Neuroanatomy: Meninges and Cerebrospinal Fluid

Meninges

• The meninges are protective membranes that enclose the brain and spinal cord.
• Inflammation of the meninges is known as meningitis.
• Pachymeningitis refers to inflammation of the dura mater, the thick outer layer of the meninges.
• Leptomeningitis refers to inflammation of the arachnoid membrane and pia mater, the thin middle and inner layers of the meninges.

Blood-Brain Barrier

• The blood-brain barrier is a barrier between the plasma within the CNS vessels and the extracellular fluid (ECF).
• The blood-brain barrier acts to protect the brain from harmful substances.

Subarachnoid Space

• The subarachnoid space is located between the arachnoid and pia mater, containing cerebrospinal fluid (CSF).

Cerebrospinal Fluid (CSF)

• CSF is a clear, colorless fluid that circulates throughout the brain and spinal cord.
• CSF samples can be collected from the lumbar cistern, an enlargement of the SAS at the caudal cerebellum and dorsal brainstem.
• CSF is rich in glucose and contains low concentrations of white blood cells (WBCs).
• CSF plays a role in regulating intracranial pressure.

Ventricular System

• The ventricular system consists of a series of interconnected cavities within the brain filled with CSF.
• The lateral ventricles are the first and second ventricles, located on either side of the brain.
• The mesencephalic aqueduct connects the third ventricle to the fourth ventricle.

CSF Flow

• CSF flows from the lateral ventricles through the interventricular foramen into the third ventricle.
• CSF then flows through the mesencephalic aqueduct into the fourth ventricle.
• CSF then flows into the subarachnoid space via foramina found within the fourth ventricle.
• Blockages or a failure of CSF reabsorption can result in hydrocephalus.
• CSF is absorbed through veins to regulate intracranial pressure.

Cells of the Nervous System

• Neuroglia, also known as glial cells, are small cells that surround and support neurons.

Neuroglia in the Peripheral Nervous System

• Satellite cells are found in the PNS.

Neuroglia in the Central Nervous System

• Astrocytes are star-shaped glial cells that provide structural support to the CNS.
• Oligodendrocytes are branched cells in the CNS that wrap around axons to form myelin.

Neuron Communication

• Neurons communicate with each other via neurotransmitters (NTs).
• Synaptic transmission of NTs is a complex process involving docking, priming, fusion, recycling, and degradation.
• Release of NTs is calcium-dependent and regulated by SNARE proteins.
• Signal termination occurs due to NT degradation, re-uptake, and diffusion.

Neuromuscular Junction (NMJ)

• The NMJ is a specialized synapse between a motor neuron and a muscle fiber.
• Acetylcholine (ACh) is the NT released at the NMJ.
• Following ACh release, depolarization of the sarcolemma triggers a cascade of events leading to muscle contraction.

Neurotransmitter Systems

• Excitatory projection neurons release glutamate.
• Inhibitory interneurons release GABA.
• Neuromodulator systems consist of clusters of neurons in subcortical and brainstem nuclei that release NTs throughout the nervous system.

Neurotransmitters (NTs)

• Central Nervous System (CNS)
  • Drivers: glutamate, glycine, GABA
  • Modulators: acetylcholine, dopamine, norepinephrine, serotonin
• Peripheral Nervous System (PNS)
  • Drivers: acetylcholine
    • Sympathetic preganglionic neurons
    • Parasympathetic preganglionic neurons
  • Modulators:
    • Norepinephrine (sympathetic postganglionic neurons)
    • Acetylcholine (parasympathetic postganglionic neurons)

NT Receptors

• Ionotropic Receptors (ion channels):
  • NMDA (N-methyl-D-aspartate) receptors: these receptors are permeable to Na+, K+ and Ca++, and also voltage-gated, blocked by Mg++.
    • NMDA receptor antagonists include: ketamine, dextromethorphan, dextrorphan, and ethanol
  • AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors: these receptors are permeable to Na+ and K+
  • GABA receptors: agonists include benzodiazepines, antagonists include bicuculline.
• Metabotropic Receptors (G protein-coupled receptors):
  • These receptors are associated with slower responses compared to ionotropic receptors
  • Glutamate receptors
    • mGluR receptors are involved in regulating glutamate availability
    • These receptors can affect both presynaptic and postsynaptic potentials.
  • GABA receptors
    • GABA-B receptors modulate K+ and Ca++ conductance.

