Nervous

• Nervous system


  Coordinates the activities of many other organ systems, activates muscles for movement, controls the secretion of hormones from glands, regulates the rate and depth of breathing, and is involved in modulating and regulating a multitude of other physiological processes
• Divisions of the nervous system

  • Central nervous system
  • Peripheral nervous system
• Central nervous system (CNS)

  • Includes the brain and spinal cord
  • Formed by neurons and supporting cells called neuroglia
  • Contains more than 100 billion neurons
  • Arranged in two layers: gray matter and white matter
• Gray matter


  Formed by nerve cell bodies and the proximal parts of nerve fibers
• White matter


  Formed by remaining parts of nerve fibers
• Brain and spinal cord

  • Surrounded by three layers of meninges: dura mater, arachnoid mater, and pia mater
  • Suspended in cerebrospinal fluid
• Major divisions of the brain

  • Prosencephalon
  • Mesencephalon
  • Rhomboencephalon
• Prosencephalon


  • Also called the forebrain, further divided into Telencephalon and Diencephalon
• Mesencephalon


  • Also called the midbrain
• Rhomboencephalon


  • Also called the hindbrain, further divided into Metencephalon and Myelencephalon
• Divisions of the Peripheral Nervous System

  • Somatic nervous system
  • Autonomic nervous system
• Somatic Nervous System


  Concerned with somatic functions, includes nerves supplying the skeletal muscles, responsible for muscular activities and movements of the body
• Autonomic Nervous System


  Concerned with regulation of visceral or vegetative functions, also called vegetative or involuntary nervous system, further divided into Sympathetic and Parasympathetic nervous systems
• Divisions of the Nervous System

  • Afferent System (sensory part)
  • Efferent System (motor part)
• Afferent System (sensory part)


  • Most activities of the nervous system are initiated by sensory experience exciting sensory receptors, transmits sensory information from the receptors of the entire body surface and from some deep structures to the central nervous system
• Efferent System (motor part)


  • Controls contraction of skeletal muscles, contraction of smooth muscle in the internal organs, and secretion of active chemical substances by both exocrine and endocrine glands
• Neuron


  The structural and functional unit of the nervous system, contains nucleus and all the organelles in cytoplasm, has branches or processes called axon and dendrites, and does not have a centrosome so cannot undergo division
• Classification of neurons


  • Depending on the number of poles
  • Depending on the function
  • Depending on the length of axon
• Structure of the neuron

  • Consists of nerve cell body, dendrites, and axon
  • Dendrites are short processes and axons are long processes
  • Dendrites and axons are called nerve fibers
• Nerve cell body


  Also known as soma or perikaryon, contains cytoplasm, a large nucleus, Nissl bodies, neurofibrils, mitochondria, and Golgi apparatus
• Nissl bodies


  Small basophilic granules found in cytoplasm of neurons, concerned with synthesis of proteins
• Neurofibrils


  Thread-like structures present in the form of a network in the soma and the nerve processes, consist of microfilaments and microtubules
• Dendrite


  Branched process of the neuron that transmits impulses towards the nerve cell body
• Axon


  Longer process of the nerve cell that extends for a long distance away from the nerve cell body and transmits impulses away from the nerve cell body
• Structure of the axon

  • Has a long central core of cytoplasm called axoplasm covered by the tubular sheath-like membrane called axolemma
  • Axoplasm contains mitochondria, neurofibrils and axoplasmic vesicles
  • Lacks Nissl bodies, so proteins needed by axons are synthesized by soma and transported to it by axonal flow
• Myelinated nerve fibers


  Nerve fibers wrapped in a thick lipoprotein sheath called the myelin sheath, which insulates the nerve fiber and allows for faster conduction of impulses
• Non-myelinated nerve fibers


  Nerve fibers without a myelin sheath
• Node of Ranvier


  The area where the myelin sheath is absent
• Internode


  The segment of the nerve fiber between two nodes of Ranvier
• Synapse


  The junction between two neurons, can be classified anatomically and functionally
• Anatomical classification of synapses

  • Axoaxonic synapse
  • Axodendritic synapse
  • Axosomatic synapse
• Functional classification of synapses

  • Electrical synapse
  • Chemical synapse
• Electrical synapse


  The synapse in which the physiological continuity between the presynaptic and the postsynaptic neurons is provided by gap junctions, allowing for direct exchange of ions and minimal synaptic delay
• Chemical synapse


  The junction between a nerve fiber and a muscle fiber or between two nerve fibers, where signals are transmitted by the release of chemical transmitters across a synaptic cleft