BIPN 100 - E1 Nervous System Organization

Nervous System Organization

• General Overview
  • The nervous system is divided into two main parts:
    • Central Nervous System (CNS): brain and spinal cord.
    • Peripheral Nervous System (PNS): somatic motor system, somatic sensory system, and autonomic nervous system (ANS).
      • Somatic motor system: controls skeletal muscles.
      • Somatic sensory system: carries sensory information from the body.
      • Autonomic nervous system (ANS): controls organs and glands.
• Cell Types and Functions
  • Neurons
    • Sensory (afferent) neurons:
      • Cell bodies are located outside the CNS.
      • Axons enter the CNS.
    • Motor (efferent) neurons:
      • Cell bodies are located inside the CNS.
      • Axons exit the CNS.
    • Interneurons:
      • Located entirely inside the CNS.
      • Facilitate connections between neurons.
  • Collections of Cell Bodies
    • In the CNS: called a "nucleus".
    • In the PNS: called a "ganglion".
  • Collections of Axons
    • In the CNS: called a "tract" or "pathway".
    • In the PNS: called a "nerve".
  • Glial Cells
    • Various types of glial cells exist.
    • Functions include:
      • Myelination (Schwann cells, oligodendrocytes).
      • Formation of the blood-brain barrier (astrocytes).
      • Development and healing processes.
• Brain Function
  • Receives sensory input.
  • Processes information.
  • Sends response output.
    • Signal: Response to changes in a regulated variable.
    • Integrating Center: Controller that receives inputs and initiates responses.
• 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 opposite side.
  • Decussation: where something crosses the midline.

Major CNS Divisions

• Brain
• Spinal Cord

Brain Structures

• Cerebral Cortex (Cerebrum)
• Thalamus
• Hypothalamus
• Pituitary Gland
• Midbrain
• Pons
• Cerebellum
• Medulla

Neuron Clusters

• Clusters of CNS Neurons
  • Nuclei: clustered groups of neurons with shared functions.
  • Tracts: bundles of axons connecting CNS regions.
• Clusters of PNS Neurons
  • Ganglia: bundles of nerve cell bodies.
  • Nerves: bundles of axons carrying information to/from CNS.

Fluid-Filled Compartments

• The CNS has hollow, fluid-filled compartments.
  • Ventricles: four fluid-filled cavities in the center of the brain.
  • Central canal: fluid-filled cavity running through the spinal cord.

Gray vs. White Matter

• Two types of tissue in the CNS:
  • Gray Matter:
    • Composed of unmyelinated neuron cell bodies, dendrites, and axons.
    • Results in slow, continuous conduction.
    • Located in the inner layers of the spinal cord and outer layers of the brain.
  • White Matter:
    • Composed of myelinated axons.
    • Results in fast, saltatory conduction.
    • Located in the outside layers of the spinal cord and inner layers of the brain.

Blood-Brain Barrier

• Blood Vessel 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 Brain Divisions

• Sensory Areas: receive sensory input and translate it into perception.
• Association Areas: integrate information and direct voluntary behaviors.
• Motor Areas: direct muscle movement.

Cerebral Cortex Lobes

• Cerebral cortex structure: outermost layer of the brain.
  • Four lobes.
  • Two hemispheres with (typically) contralateral outputs and inputs.
  • Composed of gray matter.
• Function: higher-level processing.

Limbic System

• Limbic System: Gray matter structure below the cerebral cortex.
  • Includes the amygdala, hippocampus, and cingulate gyrus.
  • Part of the gray matter of the cerebrum.
    • Amygdala: involved in emotion and memory.
    • Hippocampus: involved in learning and memory.
    • Cingulate Gyrus: plays a role in emotion.

Brain Stem Structures

• Thalamus:
  • Relay center.
  • Bundles of white matter tracts.
  • Directs sensory information.
• Midbrain:
  • Controls eye movement and auditory & visual reflexes.
  • Substantia Nigra: nucleus in midbrain with dopamine-expressing cells that control the initiation of movement.
    • Parkinson’s disease is marked by degeneration of substantia nigra cells, resulting in motor deficits.
• Cerebellum:
  • Processes bodily senses and coordinates movement.
• Medulla:
  • Controls autonomic functions and is involved in swallowing and 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 root.
• Motor Nerve Pathway: motor root.
• Cauda Equina.

