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