E1. Nervous System organization

Nervous System Organization

Learning Outcomes

• Compare and contrast the Central Nervous System (CNS) and Peripheral Nervous System (PNS).

• Compare and contrast, and diagram white and gray matter.

• Describe the blood-brain barrier.

• Compare and contrast functions of the cerebral cortex, limbic system, and brainstem structures.

• Diagram, compare, and contrast the lobes of the brain.

• Compare and contrast functions of the spinal cord.

• Diagram the anatomy of the spinal cord.

• Sequence, compare, and contrast the flow of afferent/efferent information to/from the brain via the spinal cord.

• Sequence spinal reflexes.

Lecture Outline

• Nervous System Organization

• Afferent Information Flow (Sensory Input)

• Efferent Information Flow (Motor Output)

• Spinal Cord Tracts Ascending (Afferent)

• Spinal Cord Tracts Descending (Efferent)

• Reflexes

I. Nervous System Organization

Organization of the Nervous System

• Central Nervous System (CNS): Brain and spinal cord.

• Peripheral Nervous System (PNS):

  • Somatic Motor System: Controls skeletal muscles.

  • Somatic Sensory System: Carries sensory information from the body.

  • Autonomic Nervous System (ANS): Controls organs, glands, etc.

A. Cell Types/Functions

• Neurons:

  • Sensory (Afferent):

    • Cell bodies outside the CNS.

    • Axons go into the CNS.

  • Motor (Efferent):

    • Cell bodies inside the CNS.

    • Axons go out of the CNS.

  • Interneurons:

    • Entirely inside the CNS.

    • Make lots of connections.

• Collection of cell bodies:

  • In the CNS: called a “nucleus”.

  • In the PNS: called a “ganglion”.

• Collection of axons:

  • In the CNS: called a “tract” or “pathway”.

  • In the PNS: called a “nerve”.

• Glial Cells:

  • Many different types.

  • Myelination (Schwann cells, oligodendrocytes).

  • Blood-brain barrier (astrocytes).

  • Development, healing, etc.

Brain Function

1. Receive sensory input.

2. Processing.

3. Sends response output.

• Input: Signal

• Integrating Center: Controller that receives inputs and initiates response

• Output: Produces response

• Response to changes in regulated variable

Anatomical Directions

• Superior: Closer to the head.

• Inferior: Away from the head.

• Anterior: Closer to the chest.

• Posterior: Closer to the back.

• Medial: Closer to the midline.

• Lateral: Farther from the midline.

• Ipsilateral: On the same side.

• Contralateral: On the other side.

• Decussation: Where something crosses the midline.

II. Major CNS Divisions

• Brain

• Spinal Cord

Brain Structures

• Cerebral Cortex (cerebrum)

• Thalamus

• Hypothalamus

• Pituitary

• Midbrain

• Pons

• Cerebellum

• Medulla

Clusters of CNS Neurons

• Nuclei: Clustered groups of neurons with shared functions.

• Tracts: Bundles of axons connecting CNS regions.

Clusters of PNS Neurons

• Ganglia: Bundle of nerve cell bodies

• Nerves: Bundles of axons carrying info to/from CNS

Fluid-Filled Compartments

• The CNS has hollow, fluid-filled compartments

  1. Ventricles: 4 fluid-filled cavities in the center of the brain.

  2. Central canal: Fluid-filled cavity running through the spinal cord.

Comparing Gray and White Matter

Two types of tissue in the CNS

1. Gray Matter:

   • Nervous tissue composed of unmyelinated neuron cell bodies, dendrites, axons $\rightarrow$ slow, continuous conduction.

   • Gray matter makes up the inner layers of the spinal cord & outer layers of the brain.

2. White Matter:

   • Tissue composed of myelinated axons $\rightarrow$ Fast, saltatory conduction.

   • White matter makes up the outside layers of the spinal cord & inner layers of the brain.

Blood-Brain Barrier Structure

• Blood vessel walls composed of endothelial cells.

• Tight Junctions: Cell-to-cell junctions between endothelial cells.

  • Seal leaky pores.

  • Prevent solute movement between cells.

Functional Divisions of the Brain

1. Sensory areas: Receive sensory input & translate into perception.

2. Association areas: Integrate information and direct voluntary behaviors.

3. Motor areas: Direct muscle movement.

Lobes of the Cerebral Cortex

Cerebral cortex structure: outermost layer of the brain

• 4 lobes

• 2 hemispheres with (typically) contralateral outputs and inputs

• Composed of gray matter

• Function: higher-level processing

Limbic System

Limbic System: gray matter structure below cerebral cortex
The limbic system includes the amygdala, hippocampus, and cingulate gyrus.
Anatomically, the limbic system is part of the gray matter of the cerebrum. The thalamus is shown for orientation purposes and is not part of the limbic system.