Spinal Cord Functional Organization

• Each region is divided into sub-segments with:
  • Spinal cord columns: white matter tracts, carry information up/down.
  • Bilateral pair of spinal nerves.
• Spinal Cord Horns:
  • Gray matter nuclei.
  • Motor neuron cell bodies and interneurons.
    • Dorsal Horn: sensory nuclei.
    • Lateral Horn.
    • Ventral Horn: motor nuclei.

Spinal Cord Anatomy

• White matter tracts (myelinated axons).
  • Dorsal horn.
  • Lateral horn.
  • Posterior (dorsal).
  • Anterior (ventral).
  • Left/Right.
• Gray matter (lots of cell bodies).
  • Ventral (anterior) horn.

Afferent Information Flow

• Nerves carry information from sensory receptors to ganglia.
• Afferent neurons (PNS) carry sensory information into the spinal cord (CNS).
  • Dorsal root ganglia:
    • Bundles of sensory neuron cell bodies that form swellings on the nerve just before entering the spinal cord.
  • Dorsal roots:
    • Afferent neuron axons extend from the ganglia into the spinal cord.
• Ascending Tracts:
  • (CNS) white matter columns that carry sensory information to the brain.
• Brain processing (interneurons).

Spinal Cord Roots

• Dorsal Root:
  • Sensory (afferent) neurons enter the spinal cord.
  • DRG - contains sensory neuron cell bodies.
• Ventral Root:
  • Motor (efferent) neurons leave the spinal cord.

Sensation

• Sensory Stimulus: physical energy acting on a sensory receptor.
• 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.

Sensory Receptors

• Neuronal:
  • Simple Sensory Receptors:
    • Afferent neuron (can fire action potentials).
    • Naked or “free” nerve ending.
  • Complex Sensory Receptor:
    • Afferent neuron (can fire action potentials).
    • Nerve ending encased in connective tissue.

Sensory Transduction Mechanisms

• Ion Channels:
  • Stimulus leads to ion channels opening, resulting in ion flow and a receptor potential (e.g., TRPs: transient receptor potential cation channels).
• Signal Transduction:
  • External signal triggers intracellular changes via a second messenger system & signal cascade.

Sensory Receptor Types

Sensory Receptors - How They Work

• Receptor deformation, temperature change, damage, light, chemicals.
• Change in ion permeability leads to a receptor potential.
• Receptor Potentials:
  • Graded response (not an action potential).
  • Maintain stimulus.
• Receptor Potential leads to Action Potential (AP) Frequency in Sensory Neuron:
  • If the receptor potential is greater than the threshold (VT), an action potential is fired.

Efferent Information Flow

• Brain processing.
• Descending Tracts:
  • (CNS) white matter columns that carry efferent information from the brain.
• 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.
• Ventral Root: efferent neuron axons, carries information from the spinal cord to effectors.

Sensory Ascending Tracts

• Dorsal Columns:
  • Carry mechanoreceptor information (touch, vibration, proprioception).
  • Ascend ipsilateral (same side as the receptor).
  • Synapse/decussate in the medulla.
  • Pathway: Medulla → Thalamus → Somatic Sensory Cortex.
  • Medial Dorsal Column: from the sacral and lumbar regions (lower body).
  • Lateral Dorsal Column: from the upper thoracic and cervical regions (upper body).
• Spinothalamic Tracts (Anterolateral Tracts):
  • Carry pain and temperature information.
  • Decussate with an interneuron at the spinal level.
  • Ascend contralateral (opposite side as the receptor).
  • Synapse in the thalamus.
  • Pathway: Thalamus → Somatic Sensory Cortex.

Sensory Pathways

• Left side of the body:
  • Touch:
    • Medial dorsal column (from the legs and feet).
    • Lateral dorsal column (from the shoulders, arms, hands).
  • Pain and Temperature:
    • Spinothalamic tract.

