Central Nervous System

Where is the central canal located?

at the center of the gray commissure.

What is a funiculus?

→ A major column of spinal cord white matter containing axonal pathways.

What are the meninges, and what do they protect?

Three connective tissue layers surrounding and protecting the brain and spinal cord.

Name the meninges from superficial to deep.

Dura mater

What is the dura mater?

The tough, fibrous outer meninx adjacent to the skull and vertebral canal.

What is the arachnoid mater?

A thin middle meninx with web-like trabeculae extending toward the pia mater.

What is the pia mater?

The delicate innermost meninx that closely follows the brain and spinal cord.

Where is the spinal epidural space?

Between the vertebral canal and dura mater; it contains fat and vessels.

Why is the spinal epidural space clinically important?

It is used to administer epidural anesthesia.

What is the difference between subdural and subarachnoid spaces?

Subdural is a potential space between dura and arachnoid; subarachnoid contains CSF between arachnoid and pia.

Where does an epidural hematoma occur?

Between the skull and dura mater.

Where does a subdural hematoma occur?

Between the dura and arachnoid mater.

Where does a subarachnoid hemorrhage occur?

In the CSF-filled space between arachnoid and pia mater.

What are the main functions of cerebrospinal fluid (CSF)?

Cushioning, buoyancy, chemical homeostasis, nutrient delivery, and waste removal.

How does CSF buoyancy affect the brain?

It reduces a brain weighing about 1,400 g to an effective weight of roughly 60 g.

How does CSF composition differ from blood plasma?

CSF has more water, Na⁺, and Cl⁻; less protein, glucose, K⁺, and Ca²⁺; and similar osmolarity.

Why is CSF protein concentration much lower than plasma protein?

Selective barriers restrict protein entry into CSF.

What produces most cerebrospinal fluid?

The choroid plexus within the brain ventricles.

How does active sodium transport promote CSF production?

Na⁺ moves into CSF, water follows by osmosis, and Cl⁻ moves through channels.

Approximately how much CSF is produced each day?

About 500 mL.

Approximately how much CSF is present at one time?

About 150 mL.

What is the pathway of CSF from the lateral ventricles to the fourth ventricle?

cerebral aqueduct

What do the interventricular foramina connect?

The lateral ventricles to the third ventricle.

What does the cerebral aqueduct connect?

The third ventricle to the fourth ventricle.

How does CSF leave the fourth ventricle?

Through the lateral apertures of Luschka and median aperture of Magendie.

How does CSF return to the circulation?

Mainly through arachnoid granulations into venous sinuses, with additional lymphatic drainage.

What is hydrocephalus?

Abnormal CSF accumulation, often from blocked flow or drainage, causing ventricular enlargement.

What are the lateral ventricles?

Two large C-shaped CSF-filled chambers in the brain.

What is the septum pellucidum?

A thin membrane separating the lateral ventricles.

What is the brain interstitial system?

Fluid-filled spaces around neurons and capillaries that support neuronal function.

What structures form the neurovascular unit?

Capillary endothelial cells, pericytes, astrocytes, and neurons.

What is the main role of endothelial tight junctions?

Prevent substances from passing freely between adjacent endothelial cells.

Why does the brain need a blood-brain barrier?

To maintain a stable chemical environment for neurons.

What is selective permeability?

Allowing certain substances to cross a barrier while restricting others.

What is the difference between the BBB and the blood-CSF barrier?

The BBB controls exchange at brain capillaries; the blood-CSF barrier controls exchange at the choroid plexus.

What is the role of astrocytes in BBB function?

They help regulate endothelial cells and maintain the neuronal environment.

What is the role of pericytes in the neurovascular unit?

They help support and regulate CNS capillaries.

Why is uncontrolled movement of ions into the brain dangerous?

Changes in extracellular ion concentrations can disrupt neuronal excitability.

How can glucose reach neurons despite the BBB?

Through specialized membrane transport proteins.

Why can some medications have difficulty entering the brain?

The BBB restricts the passage of many substances from blood into CNS tissue.

Why is BBB disruption clinically important?

It can expose CNS tissue to substances normally excluded from the brain.

What is the most common CNS neuron morphology?

Multipolar: several dendrites extending from the soma and one axon.

What identifies a pyramidal neuron?

A triangular cell body and extensive dendrites; common in the cerebral cortex.

Which neuroanatomist first described pyramidal neurons?

Santiago Ramón y Cajal.

How does a true unipolar neuron differ from a multipolar neuron?

True unipolar: dendrites branch from one long axon. Multipolar: several dendrites arise from the soma.

What distinguishes a pseudo-unipolar neuron?

It lacks dendrites and has peripheral and central axonal processes, with the soma between them.

Where are pseudo-unipolar sensory neuron somata commonly found?

Dorsal root ganglia in the peripheral nervous system.

What are oligodendrocyte precursor cells (OPCs)?

Dividing CNS precursor cells that can mature into myelin-forming oligodendrocytes.

How do OPCs contribute to CNS injury responses?

They are recruited to damaged regions and can differentiate into oligodendrocytes.

Which cells produce CNS myelin, and how many axons can one serve?

Oligodendrocytes; one can myelinate portions of up to approximately 50 axons.

Which cells produce PNS myelin?

