Spinal Cord

Spinal Cord Anatomy

  • The spinal cord extends from the brainstem and ends around the L1 vertebra.

  • Key structures:

    • Cervical Enlargements: Supports the upper limbs.

    • Lumbar Enlargements: Supports the lower limbs.

    • Conus Medullaris: Tapering end of the spinal cord.

    • Cauda Equina: Bundle of nerve roots extending from the spinal cord.

    • Filum Terminale: A connective tissue strand anchoring the spinal cord.

  • Pia Mater: A delicate inner meninges layer attaches to the spinal cord, forming the filum terminale.

Spinal Nerve Anatomy

  • The spinal nerve is formed from two roots:

    • Anterior (Ventral) Roots: Contain motor neurons; send signals to muscles.

    • Posterior (Dorsal) Roots: Contain sensory neurons; receive signals from the body.

  • Posterior Root Ganglion: Contains the cell bodies of sensory neurons.

  • The anterior ramus innervates the skin and muscles of the body, while the posterior ramus innervates the back.

Cross-Sectional Features of the Spinal Cord

  • Identified structures include:

    • Anterior Horn: Contains motor neuron cell bodies.

    • Posterior Horn: Contains sensory neuron axon endings.

    • Lateral Horn: Present in thoracic regions; contains autonomic neurons.

    • Gray Commissure: Contains unmyelinated axons crossing to the opposite side.

    • Central Canal: Filled with cerebrospinal fluid (CSF).

    • Funiculi (Columns): Anterior, lateral, and posterior columns contain ascending and descending tracts.

Spinal Nerve Plexuses

  • Defined as networks formed by the anterior rami of spinal nerves (except thoracic).

  • Major plexuses include:

    • Cervical Plexus: Innervates neck and diaphragm.

    • Brachial Plexus: Innervates the upper limbs.

    • Lumbar Plexus: Innervates the lower abdomen and thighs.

    • Sacral Plexus: Innervates the pelvis and lower limbs.

Sensory Pathway Neuron Structure

  1. First-order Neurons: Receive sensory information; peripheral neuron terminates in the spinal cord.

  2. Second-order Neurons: Interneurons in the spinal cord extend to the thalamus.

  3. Third-order Neurons: Project from the thalamus to the sensory cortex for processing.

    • Example: Pain in the thumb is processed through this pathway.

Motor Pathway Neuron Structure

  1. Upper Motor Neurons: Originate in the primary motor cortex and descend through the brain to the spinal cord.

    • Decussates (crosses over) in the medulla oblongata.

  2. Lower Motor Neurons: Cell bodies in the anterior horn; axons exit via spinal nerves to target muscles.

Decussation Concept

  • Decussation refers to the crossing of nerve fibers from one side of the brain to the other, resulting in:

    • Right side of the brain controlling the left side of the body and vice versa.

Autonomic vs. Somatic Nervous System

  • Somatic Nervous System (SNS):

    • Controls voluntary movements via one motor neuron to muscles.

    • Uses Acetylcholine (ACh) at neuromuscular junctions.

  • Autonomic Nervous System (ANS):

    • Controls involuntary functions with a two-neuron chain:

    • Preganglionic Neuron: From CNS to a ganglion.

    • Postganglionic Neuron: From the ganglion to the target organ.

    • Sympathetic Division: Involves fight-or-flight responses (uses Norepinephrine, NE).

    • Parasympathetic Division: Promotes rest-and-digest functions (releases ACh).

Neurotransmitters in the ANS

  • Acetylcholine (ACh):

    • Used in autonomic ganglia and neuromuscular junctions (NMJs) for somatic functions.

    • Has nicotinic and muscarinic receptors.

  • Norepinephrine (NE):

    • Released at sympathetic NMJs, involved in stress response.

  • Epinephrine (Adrenaline):

    • Hormone released by the adrenal medulla during stress responses.