Spinal Cord Anatomy Overview

Spinal Cord Anatomy:

• The spinal cord is made up of white and gray matter tracts extending along the vertebral column.
• Spinal nerves exit the spinal cord in the dorsal and ventral portions, forming nerves that connect to receptors and muscles.
• Dorsal root ganglia are enlargements in the dorsal portion of spinal nerves, containing cell bodies of A-alpha and A-beta fibers.
• These fibers carry proprioceptive and fine touch information.
• Axons from these neurons extend from the periphery to the cell body and continue as continuous axons that ascend to the medulla through the dorsal columns.
• Sensory axons also have collaterals that terminate in the dorsal gray matter of the spinal cord, allowing information about muscle stretch to be represented at the spinal cord level.

Dorsal Columns:

• Gracile funiculus (gracile dorsal column) extends from the fifth sacral vertebra to the seventh thoracic vertebra, carrying information from legs and feet.
• In the dorsal columns, information from the hands is lateral and feet is medial.
• Cuneate funiculus is present at C5, carrying information from the upper limbs.
• Only the gracile funiculus is present at L2, below the level of upper limb termination.

Medial Lemniscus:

• First-order neurons carry fine touch and proprioception information from the periphery to the central nervous system.
• Internal capsule is a bundle of white matter fibers that carries information between the thalamus and the cortex.

Somatotopic Map and Homunculus:

• Somatotopic map is a representation of the body in the primary somatosensory cortex, known as homunculus.
• Medial portion: Represents the legs and feet.
• Central portion: Represents the arm and hand.
• Lateral portion: Represents the face and oral pharynx.
• In Vivo Intercortical Recordings: Stimulating or recording activity in different parts of the cortex reveals which body part each region represents.

Plasticity of the Somatosensory Cortex:

• Homunculus is not static and can change throughout life, demonstrating plasticity.
• Loss of sensory input: Adjacent brain areas can take over the representation of the lost body part.
• Phantom limb sensation: Sensations in a missing limb due to the brain's representation of the limb still existing.

Mirror Box Therapy:

• Phantom limb pain: Pain or discomfort in a missing limb.
• Mirror box therapy: Uses a mirror to create a visual illusion of the missing limb and reduce pain by stimulating adjacent areas on the cortex.

Patellar Tendon Reflex Pathway:

• Sensory input: Striking the patellar tendon stimulates muscle spindles.
• First-order neuron activation: Stimulated muscle spindle sends a signal via type 1A sensory neurons to the spinal cord.
• Two pathways:
  • One pathway goes to the gracile nucleus in the medulla.
  • The other pathway directly synapses onto an alpha motor neuron in the spinal cord's anterior horn gray matter.
• Motor response: Alpha motor neuron sends a signal back to the quadriceps muscle, causing contraction and leg kick.
• Significant points:
  • Gamma motor neuron system is not involved in this reflex.
  • This reflex does not require a spinal cord interneuron.

Type 1A Sensory Axon:

• This is the first instance of axon collateralization observed.
• A single fiber can synapse at multiple locations, even if only coming from a single muscle spindle.

Golgi Tendon Organ Reflex:

• Responds to muscle load and is protective.
• Characteristics:
  • Polysynaptic: Involves multiple synapses, including an interneuron in the spinal cord.
  • Heterogenic: Response can occur in multiple muscles.
  • Ipsilateral: Reflex action occurs on the same side as the stimulus.
• Mechanism:
  • Stimulus: Overloading the muscle activates the Golgi tendon organ.
  • Golgi Tendon Organ Activation: The Golgi tendon organ (located at the muscle-tendon junction) sends a signal via type 1B sensory neurons to the spinal cord.
  • Signal Pathway: The sensory neuron collateralizes and synapses onto an inhibitory interneuron in the spinal cord.
  • Motor Response: Inhibitory interneuron suppresses the activation of the alpha motor neuron, stopping muscle contraction and facilitating antagonist muscle contraction for load control.
• Important Points: The Golgi Tendon Organ Reflex prevents the application of excessive force to the muscle.

Taste System (Gustatory System):

• Basic Taste Sensations: There are five basic tastes: salty, sour, sweet, bitter, and umami.
• Taste Bud Anatomy: Taste buds are located within papillae on the tongue, soft palate, and epiglottis.
• Taste Receptor Cells: These cells are specialized epithelial cells that detect specific tastants.
• Sensory Transduction: Taste receptors transduce chemical stimuli into electrical signals.
• Taste Cell Types:
  • Type I (Glial-like): Detect salty taste via ion channels, also involved in ATP processing.
  • Type II (Receptor Cells): Most common, detect sweet, bitter, and umami tastes through G-protein coupled receptors.
  • Type III (Presynaptic Cells): Detect sour taste and release various neurotransmitters.
• Transduction Pathways For Different Tastes: Salty and sour tastes use direct ion channels. Sweet, bitter, and umami tastes use G-protein coupled receptors.
• Taste Cell Replacement: Taste cells are frequently replaced by stem cells, leading to a continuous renewal of the epithelium.

Sensory Pathway to the Brain

• Cranial Nerve Involvement: Taste information is carried by three cranial nerves: VII (facial), IX (glossopharyngeal), and X (vagus) to the nucleus of the solitary tract (NTS) in the brainstem.
• NTS Processing: Neurons in the NTS respond to various stimulants, including water, salt, bitter, sweet, and acidic solutions.
• Thalamus and Gustatory Cortex: Signals are relayed to the ventral posterior medial nucleus of the thalamus and then to the gustatory cortex near the anterior insula and frontal operculum.
• Specificity at Gustatory Cortex: Neurons in the gustatory cortex exhibit more specificity for certain tastants, refining taste sensation.

Smell System (Olfactory System):

• Role of Smell in Taste: Retronasal olfaction enhances taste perception significantly, contributing to our perception of flavor.
• Smell and Taste Interaction: Taste and smell combine to create the perception of flavor.

Olfactory Anatomy:

• Olfactory Epithelium: The olfactory epithelium lines the upper part of the nasal cavity and contains specialized odorant receptor cells.
• Odorant Receptor Cells: These cells have cilia that bind odorant molecules.
• Olfactory Nerve: Axons from olfactory receptor cells form the olfactory nerve, which carries signals to the olfactory bulb.
• Olfactory Bulb: This structure processes olfactory information and relays it to higher brain regions.
• Olfactory Tract: This tract carries olfactory information from the olfactory bulb to the brain.
• Olfactory Cortex: The olfactory cortex is involved in processing and perceiving smells.
• Olfactory Receptor Genes: The olfactory system has numerous genes that code for odorant receptors, allowing detection of a wide range of odors.