Organization, Cellular Components, and Topography of the CNS
ORGANIZATION, CELLULAR COMPONENTS, AND TOPOGRAPHY OF THE CNS
MODULE 1, PART 1
INTRODUCTION
How does the Nervous System work?
Begins with the Neuron.
Neurons:
Respond to stimuli.
Convey signals.
Process neurological information.
Neurological signals (impulses) enable:
Awareness of “self.”
Memory.
Learning.
Speech.
Muscle coordination.
Glandular secretion.
NERVOUS CIRCUITS
Types of nervous circuits:
Sensory circuit:
Initiated by sensory stimulus.
Reflex circuit:
Involves a movement or response (e.g., secretion).
Relay circuit:
Leads to sensory perception from sensory stimuli.
Overlapping circuits can combine elements from the above types to enhance response mechanisms.
FUNCTIONAL PATHS
Functional paths consist of numerous nerve cell bodies and axons.
Nerve cell bodies:
May form groupings known as nuclei or ganglia.
Axons in a functional path typically form bundles termed tracts, fasciculi, or nerves.
The nervous system includes functional paths where neuronal cell bodies reside in nuclei, ganglia, or laminae, with axons in tracts or nerves.
CENTRAL NERVOUS SYSTEM VS. PERIPHERAL NERVOUS SYSTEM
Central Nervous System (CNS):
Comprises the brain and spinal cord.
Peripheral Nervous System (PNS):
Comprises cranial nerves, spinal nerves, autonomic nerves, and their ganglia.
CNS VS PNS CONT.
Functions of the CNS:
Integrates, processes, and controls the entire nervous system.
Receives input about changes in the internal and external environments.
Interprets and integrates information.
Provides output signals for activities such as movement or secretion.
Functions of the PNS:
Connects the CNS to body tissues and organs.
Responsible for conveying input and output signals to and from the CNS.
Signals going to the CNS are termed Afferent (A for “Arriving” at the spinal cord; Sensory).
Signals departing the CNS are termed Efferent (E for “Exiting” the spinal cord; Motor).
SUPPORT AND PROTECTION
Protection of the CNS:
The brain is safeguarded by the skull.
The spinal cord is shielded by vertebrae.
All of the CNS is enveloped by meninges.
MENINGES
The meninges consist of three layers (superficial to deep):
Dura mater:
Outermost layer; strong and fibrous.
Consists of two layers with a venous sinus between them.
Includes Dural Folds:
Falx cerebri: longitudinally separates cerebral hemispheres.
Falx cerebelli: longitudinally separates cerebellar hemispheres.
Tentorium cerebelli: separates the posterior cerebral hemispheres from the cerebellum.
Diaphragm sellae: covers the sella turcica where the pituitary gland resides.
Arachnoid:
Thin, delicate layer surrounding the brain and spinal cord with cobweb-like projections (trabeculae) connecting to the pia mater.
Pia mater:
Thin, highly vascular layer.
Contains small blood vessels supplying the brain and spinal cord.
MENINGEAL SPACES
Epidural space: Located between the bone and the dura mater.
Subdural space: Found between the dura and arachnoid layers.
Subarachnoid space: Located between the arachnoid and pia mater; contains Cerebrospinal fluid (CSF).
In case of trauma to the head, blood vessels passing through these spaces may tear, leading to a hematoma.
Pooling of blood between the dura and arachnoid layer results in a subdural hematoma.
CLINICAL CONNECTION
Viral or Bacterial Meningitis:
Potentially life-threatening; causes inflammation of meningeal layers.
Signs & Symptoms (S/S):
Neck stiffness (nuchal rigidity).
Headache.
Fever.
Altered consciousness.
Vomiting.
Aversion to bright light or loud noises.
In children, potential for deafness.
Diagnosis: Conducted via lumbar puncture if no elevated intracranial pressure is present.
SUPPORTING CELLS IN THE CNS
Astrocytes:
Exclusive to the CNS; most numerous cells in the CNS.
Form the glial membrane and surround capillaries.
After injuries (ischemia, trauma), astrocytes can form scars that impair their functional capacity.
Endothelial cells:
Form the blood-brain barrier (BBB), selectively regulating substances entering the CNS.
Oligodendrocytes:
Exclusive to the CNS; responsible for formation and maintenance of myelin in the CNS.
Wrap around nerves, facilitating rapid transmission of nerve impulses and supporting CNS nerves.
Disorders such as multiple sclerosis (MS) involve autoimmune attacks on myelin sheaths.
SUPPORTING CELLS IN THE PNS
Schwann cells:
Unique to the PNS; perform functions similar to oligodendrocytes but only in the PNS.
Small gaps between Schwann cell wrappings are referred to as Nodes of Ranvier.
Autoimmune reactions (e.g., Guillain-Barre Syndrome) attack PNS myelin.
Regeneration ability:
If an axon is severed, it can regrow unmyelinated immature rootlets to reconnect the two ends of the cut nerve.
Note: Axonal regeneration is restricted to the PNS (not the CNS).
NEURONS
Neurons:
Smallest functional unit of the nervous system.
Composed of:
Cell body: Contains the nucleus and cytoplasm (including mitochondria).
Dendrites.
Axon.
The area devoid of rough endoplasmic reticulum (Nissl bodies) is termed the Axon Hillock.
For comparisons of axon and dendrite functions, refer to Table 1-1 in the textbook.
