Neuroanatomy: Central Nervous System Structure

Neuroanatomy

• The Central Nervous System (CNS) structurally consists of the brain and spinal cord.
• The brain is divided into four structural components: Cerebrum, Diencephalon, Brainstem, and Cerebellum.
• Cerebrum is divided into four lobes: Frontal (Primary Motor cortex), Parietal (Pain & touch sensory), Occipital (Vision cortex), and Temporal (Auditory and speech centers).
• The Cerebral Cortex is the outer 3mm layer of cerebral hemispheres, divided into structurally distinct areas called Brodmann areas.
• The Diencephalon consists of the Thalamus, Hypothalamus, Epithalamus, and Subthalamus.
• Thalamus is the "Relay Station" that integrates and transmits sensory information to various cortical areas via separate pathways.
• Hypothalamus is the primary neurohumoral organ.

Cerebellum

• Cerebellum integrates afferent information received from other areas of CNS & PNS to be transmitted to the cerebral cortex and to lower motor neurons for muscle tone, equilibrium, and voluntary movement coordination.
• Cerebellum is connected via cerebellar peduncles which have both efferent and afferent pathways.
• Archicerebellum maintains equilibrium, Paleocerebellum regulates muscle tone, and Neocerebellum coordinates voluntary movement.

Brainstem

• Brainstem consists of midbrain, pons, and medulla.
• Brainstem is responsible for consciousness, autonomic functions, and many reflexes.
• Contains ascending and descending fiber tracts and extends to Foramen Magnum.

Electrophysiology

• There are two primary CNS cell types: Neurons and Glial Cells.
• Neurons conduct antegrade impulses from dendrites to soma to axon to synaptic terminals.
• Gray matter is composed of cell bodies, and White matter is composed of axons.
• The majority of CNS neurons are either Multipolar motor neurons or Pseudounipolar sensory neurons.

Neurons

• Nerve terminals contain neurotransmitters within vesicles.
• Upon depolarization, voltage-gated Ca++ channels open, allowing vesicles to release neurotransmitters into the synaptic cleft.
• Excitatory neurotransmitters (e.g., Glutamate) hypopolarize the postsynaptic neuron, while Inhibitory neurotransmitters (e.g., GABA, Glycine) hyperpolarize the postsynaptic neuron.

Cranial Nerves

• Cranial nerves are made up of sensory and/or motor neurons.
• Cranial nerves exit the cranial cavity through foramina in the cranium, except for CN XI which arises from the superior part of the spinal cord.

Glial Cells

• Glial cells are more abundant (5x) and supportive in nature, maintaining ionic environment, modulating action potential conduction, controlling reuptake of neurotransmitters, and repairing neurons.
• Glial cell subtypes include Astrocytes, Ependymal cells, Oligodendrocytes, and Microglia.

Blood-Brain Barrier

• The Blood-Brain Barrier maintains homeostasis, reducing electrophysical activity, decreasing Ca++ entry, glutamate release, and protein kinase activity, while stabilizing proteins and slowing enzymatic activity.

Neuroanesthesia

• Neuroanesthesia is the practice of applied cerebrovascular physiology and pharmacology, using techniques and agents to control CMR & CBF, which subsequently manipulates CBV.
• Induction goal is to prevent increases in ICP and decreases in CPP, while Maintenance goal is to provide adequate substrate delivery and control brain tension by controlling CBF/CBV.

Goodies

• Benzodiazepines are useful anesthetic adjuncts due to their anxiolytic, anticonvulsant, and amnestic effects, with reductions in CBF and CMR.
• Opioid-based anesthetic techniques are popular in neuroanesthesia because they provide hemodynamic stability and a predictable emergence, with minimal effects on CBF and CMR.
• Ketamine causes a dramatic increase in CBF (up to 60%) and a lesser increase in CMR, with impeded CSF absorption, leading to increased ICP.

Neuromuscular Blocking Agents

• Nondepolarizing muscle relaxants have a direct effect on cerebral physiology due to histamine release, which dilates the cerebral vasculature.
• Succinylcholine can produce increases in ICP, attenuated with a defasciculating dose of nondepolarizers.

Neuroanatomy

• The Central Nervous System (CNS) consists of the brain and spinal cord.
• The brain is divided into four structural components:
  • Cerebrum
  • Diencephalon
  • Brainstem
  • Cerebellum
• Cerebrum:
  • Consists of the cerebral cortex, which is responsible for cognition, movement, and sensation.
  • Divided into four lobes: frontal, parietal, occipital, and temporal.
  • Can be further divided into structurally distinct areas called Brodmann areas.
• Diencephalon:
  • Located midline between the two hemispheres.
  • Consists of the thalamus, hypothalamus, epithalamus, and subthalamus.
  • Thalamus acts as a "relay station" for integrating and transmitting sensory information to various cortical areas.
• Cerebellum:
  • Integrates afferent information from other areas of the CNS and PNS.
  • Transmits information to the cerebral cortex and lower motor neurons for muscle tone, equilibrium, and voluntary movement coordination.
  • Connected to the brain via cerebellar peduncles.
  • Divided into three parts: archicerebellum, paleocerebellum, and neocerebellum.
• Brainstem:
  • Consists of the midbrain, pons, and medulla.
  • Responsible for consciousness, autonomic functions, and many reflexes.
  • Contains ascending and descending fiber tracts.

Electrophysiology

• There are two primary CNS cell types: neurons and glial cells.
• Neurons conduct antegrade impulses from the dendrites to the soma to the axon to the synaptic terminals.
• Gray matter consists of cell bodies, while white matter consists of axons.
• The majority of CNS neurons are either multipolar motor neurons or pseudounipolar sensory neurons.
• Neurons release neurotransmitters into the synaptic cleft, which can be excitatory or inhibitory.

Cranial Nerves

• Cranial nerves are made up of sensory and/or motor neurons.
• They exit the cranial cavity through foramina in the cranium.

Intracranial Pressure

• Intracranial hypertension results in reduced cerebral perfusion pressure (CPP) and oxygen delivery.
• Cushing's triad: decreased CPP, hypertension, and bradycardia.
• Other symptoms of intracranial hypertension include headache, N/V, papilledema, pupil dilation, focal deficits, seizure, and coma.
• Intracranial hypertension can lead to cerebral ischemia and further decreased CPP.

Cerebral Ischemia

• The brain is highly sensitive to ischemia due to its high energy needs and limited storage capacity of essential substrates.
• Ischemia results in inefficient glycolysis and reduced ATP production.
• This leads to the release of excitatory neurotransmitters, further depolarization, and increased calcium influx.

Anesthetic Considerations

• Inhalation anesthetics decrease cerebral metabolic rate (CMR) in a concentration-dependent manner.
• Volatile anesthetics are potent cerebrovascular dilators, increasing cerebral blood flow (CBF).
• Isoflurane produces the greatest decrease in CMR and is the least potent cerebral vasodilator.
• Nitrous oxide increases CBF, CMR, and intracranial pressure (ICP).
• Opioid-based anesthetic techniques are popular in neuroanesthesia due to their hemodynamic stability and predictable emergence.
• Ketamine causes a dramatic increase in CBF and ICP, but provides protection against neuronal cell death.

Neuromuscular Blocking Agents

• Nondepolarizing muscle relaxants have a direct effect on cerebral physiology due to histamine release.
• Histamine directly dilates the cerebral vasculature.
• Atracurium has significant histamine release, while pancuronium demonstrates sympathetic effects on induction.
• Succinylcholine can produce increases in ICP, which can be attenuated with a defasciculating dose of nondepolarizers.