Nervous System

Nervous System

• The nervous system enables the body to respond to changes in its external and internal environments.
• The nervous system controls and integrates the functional activities of organs and organ systems.
• The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS).
  • The CNS includes the brain and spinal cord.
  • The PNS includes cranial nerves, spinal nerves, ganglia, and peripheral receptors.
  • The main component of the NS is nervous tissue (NT).
  • NT regulates and controls body functions.
  • NT is characterized by irritability and conductivity.

Anatomical Organization of the Nervous System

• The CNS consists of the brain and spinal cord, as well as tracts and nuclei.
  • Nuclei are collections of nerve cell bodies in the CNS.
  • Tracts refer to bundles of nerve fibers within the CNS.
• The PNS consists of ganglia, cranial nerves (12 pairs), spinal nerves (31 pairs), and peripheral receptors.
  • Ganglia contain the bodies of neurons located outside the CNS, glial cells, and a fibrous capsule on the surface.
  • Nerves are composed of nerve fiber fascicles and their connective tissue sheaths.

Peripheral Nerve

• Nerve fibers in the PNS are grouped in bundles, forming nerves.
• Individual nerve fibers are held together by connective tissue (CT) organized into:
  • Endoneurium, a thin layer of LCT surrounding each individual nerve fiber.
  • Perineurium, which surrounds each bundle of nerve fibers.
  • Epineurium, the irregular DCT that surrounds a peripheral nerve.
• Nerves facilitate communication between the brain and spinal cord centers, and the sense organs and effectors (muscles, glands).

Neuronal Plasticity and Regeneration

• After the maturation of the NS during childhood, the regenerative function of the NS is limited, especially in the CNS.
• This limitation stems from the high degree of specialization and differentiation of neurons in the CNS.
• NS organs are highly dependent on oxidative metabolism, requiring oxygen and nutrients from the bloodstream.
• In the event of trauma or insufficient blood supply, neurons are usually the first cells to die, typically within 4-5 minutes.
• Dead neurons are often replaced by the proliferation of glia in the CNS, leading to glial scarring.
• Despite this, some areas of the brain demonstrate the ability to reprogram themselves, engage other circuits and networks, and restore function to damaged areas. This phenomenon is known as neural plasticity.

Damage, Regeneration, and Degeneration of the Peripheral Nerve Axon

• Neurapraxia:
  • Causes a reversible defect in neural compression.
  • Regeneration occurs within hours to two weeks.
  • Axons are not interrupted, and Wallerian degeneration does not occur.
• Axonotmesis:
  • Axons are interrupted, but the nerve sheaths remain intact.
  • Wallerian degeneration occurs, but spontaneous nerve regeneration happens.
• Neurotmesis:
  • Anatomical interruption of the entire nerve occurs.
  • Spontaneous regeneration is impossible.
  • Neurosurgical intervention is required to suture the nerve.

Ganglia

• Ganglia are aggregations of neuronal cell bodies located outside the CNS.
• There are two types of ganglia: sensory and autonomic.
• Sensory ganglia are present in three cranial nerves (CNV, IX, and X) and the posterior roots of the spinal nerves.
• Autonomic ganglia transmit visceromotor information (sympathetic and parasympathetic).

Autonomic Ganglia

• Sympathetic ganglia are localized near the spinal cord, forming:
  • Paravertebral ganglia (sympathetic trunk ganglia).
  • Prevertebral ganglia (coeliac ganglia, aorticorenal ganglia, superior and inferior mesenteric ganglion).
  • The neurons of the adrenal gland medulla, which are sympathetic post-ganglionic neurons. The adrenal medulla is considered a sympathetic ganglion developmentally.
• Parasympathetic ganglia are often isolated in the vicinity of innervated organs or within the organ wall (intramural ganglia).

Sensory Ganglia

• Sensory ganglia are located just outside the CNS.
• Neurons in these ganglia carry impulses towards the CNS.
• Examples include dorsal root ganglia of spinal nerves and sensory ganglia of cranial nerves (trigeminal, facial, vestibulocochlear, glossopharyngeal, and vagus nerve).
• Sensory ganglia receive somatosensory information (proprioception, pressure, pain, temperature) and viscerosensory information (blood pressure, pH, internal organ tone).