3 MAIN CLASSIFICATIONS OF NEURONS
Unipolar neurons:
Located in ganglia of spinal nerves.
Physiological dendrites; cell body positioned between dendrite and axon.
Bipolar neurons:
Operate as sensory nerves.
Possess true dendrites; cell body situated between dendrite and axon.
Multipolar neurons:
Comprise the majority of nerve cells.
Multiple dendrites interfacing directly with the cell body, with impulses sent down the axon.
RESTING CELL MEMBRANE POTENTIAL
Ion distribution:
Na+ (sodium) ions predominantly extracellular (outside the cell).
K+ (potassium) ions predominantly intracellular (inside the cell).
Ion channels: Allow passive flow of Na+ and K+ ions across the cell membrane.
Resting membrane potential determined by the concentrations of Na+ enclosed with the cell and K+ outside, along with the activity of the Na+/K+ pump (which requires ATP).
The pump moves 3 Na+ ions out and 2 K+ ions into the cell with each cycle.
ACTION POTENTIALS OF NEURONS
Action potentials initiate when a membrane threshold is attained, leading to the opening of Na+ channels along the cell membrane.
This allows for an influx of sodium ions into the cell, resulting in depolarization (increase in positive charge).
Depolarization and repolarization propagate in one direction down the axon toward the terminal branches or synapse.
Speed increases with axon diameter and myelination.
MYELINATED VS UNMYELINATED NEURONS
Myelinated neurons:
Exhibit faster conduction speeds of nerve impulses.
Thicker myelin sheaths (larger diameters) result in expedited impulse conduction.
Myelination enables Saltatory Conduction, where impulses can “jump” between nodes on the axon.
Unmyelinated neurons:
Typically possess smaller diameters; impulse transmission is slower.
In this case, impulses undergo nonsaltatory conduction.
SYNAPSES
The junction wherein an axonal ending meets a neuron, muscle cell, or gland is termed a Synapse.
A synpatic cleft consists of an axon, a synapse, and endpoint tissue (neuron, muscle, or gland).
Impulses crossing the synaptic cleft consistently propagate in one direction across it.
Neurotransmitters: These chemicals traverse the synaptic cleft to influence the postsynaptic neuron, gland, or muscle.
At the neuromuscular junction (NMJ), neurotransmitters are typically excitatory, facilitating muscle contraction or gland secretion.
PATHOLOGICAL DISEASES OF NEUROTRANSMISSION
Pathological conditions may arise from either a reduction in presynaptic release of Acetylcholine (ACh) or by impairing postsynaptic actions of ACh.
MYASTHENIA GRAVIS
An autoimmune disease impacting acetylcholine receptors.
Consequences include muscle weakness in various regions:
Orbital.
Oropharyngeal.
Skeletal musculature.
Nerve fibers and ACh release remain normal; however, antibodies attack acetylcholine receptors at postjunctional folds.
This leads to diminished strength (amplitude) of the action potential signal, resulting in weakened muscle action.
GUILLAIN-BARRE SYNDROME
Acquired acute onset inflammatory peripheral demyelinating neuropathy with axonal sparing.
Mechanism: Myelin is stripped from PNS axons, significantly reducing nerve conduction velocities.
Manifests as gradually progressive weakness starting in the legs and ascending to arms, often described as “ascending paralysis.”
Common symptoms include difficulty in walking and rising from a chair.
If left untreated, may progress to respiratory muscles, posing a risk of respiratory failure.
MULTIPLE SCLEROSIS (MS)
The most common acquired demyelinating disease of the CNS of immunologic origin.
Resembles Guillain-Barre Syndrome, but affects the CNS rather than the PNS.
Hallmark Sign: Presence of demyelinating plaques that obstruct or slow nerve impulses.
Characterized by cycles of relapse and remission:
Remission phases show improved symptoms indicating partial remyelination of impaired axons.
Relapse phases reflect ongoing demyelination and degradation of symptoms.
OTHER COMMON DISORDERS
NERVE COMPRESSION (ENTRAPMENT)
Most frequently observed in median nerve during carpal tunnel syndrome.
Leads to muscle weakness in the hand and numbness, tingling, burning pain along the thumb and first 2.5 fingers on the palmar surface.
Caused by repetitive hand movements inflaming surrounding structures, compressing the median nerve.
Other contributing factors: obesity & pregnancy can raise pressure on the median nerve.
Common Treatments:
Splinting.
Steroid injections.
Surgical intervention (carpal tunnel release) if severe.
DISEASE-BASED NEUROPATHIES
Generally bilateral, affecting sensorimotor axons in distal lower and upper extremities.
Example: foot numbness in diabetic patients.
Initial Symptoms:
Burning sensations, tingling, numbness, and muscle weakness.
Chronic Symptoms:
Loss of sensation, decreased muscle bulk, abnormal reflexes, muscle fasciculations.
Referred to as polyneuropathies.
Diabetes Mellitus is the most prevalent cause of polyneuropathies.
AXON REGENERATION
Most body cells have regenerative abilities except for nerve cells.
Once a neuron cell body is lost, it cannot be replaced.
Axons can regenerate and regain functionality if the cell body remains intact.
Crucially, axon regeneration occurs exclusively in the PNS (not in the CNS).
Damage to neuron cell bodies in the CNS (like those in the brain and spinal cord) is irreparable.