Nervous Tissue

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Nervous Tissue
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Functions of the Nervous System

• Along with the endocrine system, the nervous
  system helps to maintain homeostasis in the body
• Homeostasis is the state of physiological balance
  that the body attempts to maintain
• The nervous system regulates body activities by
  responding rapidly using nerve impulses, or
  action potentials

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Functions of the Nervous System (continued)

• The endocrine system responds slowly by releasing
  chemical messengers called hormones
• The specific nervous system functions can be
  grouped as follows
• Sensory functiondetects stimuli, changes in the
  internal or external environment
• Integrative functionprocesses and analyzes
  sensory information so that an effective response
  can be made
• Motor functionresponds to the stimuli

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Organization of the Nervous System

• Central Nervous System (CNS)consists of the
  brain and spinal cord
• Peripheral Nervous System (PNS)includes all
  nervous tissue outside the CNS, such as cranial
  nerves (12 pair) and spinal nerves (31 pair)

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Organization of the Nervous System (continued)

• The PNS is subdivided further into the following
• Somatic Nervous System (SNS)consists of neurons
  that cause skeletal muscles to respond voluntary
• Autonomic Nervous System (ANS)consists of
  neurons that cause smooth muscle, cardiac muscle,
  and glands to respond involuntary
• Enteric Nervous System (ENS)sometimes included
  within the ANS consists of neurons that control
  responses within the digestive, or
  gastrointestinal tract involuntary

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Organization of the Nervous System (continued)

• The ANS is further subdivided into the following
• Sympathetic Divisioncontrols fight-or-flight
  responses
• Parasympathetic Divisioncontrols
  rest-and-digest responses

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Cells within the Nervous System

• Neuronslike muscle cells, they are excitable
  (can produce action potentials/nerve impulses in
  response to stimuli)
• Neurogliasupport, nourish, and protect the
  neurons

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Neurons

• Cell Bodycontains the nucleus, cytoplasm, and
  typical cellular organelles
• Dendritesmultiple processes that receive
  impulses and conduct them to the cell body
• Axonsusually single processes that carry
  impulses away from the cell body to an effector
  (another neuron, a muscle fiber, or a gland)

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Types of Neurons (Structural)

• Multipolar Neuronsusually have several dendrites
  and one axon most neurons in the brain and
  spinal cord are this type have many processes
• Bipolar Neuronshave one main dendrite and one
  axon found in the retina of the eye, the inner
  ear, and in the olfactory area of the brain have
  two processes
• Unipolar Neuronshave one main process

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Types of Neurons (continued)

• Myelinated neuronsneurons whose axons are
  surrounded by a myelin sheath, a multilayered
  lipid and protein covering the myelin insulates
  the axon and increases the speed of impulse
  conduction gaps in the myelin sheath are called
  nodes of Ranvier
• Unmyelinated neuronsneurons whose axons do not
  have a myelin sheath

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Types of Neurons (Functional)

• Sensory neuronsneurons that carry sensory
  information to the spinal cord and/or brain
  afferent neurons
• Interneuronsneurons within the CNS (brain and
  spinal cord) that process sensory information and
  decide the appropriate response most neurons in
  the body are of this type have short axons also
  called association neurons
• Motor neuronsneurons that carry response
  information from the brain and/or spinal cord to
  effectors efferent neurons

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A Reflex Arc

• The most basic conduction pathway through the
  nervous system is a reflex arc
• It consists of 5 components
• Receptor
• Sensory neuron
• CNS integrating center (usually consisting of one
  or more interneurons either in the brain or
  spinal cord)
• Motor neuron
• Effector

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Neuroglia

• Make up about half the volume of the CNS
• Nerve glue that holds nervous tissue together
• Generally smaller than neurons but more numerous
• Neuroglia are not able to transmit nervous
  impulses
• They can multiply and divide in the mature
  nervous system, though
• Most brain tumors involve neuroglia

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Types of Neuroglia

• Central Nervous System
• Astrocytes
• Oligodendrocytes
• Microglia
• Ependymal cells
• Peripheral Nervous System
• Schwann cells
• Satellite cells

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Astrocytes

• Star-shaped cells with many processes
• Help maintain appropriate chemical environment
  for the generation of nerve impulses nourish
  neurons take up excess neurotransmitters assist
  in brain development help form the blood-brain
  barrier

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Oligodendrocytes

• Smaller than astrocytes, with fewer processes
• Produce myelin sheaths within the CNS

