chapter 12 introduction to the nervous system
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Chapter 12 – Introduction to the Nervous System. Organization Cell Types. Review. What 3 parts make up the nervous system? Brain Spinal cord Nerves. Functions of the Nervous System.

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What 3 parts make up the nervous system?

  • Brain
  • Spinal cord
  • Nerves
functions of the nervous system
Functions of the Nervous System
  • Detect changes (stimuli) in the internal or external environment
  • Evaluate the information
  • Initiate a change in muscles or glands

Goal – maintain homeostasis

What does this remind you of??

organization of the nervous system
Organization of the Nervous System
  • Central nervous system (CNS)
    • Brain and spinal cord
  • Peripheral nervous system (PNS)
    • Nervous tissue in the outer regions of the nervous system
    • Cranial nerves: originates in the brain
    • Spinal nerves : originates from the spinal cord
    • Central fibers: extend from cell body towards the CNS
    • Peripheral fibers: extend from cell body away from CNS
afferent vs efferent
Afferent vs Efferent

Nervous pathways are organized into division based on the direction they carry information

  • Afferent division: incoming information (sensory)
  • Efferent division: outgoing information (motor)

(Efferent = Exit)

somatic autonomic nervous systems
Somatic & Autonomic Nervous Systems

Nervous pathways are also organized according to the type of effectors (organs) they regulate

  • Somatic nervous system (SNS)
    • Somatic sensory division (afferent)
    • Somatic motor division (efferent)
somatic autonomic nervous systems cont
Somatic & Autonomic Nervous Systems cont…
  • Autonomic nervous system (ANS): Carry information to the autonomic or visceral effectors (smooth & cardiac muscles and glands)
    • Visceral sensory division (afferent)
    • Efferent pathways
      • Sympathetic division – “fight or flight”
      • Parasympathic division – “rest and repair”

What are the two main cell types in the nervous system?

(Hint: we talked about this when we covered tissue types)

Answer: neurons and glia

cells of the nervous system
Cells of the Nervous System

Neurons: excitable cells that conduct information

Glia (also neuroglia or glial cells): support cells, do not conduct information

  • Most numerous
  • Glia = glue
types of glia
Types of Glia

Five major types:

  • Astrocytes
  • Microglia
  • Ependymal cells
  • Oligodendrocytes
  • Schwann cells
astrocytes 12 3a
Astrocytes (12-3A)
  • Star-shaped, largest, most numerous
  • Cell extension connect neurons and capillaries
    • Transfer nutrients from blood to neuron
    • Help form blood-brain barrier (BBB)

blood brain barrier
Blood-Brain Barrier
  • Helps maintain stable environment for normal brain function
  • “feet” of astrocytes wrap around capillaries in brain
  • Regulates passage of ions
  • Water, oxygen, CO2, glucose and alcohol pass freely
  • Important for drug research
    • Parkinson’s Disease
microglia 12 3b
Microglia (12-3B)
  • Engulf and destroy cellular debris (phagocytosis)
  • Enlarge during times of inflammation and degeneration
ependymal cells 12 3c
Ependymal cells (12-3C)
  • Similar to epithelial cells
  • Forms thin sheets that line the fluid-filled cavities of the brain and spinal cord
  • Some cells help produce the fluid that fills these cavities (cerebral spinal fluid - CSF)
  • Cilia may be present to help circulate fluid

oligodendrocytes 12 3d
Oligodendrocytes (12-3D)
  • Hold nerve fibers together
  • Produce myelin sheaths in CNS

multiple sclerosis ms
Multiple Sclerosis (MS)
  • Most common myelin disorder
  • Characterized by:
    • myelin loss and destruction  injury and death  plaque like lesions
    • Impaired nerve conduction  weakness, loss of coordination, vision and speech problems
    • Remissions & relapses
  • Autoimmune or viral infection
  • Women 20-40 yrs
  • No known cure
multiple sclerosis ms1
Multiple Sclerosis (MS)

schwann cells 12 3e
Schwann cells (12-3E)
  • Only in PNS
  • Support nerve fibers & form myelin sheaths
  • Satellite cells (12-3G)
    • Types of schwann cell that covers a neuron’s cell body

All neurons have 3 parts:

