TR

Nervous System

Representative Nervous Systems

  • All animals have a nervous system except for sponges

  • Nerve net

    • Simplest nervous system
    • Cnidarians (jellyfish, hydras, anemones)
    • Neurons connect to each other in a network
    • Activation of neurons in one area leads to activation of all or most neurons

  • Echinoderms

    • Slightly more complicated
    • Nerve ring around mouth connected to larger radial nerves extending to arms
    • Mouth and arms operate independently

  • Planaria

    • Nerve cords extend length of animal connected by transverse nerves
    • Collection of neurons in head form cerebral ganglia
    • Basic integration of sensory input and motor output

  • Annelids

    • Same basic structure
    • More neurons
    • Ventral nerve cords have ganglia in each segment
    • Rudimentary brain

  • Simple mollusks

    • Similar to annelids
    • Pair of anterior ganglia
    • Paired nerve cords

  • Trend toward cephalization: increasingly complex brain in the head

  • Insects – Drosophila

    • Brain has several subdivisions with separate functions

  • Advanced mollusks

    • Well- developed brain with subdivisions

  • Chordates

  • CNS: brain and nerve cord

    • Brain and spinal cord in vertebrates

  • PNS: all neurons that are outside of the CNS

  • In certain invertebrates with a simple nervous system, the distinction is less clear

Two Divisions of PNS

  1. Somatic nervous system: primary function to sense external environment and control skeletal muscles
    • Sensory neurons receive stimuli – heat, vision, smell, taste, hearing, touch – and transmit to CNS
    • Motor neurons control skeletal muscles
    • Controls many voluntary responses
  2. Autonomic nervous system: regulates homeostasis and organ function
    • Predominantly composed of motor neurons
      • Involuntary – usually cannot be consciously controlled
    • Sensory neurons detect internal body conditions
    • Efferent nerves of autonomic nervous system are further divided
      • Act on same organs with opposing actions
    • Sympathetic division
      • Fight-or-flight
      • Increased heart rate, faster breathing, relaxed airways
    • Parasympathetic division
      • Rest-or-digest
      • Slow heart rate, promote digestion

Hindbrain

  • Medulla oblongata
    • Coordinates basic reflexes and bodily functions that maintain normal homeostatic processes
    • Controls heart rate, breathing, cardiovascular function, digestion, swallowing, and vomiting
  • Cerebellum
    • Overall function of the cerebellum is to maintain balance and coordinate hand-eye movements
    • May have significant cognitive functions
  • Pons
    • Serves as a relay between the cerebellum and other areas of the brain
    • Regulates rate and depth of breathing

Midbrain

  • Processes sensory inputs of several types, including vision, olfaction, and audition
  • Tracts pass this information to other parts of the brain for further processing and interpretation
  • Brainstem: medulla oblongata, pons and midbrain
    • Reticular formation
    • All three parts of brainstem contain nuclei contributing to the reticular formation
    • Maintains and controls alertness and sleep
    • Regulation of respiration and cardiovascular systems

Forebrain - Diencephalon

  • Thalamus
    • Relays sensory information to the cerebrum 
    • Also sends outputs from the cerebrum to other parts of the brain
    • Gets input from all sensory systems except olfaction
  • Hypothalamus
    • Produces hormones regulating pituitary gland, which regulates hormone secretion from other glands
    • Great importance for homeostasis of the body and the control of behavior
  • Epithalamus
    • Structures with various roles in the production of cerebrospinal fluid, control of food and water intake, and rhythmic and seasonal behaviors
    • Pineal gland produces melatonin
  • Cerebral cortex
    • Surface layer of gray matter on the cerebrum
    • Contains 10% of neurons in the brain
    • Integrate information from other nervous system structures and creates outgoing signals
  • Brain has two halves or hemispheres with four lobes each
    • Frontal lobe: conscious thought and social awareness
    • Parietal lobe: receives and interprets sensory input from visual pathways and somatic pathways
    • Occipital lobe: vision and color recognition
    • Temporal lobe: language, hearing, and some types of memory
    • Corpus callosum: connects the cerebral hemispheres
    • Severing this connection was used in the past to treat severe epilepsy
    • Hemispheres can function independently
    • Process different types of information
    • Left hemisphere: understanding language and producing speech
    • Right hemisphere: nonverbal memories, recognizing faces. and interpreting emotions

Cerebrum - Telencephalon

  • Basal nuclei
    • Involved in planning, learning, and fine-tuning movements
    • Complex circuitry to initiate or inhibit movements
    • Affected in Parkinson disease
    • People with disease have trouble initiating movement
  • Limbic system
    • Include the olfactory bulbs, amygdala, and hippocampus
    • Primarily involved in formation and expression of emotions
    • Role in learning, memory, and perception of smells
    • amygdala – understand and remember emotional situations, recognize emotional expressions in others
    • hippocampus – establish memories for spatial locations, facts, and sequences of events

Three Main Types of Neurons

  • Sensory Neurons: detect information from the outside world or internal body conditions
    • Afferent neurons – transmit to CNS
  • Motor Neurons: send signals away from CNS (efferent neurons) to elicit response
  • Interneurons or Association Neurons: form interconnections between other neurons

Reflex Arc

  • Stimulus from sensory neurons sent to CNS, little or no interpretation (few or no interneurons), signal transmitted to motor neurons to elicit response
  • Quick and automatic response