Sympathetic Nervous System (SNS)

• The SNS innervates the thoracolumbar region of the spinal cord.
• SNS fibers release norepinephrine (NE) to adrenergic receptors.
• SNS activation leads to the "fight or flight" response.
• Sympathomimetic drugs mimic the SNS.
• Sympatholytic drugs oppose the SNS.

Parasympathetic Nervous System (PNS)

• The PNS innervates cranial and sacral regions of the nervous system.
• The PNS promotes "rest and digest" functions.
• The PNS is innervated by CN III, VII, IX, and X, and the sacral nerves S2-4.
• The PNS uses acetylcholine (ACh) as its neurotransmitter.

Thoracic Cavity

• The thoracic cavity is bordered dorsally by the thoracic vertebrae and ventrally by the sternum.
• The cranial inlet is formed by the first rib and costal cartilages.
• The caudal outlet is bordered by T13, the 13th rib, and the xiphoid process.
• The diaphragm separates the thoracic cavity from the abdominal cavity.

Abdominal Cavity

• The abdominal cavity is bordered dorsally by the lumbar vertebrae and hypaxial muscles, ventrally by the abdominal wall muscles, cranially by the diaphragm, and caudally by the pelvic inlet.
• The diaphragm has three openings:
  • Aortic hiatus
  • Esophageal hiatus
  • Caval foramen

Pelvic cavity

• The pelvic cavity is cranial to the pelvic inlet.
• The pelvic cavity is innervated by the hypogastric nerves (sympathetic) and pelvic nerves (parasympathetic).

Micturition

• Micturition is the process of urination.
• The detrusor muscle (parasympathetically controlled) contracts to expel urine.
• The internal urethral sphincter (sympathetically controlled) relaxes.
• The external urethral sphincter (somatically controlled) relaxes.

Reflex Arcs

• Reflex arcs are neural pathways that mediate reflex actions.
• Reflex arcs involve sensory neurons, interneurons (integration center), motor neurons, and effectors.
• Somatic reflexes activate skeletal muscles.
• Autonomic reflexes involve smooth muscles, cardiac muscles, and glands.
• The hypothalamus is the main integrative center for both somatic and autonomic reflexes.

Thoracic Limb Innervation

• Digit I (radial nerve)
  • Lateral distal brachium
  • Dorsal proximal antebrachium
  • Dorsal manus
• Digit V (ulnar nerve)
  • Ventral antebrachium
  • Caudal distal brachium and antebrachium
• Musculocutaneous nerve
  • Dorsal brachium
  • Medial distal brachium
  • Proximal antebrachium

Ungulate Limb Innervation

• Thoracic Limb
  • Subscapular nerve
  • Suprascapular nerve
  • Cranial pectoral nerve
  • Musculocutaneous nerve
  • Median nerve
  • Ulnar Nerve
  • Radial nerve
  • Lateral thoracic nerve
  • Axillary nerve
  • Thoracodorsal nerve

Thoracic Limb Nerve Blocks

• Palmar nerve block
  • The palmar digital nerve block desensitizes the palmar foot, DIPJ, and sole of the foot.

Pelvic Limb Innervation

• Femoral nerve
  • Innervates the quadriceps muscle
• Obturator nerve
  • Innervates the adductor muscles
• Common fibular (peroneal) nerve
  • Innervates the extensor muscles of the digits
• Tibial nerve
  • Innervates the flexor muscles of the digits
• **Pudendal nerve **
  • Innervates the external anal sphincter muscle

Femoral Nerve Paralysis

• Femoral nerve paralysis can lead to an inability to extend the stifle joint.

This quiz covers key concepts related to the meninges, blood-brain barrier, subarachnoid space, and cerebrospinal fluid (CSF). Participants will explore the structure and function of these essential components of the central nervous system. Test your knowledge on inflammation conditions and the protective roles of these membranes.