Nervous System

• Nervous system


  Controls all physiological and psychological reactions, even the endocrine system
• The nervous system is the most distinctive feature of the human species
• Principal functions of the nervous system

  • Sensory input
  • Integration
  • Motor output
• Central nervous system


  Brain and spinal cord - the main control center
• Peripheral nervous system


  Nerves that branch off from the brain and spine to communicate with the rest of the body
• Divisions of the peripheral nervous system

  • Sensory (afferent) division
  • Motor (efferent) division
• Components of the motor (efferent) division

  • Somatic (voluntary) nervous system
  • Autonomic (involuntary) nervous system
• Divisions of the autonomic nervous system


  • Sympathetic division
  • Parasympathetic division
• Nervous tissue


  Densely packed with cells, less than 20% extracellular space
• Neurons


  Nerve cells that respond to stimuli and transmit signals
• Neuroglia/Glial cells


  Cells that surround and support neurons, make up about half the mass of the brain
• Types of glial cells

  • Astrocytes
  • Microglial cells
  • Ependymal cells
  • Oligodendrocytes
  • Satellite cells
  • Schwann cells
• Neurons


  • Some of the longest-lived cells in the body
  • Irreplaceable - cannot divide once they take on their roles
  • Have a high metabolic rate and require abundant glucose and oxygen
• Types of neurons by number of processes

  • Multipolar (3+ processes)
  • Bipolar (2 processes)
  • Unipolar (1 process)
• Neuron types by function

  • Sensory (afferent)
  • Motor (efferent)
  • Interneurons (association)
• Nervous system response to a spider on the knee

  1. Sensory neurons in the skin detect the spider

  2. Signal travels to the spinal cord and brain

  3. Interneurons process the signal and trigger a response (kick leg, scream, remain calm)
• Action potential


  The electrical impulse that zips down a neuron's axon to its neighboring neurons
• Action potential


  • It only transmits at one uniform strength and speed
  • The frequency or number of pulses can vary
• Neuron


  A cell that can fire an electrical impulse (action potential)
• Voltage


  The measure of potential energy generated by separated charges, measured in millivolts
• Membrane potential


  The difference in charge between the inside and outside of a cell
• Resting membrane potential


  The membrane potential of a neuron at rest, around -70 millivolts
• How neurons maintain their resting membrane potential


  1. Sodium-potassium pump pumps out 3 sodium ions for every 2 potassium ions pumped in

  2. Creates an electrochemical gradient
• Ion channels


  Proteins in the cell membrane that allow the passage of ions
• Graded potential


  A small, localized change in membrane potential
• Action potential threshold


  The membrane potential of around -55 millivolts that triggers an action potential
• Generation of an action potential

  1. Stimulus depolarizes membrane to threshold

  2. Voltage-gated sodium channels open

  3. Massive influx of sodium ions

  4. Membrane potential briefly goes positive

  5. Voltage-gated potassium channels open to repolarize membrane

  6. Refractory period where neuron cannot fire again
• Action potentials


  • Always have the same initial threshold and peak amplitude
  • Vary in frequency to encode information
• Conduction velocity


  The speed at which an action potential travels down an axon
• Myelinated axons


  • Allow faster conduction velocity through saltatory conduction
• The synapse is the meeting point between two neurons, and is 1000 times thinner than a piece of paper, and more numerous in the human body than grains of sand on a beach
• Synapse


  A junction or crossroads where an action potential sends an electrical message to the end of an axon, which then translates or converts it into a different type of signal and sends it to another neuron
• The human brain has 100 billion neurons, each with 1000 to 10,000 synapses, resulting in 100 to 1,000 trillion synapses in the brain
• Synapses


  • They are like tiny computers, capable of running different programs simultaneously and able to change and adapt in response to neuron firing patterns, strengthening or weakening over time depending on usage
• Electrical synapses


  Use ion currents flowing directly from the cytoplasm of one nerve cell to another through small windows called gap junctions, allowing for fast, synchronous signaling
• Chemical synapses


  Use neurotransmitters that diffuse across a synaptic gap to deliver messages, allowing for more control and selectivity in signaling
• Chemical synapse signaling

  1. Action potential activates voltage-gated calcium channels in presynaptic terminal

  2. Calcium influx causes synaptic vesicles to release neurotransmitters into synaptic cleft

  3. Neurotransmitters bind to receptors on postsynaptic neuron

  4. This opens ion channels, converting chemical signal back to electrical
• Excitatory neurotransmitters


  Depolarize the postsynaptic neuron, bringing it closer to action potential threshold and making it more likely to fire
• Inhibitory neurotransmitters


  Hyperpolarize the postsynaptic neuron, driving its charge away from the threshold and making it less likely to fire
• Neurotransmitters don't stay bonded to receptors for long, and are either recycled, broken down, or diffuse back across the synapse