Nervous System Overview and Development

Nervous System Overview

• The nervous system is responsible for controlling bodily functions and communication.
• It has two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS includes the brain and spinal cord. The PNS includes nerves that connect the CNS to the rest of the body.

Development of the Nervous System

• The CNS begins to form during the third week of embryonic development.
• The neural plate forms, then develops into the neural tube.
• The neural tube gives rise to the brain and spinal cord.
• The peripheral nervous system (PNS) develops from neural crest cells, neural tube, and mesoderm.

Development of the Neural Tube

• Neural plate folds inward to form neural groove, then neural folds.
• Neural folds fuse to form neural tube.
• Anterior and posterior neuropores close at different times.
• Closure of these openings creates a continuous neural tube.
• The neural tube differentiates into brain vesicles which develop into the brain structures.

Development of Brain Vesicles

• Three primary vesicles, prosencephalon, mesencephalon, and rhombencephalon, develop.
• The primary vesicles further subdivide to form the five secondary brain vesicles, telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon.
• These vesicles give rise to structures like the cerebral hemispheres, thalamus, hypothalamus, midbrain, pons, and cerebellum.

Spinal Cord Development

• The caudal end of the neural tube develops into the spinal cord.
• Caudal part develops into the spinal cord, with its associated spinal nerves diverging throughout the vertebral column.
• The spinal cord extends down the vertebral column initially, but it does not grow as quickly as the vertebral column, and as a result the spinal cord is shifted upwards to higher levels.

Other Important Structures

• Neural crest cells: give rise to various structures outside the CNS, including certain ganglia (clusters of nerve cells), Schwann cells (supporting cells for PNS nerves), and melanocytes (pigment cells).
• Placodes: localized thickenings of the surface ectoderm that give rise to some sensory organs and nerves.
• Olfactory and otic placodes are examples of placodes.

Neural Tube Wall Layers

• The neural tube wall has three layers: neuroepithelial (innermost), mantle, and marginal (outermost layers).
• These layers give rise to later neuronal and glial (support) cell populations.
• The alar and basal plates form the dorsal and ventral horns of the spinal cord respectively.

Spinal Cord Positional Changes

• The spinal cord grows slower than the vertebral column.
• As a result, the lower end of the spinal cord is located higher than the vertebral column's lower spine by adulthood.
• This creates the cauda equina (horse's tail) where nerve roots descend in the spinal canal.

Congenital Anomalies

• Spina bifida: a defect where the vertebral arches fail to fuse properly. Types include occulta, meningocele, and myelomeningocele.
• Hydrocephalus: excessive cerebrospinal fluid in the brain.
• Anencephaly: a defect where a substantial part of the brain fails to develop normally.

Sensory Part of the Nervous System

• Receptors are initiated by sensory experience detecting stimuli (temperature, pressure, light).
• The information of sensory events from receptors transmits to spinal cord (multiple locations), cerebellum and specific regions of the brain through peripheral nerves.
• This information is interpreted in the brain, causing a response if needed.

Motor Part of the Nervous System

• The nervous system controls: skeletal muscle contraction, smooth muscle contraction, and gland secretion.
• Skeletal muscles can be controlled from various levels of the CNS, including the spinal cord, brainstem, basal ganglia, cerebellum, and motor cortex.
• The autonomic nervous system is responsible for involuntary functions like heart rate and digestion.

Synaptic Transmission

• Synapses are junctions between neurons.
• Chemical synapses: rely on neurotransmitters (chemical messengers) to transmit signals across the synaptic cleft between neurons.
• Electrical synapses: transmit signals through direct ionic current, connecting adjacent cells directly.
• Synaptic activity is influenced by multiple factors like presence or absence of various ions, and the amount of neurotransmitters.
• Postsynaptic potentials can be either excitatory (depolarizing) or inhibitory (hyperpolarizing).

Nervous System Integration

• Information processing is crucial for initiating appropriate responses: meaning that the nervous system must interpret and process information to ensure appropriate responses.
• Some information is considered irrelevant by the brain and is often ignored or relegated to a subconscious level.
• Three major levels of CNS function: spinal cord, lower brain, higher cortical level.

Action Potential Generation and Transmission

• Action potential: a rapid change in membrane potential, essential for transmitting impulses along nerve fibers and between neurons.
• The action potential is initiated at the axon hillock, and then travels across the fiber's myelinated or unmyelinated portions.