Spinal Cord Anatomy and Function

Spinal Cord (CNS)

• Conus Medullaris: Tapered lower end of the spinal cord (around L1-L2), marking its termination.
• Cervical Enlargement: Thickened area in the cervical region (C4-T1), containing motor neurons for upper limb control.
• Lumbar Enlargement: Thickened area in the lumbar region (T11-L1), containing motor neurons for lower limb control.
• Cauda Equina: Bundle of spinal nerves and roots extending from the conus medullaris.
• Sensory Neurons: Cell bodies in dorsal root ganglia; axons enter the dorsal horn of gray matter.
• Interneurons: Found within the spinal cord's gray matter, processing sensory and motor information (dorsal and ventral horns).
• Somatic Motor Neurons: Located in the ventral horn, controlling voluntary muscles.
• Visceral Motor Neurons (Autonomic): Located in the lateral horn (thoracic and lumbar segments), controlling involuntary muscles and glands.
• Dorsal Column-Medial Lemniscal Pathway: Carries fine touch, proprioception, and vibration sensations, crossing over in the medulla.
• Spinothalamic Pathway: Carries pain and temperature sensations, crossing over in the spinal cord.
• Corticospinal Pathway: Carries voluntary motor commands from the brain, crossing over in the medulla (lateral corticospinal) or spinal cord (anterior corticospinal).
• Upper Motor Neuron (UMN) Lesion: Damage to motor neurons in the brain or spinal cord before the spinal cord synapse; results in spasticity, hyperreflexia, and weakness.
• Lower Motor Neuron (LMN) Lesion: Damage to motor neurons directly innervating muscles; results in flaccid paralysis, muscle atrophy, and hyporeflexia.
• Damage to Dorsal Horn: Results in sensory loss (pain, temperature, proprioception).
• Damage to Dorsal Root: Causes sensory loss or numbness.
• Damage to Ventral Horn/Root: Results in motor deficits (weakness or paralysis).
• Damage to Corticospinal Tract (UMN): Results in spastic paralysis and hyperreflexia.

Peripheral Nervous System (PNS)

• Interoceptors: Detect internal stimuli (e.g., stretch receptors in digestive organs).
• Exteroceptors: Detect external stimuli (e.g., touch, temperature, vision).
• Thermoreceptors: Respond to temperature changes.
• Mechanoreceptors: Respond to mechanical pressure or deformation (e.g., Pacinian corpuscles, Merkel discs).
• Nociceptors: Detect pain (mechanical, thermal, chemical).
• Knee-Jerk (Patellar Reflex): Monosynaptic reflex; maintains posture by stretching quadriceps muscle.
• Crossed Extensor Reflex: Reflex that occurs with a withdrawal reflex; supports body weight on opposite limb during withdrawal.
• Withdrawal Reflex: Protective reflex removing a body part from a harmful stimulus.
• Spinal Reflex Arc: Stimulus → Receptor → Sensory Neuron → Integration Center (spinal cord) → Motor Neuron → Effector (muscle/gland).
• Perceptual Processing: Thalamus relays sensory information to the primary sensory cortex for processing.
• Stimulus Strength Communication (Frequency Code): Strength of a stimulus is communicated by frequency of action potentials. Stronger stimulus = higher frequency.
• Receptor Ending Types:
  • Nociceptors: Pain receptors.
  • Pacinian/Lamellar Corpuscles: Detect vibration and pressure.

Autonomic Nervous System (ANS)

• Sympathetic Nervous System (SNS): Prepares body for "fight or flight"; short preganglionic, long postganglionic neurons; neurotransmitter: norepinephrine (NE).
• Parasympathetic Nervous System (PNS): Promotes "rest and digest"; long preganglionic, short postganglionic neurons; neurotransmitter: acetylcholine (ACh).
• Sympathetic Functions: Dilated pupils, increased heart rate, bronchodilation, vasoconstriction (except in skeletal muscles), inhibited digestion, etc.
• Parasympathetic Functions: Constricted pupils, slowed heart rate, promoted digestion, constricted airways, etc.
• Neurotransmitters:
  • Acetylcholine (ACh): Released by all preganglionic neurons & PNS postganglionic neurons.
  • Norepinephrine (NE): Released by most SNS postganglionic neurons.
• Receptor Types:
  • Cholinergic receptors (ACh): Nicotinic (postganglionic neurons), muscarinic (target organs in PNS).
  • Adrenergic receptors (NE): Alpha (e.g., alpha-1 for vasoconstriction), beta (e.g., beta-1 for heart rate, beta-2 for bronchodilation).
• Tables 14.3 & 14.4: Crucial for understanding neurotransmitters, receptors, and their effects, as well as drug interactions.

Special Senses

• Eye Anatomy:
  • Fovea Centralis: Central retina for sharp central vision.
  • Retina: Contains photoreceptors (rods for low-light, cones for color).
  • Humors: Aqueous (front) and vitreous (behind) humor maintain shape and refract light.
  • Optic Disc: "Blind spot" where optic nerve exits the eye.
• Ear Anatomy:
  • Tympanic Membrane (eardrum): Vibrates in response to sound waves.
  • Otoliths: Small crystals that detect head position (vestibular system).
  • Semicircular Canals: Detect rotational head movement.
  • Pharyngotympanic Tube (Eustachian tube): Equalizes pressure between middle ear and exterior.
• Special Senses Characteristics:
  • Photoreceptors (eye): Rods and cones detect different light.
  • Taste (gustatory): Taste cells respond to food chemicals.
  • Olfaction (smell): Chemoreceptors in nose detect odor molecules.

This quiz covers key concepts related to the spinal cord's structure and function, including the conus medullaris, cervical and lumbar enlargements, and the types of neurons present. Test your understanding of the sensory and motor pathways and their roles in the central nervous system.