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Microglia

• Small cells with few processes
• Protect CNS cells from disease by phagocytosis of
  the invading microbes
• Clean up dead or injured nerve tissue

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Ependymal Cells

• A single layer of epithelial cells
• Line the ventricles of the brain and the central
  canal of the spinal cord, spaces filled with
  cerebrospinal fluid
• Produce cerebrospinal fluid

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Schwann Cells

• Flattened cells that surround PNS axons
• Form myelin sheaths around axons of the PNS by
  wrapping around them many times
• Help in regeneration of PNS axons

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Satellite Cells

• Flattened cells arranged around the cell bodies
  of neurons in ganglia, groups of cell bodies
  outside of the CNS
• Support neurons in PNS ganglia

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Gray vs. White Matter

• Gray Matterregions of the brain and spinal cord
  that appear gray in color consists of neuronal
  cell bodies, dendrites, unmyelinated axons, and
  neuroglia
• White Matterregions of the brain and spinal cord
  that appear white in color consists of many
  myelinated axons and some unmyelinated axons

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Membrane Potentials

• The plasma membrane of each body cell has a
  membrane potential, a voltage difference that
  exists between the inside and the outside of the
  membrane due to various concentrations of charged
  atoms, or ions, in each location
• Muscle cells and neurons are excitable these
  cells have action potentials when responding to
  stimuli and resting potentials when not responding

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Membrane Potentials (continued)

• A membrane potential occurs partly because the
  plasma membrane contains ion channels (proteins)
  which allow various ions, or charged atoms, to
  pass into/out of a cell
• Cells vary in their number and types of ion
  channels
• Ion channels may be open at some times and
  closed at other times
• Ions move into/out of open ion channels by the
  process of diffusion
• In diffusion, substances move from an area of
  greater concentration to an area of lesser
  concentration
• Diffusion is a passive process, which means that
  the cell does not use ATP energy when it occurs

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Membrane Potentials (continued)

• In addition to ion channels, the plasma membranes
  of cells contain proteins that transport ions
  into/out of cells by active transport, which does
  require a cell to use ATP energy
• The most common example of active transport is
  the sodium-potassium pump, which consists of
  proteins located in the plasma membrane
• K ions are pumped into a cell and Na ions out
  of a cell against the concentration gradient
  (from lesser to greater concentration)

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Membrane Potentials (continued)

• Three Na ions are pumped out of the cell to
  every two K ions that are pumped in
• In addition, the cytoplasm of a cell contains
  many negatively charged anions, such as those
  attached to large proteins
• As a result, the cytoplasm just inside the plasma
  membrane of a cell is negative compared to the
  extracellular fluid just outside the membrane,
  which is positive
• In neurons, the typical value of the resting
  membrane potential is -70 millivolts (mV)

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Membrane Potentials (continued)

• A cell that has a membrane potential is said to
  be polarized most body cells are polarized, but
  the millivolt values vary
• The membrane potential of excitable cells
  (neurons and muscle cells) can change in response
  to a stimulus
• A stimulus may cause ion channels to open or
  close differently than under normal (resting)
  conditions

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Graded Potentials vs. Action Potentials

• A graded potential is a small change in membrane
  potential in a localized area of the plasma
  membrane
• An action potential is a change in membrane
  potential that propagates the length of the
  plasma membrane also called a nerve impulse

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Action Potentials

• The events of an action potential occur in two
  phases depolarization and repolarization
• During depolarization, the negative membrane
  potential decreases toward 0 mV and eventually
  becomes positive (about 30 mV)
• During repolarization, the resting membrane
  potential is restored (back to its negative value
  of -70 mV)

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Action Potentials (continued)

• During an action potential, ion channels open
  first to allow Na ions to move into the cell
• This causes depolarization as the membrane
  potential changes from -70 mV to 30 mV
• Next, ion channels open to allow K ions to move
  out of the cell
• This causes repolarization as the membrane
  potential changes back from 30 mV to -70 mV
• If K continues to flow out of the cell, the
  membrane potential may reach -90 mV and
  hyperpolarization occurs
• Eventually, the resting membrane potential is
  restored

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Action Potentials (continued)

• Neurons, like muscle cells, go through a
  refractory period after generating an action
  potential
• The refractory period is a short period of time
  during which an excitable cell cannot respond to
  a second stimulus

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All-or-None Principle

• Action potentials arise according to the
  all-or-none principle
• When depolarization reaches threshold level,
  which is about -55 mV, an action potential occurs
• The action potential is always the same size, no
  matter how strong the stimulus is
• The frequency of a stimulus is the main factor
  that varies in the conduction of a nerve impulse