Organization of the CNS

• The CNS (brain, spinal cord) contains regions of:
  • White matter, composed of myelinated axons and myelin-producing oligodendrocytes.
  • Grey matter, consisting of neuronal cell bodies, dendrites, initial unmyelinated portions of axons, and glial cells.
  • Neuropil, a part of the grey matter of the brain, is characterized by the projections of neurons and neuroglia, filling the space between neuronal bodies and vessels.
  • Grey matter is found at the surface of the cerebrum and cerebellum, forming the cortex.
  • Aggregates of neuronal cell bodies form islands of grey matter known as nuclei.
  • The central canal, in the middle of the grey matter, is present in the spinal canal.

Spinal Cord

• The spinal cord contains:
  • White matter, primarily consisting of myelinated fibers and myelin-producing oligodendrocytes.
  • Grey matter, composed of cell bodies, unmyelinated axons, and neuroglia.
• In the spinal cord, grey matter is located centrally, while white matter is found peripherally.

Horns of the Spinal Cord

• Grey matter in the spinal cord is subdivided into horns:
  • Anterior (ventral) horns are short, broad, and directed forward. They contain large multipolar somatomotor neurons.
  • Posterior (dorsal) horns are narrow, elongated, and directed backwards. They contain small, multipolar somatosensory and interneurons.
  • Lateral horns, present in the T1 to L2 segments, contain visceromotor neurons in the segments C8-L3 and S2-S4.

White Matter of the Spinal Cord

• White matter is divided into six columns (funiculi), which contain tracts:
  • Ascending tracts relay information from the spinal cord to the brain.
  • Descending tracts carry information from the brain to the spinal cord.

Cerebellum

• The cerebellum (small brain) is highly folded to accommodate millions of neurons in a compact area.
• The cerebellar cortex is responsible for maintaining balance and equilibrium, muscle tone, and coordination of skeletal muscles.

Structure of the Cerebellum

• External Structure:

  • Cerebellar vermis: The worm-like, unpaired central part.
  • Cerebellar hemispheres: The paired cerebellar hemispheres.
    • Cerebellar lobes: Three lobes.
    • Cerebellar lobules: Ten lobules.
    • Cerebellar folia: Layers of the cerebellar cortex.
    • Cerebellar fissures: Grooves that separate the lobes and sheets.

• Internal Structure:

  • Grey matter:
    • Cerebellar cortex: Cortex of the cerebellar hemispheres and vermis.
    • Cerebellar nuclei: Nuclei within the cerebellum.
  • White matter:
    • Medullary cerebellar body: Cerebellar medulla consists primarily of myelinated fibers.

Cerebellar Cortex

• Layers:
  • Outer molecular layer: Surface molecular layer.
  • Middle Purkinje cell layer: A narrow medium layer containing Purkinje cells.
  • Inner granule cell layer: The deepest granular layer.

Cerebellar Medulla

• Composed of:
  • Climbing fibers.
  • Mossy fibers.
  • Purkinje cell axons.
  • Deep cerebellar nuclei.
• Mossy fibers: Afferent axons from the cerebral cortex, pons, spinal cord, and vestibular nuclei to the cerebellum. They terminate on granule cells, sending excitatory stimuli to Purkinje cells.
• Climbing fibers: Afferent axons from the inferior olivary nuclei of the medulla that terminate on Purkinje cells.
• Deep cerebellar nuclei: Four nuclei (from lateral to medial): dentate, emboliform, globose, and fastigial.

Cerebellar Glomerulus

• Mossy fibers end as dilated terminals, forming rosettes.
• These rosettes are surrounded by dendrites of granule cells and axons of Golgi cells.
• The mossy fiber terminal, with surrounding dendrites and axons, forms lightly stained areas in the granular layer, known as cerebellar glomeruli.

Cerebellar Nuclei

• Embedded within the central core of white matter, masses of grey matter constitute the cerebellar nuclei (dentatus, emboliformis, globosus, and fastigius).

Cerebrum

• The cerebral hemispheres consist of a convoluted cortex of grey matter.

