Neurons

• Nervous system


  Specialised cells that: Gather information about an organism's condition and external environment, Process the information, Respond by issuing commands to muscles and glands
• Sponges may well show us the beginning stages of the evolutionary development of a nervous system
• Neurons


  Receive and transmit signals
• Glial cells


  Support and nourish neurons
• Organisation of the nervous system


  1. Sensory Receptors

  2. Processing in the Central Nervous System (CNS)

  3. Motor Effectors in the Peripheral Nervous System (PNS)
• Central Nervous System (CNS)


  • Brain
  • Spinal cord
  • Processing
• Peripheral Nervous System (PNS)

  • Sensory nerves, sense organs
  • Motor systems
• Sensory (afferent) system


  Sensory neurons carry impulses from receptors to CNS
• Motor (efferent) system


  Motor neurons carry impulses from CNS to effector organs
• Somatic Nervous System (SNS)


  • Voluntary efferent neurons
  • Skeletal muscle stimulated
  • Cell bodies in brain stem
  • Axons synapse at skeletal muscle cells
• Autonomic Nervous System (ANS)


  • Involuntary efferent neurons
  • Visceral muscle, heart muscle, glandular tissue
  • Sympathetic
  • Parasympathetic
• Sympathetic


  • Fight or Flight
  • Response to stress / danger / embarrassment / emotion
  • Increases respiration, releases hormones
  • Decreases digestion
• Parasympathetic


  • Rest and Digest
  • Body is relaxed, resting or feeding
  • Reverses effects of Fight or Flight response
  • Decreases respiration
  • Increases digestion, waste elimination
• Enteric Nervous System (ENS)

  • A division of ANS
  • Regulates digestion
  • Receives signals from Sympathetic and Parasympathetic
  • Largely independent of CNS
  • Large numbers of neurons
  • "Second brain" / "Brain of the gut"
• Neuron


  Nerve cell that uses electrochemical signals to transmit information
• Neuron: Cell Body


  • Cytoplasm
  • Nucleated
  • Organelles
• Neuron: Dendrites


  • Extend from cell body
  • Short & branched
  • Receive signals from axons
  • Dendritic Spines increase surface area
• Neuron: Axon


  • Extends from cell body
  • 1 neuron - 1 axon
  • May branch
  • Diameter 1µm - 1mm
  • Up to 3m long
  • Electrical signal away from cell body
• Neuron: Axon Terminal


  • Connects at synapse to other nerve or effector cell
  • Often branched: one neuron can have many synapses
• Neuroglia


  Supporting cells that support neurons
• Neuroglia: Functions


  • Nutrient supply
  • Waste removal
  • Schwann cells (PNS)
  • Oligodendrocytes (CNS)
  • Produce myelin sheath
• Albert Einstein's brain had higher levels of neuroglial cells
• Neuron: Axon - Myelin Sheath


  • Wrap round axon several times
  • Myelinated / Unmyelinated
  • CNS: Myelinated = white matter, Non-myelinated = grey matter
  • PNS: Myelinated axons bundled together = nerves
• Neuron: Axons - Myelin Sheath - Nodes of Ranvier


  • Constrictions in myelin sheath at 1-2µm intervals
  • Protection
  • Insulation
  • Speeds up impulse transmission
• Meninges


  • Protective coverings of CNS
  • Dura mater - dense connective tissue, blood vessels, protection
  • Arachnoid mater - thinner, delicate, lines inside Dura mater
  • Pia mater - thin, delicate layer on outside of brain & spinal cord, blood vessels
• Cerebrospinal Fluid (CSF)


  • Formed from blood plasma
  • Fills space surrounding brain and spinal cord
  • Functions: Shock absorber, Maintains homeostasis, Waste product removal
• Special Senses


  Vision, taste, hearing, balance, smell
• Sensory Receptors


  Connected to afferent neurons
• Neurons - Classification


  • Sensory Neurons
  • Motor Neurons
  • Interneurons
• Afferent Neurons


  Carry information from receptors to CNS
• Efferent Neurons


  Carry information from CNS to effectors (muscles, glands)
• Interneurons


  Complex networks in CNS that integrate received information and direct body function
• Nerves


  • Collection of neurons in Peripheral Nervous System
  • Bundles of axons
  • Carry information between CNS and rest of body
• Nerves: Classification


  • Afferent Nerves - carry info to CNS
  • Efferent Nerves - carry info from CNS
  • Mixed Nerves - contain both afferent and efferent axons
• Cranial Nerves


  • Direct connection to the brain for sense organs, muscles of head/neck/shoulders, heart, GI tract
• Spinal Nerves


  • Carry sensory & motor signals between spinal cord and rest of body