• Cingulate gyrus plays a role in emotion.

• Hippocampus is involved in learning and memory.

• Amygdala is involved in emotion and memory.

Brain Stem

Thalamus: Relay Center

• Thalamus: bundles of white matter tracts
  Function: direct sensory information

Midbrain: controls eye movement and auditory & visual reflexes

• Substantia nigra: nucleus in midbrain with dopamine-expressing cells that control initiation of movement
  Brain Stem: Midbrain

Parkinson’s disease is marked by degeneration of substantia nigra cells, resulting in motor deficits

Brain Stem: Cerebellum

• Cerebellum: processes bodily senses, coordinates movement

Brain Stem: Medulla

• Medulla: autonomic control, swallowing, nausea/vomiting

Spinal Cord

• Cervical nerves (8 pairs): Head and Neck, Diaphragm, Deltoids, Biceps, Wrist Extenders, Triceps, Hand

• Thoracic nerves (12 pairs): Chest Muscles, Abdominal Muscles

• Lumbar nerves (5 pairs): Leg Muscles

• Sacral nerves (5 pairs): Bowel, Bladder, Sexual Function

• Coccygeal nerve (1 pair)

Spinal Cord Structure

• Sensory nerve pathway: dorsal

• Motor nerve pathway: Motor root

Spinal Cord Functional Organization

Each region divided into sub-segments with:

1. Spinal cord columns: white matter tracts, carry info up/down

2. Bilateral pair of spinal nerves

3. Spinal cord horns: gray matter nuclei, motor neuron cell bodies and interneurons

   • Gray matter consists of sensory and motor nuclei.

     • Visceral sensory nuclei

     • Dorsal horn

     • Lateral horn

     • Ventral horn

     • Somatic sensory nuclei

     • Autonomic efferent nuclei

     • Somatic motor nuclei

Spinal Cord Anatomy

• white matter tracts (myelinated axons)

  • dorsal horn

  • lateral horn

  • posterior (dorsal)

  • anterior (ventral)

  • left right

• grey matter (lots of cell bodies)

  • ventral (anterior) horn

II. Afferent Information Flow

Flow of Afferent Information

1. Nerves carry information from sensory receptors to ganglia.

2. Afferent neurons (PNS) carry sensory information into the spinal cord (CNS)

   • Dorsal root ganglia: Bundles of sensory neuron cell bodies form swellings on nerve just before entering SC.

   • Dorsal roots: Afferent neuron axons extend from ganglia into the spinal cord.

   • Spinal cord is CNS

   • Roots are PNS

     1. Dorsal roots: sensory (afferent) neurons enter the spinal cord

        • DRG - contains sensory neuron cell bodies

     2. Ventral root: motor (efferent) neurons leave the spinal cord

3. Ascending tracts: (CNS) white matter columns, carry sensory info to brain

4. Brain processing (interneurons)Sensory Ascending Dorsal

Sensation

1. Sensory stimulus: physical energy acting on a sensory receptor

2. Sensory transduction at sensory receptors

   • Sensory receptors: specialized cells that detect physical events

   • Sensory transduction = sensation!

     • Conversion of stimulus energy into electrochemical signals that can be processed by the nervous system

     • Produces a receptor potential: graded potential in a sensory receptor

Types of Sensory Receptors

1. Neuronal:

   • Simple sensory receptors

     • Afferent neuron (can fire action potentials)

     • Naked or “free” nerve ending

   • Complex sensory receptorAfferent neuron (can fire action potentials)

     • Nerve ending encased in connective tissue

Two Mechanisms of Sensory Transduction

1. Ion channels

   • Stimulus $\rightarrow$ Ion channels open $\rightarrow$ Ion flow $\rightarrow$ Receptor Potential (ex: TRPs are transient receptor potential cation channels)

2. Signal transduction: external signal triggers intracellular changes via second messenger system & signal cascade

Sensory Receptors

A). Receptor Types

Sensory Receptors: How they work

• receptor $\rightarrow$ CNS

• deformation temperature damage light

• chemicals change Pions Receptor Potential

• Receptor Potentials

  1. graded response (not an action potential)

  2. maintain or

• stimulus $\rightarrow$ receptor potential $\rightarrow$ AP frequency in sensory neuron