Cerebral Cortex

• Cytoarchitectonic:
  • Molecular layer (horizontal cells of Cajal; their axons form tangential plexus).
  • External granular layer (small pyramidal cells and stellate (granular) cells).
  • External pyramidal layer (medium-sized pyramidal cells).
  • Internal granular layer (densely packed stellate and pyramidal cells).
  • Internal pyramidal (ganglionic) layer (large pyramidal cells, and in the motor cortex, the giant pyramidal neurons of Betz).
  • Multiform layer (cells of various shapes).

Types of Cerebral Cortex

• The development of specific layers in the cerebral cortex, which are connected to various functions, leads to the formation of different types of cerebral cortex.
• The agranular type of cerebral cortex features the greatest development of layers III, V, and VI, with weak development of layers II and IV (granular layers), mainly found in motor centers.

Nervous System Overview

• The Nervous System (NS) enables the body to respond to internal and external changes.
• Controls and integrates the functional activities of organs and organ systems.
• Divided into the Central Nervous System (CNS) and Peripheral Nervous System (PNS).

Central Nervous System (CNS)

• Consists of the brain and spinal cord.
• Contains tracts (bundles of nerve fibers) and nuclei (collections of nerve cell bodies).

Peripheral Nervous System (PNS)

• Consists of ganglia (collections of neuronal cell bodies outside the CNS), cranial nerves (12 pairs), spinal nerves (31 pairs), and peripheral receptors.
• Nerves are made up of nerve fiber fascicles and their connective tissue sheaths.

Peripheral Nerve

• Nerve fibers are grouped into bundles to form nerves.
• Individual nerve fibers are held together by connective tissue (CT) organized into:
  • Endoneurium: thin layer of loose CT surrounding each individual nerve fiber.
  • Perineurium: surrounds bundles of nerve fibers.
  • Epineurium: irregular dense CT that surrounds a peripheral nerve.

Neuronal Plasticity & Regeneration

• The regenerative function of the NS is limited, especially in the CNS, due to specialization and differentiation.
• The NS is highly dependent on oxidative metabolism and blood supply for oxygen and nutrients.
• Neurons are the first cells to die in case of trauma or insufficient blood supply (within ~4-5 minutes).
• Dead neurons are replaced by glial cells, forming a glial scar in the CNS.
• Some areas of the brain can reprogram and replace functions of damaged areas, referred to as neural plasticity.

Damage, Regeneration & Degeneration of Peripheral Nerve Axon

• Neurapraxia: reversible defect in neural compression, axons are not interrupted, regeneration possible within hours to two weeks.
• Axonotmesis: axons are interrupted but nerve sheaths remain intact, Wallerian degeneration occurs but the nerve regenerates spontaneously.
• Neurotmesis: complete interruption of the whole nerve, spontaneous regeneration is not possible, requires surgical intervention.

Ganglia

• Aggregations of neuronal cell bodies located outside the CNS.
• Two types: sensory and autonomic.

Sensory Ganglia

• Found in three cranial nerves and the posterior roots of spinal nerves.
• Carry impulses towards the CNS.
• Contain pseudounipolar neurons (in most sensory ganglia) and bipolar neurons (in the cochlear & vestibular ganglia).

Autonomic Ganglia

• Transmit visceromotor information (sympathetic & parasympathetic).
• Sympathetic Ganglia:
  • Paravertebral ganglia (sympathetic trunk ganglia).
  • Prevertebral ganglia (in front of the aorta).
  • Adrenal gland medulla neurons.
• Parasympathetic Ganglia:
  • Often located near innervated organs or in the organ wall (intramural ganglia).

Sensory Ganglia: Differences

• Found just outside the CNS.
• Receive somatosensory information (position, pressure, pain, temperature) & viscerosensory information (blood pressure, blood pH, organ tone).
• Contain pseudounipolar and bipolar neurons.

Spinal Root Ganglion

• Has a thin CT capsule surrounding nerve cells peripherally.
• Each ganglion cell is surrounded by satellite cells.

