Nervous System

• Nervous System


  The master controlling and communicating system of the body
• Functions of the Nervous System

  • Sensory input
  • Integration
  • Motor output
• Sensory input


  • Sensory receptors monitor stimuli occurring inside and outside the body that may be interpreted as touch, temperature, taste, smell, sound, blood pressure and body position
  • Action potentials from sensory receptors travel along nerves to the spinal cord and brain where they are interpreted
• Integration


  • Interpretation of sensory input by the processing centers brain and spinal cord to exhibit response that may be stored as memory, or may be ignored
• Motor output


  • Response to stimuli by activating effector organs (muscles and glands)
• Other functions of the Nervous System

  • Homeostasis
  • Mental Activity
• Homeostasis


  • Depends on the ability of the nervous system to detect, interpret, and respond to changes in internal and external conditions
  • The nervous system can stimulate or inhibit the activities of other systems to maintain constant internal environment
• Mental Activity


  The brain is the center of mental activity, including consciousness, memory, and thinking
• Parts of the Nervous System

  • Peripheral NS
  • Central NS
• Peripheral NS

  • Somatic
  • Autonomic
• Central NS


  • Brain
  • SC
• Cells of the Nervous System

  • Neurons
  • Neuroglia
• Neurons


  • Nerve cells that receive and transmit action potentials to other neurons or effector organs
  • Each neuron contains one cell body and processes: axon and dendrites
• Parts of a Neuron

  • Cell Body
  • Dendrites
  • Axon
• Cell Body


  • Contains the nucleus and a nucleolus
  • Major biosynthetic center (protein synthesis)
  • Focal point for the outgrowth of neuronal processes
  • There are no centrioles (hence its amitotic nature)
  • Surrounded by well developed Nissl bodies (rough ER), GA & mitochondria
  • Axon hillock – cone-shaped area from which axons arise
• Dendrites


  • Short, tapering, and diffusely branched processes
  • They are the receptive, or input, regions of the neuron (receive information from other neurons or from sensory receptors and transmit the information toward the neuron cell body)
  • Electrical signals are conveyed as graded potentials (not action potentials)
• Axon

  • Slender processes of uniform diameter arising from the hillock
  • Long axons are called nerve fibers
  • Usually there is only one unbranched axon per neuron
  • Rare branches, if present, are called axon collaterals
  • Axonal terminal – branched terminus of an axon
  • Surrounded by neuroglia called Schwann Cells forming the insulator myelin sheath
  • Generate and transmit action potentials
  • Secrete neurotransmitters from the axonal terminals
• Neuron Classification


  • Structural: Multipolar, Bipolar, Unipolar
  • Functional: Sensory (afferent), Motor (efferent), Interneurons (association neurons)
• Neuroglia


  • Astrocytes
  • Microglia
  • Ependymal Cells
  • Oligodendrocytes
  • Schwann Cells
  • Satellite Cells
• Astrocytes


  • Most abundant, versatile, and highly branched glial cells and serves as the major supporting tissue in the CNS
  • They cling to neurons and cover capillaries – (blood –brain barrier)
  • Functionally, they: Support and brace neurons, Anchor neurons to their nutrient supplies, Guide migration of young neurons, Control the chemical environment
• Microglia


  • Small, ovoid cells with spiny processes
  • Phagocytes that monitor the health of neurons (help remove bacteria and cell debris from the CNS)
• Ependymal Cells


  • Squamous- to columnar-shaped cells
  • They line the central fluid filled cavities of the brain and spinal column
  • Produce and move CSF to CNS
• Oligodendrocytes


  Branched cells (dendritic) that wrap CNS nerve fibers
• Schwann Cells


  Surround fibers of the PNS
• Satellite Cells


  Surround neuron cell bodies with ganglia
• Myelin Sheath


  • Axons are surrounded by oligodendrocytes in the CNS and Schwann cells in the PNS
  • Myelinated axon have specialized sheath called myelin sheath which is an excellent insulator
  • It contain gaps called nodes of Ranvier
• Neurophysiology


  • Irritability
  • Conductivity
• Irritability


  Ability to respond to a stimulus and convert it into nerve impulse
• Conductivity


  Ability to transmit impulse to other neurons, muscles or glands
• Resting Membrane Potential


  • Outside of cell membrane is positively charged and negatively charged inside
  • The charge difference across membrane of an unstimulated cell is called the resting membrane potential and the membrane is said to be polarized
• Polarized


  Plasma membrane is resting, or inactive which means there are fewer positive ions (K+)on the inner face of the neuron's plasma membrane than there are on its outer surface(Na+)
• Depolarization


  • When neuron is stimulated, the sodium gates in the membrane open (because of its higher Na+ concentration outside the cell) and the Na+ will diffuse quickly into the neuron
  • The inward rush of Na+ ions changes the polarity of neuron's membrane; the inside is now more positive and the outside is less positive
• Repolarization


  • After Na+ ions rush into the neuron, the membrane permeability changes again, becoming impermeable to sodium ions
  • Potassium ions are allowed to diffuse out of the neuron into the tissue fluid rapidly
  • This outflow of K+ ions from the cell restores the electrical conditions at the membrane to the polarized or resting state called repolarization
  • After repolarization the concentration of both ions in and out of the cell are restored by the activation of sodium potassium pump which uses ATP to pump excess Na+ out and bring K+ back into it
• Saltatory Conduction


  • Current passes through a myelinated axon only at the nodes of Ranvier
  • Action potentials are triggered only at the nodes and jump from one node to the next
  • Much faster than conduction along unmyelinated axons
• Conductivity


  The conductivity of neuronal functioning can be explained by the release of neurotransmitter chemical at the axonal terminals
• Synapse


  • A junction that mediates information transfer from one neuron to another neuron or to an effector cell
  • Presynaptic neuron – conducts impulses toward the synapse
  • Postsynaptic neuron – transmits impulses away from the synapse
• Synaptic Cleft


  • Fluid-filled space separating the presynaptic and postsynaptic neurons
  • Prevent nerve impulses from directly passing from one neuron to the next
  • Transmission across the synaptic cleft is a chemical event (as opposed to an electrical one)
  • Ensures unidirectional communication between neurons
• Information Transfer at Synapses

  1. Nerve impulse reaches axonal terminal of the presynaptic neuron

  2. Neurotransmitter is released into the synaptic cleft

  3. Neurotransmitter crosses the synaptic cleft and binds to receptors on the postsynaptic neuron

  4. Postsynaptic membrane permeability changes, causing an excitatory or inhibitory effect
• Central Nervous System (CNS)

  • Made up of brain and spinal cord
  • The brain is housed within braincase or skull and the spinal cord within vertebral column
  • The brain and spinal cord are covered by three protective coverings known as meninges
  • The space between the two inner meninges are filled with cerebrospinal fluid (CSF)
  • It cushions the brain and spinal cord against shock and removes waste materials from the brain
  • The outer portion of the brain tissue is gray and the inner is white
  • In contrast, the spinal cord appears white on the outside and gray on the inside
• Parts of the Central Nervous System

  • Brain
  • Spinal Cord