Nervous System

• Nervous system (NS)


  Body's control center and communication network
• Nervous system


  • Allows us to interpret what is occurring in our external environment and helps us to decide how to react to any environmental change or stimulus by causing muscular contractions
  • Shares in the maintenance of homeostasis (the internal environment of our bodies) with the endocrine system by controlling the master endocrine gland (the pituitary) through the hypothalamus of the brain

  
• Functional division of the NS

  • Peripheral Nervous System (PNS)
  • Central Nervous System (CNS)

  
• Afferent division (PNS)


  Brings sensory information to the CNS from receptors in peripheral tissues and organs

  
• Efferent division (PNS)


  Carries motor commands from the CNS to muscles, glands, and adipose tissue

  
• Somatic nervous system (SNS)


  • Controls skeletal muscle contractions
  • Voluntary contractions - under conscious control
  • Involuntary contractions - automatic responses or complex movements, but they are controlled at the subconscious level, outside your awareness
• Autonomic nervous system (ANS)


  • Automatically regulates smooth muscle, cardiac muscle, glandular secretions, and adipose tissue at the subconscious level
  • Sympathetic - stimulates or speeds up activity and thus involves energy expenditure and uses norepinephrine as neurotransmitter
  • Parasympathetic - stimulates or speeds up the body's vegetative activities such as digestion, urination, and defecation and restores or slows down other activities; uses acetylcholine as neurotransmitter

  
• Enteric nervous system (ENS)


  • Extensive network of neurons and nerve networks in the walls of the digestive tract
  • Although the sympathetic and parasympathetic divisions both influence ENS activities, the ENS initiates and coordinates many complex visceral reflexes locally and without instructions from the CNS

  
• Classification of nerve cells

  • Neurons
  • Neuroglia cells

  
• Neurons


  Transmit information called nerve impulses in the form of electrochemical changes

  
• Nerve


  A bundle of nerve cells or fibers
• Neuroglia cells


  • Astrocytes
  • Oligodendroglia
  • Microglia
  • Ependymal cells
  • Schwann cells or neurolemmocytes

  
• Astrocytes


  • Star-shaped cells that wrap around nerve cells to form a supporting network in the brain and spinal cord; helping regulate nutrients and ions that are needed by the nerve cells

  
• Oligodendroglia


  • Produce the fatty myelin sheath on the neurons of the brain and spinal cord of the CNS

  
• Microglia


  • Engulf and destroy microbes like bacteria and cellular debris
• Ependymal cells


  • Produce cerebrospinal fluid and others with cilia move the fluid through the CNS; line the fluid-filled ventricles of the brain

  
• Schwann cells or neurolemmocytes


  • Form myelin sheaths around nerve fibers in the PNS

  
• Types of neurons


  • Multipolar neurons
  • Bipolar neurons
  • Unipolar neurons

  
• Multipolar neurons


  Brain and spinal cord are this type

  
• Gaps in the myelin sheath

  Nodes of Ranvier also called neurofibril nodes which allow ions to flow freely from the extracellular fluids to the axons, assisting in developing action potentials for nerve transmission

  
• Bipolar neurons


  Have one dendrite and one axon. They function as receptor cells in special sense organs – retina, inner ear, olfactory area

  
• Unipolar neurons


  Have only one process extending from the cell body. Sensory neurons are mostly of this type
• Functional classification of neurons

  • Receptors
  • Sensory or afferent neuron
  • Internuncial or association neurons
  • Motor or efferent neuron

  
• Receptors


  The peripheral nerve endings of sensory nerves that respond to stimuli

  
• Sensory or afferent neuron


  First nerve cell receiving this impulse directly from a receptor

  
• Internuncial or association neurons

  Found in the brain and the spinal cord; transmit the sensory impulse to the appropriate part of the brain or spinal cord for interpretation and processing

  
• Motor or efferent neuron


  Brings about the reaction to the final nerve whether muscles or glands

  
• Threshold


  The membrane potential at which an action potential begins (between -60 mV and -55 mV) corresponding to a depolarization of 10 to 15 mV

  
• All-or-None Principle


  All stimuli that bring the membrane to threshold generate identical action potentials; a given stimulus either triggers a typical action potential, or none at all
• Synapses


  • Areas where the terminal branches of an axon (the axon terminals) are anchored close to, but not touching, the ends of the dendrites of another neuron
  • One-way junctions that ensure that the nerve impulse travels in only one direction. This area is called a synaptic cleft

  
• Acetylcholine


  Allows the impulse to travel across the synaptic cleft to the postsynaptic neuron

  
• Acetylcholinesterase


  Immediately begins to break down the acetylcholine after it performs its function
• Neurotransmitters


  Acetylcholine and norepinephrine are the best. Some others are serotonin, dopamine, and the endorphins
• Components of a reflex arc

  • Sensory receptor in the skin
  • Sensory or afferent neuron
  • Association or internuncial neurons within the spinal cord
  • Motor or efferent neuron
  • Effector organ like a muscle

  
• Spinal cord


  • Carries sensory and motor information between the brain and most other parts of the body
  • Approx 16 – 18 in long in an adult spinal cord extending inferiorly from the brain and ends between vertebrae L1 and L2
  • Begins as a continuation of the medulla oblongata of the brainstem
  • Made up of a series of 31 segments, each giving rise to a pair of spinal nerves
  • Protected by the meninges

  
• Laboratory exercise: Knee-jerk reflex (patellar reflex). Have your laboratory partner sit on a table (or sturdy chair) with legs relaxed and hanging free
• Spinal cord


  • Carries sensory and motor information between the brain and most other parts of the body
  • Approx 16 – 18 in long in an adult spinal cord extending inferiorly from the brain and ends between vertebrae L1 and L2
  • Begins as a continuation of the medulla oblongata of the brainstem
  • Made up of a series of 31 segments, each giving rise to a pair of spinal nerves
  • Protected by the meninges

  
• Knee-jerk reflex (patellar reflex)

  1. Have your laboratory partner sit on a table (or sturdy chair) with legs relaxed and hanging freely over the edge without touching the floor

  2. Gently strike your partner's patellar ligament (just below the patella) with the blunt side of a rubber percussion hammer

  3. The normal response is a moderate extension of the leg
• Ankle-jerk reflex


  1. Have your partner kneel on a chair with back toward you and with feet slightly dorsiflexed over the edge and relaxed

  2. Gently strike the calcaneal tendon (just above its insertion on the calcaneus) with the blunt side of the rubber hammer

  3. The normal response is plantar flexion of the foot

  
• Biceps-jerk reflex


  1. Have your partner place bare arm bent about 90° at the elbow on the table

  2. Press your thumb on the inside of the elbow over the tendon of the biceps brachii, and gently strike your finger with the rubber hammer

  3. Watch the biceps brachii for a response

  4. The response might be a slight twitch of the muscle or flexion of the forearm at the elbow joint