Unit 7.8

• Sections of the study material


  • Evolution of the Nervous System
  • Nervous Tissue
  • The Central Nervous System
  • The Peripheral Nervous System

  
• Invertebrate Nervous System Organization


  • Nerve net composed of neurons in contact with one another
  • Also in contact with contractile cells in the body wall

  
• Planarians


  • Ladderlike nervous system
  • Cephalization - a concentration of ganglia and sensory receptors in the head

  
• Annelids, Arthropods and Molluscs


  • Complex animals
  • True nervous systems
• Vertebrate Nervous System Organization

  • Central nervous system develops from an embryonic dorsal neural tube
  • Cephalization and bilateral symmetry result in several paired sensory receptors
  • Eyes, ears, olfactory structures

  
• Central Nervous System (CNS)


  Includes the brain and spinal cord

  
• Peripheral Nervous System (PNS)


  • Consists of all nerves and ganglia that lie outside the CNS
  • Somatic nervous system - sensory and motor functions that control skeletal muscle
  • Autonomic nervous system - controls smooth muscle, cardiac muscle, and glands

  
• Neurons (Nerve Cells)


  • Cell body contains nucleus and organelles
  • Dendrites receive signals from sensory receptors or other neurons
  • Axon conducts nerve impulses to another neuron or to other cells
  • Covered by myelin sheath

  
• Types of Neurons


  • Motor (efferent) neurons
  • Sensory (afferent) neurons
  • Interneurons

  
• Resting Potential


  • The membrane potential (voltage) when the axon is not conducting an impulse
  • The inside of a neuron is more negative than the outside, around -70 mV
  • Due in part to the activity of the sodium-potassium pump
• Action Potential


  • A rapid change in polarity across a portion of an axonal membrane
  • An action potential is generated only after a stimulus larger than the threshold
  • Gated channel proteins allow sodium to enter and potassium to leave the cell
• Propagation of Action Potentials


  • In nonmyelinated axons, the action potential travels down an axon one small section at a time
  • In myelinated fibers, an action potential at one node causes an action potential at the next node (Saltatory Conduction)
  • Conduction of a nerve impulse is an all-or-nothing event
  • Intensity of signal is determined by how many impulses are generated within a given time span

  
• Transmission Across a Synapse

  • A synapse is a region where neurons nearly touch
  • Transmission across a synapse is carried out by neurotransmitters
  • Sudden rise in calcium in the axon terminal of one neuron stimulates synaptic vesicles to merge with the presynaptic membrane and release neurotransmitter molecules into the synaptic cleft
• Synaptic Integration


  • A single neuron is on the receiving end of many excitatory and inhibitory signals
  • Integration is the summing of excitatory and inhibitory signals
  • The receiving dendrite must receive enough + or - signal to cross a threshold and continue propagating action potential

  
• The Spinal Cord


  • Center for many reflex actions
  • Means of communication between the brain and spinal nerves
  • Composed of grey matter and white matter

  
• Parts of the Brain

  • Cerebrum
  • Cerebral Cortex
  • Basal nuclei
  • Diencephalon
  • Brain Stem
  • Reticular Activating System
  • Limbic System

  
• Learning and Memory


  • Memory is the ability to hold a thought in mind or recall events from the past
  • Learning takes place when we retain and use past memories
  • Short term memory is associated with the prefrontal area of the frontal lobe
  • Long term memory is associated with the hippocampus
• Somatic System


  • Includes cranial nerves and spinal nerves
  • Gathers information from sensors and conducts decisions to effectors
  • Controls the skeletal muscles

  
• Autonomic System


  • Controls the smooth muscles, cardiac muscles, and glands
  • Innervates all internal organs
  • Usually involuntary
  • Divided into Sympathetic and Parasympathetic divisions
• Reflex Arc


  1. Sensory receptors generate a nerve impulse that moves along sensory axons through a dorsal root ganglion toward the spinal cord

  2. Sensory neurons pass signals on to many interneurons in the gray matter of the spinal cord

  3. Nerve pulses travel along motor axons to an effector, which brings about a response to the stimulus

  
• Sympathetic Division


  • Especially important during fight or flight responses
  • Accelerates heartbeat and dilates bronchi
  • Norepinephrine main neurotransmitter
• Parasympathetic Division


  • Promotes all internal responses associated with a relaxed state
  • Promotes digestion and retards heartbeat
  • Acetylcholine main neurotransmitter