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Continuous vs. Saltatory Conduction

• Continuous conductionthe step-by-step
  depolarization and repolarization of the plasma
  membrane described previously occurs in muscle
  fibers and unmyelinated axons
• Saltatory conductiona special type of nerve
  impulse conduction that occurs in myelinated
  axons the impulse leaps from one node of Ranvier
  to another, resulting in much faster conduction
  of an impulse

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Speed of Conduction

• Myelinated axons conduct impulses faster than
  unmyelinated axons
• Axons with large diameters conduct impulses
  faster than axons with small diameters

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Transmission at Synapses

• When an action potential is propagated from a
  neuron to a skeletal muscle cell, the signal must
  be transmitted across a type of synapse called a
  neuromuscular junction
• In this situation, the impulse is transmitted
  electrically along an axon and chemically
  across a synapse (because of the neurotransmitter
  acetylcholine)

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Transmission at Synapses (continued)

• As an action potential is transmitted from one
  neuron to another, it usually must cross a
  different type of synapse, but the basic process
  is similar to that involving the neuromuscular
  junction
• The neuron that sends the impulse is called the
  presynaptic neuron the neuron that receives the
  impulse is called the postsynaptic neuron
• The two neurons are separated by a synaptic
  cleft, which is a tiny space filled with
  interstitial fluid

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Transmission at Synapses (continued)

• When a nerve impulse arrives at a synaptic end
  bulb of a presynaptic neuron, ion channels in
  that neurons plasma membrane open and allow Ca2
  ions to flow into the cell
• This increase in Ca2 in the presynaptic neuron
  triggers exocytosis of some of the synaptic
  vesicles, causing the release of neurotransmitter
  molecules into the cleft each vesicle contains
  thousands of molecules of neurotransmitter

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Transmission at Synapses (continued)

• The neurotransmitter molecules diffuse across the
  cleft and bind to receptors in the postsynaptic
  neurons plasma membrane
• This opens ion channels in the postsynaptic
  neuron and allows specific ions to move into that
  cell from the synaptic cleft

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Transmission at Synapses (continued)

• As the ions flow into the postsynaptic neuron,
  the membrane potential there changes
• If threshold level (-55 mV) is reached,
  depolarization occurs hyperpolarization may also
  occur
• The neurotransmitter is then removed from the
  synaptic cleft, so that it will not continuously
  influence the postsynaptic neuron
• This occurs through the action of enzymes,
  reuptake by the neuron that released the
  substance, or by the diffusion of the
  neurotransmitter away from the cleft

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Excitatory and Inhibitory Postsynaptic Potentials

• If a neurotransmitter depolarizes the
  postsynaptic membrane, an excitatory postsynaptic
  potential (EPSP) results
• If a neurotransmitter hyperpolarizes the
  postsynaptic membrane, an inhibitory postsynaptic
  potential (IPSP) results

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Summation of Postsynaptic Potentials

• Most CNS neurons receive input from many synapses
• Integration of these inputs is called summation
• When summation results from the buildup of
  neurotransmitter released by several presynaptic
  end bulbs at the same time, it is called spatial
  summation
• When summation results from the buildup of
  neurotransmitter released by a single presynaptic
  end bulb two or more times in rapid succession,
  it is called temporal summation

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Summation (continued)

• A single postsynaptic neuron receives both
  excitatory and inhibitory information at the same
  time
• If the total summation effect is excitatory and
  threshold level is reached, an EPSP results and
  creates a nerve impulse
• If the total summation effect is inhibitory, an
  IPSP results and no impulse is generated
• If the total summation effect is excitatory but
  threshold level is not reached, an EPSP results
  however, an impulse is not generated until a
  subsequent stimulus causes depolarization

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Neurotransmitters

• Many substances act as neurotransmitters
• Neurotransmitters can be excitatory or inhibitory
• A neurotransmitter can even be excitatory in some
  locations and inhibitory in others
• Examples include the following
• Acetylcholine (Ach)
• Gamma aminobutyric acid (GABA)inhibitory
• Norepinephrine (NE)
• Epinephrine
• Dopamine (DA)
• Serotonin
• Nitric oxide (NO)
• Endorphinsthe bodys natural painkillers

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Regeneration and Repair of Nervous Tissue

• Human neurons have very limited ability to repair
  themselves or regenerate
• In the PNS, repair may generally occur if the
  cell body and Schwann cells are not damaged
• In the CNS, little or no repair may occur