signal and passes it down the next axon.

The soma is the cell body of the neuron, which synthesizes proteins and integrates incoming signals. Nodes of Ranvier are regions along myelinated axons that allow for faster action potential conduction.

A fatty substance called myelin wraps arounds the axon of a neuron, forming an insulating layer called the myelin sheath. Gaps in the myelin coating create small openings, called the nodes of Ranvier, where the cell membrane is exposed. During an action potential, the electrical signal is able to jump from on node to the next, skipping portions of the axon. This speeds up the conduction of the action potential signal. Instead of travelling in a constant wave down the axon, the signal can jump or bounce past segments of it. This process is known as saltatory conduction.

Many neurological diseases and disorders arise from the degeneration of the myelin sheath, slowing the propagation of action potentials and hindering neural functionality.

Glial cells are cells that make up nervous tissue and provide support, protection, and nutrients for the neurons in the brain and nervous system. While neurons are the cells responsible for actually generating, sending, and receiving neural impulses, glial cells are capable of enhancing the speed at which these impulses can be transmitted. In particular, Schwann cells and oligodendrocytes provide myelin sheaths to neurons. Myelin acts as an insulator and helps the action potential travel more quickly down the neural axon.

The inside of a neuron always has a net negative charge at resting state and only becomes positive briefly when threshold is reached causing an action potential. This is due to the concentration and flow of positive sodium and potassium ions that exist at differing gradients when the neuron is at rest.

Influx of sodium ions causes depolarization in a cell. Influx of sodium ions causes the membrane potential to become more positive and leads to activation of cell activity. Repolarization is done by the efflux of potassium ions. Hyperpolarization is a state in which a cell cannot conduct another action potential. It must reach its resting membrane potential before it can cause another action potential to occur.

Astrocytes are responsible for proliferating in regards to brain injury. They also help to make the blood brain barrier (BBB) and are a glycogen fuel reserve buffer. Microglial cells are phagocytes of the central nervous system. They respond to tissue damage by differentiation into large phagocytic cells. Myelin helps to wrap and and insulate axons. It increases space constant (length constant) and increases conduction velocity. Oligodendrocytes myelinate the axons of neurons in the central nervous system.

Nervous tissue contains a variety of support cells in order to preserve the neurons in the brain, known as glial cells. Microglia are similar to monocytes, a type of white blood cell. They are used to remove debris and pathogens from the central nervous system.

Schwann cells, oligodendrocytes, and astrocytes are all part of the neural glia. Schwann cells produce myelin around the axons of neurons in the peripheral nervous system, while oligodendrocytes produce myelin for the axons of neurons in the central nervous system. Astrocytes around found in the central nervous system and help create the blood-brain barrier.

There are two types of cells responsible for myelinating axons in the nervous system: oligodendrocytes and Schwann cells. They only differ by the division of the nervous system in which they are found. Oligodendrocytes myelinate axons in the central nervous system, while Schwann cells myelinate axons in the peripheral nervous system.

Ependymal cells and astrocytes are other types of neuroglia. Ependymal cells secrete cerebrospinal fluid (CSF) into the central nervous system. Astrocytes play a key role in creating the blood-brain barrier in the central nervous system.

The cerebrum is the portion of the brain above the middle of the brain, or diencephalon. The thalamus is a part of the diencephalon, and it is intimately associated with motor behaviors, such as walking or flying (in birds). The cerebrum is divides into four lobes based on location and function. The frontal lobe contains the motor cortex and pre-frontal cortex, as well as Broca's area. It is associated with thought and higher brain function. The parietal lobe contains the sensory cortex and processes tactile input. The temporal lobe contains the hippocampus and auditory cortex. The occipital lobe contains the visual cortex.

Sensory neurons respond to stimuli, such as pain. Association neurons receive signals from the sensory neurons and, on the basis of that input, activate motor neurons. Motor neurons activate muscles or glands. Effectors are the muscles or glands that perform the response directed by the nervous system.

Connective tissue consists of bone, cartilage, blood, and fat and is not involved in this process. Connective neurons is a misnomer. Interneurons, however, can be used to relay signals between sensory and motor neurons in the spinal cord.

White matter is white due to the myelin sheath. Since the myelin sheath only covers the axon, neural tissue that is referred to as white matter is only located on a neuron's axon. Grey matter actually refers to the cell body due to its grayish appearance. Accordingly, one neuron is actually part of both the white and the gray matter in the nervous system.