Biology 325 Fall 2004 Neurophysiology I nervous tissue neurons signals in neurons I Introduction the nervous system NS is the master controlling and communicating system of the body together with the endocrine system it is responsible for maintenance of homeostasis A Functions of the nervous system 1 monitors changes occurring inside outside of the body sensory input 2 processes interprets sensory input integration 3 causes a response in effector organs muscles glands motor output B Organization of the nervous system the NS is divided into two major parts CNS brain spinal cord integrative control centers interprets incoming information dictates responses PNS spinal nerves cranial nerves it is divided into two parts functionally sensory division conducts impulses from receptors to CNS motor division conducts impulses from CNS to effectors has two major subdivisions somatic division conducts impulses from CNS to skeletal muscles voluntary autonomic division ANS conducts impulses from CNS to cardiac and smooth muscle and glands involuntary has two final subdivisions sympathetic division fight or flight parasympathetic division non emergency vegetative body functions II Histology of nervous tissue nervous tissue is highly cellular cells densely packed intertwined two major cell types neurons excitable cells and supporting cell smaller wrap around neurons A Supporting cells six types total 4 CNS 2 PNS provide protective scaffolding for neurons 1 CNS supporting cells neuroglia branching processes with central body outnumber neurons 9 1 a astrocytes star shaped most common radial projections cling neurons to capillaries role in making exchanges between neurons and capillaries b microglia the macrophages of the CNS c ependymal cells range from squamous to columnar line central cavities of brain spinal cord d oligodendrocytes wrap themselves around thicker neuron fibers of CNS forming insulating covering myelin sheath 2 PNS supporting cells satellite cells Schwann cell similar structures differ mostly in location a satellite cells surround neuron cell bodies in ganglia role in chemical environment control b Schwann cells wrap themselves around thicker nerve fibers form myelin sheaths 3 Myelin sheath white fatty material that covers large diameter axons in a segmented fashion protects insulates fibers from one another increases conduction velocity a Formation of myelin sheaths in PNS Schwann cells wrap plasma membranes around axon in successive layers the cytoplasm is squeezed out of plasma membrane layers forming a structure consisting of many concentric layers of Schwann cell plasma membrane wrapped around a single axon nucleus and cytoplasm of Schwann cell lie immediately below the outer leaflet of plasma membrane outside concentric rings of empty membrane neurilemma one Schwann cell myelinates only a segment of one axon thus various Schwann cells myelinate a single axon gaps in myelin sheath produced by adjacent Schwann cells are called Nodes of Ranvier axolemma exposed at these areas basis of saltatory conduction unmyelinated axons are also associated with Schwann cells however Schwann cell does not wrap its membrane around nerve fiber just encloses it in this case a single Schwann cell can enclose several axons b Formation of myelin sheaths in CNS oligodendrocyte produces myelin sheath a single oligodendrocyte can wrap its plasma membrane around several nerve fibers cell extensions doing the coiling rather than whole cell itself B Neurons nerve cells structural functional units of NS very specialized cells conduct messages in form of electrical impulses general characteristics extreme longevity amitotic high metabolic rate very diverse structurally but idealized neuron contains a cell body from which one or more processes emerge all neurons have three functional regions a receptive region input receives information a conducting component generates and transmits nerve impulses or action potentials and a secretory component neurotransmitter release 1 Neuron cell body soma biosynthetic center of cell contains very well developed RER also called Nissl bodies well developed Golgi apparatus numerous neurofilaments part of receptive component 2 Neuron processes cytoplasmic extensions projecting from cell bodies two major types axons and dendrites a dendrites short diffuse branching processes usually close to soma receptive region conducts electrical signals to cell body these signals are not nerve impulses action potentials they are short lived electrical currents graded potentials b axon single large process arises at axon hillock can be short or very long long axon called a nerve fiber may give rise to axon collaterals and terminates in many numerous telodendria that have bulbous ending called axon terminals are the conducting component of a neuron generate nerve impulses action potentials AP at the trigger zone and propagate them away from soma to axon terminals also are the secretory component of neurons AP reaches axon terminals causes neurotransmitter NT release contains same organelles as soma except no Nissl bodies hence depends on soma to renew necessary membrane protein components and other enzymes these substances are transported down axon by axonal transport anterograde retrograde a function or MT microfilaments neurofilaments 3 Classification of neurons a Structural classification based on number of processes extending from soma i multipolar neurons three or more processes many dendrites one axon ii bipolar neurons two processes one axon one dendrite extending from opposite sides of the soma very rare receptors in sense organs iii unipolar neurons pseudounipolar a single process emerges from soma divides T like into a peripheral process usually associated with a sensory receptor and a central process enters CNS b Functional classification according to direction of impulse propagation relative to CNS i sensory afferent neurons conduct sensory input towards CNS mostly unipolar cell bodies in sensory ganglia outside CNS ii motor efferent neurons conduct nerve impulses away from CNS to effector organs multipolar cell bodies in CNS except some ANS axons bunched as nerves iii interneurons association lie between sensory and motor neurons in neural pathways multipolar usually confined to CNS III Neurophysiology neurons are highly irritable responsive to stimuli that is when a neuron is adequately stimulated an electrical impulse will be generated at and conducted along the axon AXON