Organization of the CNS

• The CNS (brain & spinal cord) contains:
  • White Matter: myelinated axons and oligodendrocytes.
  • Grey Matter: neuronal cell bodies, dendrites, unmyelinated axons, and glial cells.
  • Neuropil: part of the grey matter consisting of neuronal projections and neuroglia, filling spaces between cell bodies and vessels.
• Grey matter is located at the surface of the cerebrum and cerebellum (cortex) and forms islands in the CNS (nuclei).
• The central spinal canal is in the middle of the grey matter.

Spinal Cord Structure

• White matter is mainly myelinated fibers and oligodendrocytes.
• Grey matter is cell bodies, unmyelinated axons, and neuroglia.
• Grey matter is central, white matter is peripheral.

Horns of the Spinal Cord

• Grey matter is subdivided into horns:
  • Anterior (ventral) horns: short, broad, contain large multipolar somatomotor neurons.
  • Posterior (dorsal) horns: narrow, elongated, contain small multipolar somatosensory and interneurons.
  • Lateral horns (in T1 - L2 segments): visceromotor neurons.

White Matter of the Spinal Cord

• Divided into six columns (funiculi) containing tracts:
  • Ascending tracts relay information from the spinal cord to the brain.
  • Descending tracts carry information from the brain to the spinal cord.

Cerebellum

• Highly folded to accommodate many neurons in a small area.
• The cerebellar cortex maintains balance, equilibrium, muscle tone, and skeletal muscle coordination.

Cerebellum: External Structure

• Cerebellar vermis: unpaired central part.
• Cerebellar hemispheres: geminate hemispheres.
  • Cerebellar lobes: three lobes.
  • Cerebellar lobules: ten lobules.
  • Cerebellar folia: layers of the cerebellar cortex.
  • Cerebellar fissures: grooves separating the lobes and sheets.

Cerebellum: Internal Structure

• Grey matter:
  • Cerebellar cortex: cortex of the cerebellar hemispheres and vermis.
  • Cerebellar nuclei: nuclei within the cerebellum.
• White matter:
  • Medullar cerebellar body (cerebellar medulla): consists mainly of myelinated fibers

Cerebellar Cortex

• Layers:
  • Outer molecular layer: surface layer.
  • Middle Purkinje cell layer: narrow layer of Purkinje cells.
  • Inner granule cell layer: deepest granular layer.

Cerebellar Medulla

• Composed of:
  • Climbing fibers.
  • Mossy fibers.
  • Purkinje cell axons.
  • Deep cerebellar nuclei.
• Mossy fibers: afferent axons from cerebral cortex, pons, spinal cord, and vestibular nuclei to the cerebellum.
• Climbing fibers: afferent axons from the inferior olivary nuclei of the medulla.
• Four deep cerebellar nuclei: dentate, emboliform, globose, and fastigial nucleus.

Cerebellar Glomerulus

• Mossy fibers end as a dilated rosette.
• The rosette is surrounded by dendrites from granule cells and axons of Golgi cells.
• Mossy fiber terminals and surrounding dendrites/axons form light-stained glomeruli in the granular layer.

Cerebellar Nuclei

• Masses of grey matter embedded within the white matter.
• Four nuclei: dentate, emboliform, globosus, and fastigius.

Cerebrum

• Cerebral hemispheres consist of a convoluted cortex of grey matter.

Cerebral Cortex Cytoarchitectonic

• Molecular layer: horizontal cells of Cajal; axons form tangential plexus.
• External granular layer: small pyramidal cells and stellate cells.
• External pyramidal layer: medium-sized pyramidal cells.
• Internal granular layer: densely packed stellate and pyramidal cells.
• Internal pyramidal (ganglionic) layer: large pyramidal cells and giant pyramidal neurons of Betz in the motor cortex.
• Multiform layer: cells of various shapes.

Types of Cerebral Cortex

• Different areas of the cerebral cortex are involved in different functions.
• The Agranular type of cerebral cortex is characterized by the greatest development of layers III, V, and VI with weak development of granular layers II and IV (motor centers).

This quiz explores the structure and function of the nervous system, including its two main divisions: the central nervous system and the peripheral nervous system. Key concepts such as nervous tissue characteristics and anatomical organization are covered. Test your understanding of how the nervous system responds to environmental changes!