• receptor potential > VT $\rightarrow$ AP

III. Efferent Information Flow

Flow of Efferent Information

1. Brain processing

2. Descending tracts: (CNS) white matter columns, carry efferent information from the brain

3. Efferent (PNS) neuron cell bodies originate in the spinal cord

   • Ventral horns: gray matter bundles of somatic motor neuron cell bodies

   • Lateral horns: gray matter bundles of autonomic sympathetic neuron cell bodies

   • Gray matter interneurons

   • See: motor pathways and ANS lectures

4. Ventral root: efferent neuron axons, carries info from spinal cord to effectors

IV. Sensory Ascending Tracts

VI. Sensory Pathways

A). Dorsal Columns

• carry mechanoreceptor information

  • touch, vibration, proprioception

  • ascend ipsilateral (same side as the receptor)

  • synapse/decussate in the medulla

  • medulla $\rightarrow$ thalamus $\rightarrow$ somatic sensory cortex

B). Spinothalamic Tracts (also called “anterolateral tracts”)

• left hand

• carry pain, temperature

• decussate with interneuron at the spinal level

• ascend contralateral (opposite side as the receptor)

• synapse in the thalamus

• thalamus $\rightarrow$ somatic sensory cortex

SENSORY PATHWAYS (for left side of the body)

• RIGHT SIDE LEFT SIDE

• thalamus medulla

• pain, temp pain, temp

• pain, temp touch touch touch

• lateral dorsal column medial dorsal column spinothalamic

• from the shoulders, arms, hands from the legs from the feet

IV. Motor Descending Tracts

Motor Pathways

A). Corticospinal Tracts

1. Lateral Corticospinal (also called the “pyramidal tract”)

   • decussate in the pyramids (part of the medulla)

   • descend in the lateral part of the spinal cord

   • control fine muscle movement

2. Anterior Corticospinal

   • descend in the anterior part of the spinal cord

   • decussate at the spinal level

   • control bilateral “posture” muscles (mainly in the back)

     • “upper motorneurons”

     • “lower motorneurons”

VI. Spinal Reflexes

Spinal Reflexes

• Spinal Reflexes: behavioral response produced by spinal cord, without input from the brain

• See: motor pathways

Sequence Spinal Reflexes

1. Stimulus

2. Afferent neuron fires

3. Chemical signaling at motor neuron and/or interneuron

4. Motor neuron fires

5. Chemical signaling at effector

6. Response

Stretch Reflex Deep Tendon Reflex (DTR)

1. Tap tendon with a rubber hammer causes quick muscle stretch

2. Muscle stretch causes the muscle to contract requires:
   a stretch receptor that recognizes that the muscle has been stretched connections to the muscle that can trigger a contraction extrafusal fibers contract generate tension innervated by A$\alpha$ motorneurons

3. intrafusal fibers don’t generate tension innervated by A$\gamma$ motorneurons 2 types:
   a. “static” muscle length AP frequency in sensory neuron
   b. “dynamic” dL/dt AP frequency in sensory neuron Skeletal muscle Stretch receptors (muscle spindles) - an intrafusal fiber wrapped with a sensory neuron

Tx: $Mg^{++}$ sulfate neuron excitability seizures
DTRs sensory neurons

Reflex Arc

EPSP

1. Hit with hammer stretch muscle ( L and dL/dt of intrafusal fibers)

2. AP frequency in the sensory neurons

3. Sensory neurons synapse onto A$\alpha$ motorneurons in anterior horn (NT = glutamate) EPSP

4. A$\alpha$ motorneuron sends APs to NMJ (NT = ACh) EPP

5. Muscle contracts (and synergist excitation – similar muscles also stimulated to contract) Clinical uses:
   a. Peripheral nerve damage DTRs
   b. Pre-eclampsia (occurs near the end of pregnancy) seizures
   c. Brain injury normal - brain inhibits DTRs A$\alpha$ motorneurons brain injury inhibition DTR

Antagonist Inhibition

• muscles that do the opposite action are inhibited

• Quadriceps (extensors) the synergists

• Hamstrings (flexors) the antagonists

• EPSP EPSP IPSP

The patellar (knee-jerk) reflex-an example of a stretch reflex

• Tapping the patellar ligament stretches the quadriceps and excites its muscle spindles.

• Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons

• The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee.

• The interneurons (green) make inhibitory synapses with ventral horn neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps.

Golgi tendon receptor (apparatus)

• receptor is in a tendon that is attached to a muscle

• when muscle pulls on the tendon increases AP frequency in the receptor’s sensory neuron

  1. As muscle contracts increases the tension in the tendon

  2. Golgi receptor AP frequency 4). Uses:
     a. sends information to the brain about how hard muscles are contracting
     b. may help prevent excessive muscle contraction (decreases risk of muscle/tendon injuries)

  3. Inhibits the muscle Activates the antagonist opposite of the muscle spindles

     • Synergist inhibition

     • Antagonist excitation