Nervous System Structure and Neurons

The nervous system is composed of two primary cell types: neurons and glial cells.
Neurons are the functional units of the nervous system, communicating via synapses.
Glial cells support and protect neurons.

Neuron structure includes a cell body (soma), a single axon, and dendrites.
The soma contains the nucleus and cytoplasm, with organelles like mitochondria and Nissl bodies.
The axon transmits information to other neurons, and can be myelinated or unmyelinated.
Myelin sheath acts as insulation, increasing speed of nerve impulse transmission.
Dendrites receive impulses from other neurons.

Classified by size, function, and number of processes.
Size: Golgi type I neurons are large with long axons, while Golgi type II neurons are small with short axons.
Function: Motor neurons control movement, sensory neurons transmit sensory information, and interneurons connect neurons.
Number of Processes:
- Unipolar neurons have one process, with true unipolar neurons in the retina, and pseudounipolar neurons in the spinal ganglia.
- Bipolar neurons have two processes, an axon and a dendrite, found in sense organs.
- Multipolar neurons have one axon and many dendrites, comprising most nerves in the nervous system.

Central neuroglia are found in the CNS:
- Astrocytes support neurons and transport metabolites.
- Oligodendrocytes create myelin in the CNS.
- Microglia are defense cells against infection.
- Ependymal cells line the ventricles of the brain.
Peripheral neuroglia are found in the PNS:
- Schwann cells form myelin in the PNS.
- Satellite cells surround nerve cells.

Synapses are the contact points between two neurons.
Chemical synapses transmit information using neurotransmitters.
Electrical synapses transmit information directly without a chemical intermediate.
Chemical synapses are composed of three parts:
- Presynaptic element: the axon terminal with synaptic vesicles and mitochondria.
- Synaptic cleft: a 20-nm space between neurons.
- Postsynaptic element: usually a dendrite.
Synapses can be axodendritic, axoaxonic, axosomatic, or at the neuromuscular junction.

Central Nervous System (CNS): brain and spinal cord.
Peripheral Somatic Nervous System (PNS):
- Cranial nerves: 12 pairs originating from the brain.
- Spinal nerves: 31 pairs originating from the spinal cord.
- Associated ganglia: spinal and cranial ganglia.
Peripheral Autonomic Nervous System (ANS):
- Sympathetic part: thoracolumbar outflow.
- Parasympathetic part: craniosacral outflow.

Divisions:
- Forebrain:
  - Cerebral hemispheres: contain higher motor and sensory centers.
  - Diencephalon: consists of the thalamus (sensory relay station) and hypothalamus (autonomic and endocrine control center).
- Midbrain:
- Hindbrain:
  - Pons: middle portion of the brainstem.
  - Medulla oblongata: lower portion of the brainstem.
  - Cerebellum: coordinates muscle movement and maintains balance.
Alternative Division:
- Cerebrum: includes the forebrain (cerebral hemispheres and diencephalon).
- Cerebellum:
- Brainstem: consists of the midbrain, pons, and medulla oblongata.

Distribution: 31 pairs of spinal nerves:
- 8 cervical nerves.
- 12 thoracic nerves.
- 5 lumbar nerves.
- 5 sacral nerves.
- 1 coccygeal nerve.
Formation: Each spinal nerve arises from two roots:
- Ventral root: motor, containing axons of anterior horn cells.
- Dorsal root: sensory, containing processes of pseudounipolar cells in the spinal ganglion.
Branches: The spinal nerve divides into two rami:
- Anterior (ventral) ramus: runs anteriorly and forms plexuses.
- Posterior (dorsal) ramus: runs posteriorly to innervate muscles and skin of the back.
Lesions:
- Dorsal root lesion: sensory loss.
- Ventral root lesion: motor paralysis.
- Spinal nerve lesion: sensory loss and motor paralysis.

12 pairs of nerves connecting to the brain, numbered I to XII.
All cranial nerves originate from the brainstem except the olfactory (I) and optic (II) nerves.

Divisions:
- Sympathetic system: thoracolumbar outflow.
- Parasympathetic system: craniosacral outflow.

Originates from the lateral horn cells of the spinal cord segments T1 to L2 or L3.

Cranial outflow: arises from the brainstem through cranial nerves III, VII, IX, and X.
Sacral outflow: arises from spinal cord segments S2, S3, and S4.

Sympathetic system:
- Inhibits most functions except for heart stimulation.
- Decreases gut peristalsis.
- Contracts sphincters.
- Accelerates heart rate.
Parasympathetic system:
- Stimulates most functions except for heart inhibition.
- Increases gut peristalsis.
- Relaxes sphincters.
- Slows heart rate.