Nervous System

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Chapter 13

• Nervous System

An electro -
- chemical system.
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Review

• Overview of the Nervous System
• Central Nervous System
• The Spinal Cord
• The Brain
• The Limbic System and Higher Mental Functions
• Peripheral Nervous System
• Somatic / Autonomic Systems
• Health Focus Degenerative Brain Disorders
• Drug Abuse

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• Nervous system function?
• allows us to receive, integrate (process), and
  react to environmental stimuli to maintain
  homeostasis.
• Component parts?
• Central nervous system (CNS)
• includes the brain and spinal cord
• Peripheral nervous system (PNS)
• Somatic sensory and motor neurons to skeletal
  muscles (voluntary)
• Autonomic sensory and motor neurons go to smooth
  muscles and glands (involuntary)

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Nervous Tissue

• Nervous Tissue contains two types of cells.
• Neurons transmit nerve impulses between parts of
  the nervous system.
• Neuroglia support, protect, and nourish neurons.
  (neuroglial or glial cells)
• Have different specific names based on their
  shape, location, function.

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Neuron Structure

• Neurons are classified according to function.
• Sensory neurons take impulses from a sensory
  receptor to the CNS. (afferent, at)
• Interneurons receive input from sensory neurons,
  and other neurons, and then communicate with
  motor neurons. (association, connector)
• Motor neurons take nerve impulse away from the
  CNS to an effector that carries out responses to
  environmental change. (efferent, exit)

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Types of Neurons
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Neuron Structure

• Neurons contain three basic parts.
• Cell body contains nucleus and other organelles.
• Dendrites receive signals from sensory receptors
  or other neurons.
• Axon conducts nerve impulses away for the cell
  body.

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Myelin Sheath

• Some axons are covered by a protective myelin
  sheath.
• Formed by Schwann cells containing myelin in
  plasma membranes.
• (neurolemmocytes)
• Nodes of Ranvier are gaps on the axon with no
  myelin sheath. (neurofibril node)

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Myelin Sheath
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The Nerve Impulse

• The nervous system uses the nerve impulse to
  convey information.
• Resting potential is the voltage level when an
  axon is not conducting an impulse.
• Sodium-potassium pump causes greater
  concentration of Na outside the axon, and
  greater concentration of K inside the axon.
• Unequal ion distribution causes inside of axon to
  be negative relative to the outside.

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Action Potential

• An action potential is a rapid change in polarity
  across an axomembrane as the nerve impulse
  occurs.
• All-or-none once threshold is reached.
• Sodium gates open, allowing Na to move inside
  the axon.
• Potassium gates open, allowing K to move outside
  the axon.
• The charge flips becomes on the inside, (-)
  on the outside . . .

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Propagation of an Action Potential

• Each preceding portion causes an action potential
  in the next portion of an axon.
• As soon as an action potential has moved on, the
  previous portion of an axon undergoes a
  refractory period in which the sodium gates are
  unable to open.

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Transmission across a Synapse

• Transmission across a synaptic cleft is carried
  out by neurotransmitters stored in synaptic
  vesicles - norepinephrine (NE) or acetylcholine
  (ACh).
• Depending on the neurotransmitter and the
  receptor, the response of the postsynaptic neuron
  can be towards excitation or inhibition summary
  (net) effect.
• Integration is the summing of signals received by
  a postsynaptic neuron.

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Integration
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The Central Nervous System

• The central nervous system (CNS) is made up of
  the spinal cord and the brain.
• Both are wrapped in protective membranes,
  meninges, with spaces between meninges filled
  with cerebrospinal fluid.
• CNS is composed of two types of nervous tissue.
• Gray matter Short, nonmyelinated fibers.
• White matter - Myelinated axons.

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Computer Assisted Tomography (CAT) scan
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Magnetic Resonance Imaging (MRI) scan
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The Spinal Cord

• The spinal cord extends from the base of the
  brain through the foramen magnum into the
  vertebral canal.
• Components
• Central canal filled with cerebrospinal fluid
• Grey matter butterfly shaped
• White matter contains interneurons grouped in
  tracts (ascending and descending) which cross
  over so that left side of brain controls right
  side of body.

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Functions of the Spinal Cord

• The spinal cord (1) provides a means of
  communication between the brain and the
  peripheral nerves that leave the cord
• and (2) is a center for reflex actions.

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The Brain

• 2 parts?
• Subconscious controls, channels
  Conscious largest part . . . . .
• The Cerebrum.
• The cerebrum (or telencephalon) is the largest
  portion of the human brain.
• Communicates with, and coordinates activities of,
  other parts of the brain.
• Divided into left and right cerebral hemispheres.
• Divided by longitudinal fissure.

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The Human Brain
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The Brain

• Gray matter of cerebrum consists of cerebral
  cortex and basal nuclei.
• Cerebral cortex is a highly convoluted outer
  layer that covers the cerebral hemispheres.
• Basal nuclei are masses located deep within white
  matter.
• White matter consists of long myelinated axons
  organized into tracts.
• The two hemispheres are connected by the corpus
  callosum.

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Lobes of Cerebral Hemisphere

• Insula not much known about, lies beneath the
  surface.