  • Cell body (soma)
  • Axon
  • One or more dendrites
neuron anatomy
Neuron Anatomy
  • Soma resembles other cells
  • Nissl bodies – part of rough ER; contain proteins necessary for nerve signal transmission & nerve regeneration
  • Dendrites – branch out from soma; receptors; conduct impulse towards soma
  • Axon – process that extends from the soma at a tapered portion called the axon hillock
    • Axon collaterals: side branches
    • Telodendria: distal branches of axon
    • Synaptic knob: ends of telodendria
neuron anatomy1
Neuron Anatomy
  • Myelin sheaths: areas of insulation produced by Schwann cells; increases speed of nerve impulse
    • Myelinated = white matter
    • Unmyelinated = gray matter
  • Nodes of Ranvier: breaks in myelin sheath btwn Schwann cells
  • Synapse: junction btwn two neurons or btwn a neuron and an effector
structural classification of neurons
Structural Classification of Neurons
  • Multipolar
    • One axon, several dendrites
    • Most numerous
  • Bipolar
    • One axon, one dendrite
    • Least numerous
    • Retina, inner ear, olfactory pathway
  • Unipolar
    • Axon is a single process that branches into a central process (towards CNS) and a peripheral process (towards PNS)
    • Dendrites at distal end of peripheral process
    • Always sensory neurons

functional classification of neurons
Functional Classification of Neurons
  • Afferent
    • Sensory
    • Towards CNS
  • Efferent
    • Motor
    • Towards muscles & glands
  • Interneurons
    • Connect afferent & efferent neurons
    • Lie within CNS
examples of reflex arcs
Examples of Reflex Arcs
  • Ipsilateral
  • Contralateral
  • intersegmental
nerves vs tracts
Nerves vs Tracts
  • Nerves – bundles of parallel neurons held together by fibrous CT in the PNS
  • Tracts – bundles of parallel neurons in the CNS
nerve fibers
Nerve Fibers
  • Remember the difference between nerves and tracts?
    • Endoneurium: surrounds each nerve fiber
    • Perineurium: surrounds fascicles (bundles of nerve fibers
    • Epineurium: surrounds a complete nerve (PNS) or tract (CNS)
review gray vs white matter
Review: Gray vs White Matter
  • White matter – myelinated nerve fibers
    • Myelin sheaths help increase the speed of an action potential
  • Gray matter – unmyelinated nerve fibers & cell bodies
    • Ganglia: regions of gray matter in PNS
nerve fiber repair
Nerve Fiber Repair
  • Nervous tissue has a limited repair capacity b/c mature neurons are incapable of cell division
  • Repair can take place if soma and neurilemma remain intact
steps of nerve fiber repair
Steps of Nerve Fiber Repair
  • Injury
  • Distal axon and myelin sheaths degenerates
  • Remaining neurilemma & endoneurium forms a “tunnel” from the injury to the effector
  • Proteins produced in the nissl bodies help extend a new axon down the tunnel to the effector
nerve impulses
Nerve Impulses
  • Neurons are specialized to initiate and conduct signals  nerve impulses
    • Exhibit excitability & conductivity
    • Nerve impulse wave of electrical fluctuation that travels along the plasma membrane
membrane potentials
Membrane Potentials
  • Difference in charges across the plasma membrane
    • Inside slightly negative
    • Outside slightly positive
  • Result in a difference in electrical charges  membrane potential
    • Stored potential energy
    • Analogy = water behind a dam
membrane potentials1
Membrane Potentials
  • Membrane potential creates a polarized membrane
    • Membrane has – pole & + pole
  • Potential difference of a polarized membrane is measured in millivolts (mV)
    • The sign indicates the charge of the inside of a polarized membrane
resting membrane potential rmp
Resting Membrane Potential (RMP)
  • When not conducting electrical signals, a membrane is “resting”
    • -70mV
  • RMP maintained by ionic imbalance across membrane
    • Sodium-Potassium Pump
      • Pumps 3 Na+ out for every 2 K+ pumps in
      • Creates an electrical gradient (more positive on outside)
local potential
Local Potential
  • Local potential - The slight shift away from the RMP
    • Isolated to a particular region of the plasma membrane
  • Stimulus-gated Na+ channels open  Na+ enters  membrane potential to moves closer to zero (depolarization)
  • Stimulus-gated K+ channels open  K+ exits  membrane potential away from zero (hyperpolarization)
  • **Local potentials do not spread to the end of the axon**
action potentials
Action Potentials


