Ch. 12 The Nervous System

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Ch. 12The Nervous System
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Nervous System Functions

• The nervous system takes information in,
  integrates the information, and sends information
  out.
• Sensory function designed to detect both
  internal and external stimuli.
• Internal maintaining homeostasis
• Heart rate, blood pressure, body temperature,
  blood sugar, blood oxygen, etc.
• External
• Smell, touch, amount of light, noise levels, etc.

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Nervous System Functions

• Integrative function designed to process sensory
  information and use it in the decision making
  process.
• Brain determines what information is important
  and unimportant for maintaining homeostasis and
  responding to stimuli.
• This interpretation differs from person to
  person.

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Nervous System Functions

• Motor function designed to initiate skeletal
  muscle contractions and influence autonomic
  responses in the body.
• Can be voluntary or involuntary
• Voluntary- picking up a pencil, walking, raising
  your hand, etc.
• Involuntary- function to maintain homeostasis or
  prevent injury
• regulating heart rate and respiration rate
• reflexes to prevent overstretching a muscle

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Nervous System Organization

• Because of the massive size of the body, the
  nervous system is broken down into smaller
  sections to help with the workload.

Nervous System
Peripheral Nervous System
Central Nervous System
Enteric
Somatic
Autonomic
Parasympathetic
Sympathetic
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Central Nervous System (CNS)

• Consists of the brain and the spinal cord.
• Brain found within the cranial cavity.
• Functions to process incoming information and to
  make you who you are (personality, emotion,
  judgement, memories).
• Spinal cord found within the vertebral column.
• Functions as a bridge between your body and
  brain. It is also the center for reflexes
  (protective mechanisms).

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Peripheral Nervous System (PNS)

• Consists of the cranial nerves and the spinal
  nerves.
• Cranial nerves
• 12 pairs
• Carry information to and from the brain
• Control sensory organs of the head and muscles of
  the head/neck
• Only 1 controls below the neck
• Spinal nerves
• 31 pairs
• Carry information to and from the spinal cord

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Divisions of the PNS

• Somatic nervous system (SNS) monitors and
  regulates the activity of the skeletal muscles.
• Also monitors proprioception- awareness of the
  bodys position in 3D space (lying, sitting,
  standing, upside down, etc.)
• Amputees and mirror therapy
• Enteric nervous system (ENS) monitors and
  regulates the activity of the digestive system.
• Embedded in the digestive system lining ranging
  from the esophagus to the colon.
• Called the 2nd brain because it is so complex.
• Monitors pressure, nutrients and dangerous
  invaders (like bacteria).

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Divisions of the PNS

• Autonomic nervous system (ANS) monitors and
  regulates all of the automatic functions that
  take place within the body.
• Two divisions- Necessary to either speed up the
  ANS or slow it down.
• Sympathetic division
• Initiates the fight or flight response
• Increases heart rate and respiration rate
• Pupil dilation
• Shuts down the gut
• Hearing shuts down

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Divisions of the PNS

• Parasympathetic division
• Shuts down the fight or flight response and
  returns the body to normal conditions

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Histology of Nervous Tissue

• Types of Nervous Tissue Cells
• Neurons responsible for most of the specialized
  functions of the nervous system.
• General information
• 100 billion neurons make up nervous tissue
• Amitotic
• All neurons exist before a person is born
• Can generate and transmit electric signals called
  action potentials.

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Histology of Nervous Tissue

• Characteristics of neurons
• Irritability The ability of a neuron to respond
  to a stimulus. Neurons only respond to certain
  stimuli.
• Chemoreceptors- chemicals
• Photoreceptors- light
• Mechanoreceptors- pressure
• Pain receptors- tissue damage
• Electrical Excitability Ability of a neuron to
  generate and transmit an action potential when
  properly stimulated.

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Draw a Neuron
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Parts of a Neuron

• Cell body the main part of a neuron.
• Where the nucleus and all other organelles are
  found.
• Dendrites the input portion of a neuron.
• Arise from the cell body.
• Short, highly branching extensions
• Axon the output portion of a neuron.
• Arises from the cell body
• Long, single extension
• Synaptic end bulbs connect one neuron to another
  neuron or to an effector in the body.
• Contain chemicals (neurotransmitters) that
  forward an action potential onto another neuron,
  muscle and/or gland.

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Neuroglial cells

• The cells responsible for supporting and
  assisting the neurons.
• General information
• 10-50 trillion in nervous tissue
• Mitotic
• Cannot transmit action potentials
• Provide an environment suitable for neuron
  function

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Neuroglial cells

• Astrocytes star-shaped cells that possess
  multiple processes and are the largest and most
  numerous neuroglia.
• Function to support the neuron physically by
  creating a scaffolding on which a neuron can
  rest.
• Schwann cells neuroglial cells that form the
  myelin sheath around axons.
• Myelin sheath a multilayered lipid and protein
  covering surrounding certain axons.
• Acts as an electrical insulator and keeps close
  axons separated.
• Nodes of Ranvier gaps found in the myelin
  sheath.
• Help speed up the rate of an action potential.

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Neuroglial cells

• Oligodendrocytes neuroglial cells that form the
  myelin sheath around axons found in the CNS.
• Separate axons of different neurons.

