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”

Figure 12-2



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

Reflex arc
Reflex Arc

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)

Resting membrane potential rmp1
Resting Membrane Potential (RMP)

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**

Local potentials
Local Potentials

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 period
Refractory Period

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