  • Membrane potential of an active neuron (one that is conducting an impulse
  • Action potential = nerve impulse
  • An electrical fluctuation that travels along the plasma membrane
steps of producing an action potential table 12 1
Steps of Producing an Action Potential (table 12-1)
  • A stimulus triggers stimulus-gated Na+ channels to open  Na+ diffuses inside the cell  depolarization
  • Threshold potential is reached (-59mV)  voltage-gated Na+ channels open  depolarization continues
  • Action potential peaks at +30mV, voltage-gated Na+ channels close
  • Voltage-gated K+ channels open  K+ diffuses outward  repolarization
  • Brief period of hyperpolarization (below -70mV)  RMP is restored by Na+/K+ pump
refractory period1
Refractory Period
  • Period of time where the neuron resists restimulation (AP cannot fire)
    • Absolute refractory period: half a millisecond after membrane reaches threshold potential
      • Will not respond to ANY stimulus
    • Relative refractory period: few milliseconds after absolute refractory period (during repolarization)
      • Only respond to VERY strong stimulus
refractory period what does this mean
Refractory Period – What does this mean?
  • Greater stimulus = quicker another action potential can take place
  • The magnitude of the stimulus does not affect the magnitude of the AP
    • b/c APs are “all or nothing”
    • Does cause proportional increase in frequencies of impulses
conduction of an action potential
Conduction of an Action Potential
  • During the peak of an AP, the polarity reverses
    • Negative outside, positive inside
    • Causes impulse to travel from site of AP to adjacent plasma membrane
    • No fluctuation in AP due to “all or nothing” principle
    • AP cannot travel backwards on axon due to refractory periods
conduction of an action potential1
Conduction of an Action Potential

How does myelin sheaths affect the speed of an action potential?

  • Sheaths prevent movement of ions
  • Electrical changes can only take place at Nodes of Ranvier
  • APs “leap” from node to node (current flows under sheaths)
  • Saltatory conduction
random facts
Random Facts
  • In nerve fibers that innervate skeletal muscle, impulses travel up to 130 m/s (300 mph)
  • Sensory pathways from skin  0.5 m/s (<1 mph)
  • Many anesthetics block the sensation of pain by inhibiting opening of Na+ channels
types of synapses
Types of Synapses

Electrical synapses: two cells joined end to end by gap junctions

Ex: btwn cardiac muscle cells, smooth muscles cells

types of synapses1
Types of Synapses

Chemical synapses: use neurotransmitter to send a signal from a presynaptic cell to postsynaptic cell

  • 3 Parts:
  • Synaptic knob
  • Synaptic cleft
  • Plasma membrane of postsynaptic neuron
mechanisms of synaptic transmission
Mechanisms of Synaptic Transmission
  • AP depolarizes synaptic knob
  • Voltage-gated Ca2+ channels open  Ca2+ diffuses inside the cell
  • Ca2+ triggers exocytosis of neurotransmitter vesicles
  • NTs diffuses across synaptic cleft  bind w/ receptors on postsynaptic cell
postsynaptic potentials fig 12 22
Postsynaptic Potentials (Fig 12-22)
  • Excitatory NTs cause Na+ and K+ channels to open  depolarization  excitatory postsynaptic potential (EPSP)
  • Inhibitory NTs cause K+ and Cl- channels to open  hyperpolarization  inhibitory postsynaptic potential (IPSP)
  • For every postsynaptic cell there are usually 1K-100K synaptic knobs
  • Both excitatory & inhibitory NTs are released
    • Summation of local potentials (EPSP & IPSP) occur at axon hillock
      • EPSP > IPSP  reach threshold  action potential
      • EPSP < IPSP  threshold not reached  no AP

Small-Molecule Transmitters:

  • Acetylcholine
  • Amines
    • Serotonin
    • Dopamine
    • Epinephrine
    • Norepinephrine
  • Amino Acids
    • Glutamate
    • GABA
    • Glycine

Large-Molecule Transmitters:

  • Neuropeptide
    • Endorphins