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The Brain

• Diencephalon is made up of hypothalamus and
  thalamus, and circles the third
  ventricle-integrating center that helps maintain
  homeostasis.
• Cerebellum is separated from the brain stem by
  the fourth ventricle.
• 2nd largest part, involved in muscle
  coordination, tone, and maintaining
  posture/balance.
• The brain stem contains the midbrain, pons, and
  medulla oblongata.
• Connects cerebrum with other brain parts
  regulate breathing reflex movements for head to
  respond to visual, auditory stimuli centers for
  heartbeat, respiration, vasoconstriction (blood
  pressure) etc.

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Reticular Activating System (RAS)

• Reticular formation is a complex network of
  nuclei and fibers extending the length of the
  brain stem.
• involves the thalamus
• serves as a central relay station for sensory
  impulses (except smell)
• acts as a gatekeeper to the cerebrum . . . .

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Reticular Activating System
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The Limbic System

• Limbic system is a complex network of tracts and
  nuclei.
• Blends primitive emotions and higher mental
  functions into a united whole.
• Composed of hippocampus (crucial role in learning
  and memory) and amygdala (emotional overtones to
  experiences).
• Good and bad feelings.
• It is the emotional brain.
• Involved in emotions (rage, pain, etc.),
  learning, and memory.

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Limbic System
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Higher Mental FunctionsMemory and Learning

• Memory is the ability to hold a thought in mind
  or to recall events from the past.
• Learning takes place when we retain and utilize
  past memories.
• Types of Memory
• Short-term versus long-term - is a mixture of . .
  .
• Semantic memory numbers, words, etc.
• Episodic memory persons, events, etc.
• Skill memory - can exist independent of episodic
  memory, involves performing motor activities (ie.
  riding a bike, playing a sport, etc.), initially
  conscious while learning but automatic once
  learned.

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Higher Mental FunctionsLanguage and Speech

• Depends on semantic memory and motor centers in
  the occipital and temporal lobes.
• Wernickes area ability to comprehend speech.
• Brocas area ability to speak and write.
• The visual cortex, Wernickes area and Brocas
  area must all function together. . . . .

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Language and Speech
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Language and Speech

• Left and right brain have different functions.
• Left Hemisphere
• Verbal, Logical, analytical, Rational.
• Right Hemisphere
• Nonverbal, visuospacial, Intuitive, Creative.
• Researchers believe hemispheres process the same
  information differently - left is
  more global, -
  right is more specific.

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EEGs ?

• electro/encephalo/gram
• records brains electrical activity
• subject awake with eyes closed alpha waves
• if eyes open beta waves
• during sleep, REM (rapid eye movement) theta
  waves
• occurs about 5x/night, when dream, needed for
  memory
• deep sleep delta waves

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Peripheral Nervous System

• Composed of nerves and ganglia.
• Cranial nerves take impulses to and from the
  brain. (12 prs.)
• Spinal nerves take impulses to and from the
  spinal cord. (31prs.)

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Cranial and Spinal Nerves
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Somatic System

• Somatic system includes nerves that take sensory
  information from external sensory receptors to
  the CNS and motor commands away from the CNS to
  the skeletal muscles.
• Primarily voluntary.
• The exception is . . . . .
• Reflexes are automatic, involuntary responses to
  a stimulus.
• What are its component parts?

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Reflex Arc
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Autonomic System

• Autonomic system regulates the activity of
  cardiac and smooth muscles and glands.
• Is involuntary.
• 2 component parts
• Sympathetic division brings about fight or
  flight responses. (norepinephrine)
• Parasympathetic division brings about relaxed
  responses or housekeeper system. (acetycholine)

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HEALTH FOCUSDegenerative Brain Disorders

• Alzheimer disease (AD).
• Presence of abnormal neurons.
• Plaques.
• Neurofibrillary tangles.
• Parkinson disease.
• Overactive basal nuclei due to the degeneration
  of dopamine-releasing neurons in the brain.

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Drug Abuse

• Drugs that affect the nervous system have two
  general effects.
• (1) Impact limbic system.
• (2) Promote or decrease action of a particular
  neurotransmitter.
• Drug abuse is apparent when a person takes a drug
  at a dose level and under circumstances that
  increase the potential for a harmful effect.
• Psychological dependence vs physical dependence
  (formerly addiction)

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Drug Actions at a Synapse
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Drug Abuse

• Alcohol (a depressant).
• Nicotine (a stimulant).
• Causes neurons to release dopamine.
• Excess dopamine has reinforcing effect that leads
  to dependence.
• Cocaine (a stimulant).
• Prevents synaptic uptake of dopamine.
• Continued use causes body to produce less
  dopamine.

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Drug Abuse

• Methamphetamine (a stimulant).
• Powder (speed), crystals (crystal meth or ice).
• Effects (initial elation followed by state of
  high agitation) last for 4 8 hours.
• Heroin (a depressant).
• Binds to receptors meant for endorphins.
• Continued use causes body to produce fewer
  endorphins.
• Marijuana (a psychoactive).
• Has been referred to as the gateway drug. . . .
  Why?
• Binds to receptor for anandamide.
• Brain impairment?

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Bioethical Focus

• Medical Marijuana Use

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Homeostasis

• Governance of internal organs and the regulation
  of blood and tissue fluid usually takes place
  below the level of consciousness.
• Heart Rate.
• Breathing Rate.
• You name it, and the brain is somehow involved.

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Review

• Overview of the Nervous System
• Central Nervous System
• The Spinal Cord
• The Brain
• The Limbic System and Higher Mental Functions
• Peripheral Nervous System
• Somatic / Autonomic Systems
• Health Focus Degenerative Brain Disorders
• Drug Abuse

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