Nervous Tissue

• The purpose of the chapter is to understand how the nervous system helps to keep controlled conditions within limits that maintain health and homeostasis.
• the PNS consists of nerves that extend from the CNS to all parts of the body
• the CNS consists of the brain and spinal cord
• the nervous system is made up of the central nervous system (CNS) and peripheral nervous system (PNS)
• The nervous system consists of three branches: the Central Nervous System (CNS), the Peripheral Nervous System (PNS), and the Enteric Nervous System (ENS).
• The Central Nervous System (CNS) consists of the brain and spinal cord.
• The Peripheral Nervous System (PNS) consists of cranial nerves and spinal nerves.
• Ganglia are small masses of nervous tissue, consisting primarily of neuron cell bodies, that are located outside of the brain and spinal cord.
• Nuclei are neuron cell bodies inside the Central Nervous System (CNS).
• The Enteric Nervous System (ENS) consists of the brain of the gut, which is involuntary and functions independently of the Autonomic Nervous System (ANS) and Central Nervous System (CNS) to some extent, although they also communicate with the CNS via sympathetic and parasympathetic neurons.
• The nervous system exhibits plasticity, but neurons have a limited ability to regenerate themselves.
• Plasticity is the capability to change based on experience.
• Regenerate is the capability to replicate or repair.
• In the Central Nervous System (CNS), there is little or no repair due to inhibitory influences from neuroglia, particularly oligodendrocytes, the absence of growth-stimulating cues that were present during fetal development, and the rapid formation of scar tissue.
• Neurogenesis in the Central Nervous System (CNS) is rare.
• In the Peripheral Nervous System (PNS), repair is possible if the cell body is intact, Schwann cells are functional, and scar tissue formation does not occur too rapidly.
• The steps involved in the repair process in the Peripheral Nervous System (PNS) are: Chromatolysis, Wallerian degeneration, and the formation of a regeneration tube.
• Sensory neurons monitor chemical changes within the GI tract and stretching of its walls.
• The cell body of a neuron contains a nucleus surrounded by cytoplasm that includes typical cellular organelles such as lysosomes, mitochondria, and a Golgi complex, also containing free ribosomes and prominent clusters of rough endoplasmic reticulum.
• Nissl bodies are distributed throughout the cytoplasm of the cell body, except for the region close to the axon, called the axon hillock, and are responsible for synthesizing protein.
• The cytoskeleton of a neuron consists of neurofibrils, which are bundles of intermediate filaments that provide the cell shape and support, and microtubules, which assist in moving materials between the cell body and axon.
• Lipofuscin is contained in aged neurons as a pigment that occurs as clumps of yellowish brown granules in the cytoplasm and is a harmless metabolic byproduct.
• A nerve fiber is a general term for any neuronal process (extension) that emerges from the cell body of a neuron.
• Most neurons have two kinds of processes: multiple dendrites and a single axon.
• Dendrites are receiving input receptor sites for binding chemical messengers from other cells.
• Motor neurons govern the contractions of GI tract smooth muscle to propel food through the GI tract, secretions of GI tract organs (such as acid from the stomach), and activities of GI tract endocrine cells, which secrete hormones.
• Excitable cells communicate with each other via action potentials or graded potentials.
• Sensory neurons sense changes through sensory receptors.
• Action potentials (AP) allow communication over short and long distances whereas graded potentials (GP) allow communication over short distances only.
• Production of an AP or a GP depends upon the existence of a resting membrane potential and the existence of certain ion channels.
• Leakage channels alternate between open and closed through GATES.
• K+ channels are more numerous than Na+ channels.
• Ion channels in neurons include leak channels, ligand-gated channels, mechanically-gated channels, and voltage-gated channels.
• The resting membrane potential is due to the balance of electrochemical gradient: diffusion force via concentration gradient and diffusion force via charge difference.
• Normally negative in neurons, the resting membrane potential ranges from −40 to −90 mV.
• Electrical signals in neurons are conducted by action potentials.
• Graded potentials can be added together to become larger in amplitude.
• An action potential (AP) or impulse is a sequence of rapidly occurring events that decrease and reverse the membrane potential and then eventually restore it to the resting state.
• The two main phases of an action potential are the depolarizing phase and the repolarizing phase.
• After the hyperpolarizing phase, the membrane becomes more negative than the resting membrane potential (RMP).