Anatomical and Functional Organization of the Nervous System

Functions of the Nervous System

The nervous system has three main functions:
- Sensory input: Sensory receptors monitor internal and external environments to detect changes.
- Integration: Integrates sensory information.
- Output/response: Coordinates voluntary and involuntary responses of many other cells, including mental activity, homeostasis, muscle & gland control, and behavior.

The nervous system is divided into two subdivisions:
- Central nervous system (CNS): Consists of the brain & spinal cord, which are continuous with each other at the foramen magnum.
- Peripheral nervous system (PNS): Consists of sensory receptors, cranial & spinal nerves, ganglia & plexuses.
  - Ganglia: Collections of neuronal cell bodies outside of the CNS.
  - Plexuses: Dense network of nerves adjacent to the spinal cord.

PNS is further subdivided into:
- Sensory (afferent) neurons: Carry signals (action potentials) from receptors in the periphery to the CNS.
- Motor (efferent) neurons: Carry action potentials from the CNS to the periphery (effector organs).

The motor system is further subdivided into:
- Somatic nervous system: Somatic motor neurons innervate skeletal muscle & are under voluntary control.
- Autonomic nervous system (ANS): Autonomic motor neurons innervate smooth muscle, cardiac muscle & glands & are under involuntary control.

Definition: An automatic response to a specific stimulus (stimulus-response sequence).
Awareness:
- In some reflexes, we are aware of the stimulus and/or response, e.g., withdrawal of hand from a hot stove.
- In many reflexes, we are unaware of the stimulus and/or response, e.g., CO_2 level in blood.
Learned reflexes: Many responses are learned & therefore called learned or acquired reflexes, e.g., driving your car.

Early development:
- About 2 weeks after fertilization, the three germ layers (ectoderm, mesoderm & endoderm) are formed during the process of gastrulation.
- The nervous system starts developing early during organogenesis, ± 18 days after fertilization.
- The ectoderm gives rise to all nervous tissue.
- An ectodermal sheet of cells with neural properties forms the neural plate.
Neurulation:
- During neurulation, the neural plate forms two neural folds and a neural groove.
- Superior margins of the neural folds fuse to form the neural tube.
- A specialized portion of the plate, the neural crest, separates from the tube and overlying ectoderm.
21-day-old embryo:
- The neural folds meet at the midline & fuse to form the neural tube. This fusion begins in the center of the embryo & moves both cranially & caudally.
Fate of neural tube and neural crest cells:
- The neural tube develops into the CNS:
  - Rostral neural tube → brain
  - Caudal neural tube → spinal cord
- The neural crest cells develop into the PNS:
  - Sensory neurons of dorsal root ganglia and cranial nerve ganglia
  - Sympathetic and parasympathetic motor neurons of autonomic ganglia
CNS development failures:
- Neural tube defects (anencephaly/spina bifida) hence, inclusion of folic acid in diet during early pregnancy to reduce risk.
- Interuterine infections (rubella).
- Exposure to environmental agents/toxins (alcohol).
- Abnormal neuronal migration.

Anencephaly:
- Failure of the rostral neural tube to close – cerebrum and part of brainstem never develop.
- Baby is totally vegetative, cannot see, hear or process sensory inputs, flaccid muscles, no voluntary movements.
- Death occurs soon after birth.
Spina bifida:
- Failure of the caudal neural tube to close - typically in lumbosacral region.
- Defect ranges from simple, e.g., spina bifida occulta, where part of the spinous process is missing on at least one vertebra, resulting in no clinical manifestations to more severe defects, e.g., spina bifida cystica, where a sac that may contain meninges, CSF and even part of spinal cord, protrudes out of the vertebral column, resulting paralysis of limbs or bowels & bladder, depending on where the defect occurs.

Fetal alcohol syndrome (FAS) is the most common preventable cause of mental retardation in the world today.
- First few weeks of development NB not to expose to alcohol.
- Main effect is permanent damage to CNS & decreased mental function.

The neuron doctrine = principle that individual neurons are the elementary signalling elements of the nervous system
There are an estimated 10^{12} cells in the nervous system: 10% neurons, 90% glial cells → making the nervous system the most highly organised and complex tissue known!

Neurons receive stimuli and propagate (spread) action potentials to other neurons/effector organs.
Neurons consist of three principle components:
- Cell body/soma = specialized to integrate action potentials.
- Dendrites = “branches” specialized to receive of action potentials.
- Axon = “trunk” specialized for the transmission of action potentials.

Astrocytes
- Contain processes that wrap around and anchor neurons & capillaries.
- Regulate extracellular brain fluid composition.
- Strengthen tight junctions that form at the blood-brain barrier.
Ependymal cells
- Line brain ventricles and spinal cord central canal.
- Help to form choroid plexuses that secrete cerebrospinal fluid (CSF).
- Some cells have cilia which help to circulate the CSF.
Oligodendrocytes
- Myelinate a region of many different axons.
- Insulate axons from each other.
Microglia
- Specialized macrophages ➔ migrates to dead neurons or bacteria and phagocytize them.
- NB protective role ➔ cells of the immune system cannot access CSF.

Schwann cells
- Support nerve fibers or wrap around a region of one axon.
- NB in regeneration of injured peripheral nerves.
Satellite cells
- Surround neuron cell bodies in ganglia to provide support and nutrients.

Multiple sclerosis = demyelination in the CNS.
Guillain-Barré syndrome = demyelination in the PNS.
- Auto-immune diseases where the myelin proteins are attacked by the immune system.
- Hardened lesions form which cause problems with the conduction of action potentials.