Motor Descending Tracts

• Corticospinal Tracts
  • Lateral Corticospinal (Pyramidal Tract):
    • Decussates in the pyramids (part of the medulla).
    • Descends in the lateral part of the spinal cord.
    • Controls fine muscle movement.
  • Anterior Corticospinal:
    • Descends in the anterior part of the spinal cord.
    • Decussates at the spinal level.
    • Controls bilateral “posture” muscles (mainly in the back).
  • Upper Motor Neurons & Lower Motor Neurons.

Lateral Corticospinal Tract

• Controls muscles on the left side of the body.
• Pathway: Cortex --> Medulla (decussation) --> Lateral Corticospinal Tract --> Muscles (shoulders, arms, hands, legs, feet).

Key Considerations for spinal cord damage

• Which pathway is damaged.
• Which spinal cord level is the damage at (levels above the damage are not affected).
• Where does the pathway decussate.
• What type of information is carried by the pathway.

Ascending and Descending Pathways

• Sensory and Ascending Pathways
  • Dorsal column-medial lemniscus pathway
    • Cuneate fasciculus
    • Fasciculus gracilis
  • Spinocerebellar tracts
    • Posterior spinocerebellar tract
    • Ventral spinocerebellar tract
  • Anterolateral spinothalamic tracts
    • Lateral spinothalamic tract
    • Anterior spinothalamic tract
  • Spino-olivary tract
• Motor and Descending Pathways
  • Pyramidal tracts
    • Lateral corticospinal tract
    • Anterior corticospinal tract
  • Extrapyramidal tracts
    • Reticulospinal tract
    • Rubrospinal tract
    • Vestibulospinal tract
    • Olivospinal tract

Spinal Reflexes

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

Sequence of 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, causing 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 and generate tension, innervated by A$\alpha$
       motor neurons.
     • Intrafusal fibers don’t generate tension, innervated by A$\gamma$ motor neurons.
       • Static: muscle length ∝ action potential (AP) frequency in sensory neuron.
       • Dynamic: dL/dt ∝ AP frequency in sensory neuron.
     • Skeletal muscle Stretch receptors (muscle spindles) - an intrafusal fiber wrapped with a sensory neuron.
       • Extra fusal fibers contract generate tension
       • Intrafusal fibers do not

Reflex Arc

• Clinical uses:
  • Peripheral nerve damage: ↓ DTRs.
  • Pre-eclampsia (occurs near the end of pregnancy): ↑ seizures.
  • Brain injury: Normal - brain inhibits DTRs.
    • Brain injury can lead to disinhibition and hyperactive DTRs.

Stretch Reflex Steps

1. Hit with hammer stretching muscle (ΔL and dL/dt of intrafusal fibers).
2. ↑ AP frequency in the sensory neurons.
3. Sensory neurons synapse onto A$\alpha$
   motor neurons in the anterior horn (NT = glutamate), resulting in EPSP.
4. A$\alpha$
   motor neurons sends APs to NMJ (NT = ACh), resulting in EPP.
5. Muscle contracts.

• Synergist Excitation: Similar muscles also stimulated to contract.
• Tx: $Mg^{++}$ Sulfate -neuron excitability -seizures -DTRs

Antagonist Inhibition

• Muscles that do the opposite action are inhibited.
  • Quadriceps (extensors) = the synergists.
  • Hamstrings (flexors) = the antagonists.

Golgi Tendon Receptor (Apparatus)

• Receptor is in a tendon that is attached to a muscle.
• When muscle pulls on the tendon, it increases AP frequency in the receptor’s sensory neuron.
  • As muscle contracts increases the tension in the tendon.
  • Golgi receptor AP frequency.
  • Inhibits the muscle.
  • Activates the antagonist opposite of the muscle spindles.
  • Synergist inhibition and Antagonist excitation.
• Uses:
  • Sends information to the brain about how hard muscles are contracting.
  • May help prevent excessive muscle contraction (decreases risk of muscle/tendon injuries).