Introduction to Neural Cells

Introduction to Neural Cells Introduction to Neural Cell Types: Two main types of cells in the nervous system: neurons and neuroglia (aka glial cells) Most neural cells are derived from neural stem/crest cells and arise in the ectoderm of the embryo (stem gives rise to neurons/glia in CNS and crest give rise to neurons/glia in the PNS) There are even more glia than neurons Overview of Neuron Structure: Axon can be a variety of lengths and is typically unbranched until it reaches the end (axon terminal) Target cells can be anything like muscles, another neuron, a gland cell, or even capillaries (to release hormones into the bloodstream Neural Stem Cells (can become basically any cell in the nervous system) differentiate into Neuroblasts (can only become neurons) and go away to where their somas will be eventually located after development The neuroblast then sprouts an axon with a growth cone at the end of it which follow guidance cues in the environment until it reaches its target cell The same thing happens in the PNS but instead use neural crest cells instead of neural stem cells/neuroblasts Unipolar neuron: one pole to the cell (one process coming out of the soma) and are present in humans mainly during development (but not really after) Bipolar neuron: has two processes/poles originating from the soma Multipolar neuron: multiple processes/poles originating from the soma and is the most common neuron present in adult humans Pseudounipolar neuron: has only one process coming out of it (like a unipolar neuron) but it splits up and has a different overall structure (typically an afferent neuron’s structure) Overview of Neuron Function: The main function of neurons is to process and transmit information and without any input there is a stable and relatively constant voltage difference across its cell membrane (more negative inside and more positive on the outside) and this is known as the resting membrane potential Graded potentials occur in the soma and are typically small in size and do not last very long, however, enough of a generated potential can induce the firing of an action potential The graded potentials are sparked by inhibitory or excitatory signals that are perceived through the neuron’s dendrites APs fire at a quicker rate with an increased diameter of the axon and more myelin sheath (resistance increases with myelination and capacitance decreases with myelination) 2 main types of efferent neurons: motor/somatomotor neurons (control skeletal muscle) and autonomic neurons (control smooth muscle, cardiac muscle, and gland cells) Neurons of the CNS are typically used as connection neurons known as interneurons (neurons between neurons) - Astrocytes: Astrocyte: a type of glial cell in the CNS that provides a lot of the structural support needed Form glial scar if there was an injury, meaning that astrocytes proliferate in the area and cover the injured area Also maintains homeostasis of the interstitial fluid by taking in or releasing ions in order to keep components of the interstitial fluid at optimal levels and can even release lactate to give neurons another source of energy if needed They also help form the blood-brain barrier (which keeps large molecules and other unwanted substances, including most immune cells, from entering the brain) - Also help to clear out synapses between neurons of any remaining neurotransmitters that shouldn’t be there at that time - Microglia: Microglia: derive from circulating circulating monocytes from the bone marrow that then enter the CNS and are therefore derived from the mesoderm Resting Microglia: just sit there and have their processes sampling the environment (looking primarily for inflammation through injury or infection) Active Microglia: are larger and look/act like macrophages (look for dead/damaged tissues and secrete cytotoxic factors to kill things like bacteria) and eat on debris through phagocytosis (dead cells or other things of the like) - - Ependymal Cells: Ependymal Cells: line the ventricles of the brain and produce cerebrospinal fluid (CSF) and form a simple cuboidal epithelium They also form a leaky barrier between the interstitial fluid and the cerebrospinal fluid Oligodendrocytes: Oligodendrocytes: form the myelin sheath covering axons within the central nervous system The myelin sheath is wrapped around multiple times in order to form the thick lipid covering - Schwann Cells: Schwann Cells: form the myelin sheath covering axons in the peripheral nervous system (derived from neural crest cells) There are non-myelinating Schwann Cells which provide support but don’t necessarily wrap the axon Main difference between myelin sheath in the PNS and CNS is that Schwann Cells only provide 1 the myelin sheath while oligodendrocytes can make multiple at once on different axons It is, however, still wrapped multiple times in order to form the thick lipid covering as it was in the CNS but it does have the soma/cell body on the outside (the part with the nucleus) but most of the cytoplasm is wrapped around the axon