Peripheral Nervous System (PNS)

Somatic Nervous System

Nerves connecting to voluntary skeletal muscles and sensory receptors.

There are two types of nerve fibers that carries information:

  • Afferent Nerve Fibers (incoming): Axons that carry info away from periphery to the CNS.

  • Efferent Nerve Fibers (outgoing): Axons that carry info from the CNS outward to the periphery.

Automatic Nervous System

Nerves connecting to the heart, blood vessels, smooth muscles, and glands.

Central Nervous System (CNS)

Spinal Cord

  • Local feedback loops: control reflexes (“reflex arcs”);

  • Descending motor control signals from the brain active spinal motor neurons.

  • Ascending sensory axons convey sensory information from muscles and skin back to the _brain.


  • The Hindbrain:
    • Medulla Oblongata: controls breathing, muscle tone and blood pressure.
    • Pons: Connected to the cerebellum & involved in sleep and arousal.
    • Cerebellum: Coordination and timing of voluntary movements, sense of equilibrium, language, attention …
  • Midbrain: Eye movements, visual and auditory reflexes.

  • Reticular Formation: Modulates muscle reflexes, breathing & pain perception. Also regulates sleep, wakefulness & arousal.

  • Thalamus: “Relay station” for all sensory info (except smell) to the cortex, regulates sleep/wakefulness.

  • Hypothalamus: Regulates basic needs _F_ighting, _F_leeing, _F_eeding, and _M_ating.

  • The Cerebrum: Involved in perception and motor control, cognitive functions, emotion, memory, and learning
    • Cerebral cortex: convoluted surface of cerebrum, about 1/8th of an inch thick. Approximately 30 billion neurons; each neuron makes about 10,000 synapses, approximately 300 trillion connections in total. Consists six layers of neurons, they have relatively uniform in structure.

  • basal ganglia
  • hippocampus
  • amygdala

Neural V.S. Digital Computing

  • Device count:
    • Brain: \(10^{11}\) neurons (each neuron \(\sim 10^{4}\) connections)
    • Silicon Chip: \(10^{11}\) transistors with sparse connectivity
  • Device speed:
    • Biology has \(100\mu s\) temporal resolution
    • Digital circuits are approaching a \(100 ps\) clock (\(10 GHz\))
  • Computing paradigm:
    • Brain: massively parallel computation & adaptive connectivity
    • Digital computers: sequential information processing via CPUs with fixed connectivity
  • Capabilities:
    • Brain: better at solving ill-posed problems (speech, vision)
    • Digital computers: excel in math & symbol processing …