Introduction to the Nervous System
Divisions of the Nervous System
The nervous system in humans can be divided into two (2) parts: The Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS contains the brain and spinal cord while the PNS contains everything outside of “peripheral” to the CNS, including the autonomic nervous system.
- The CNS and PNS develop almost exclusively from ectoderm, beginning in the third week of embryonic life
- The development of the nervous system begins after gastrulation
- Gastrulation is the formation of the 3 primary germ layers: ectoderm, mesoderm, and endoderm
- These three germ layers give rise to virtually all the tissues of the body.
- The notochord is a rod-like structure that develops in the mesoderm in the midline.
- The notochord produces signaling molecules such as retinoic acid (a derivative of vitamin A) and peptide hormones such as sonic hedgehog (SHH) that induce the formation of the nervous system.
- Neurulation begins when the notochord sends signals that induce the overlying ectoderm to form the Neural Plate (i.e., the initial appearance of the nervous system)
- The neural plate is a thickening of ectoderm (now neuroectoderm)
- Lateral thickenings of the neural plate form a pair of neural folds separated by the neural groove.
- The dorsolateral parts of the neural folds rotate and meet in the dorsal midline, forming the neural tube that surrounds the neural canal (which forms the ventricles of the brain, the cerebral aqueduct, and the central canal.
- Middle of the neural tube closes first (by day 22), then the cranial neuropore closes (by days 24–25), then the caudal neuropore (by day 26-27)
- The neural tube separates from the surface ectoderm, and the surface ectoderm grows over the midline at the site of neurulation.
- Sonic hedgehog induces the ventral half of the neural tube to form the Floor plate and Basal plate. The Basal Plate gives rise to motor neurons and preganglionic autonomic neurons
- Bone morphogenetic proteins (BMPs) expressed by ectoderm cells induce the dorsal half of the neural tube to form the Roof plate and Alar plate. The Alar plate gives rise to sensory neurons that respond to primary sensory neurons in the periphery
- Cells at the dorsolateral margins of the neural plate form neural crest cells (NCCs). NCCs give rise to primary sensory neurons and post ganglionic autonomic neurons. NCCs migrate into many parts of the body and develop into a variety of nonneuronal cell types
- Failure of neural crest cells to migrate to specific locations or differentiate properly may result in a variety of cardiac, craniofacial, or neurological defects.
- Vitamin A deficiency (or toxicity) and Folate deficiency can impair failure of the neural tube leading to neural tube defects
- There are 6 major components of the CNS, 5 of which develop from the 5 secondary vesicles of the neural tube
- The 5 secondary vesicles are the telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon.
- The 6th component of the CNS is the spinal cord
- Spinal Cord extends inferiorly from the medulla but is not derived from a secondary vesicle; the dorsal and ventral roots of 31 pairs of spinal nerves emerge segmentally from the spinal cord.
The 5 Vesicle Stage (5 weeks) and corresponding brain regions developing from each secondary vesicle
Derivatives of the Neural tube:
- Neurons of CNS
- Supporting cells of CNS
- Somatomotor neurons of the PNS
- Presynaptic autonomic neurons of the PNS
Derivatives of the Neural crest:
- Sensory neurons in the PNS
- Postsynaptic autonomic neurons of the PNS
- Schwann cells
- Adrenal Medulla cells
- Meninges: Arachnoid and Pia matter only (Dura is from mesoderm)
Organization of the Adult Brain
Terms Referring to Collections of Axons
- White Matter: A general term for axon groups in the CNS. The gross brain appears white at these sites from the lipid rich myelin sheaths.
- Tract: A collection of axons with a common origin and a common destination
- Capsule: A group of axons connecting the cerebrum and brain stem
- Commissure: A collection of axons connecting one side of the brain to the other
- Lemniscus: A ribbon-like tract
- Nerve: A bundle of axons in the peripheral nervous system (except the optic nerve)
Terms Referring to Collections of Neurons
- Gray Matter: A general term for neuronal cell bodies in the CNS. The gross brain appears darker at these sites.
- Cortex: Thin sheets of neurons, usually at the brain surface and most often used in reference to the cerebral cortex (but there are others)
- Nucleus/Nuclei: A clearly defined mass of neurons, usually fairly large and deeply placed in the brain
- Locus/Loci: Clearly defined groups of neurons, but smaller than a nucleus (e.g., locus ceruleus)
- Substantia: A less well-defined group of neurons
- Ganglion/Ganglia: Applied to collections of neurons in the Peripheral Nervous System (except the Basal Ganglia in the CNS)
Afferent vs Efferent
- Pathways carrying signals TOWARD the Central Nervous System are called AFFERENTS (Afferents Arrive)
- Pathways carrying signals AWAY FROM the Central Nervous System are called EFFERENTS (Efferents Exit)
Orientation and Planes
Note that the Superior and Inferior orientation stays consistent throughout the body while the Rostral, Caudal, Dorsal, and Ventral orientations change above and below the meso-diencephalic border (i.e., the red/blue double line between midbrain and diencephalon).
Note the various planes used to describe Neuroanatomy in 3 dimensions (mainly used in neuroimaging terminology)
The Spinal Cord & Spinal Nerves
- The human nervous system develops in segments, like segmented worms, as discussed in the previous sections.
- Segments in head fuse to form the cerebral hemispheres + brainstem.
- 12 Pairs of Cranial Nerves (part of PNS) exit the segments in head
Spinal Nerves arise from spinal cord segments
- Each segment gives rise to both sensory and motor nerve roots on each side of the body
- Segments and nerve roots are named according to the level at which they exit the vertebral canal: Cervical, Thoracic, Lumbar, Sacral
- Sensory and motor nerve roots join together outside SC to give rise to “spinal nerves”
- Bony vertebral canal extends faster than spinal cord during development which results in the spinal cord ending at level L1 or L2
- Cauda Equina (horse’s tail) is a collection of nerve roots after spinal cord ends. The nerve roots continue down to exit points
- The Brachial plexus and lumbosacral plexus are a network of motor and sensory nerves controlling the upper (brachial plexus) and lower extremities (lumbosacral plexus)
- Cervical and Lumbosacral enlargements result from the higher density of gray matter in these segments as they contain the neuronal cell bodies for motor control of the upper and lower extremities.
THEME TO REMEMBER:
- Anterior/Ventral parts of CNS/Spinal Cord: Motor System
- Posterior/Dorsal parts of CNS/Spinal Cord: Sensory
- A dermatome is an area of skin supplied by afferent nerve fibers from a single dorsal root of a spinal nerve. Dermatomes help neurologists identify potential locations of lesions in the spinal cord.
Autonomic Nervous System
Review Video: Divisions of The Nervous System
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