The Brain Has Been Called a 3 Pound Universe

• The brain weighs about 3 lbs (~1400 gm)
• All sensation and consciousness originates in the brain
• We know a lot about the anatomy, chemistry & electricity of the brain, but almost nothing about what sensations are

The Nervous System Has Peripheral and Central Units

• The central nervous system (CNS) is the brain and spinal column
• The peripheral nervous system (PNS) consists of nerves outside of the CNS
• There are 31 pairs of spinal nerves (mixed motor & sensory)
• There are 12 pairs of cranial nerves (some are pure sensory, but most are mixed)
• The pattern of innervation plotted on the skin is called a dermatome

The Central Nervous System Has Several Patterns of Organization

• Sensory/motor:
  • Sensory nerves enter the spinal cord by the dorsal root, their cell bodies are in ganglia outside the spinal cord (dorsal root ganglia)
    • Sensory nerves, which go to the brain, are referred to as afferent
  • Motor nerves leave the spinal cord by the ventral root
    • Motor nerves, which come from the brain, are referred to as efferent
      
• White matter/gray matter:
  • White matter consists of the myelinated axons of nerves, usually going up and down
  • Gray matter contains the cell bodies (containing the nucleus) , dendrites with synapses and blood vessels
  • In both the spinal cord and brain cell bodies are clustered into ganglia and nuclei

• Decussations & commisures:
  • There is a tendency for sensory and motor nerves and association fibers to cross from one side of the brain to the other
  • Most motor nerves cross in the medulla oblongata; a few do not cross
  • Most sensory nerves also cross in the medulla (touch, pressure, proprioception); others cross in the spinal cord (pain, temperature)
  • The corpus callosum is a major tract connecting the left and right hemispheres of the brain

Spinal Cord Injuries Disconnect Parts of the Body from the Brain

• Cutting the spinal cord "disconnects" the body from the brain below the cut
  • Sensory impulses from areas below cut cannot reach the brain: loss of sensation
  • Motor impulses from the brain cannot reach muscles whose nerves are below the cut: voluntary control of these muscles is lost (paralysis)
• After the spinal cord recovers from the trauma of the wound, reflex contraction of muscles below the cut reappears
  • Reflex activity may be more vigorous than in a normal person
  • Nerves from brain control the sensitivity of spinal reflexes
• If the spinal cord is cut above cervical nerves 3, 4 & 5 respiration will be lost
  • The respiratory rhythm is generated in the brain and is sent to the diaphragm through the phrenic nerve
  • Loss of the phrenic nerve will cause death unless artificial respiration is used

Glial Cells Support the Brain in Many Ways

• About 50% of the weight of the brain is glia cells (several types)
• Glial cells do not conduct: support brain in other ways
• Some glial cells produce the myelin sheaths of nerves (oligodendroglia, Schwann cells)
• Other glial cells secrete cerebrospinal fluid, defend against bacteria and regulate ions

The Brain is Immersed in Cerebrospinal Fluid

• The brain is a hollow tube with bulges: has 4 interconnected fluid-filled reservoirs (ventricles) filled with cerebrospinal fluid (CSF)
• Total volume ~150 mL
• CSF circulates: secreted into ventricles by the choroid plexus (about 500 mL/day)-> passes from 4th ventricle into subarachnoid space -> then absorbed into veins
• Some CSF passes into central canal of spinal cord
• Brain floats in CSF; acts as a cushion and reduces injury

There is a Special Barrier Between the Brain and the Blood

• There is a "blood brain barrier"- brain capillaries are tighter and less permeable than those in the rest of the body; protects brain from many chemicals and bacteria
• Hydrophobic compounds cross the blood brain barrier more readily than hydrophilic ones
  • Example: people with Parkinson's disease have low levels of the neurotransmitter, dopamine (DOPA)
  • DOPA is too hydrophilic to cross the blood brain barrier
  • Parkinson's patients are treated with levadopa (L-DOPA) instead
  • L-DOPA is hydrophobic and crosses the blood brain barrier
  • Brain later converts L-DOPA into DOPA

The Brain is Covered by Tough Meninges and Protected by Bone

• Three tough membranes cover the brain:
  • 1) dura mater: outermost- firmly attached to skull
  • 2) arachnoid: middle layer
  • 3) pia mater: bottom layer, firmly attached to brain, contains many blood vessels
• Inflammation of brain meninges = meningitis

The Brain Has a Hierarchical Organization

• Upper centers control lower centers
• Cortex tends to dominate the spinal cord
• But many basic life support functions (i.e., respiration, blood pressure) are under control of "lower" centers

The Brain Uses Energy at a Rate of 10 Watts

• The brain functions at about 10 watts - this is equivalent to a "dim bulb"
• But 10 watts is a fairly high proportion of the total body energy consumption rate of 80 watts
  • Brain is 2% of body weight but uses 12% of body energy
  • 14% of the blood flow goes to the brain
  • The blood flow per kilogram is equal to that of a muscle doing heavy exercise
• Thinking does not significantly raise the energy consumption of the whole brain, but circulation increases to specific areas being used (this can be seen with PET scans); presumably these areas have higher levels of energy use
  
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