Schwann cells; they also support regeneration of damaged peripheral axons.

What functions does CNS myelin serve?

It increases action-potential conduction speed and contributes to neuronal growth and repair.

Which CNS neuroglia are described as the most common?

Astrocytes.

How do astrocytes regulate neuronal excitability?

They absorb extracellular neurotransmitters, water, and ions, stabilizing the neuronal environment.

What happens if astrocytes fail to clear neurotransmitters?

Extracellular transmitters can accumulate, altering neuronal excitability.

How do astrocytes support neuronal energy metabolism?

They store glycogen and provide metabolic substrates, including glucose and lactate.

What is the structural role of astrocytes?

They help hold neurons in place and maintain the brain's three-dimensional organization.

How do astrocytes influence the blood-brain barrier?

They help regulate capillary endothelial cells and barrier function.

What vascular function do astrocytes perform?

They help regulate cerebral blood flow.

How can astrocytes stimulate CNS myelination?

They release ATP that stimulates oligodendrocytes to form myelin.

How are spinal astrocytes associated with pain hypersensitivity?

Astrocyte hypertrophy is linked to heightened pain sensitivity in animal models.

What are the main immune functions of microglia?

Detect pathogens and damaged cells, alert immune cells, and phagocytose debris.

What abnormal materials can microglia detect?

Protein tangles, plaques, DNA fragments, and damaged neuronal or glial material.

Where are ependymal cells located?

They line the brain ventricles and spinal cord central canal.

What are the principal functions of ependymal cells?

They contribute to CSF production and support the CNS fluid environment.

What do satellite cells do in the PNS?

Surround neuronal somata in sensory and autonomic ganglia and regulate their local environment.

What does asymmetric neural stem-cell division produce?

Another neural stem cell and a more specialized progenitor cell.

What occurs during the first six embryonic weeks?

According to the text, neural stem cells produce OPCs that migrate into CNS regions.

Why does fetal development produce excess neurons?

Neurons compete to reach suitable targets; those failing to connect can be eliminated.

What is apoptosis in developing neural tissue?

Programmed cell death that removes developing cells failing to establish appropriate connections.

By approximately which fetal week does the text describe this selection?

By the end of week 16.

Approximately how many neurons and synapses does the passage report?

About 10¹¹ neurons and 6 × 10¹³ synapses.

What influences neural stem-cell differentiation?

Cell orientation and cell-to-cell contacts in the developing nervous system.

How does radial neuronal migration occur?

Developing neurons travel along radial glial fibers that act as scaffolds.

What proportion of fetal neurons does the text associate with radial migration?

Approximately 90%.

How is adult tangential migration guided?

By signaling molecules released by astrocytes and other CNS cells.

Why is adult CNS repair difficult despite neurogenesis?

New neurons must recreate appropriate developmental connections and functional synapses.

What evidence supported neurogenesis in the adult hippocampus?

Radioactive labeling of dividing cells, including bromodeoxyuridine and atmospheric ¹⁴C evidence.

What is a neuronal growth cone?

A mobile, sensitive structure at the tip of a growing axon or dendrite.

How do guidance cues direct growing axons?

Attractive and repulsive extracellular molecules act on growth-cone receptors.

What is neuroplasticity?

Lifelong modification of neuronal structure and synaptic connections, often driven by activity.

What does "neurons that fire together, wire together" mean?

Coordinated neuronal activity tends to strengthen connections between neurons.

What is a divergent neural circuit?

One neuron distributes signals to multiple targets.

What is a convergent neural circuit?

Many neurons signal to fewer neurons, increasing sensitivity but reducing source specificity.

What is a rhythm-generating circuit?

An interconnected circuit producing repeating activity, such as breathing or walking.

What is a parallel after-discharge circuit?

Multiple neuronal chains feed a final neuron, supporting complex information processing.

What are emergent properties of neural circuits?

Complex functions arising from network interactions that single-neuron behavior cannot predict.

What are the components of a basic reflex arc?

Sensory afferent neuron, motor efferent neuron, and target organ or muscle.

How do monosynaptic and polysynaptic reflexes differ?

Monosynaptic: one afferent–efferent synapse. Polysynaptic: one or more CNS interneurons.

Why is the patellar reflex especially rapid?

Its monosynaptic pathway avoids additional interneuronal synapses and brain processing.

What is the main function of the spinal cord?

Carry sensory information toward the brain and motor information away from the brain.

Where does the spinal cord begin?

At the foramen magnum, where it continues from the brainstem.

What protects the spinal cord?

The vertebral column and three meninges: dura mater, arachnoid mater, and pia mater.

How are gray and white matter arranged in the spinal cord?

Gray matter is central and butterfly-shaped; white matter surrounds it.

What does spinal cord gray matter primarily contain?

Neuronal cell bodies.

What does spinal cord white matter primarily contain?

Axons organized into nerve tracts.

Which two structures divide the spinal cord into right and left halves?

The dorsal medial sulcus and ventral medial fissure.

What is the dorsal medial sulcus?

A groove along the posterior midline of the spinal cord.

What is the ventral medial fissure?

A deeper groove along the anterior midline of the spinal cord.

What are the three gray matter horns?

Dorsal, lateral, and ventral horns.