Organisation of the Nervous System

• What makes up the nervous system

  • PNS:
  • Cranial nerves
  • Spinal nerves
  • Ganglia
  • CNS:
  • Brain
  • Spinal cord
• Structure of neurons


  • Cell body
  • Dendrite
  • Axon
  • Myelin sheath
  • Schwann cells
  • Oligodendrocytes
• Nodes of Ranvier


  • Important in the propagation of the nerve impulse
• Neurons are classified according to

  • Type of information carried
  • Size
  • Degree of myelination
  • Resultant speed of conduction of impulses along the nerve
• Large diameter axons which are myelinated


  These axons have the highest conduction velocity
• Small diameter axons which are myelinated

  These axons have an intermediate conduction velocity
• Small diameter axons which are unmyelinated


  These axons have low conduction velocity
• White matter


  • Bundles of axons each coated with a sheath of myelin
• In the spinal cord, the white matter is at the surface, the grey matter inside. The opposite is true for the brain
• Components of the meninges

  • Dura mater
  • Arachnoid mater
  • Pia mater
• Dura mater


  Pressed against the bony surface of the interior of the vertebrae and the cranium
• Arachnoid mater


  Region between the arachnoid mater and pia mater is filled with cerebrospinal fluid
• Pia mater


  Innermost layer of the meninges
• Divisions of the peripheral nervous system

  • Somatic nervous system
  • Autonomic nervous system
  • Enteric nervous system
• Types of axons in the peripheral nervous system

  • Afferent – axons carry information into the CNS
  • Efferent – axons carry information away from the CNS
• Enteric nervous system


  Affects smooth muscles, glands and endocrine cells of the GI tract, the function is purely digestive
• Autonomic nervous system


  • Major function = homeostasis
  • Faster acting then the endocrine system
• Divisions of the autonomic nervous system


  • Sympathetic
  • Parasympathetic
• Sympathetic nervous system


  • Responds to danger/stress
  • Maintains blood pressure
  • Increases blood flow to active muscles
  • Increases heartbeat
  • Dilates bronchioles
• Parasympathetic nervous system


  • Sedentary state/voiding activities
  • Constriction of the pupil
  • Slowing of the heart
  • Dilation of blood vessels
  • Stimulation of the digestive system
• Motor control theory

  1. Sensory receptor – responds to a stimulus by producing a generator or receptor potential

  2. Sensory neuron – axon conducts impulses from the receptor to the integrating centre

  3. Integrating centre – one or more regions within the CNS that relay impulses from the sensory to motor neurons

  4. Motor neurons – axon conducts impulses from the integrating centre to the effector

  5. Effector – muscle or gland that responds to motor impulses
• Sensory receptors in the skin

  • Hair follicle endings
  • Merkel cells or disks
  • Ruffini ending
  • Meissner's corpuscles
  • Free nerve endings
• Sensory receptors in the muscle

  • Golgi tendon organs
  • Muscle spindles
  • Free nerve endings
• Sensory receptors in the joint/ligament

  • Ruffini ending
  • Tendon organ like receptors
  • Pacinian corpuscle
• Nociceptor


  Responds to pain, heat, cold
• Pacinian

  Responds to vibration and deep pressure
• Merkel disc


  Responds to light touch
• Functions of the brain

  • Conducts sensory (afferent) information from the peripheral nervous system to the brain
  • Conducts motor (efferent) information from the brain to other various effectors
• Effectors

  • Skeletal muscles
  • Cardiac muscle
  • Smooth muscle
  • Glands
• Dorsal column


  Carries info from the skin receptors, joint and muscle receptors to the cortex
• Sections of the spinothalamic tract

  • Lateral spinothalamic tract – carries pain and temperature sensations
  • Anterior spinothalamic tract – itch, tickle, pressure, vibrations and crude sensations
• Sections of the spinocerebellar tract

  • Posterior spinocerebellar tract
  • Anterior spinocerebellar tract
• Spinocerebellar tract


  Carries proprioceptive information from upper or lower limb from joints, tendons and ligaments
• Types of output neurons

  • Motor neurons – receive signals from the brain and the spinal cord, stimulate muscles and some glands
  • Interneurons – connect neurons to other neurons within the brain and spinal cord
• Sections of the corticospinal tract

  • Lateral cortico-spinal tract – carries information essential for control of the extremities
  • Anterior cortico-spinal tract – carries information essential for control of the axial skeleton
• Neuromuscular junction


  The nervous connection between a motor neuron and a muscle
• Electrical transmission


  1. Cell membranes of voltage and ion gated channels

  2. The normal voltage of an ion is -70mV

  3. As the voltage of the ion increases the sodium channels open because of this the voltage becomes more positive

  4. When a threshold of -50mV is met the sodium ions are released from the channels

  5. Because of this there is a huge influx of sodium ions into the cell, making the ion negative

  6. The sodium gated channels then close, causing the potassium gated channels to open

  7. This makes the cell negative

  8. The neuron then returns to its resting state
• Propagation of action potential in unmyelinated axons

  1. AP generated at the axon hillock

  2. AP moves along axon away from the hillock

  3. The signal continues in the same direction until it reaches the synaptic end plate (terminal)
• Propagation of action potential in myelinated axons

  1. AP generated at the axon hillock

  2. AP moves along axon away from the hillock

  3. Myelin acts as an insulator so the signal and as such the signal jumps to the next part of the axon that has no myelin sheath (Node of Ranvier)

  4. This is called Saltatory Conduction and this leads to the signal travelling down the axon at a much quicker speed

  5. This also uses less ATP to reset the smaller areas of the cell
• Mechanisms of reaching threshold

  1. Spatial summation – many short signals arrive at multiple dendrites

  2. Temporal summation – series of signals arrive at one dendrite