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Function of a neuron

• To generate and transmit an action potential.
• Action Potential a sequence of rapidly occurring
  events that decrease, reverse and eventually
  restore the resting membrane potential of a
  neuron.
• Stimulates an effector (neuron, muscle, gland) to
  alter its function.
• In order for this to happen, a neuron has to be
  sufficiently stimulated.
• Threshold stimulus the minimum amount of
  stimulus necessary to initiate an action
  potential.
• In humans, this is -70 millivolts.

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Function of a neuron

• Resting membrane potential an electrical voltage
  difference established across a neuron cell
  membrane.
• Created by integral proteins pumping ions up
  their concentration gradients (active transport).
• Creates a purposeful imbalance of Na and K ions
  inside and outside a neuron.

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Steps in an Action Potential

• Depolarization the application of a threshold
  stimulus causes sodium pumps on the neuron cell
  membrane to open.
• Started by a threshold stimulus.
• Na pump that is closest to the stimulus turns
  off which causes Na to rush back into the cell.

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Steps in an Action Potential

• Reverse polarization the inside of a neuron cell
  membrane becomes positive, while the outside of
  the neuron cell membrane becomes negative.
• Na pumps adjacent to each other turn off 1 at a
  time due to the environment changes.
• Eventually all Na are on the inside of the
  neuron.

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Steps in an Action Potential

• Repolarization sodium pumps close and begin
  pumping sodium to the outside of the neuron cell
  membrane.
• The first Na pump closes causing the next pump
  to close and so on.
• This re-establishes the resting potential.
• Repolarization starts before depolarization ends
  which allows us to transmit multiple action
  potentials one right after another.

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Propagation of a Nerve Impulse

• The special mode of action potential
  transmission.
• The way that an action potential is propagated
  depends on the presence/absence of a myelin
  sheath.
• Continuous conduction the step-by-step
  depolarization that occurs in unmyelinated axons.
• An action potential must take place at every
  point along the length of a neuron.
• SLOW- 0.5 m/s
• This only occurs in the CNS because here is where
  some unmyelinated axons are.

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Propagation of a Nerve Impulse

• Saltatory conduction a jumping depolarization
  that occurs in myelinated axons.
• Action potentials only occurs at the nodes of
  Ranvier.
• FAST 130 m/s
• Only type of conduction in the PNS.
• This is how reflexes occur before the brain can
  process the stimulus.

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Refractory Periods

• The period of time during which an excitable cell
  cannot generate another action potential.
• The length or type of refractory period is
  determined by how close the action potential is
  to the threshold stimulus.
• Absolute refractory period a second action
  potential cannot be initiated, no matter how
  strong the threshold stimulus.
• Neuron has not had time to reset itself.

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Refractory Periods

• Relative refractory period the period of time
  during which a second action potential can be
  initiated, but only with a stimulus that is
  larger than a threshold stimulus.
• The larger stimulus is needed to stop the Na
  pumps from moving Na out of the neuron- getting
  them to open up and allow Na back in.

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Synapses

• Functional junction between neurons.
• Draw a synapse including the presynaptic neuron,
  synapse and postsynaptic neuron

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Types of Synapses

• Determined by the presence or absence of a gap.
• Electrical action potentials are conducted
  directly between adjacent neurons through gap
  junctions.
• Pre- and postsynaptic neurons are physically
  touching.
• Most neurons that synapse this way use continuous
  conduction.

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Types of Synapses

• Chemical action potentials must be converted
  into a chemical signal in order to cross a
  synapse.
• Most numerous type of synapse.
• Lose a little speed in the transmission process
  when an action potential is converted into a
  chemical signal.
• Neurotransmitters chemical messengers released
  from the presynaptic neuron in order to stimulate
  postsynaptic neurons.
• Acetylcholine is the most common.

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Neuronal Circuits

• Complicated networks over which nerve impulses
  are conducted.
• Can be formed at any time during our lifespan.
• Neurons cannot reproduce once they are mature,
  but they can produce more dendrites and axons.

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Neuronal Circuits

• Simple series a presynaptic neuron stimulates a
  single postsynaptic neuron.
• Not common
• Allows for rapid transmissions
• Often used in reflexes which allows the brain to
  be bypassed.

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Neuronal Circuits

• Diverging circuit the nerve impulse from a
  single presynaptic neuron causes the stimulation
  of several postsynaptic neurons.
• Several extensions come off an axon and each
  stimulates a different postsynaptic neuron.
• Allows for the coordination of several muscle
  groups.

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Neuronal Circuits

• Converging circuit the postsynaptic neuron
  receives impulses from several different
  presynaptic neurons.
• Reverse of a diverging circuit.
• Allows us to sense different levels of intensity.
• Sending an action potential down more than 1
  presynaptic neuron causes a more intense
  sensation because the brain is inundated with
  signals.

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Neuronal Circuits

• Reverberating circuit the presynaptic neuron
  stimulates a series of postsynaptic neurons.
• The initial action potential is directed back to
  the beginning of the loop.
• This is how learning and memories form and are
  retrieved.
• Neurons create new extensions trying to create a
  reverberating circuit.
• If these neurons are not frequently used, the
  body redirects the neurons. (Use it or lose it!)

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Regeneration of Nervous Tissue
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ACT - UP
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Look at the normal neuron (top) and the neuron
after regeneration of the axon has taken place.

• 1) How would the rates of action potential
  transmission be affected?
• 2) Choose two body system and identify how this
  could affect the function of that body system.