Nervous System Divisions lecture 17

Divisions of the Nervous System

• Information in the nervous system flows in two directions: afferent (sensory, in) and efferent (motor, out).
• The Central Nervous System (CNS) integrates and coordinates information.
• The Peripheral Nervous System (PNS) includes spinal nerves.
• Effectors respond to motor commands, resulting in action.

Types of Information Transmitted

• Somatic information includes things people are aware of with voluntary muscle control via somatic efferent (motor) pathways and sensory awareness via somatic afferent pathways.
• Autonomic information includes involuntary muscle control via autonomic efferent pathways.
• Autonomic information includes sensory information that people are not aware of via autonomic afferent pathways.

Somatic Efferent Division: Anatomical Organization

• The upper motor neuron: cell body is located in the brain and the axon is in the spinal cord.
• The lower motor neuron: cell body is in the spinal cord and the axon is in the spinal nerve.
• Both axons of the upper and lower motor neurons are myelinated.

Somatic Efferent Division: Communication with Effector Cells

• Acetylcholine (ACh) is the neurotransmitter for somatic efferent neurons
• Communication with effector cells happens at the neuromuscular junction to cause muscular contractions

Summary of Somatic Efferent Division

• Somatic efferent division controls voluntary movement.
• It consists of two neurons between the brain and the effector, axons are myelinated.
• Acetylcholine is the neurotransmitter and the effector is skeletal muscle.

Autonomic Efferent Nervous System

• The autonomic efferent nervous system controls involuntary actions.
• The autonomic efferent nervous system has two divisions: sympathetic and parasympathetic.
• Effectors in the autonomic efferent nervous system include: smooth muscle, cardiac muscle, glands, and adipose (fat) tissue.
• There are three neurons between the brain and the effector: neuron #1, neuron #2, and neuron #3

Autonomic Nervous System: Basic Anatomical Features (Neurons 2 & 3)

• Neuron #2 (pre-ganglionic): cell body in the CNS, myelinated axon extends into the PNS, and synapses in the autonomic ganglion.
• Neuron #3 (post-ganglionic): cell body in PNS autonomic ganglion, an unmyelinated axon extends in PNS to the effector organ and synapses on the effector organ.

Autonomic Nervous System: Synaptic Neurotransmitters (Neurons 2 & 3)

• Neuron #2 uses acetylcholine as a neurotransmitter and neuron #3 uses acetylcholine or norepinephrine as a neurotransmitter.

Subdivisions of the Autonomic Nervous System: Sympathetic

• The sympathetic division prepares the body for acute/stress responses, such as "fight or flight".
• Effects include increased heart rate, constricting blood vessels to skin and viscera (increased blood flow to muscles), decreased gastric motility and salivation, increased pupil size and sweating.

Subdivisions of the Autonomic Nervous System: Parasympathetic

• The parasympathetic division prepares the body for restful situations, for "rest and digest".
• Effects include decreased heart rate, increased gastric motility and salivation, and decreased pupil size.

Structural Differences: Sympathetic vs. Parasympathetic

• Sympathetic division features a short axon in the preganglionic neuron and a long axon in the post-ganglionic neuron; ganglia are close to the CNS
• Parasympathetic division features a long axon in the preganglionic neuron and a short axon in the post-ganglionic neuron; the ganglia are distant from the CNS.

Sympathetic Nervous System: Exit from CNS and Position of Ganglia

• Cell bodies of preganglionic neurons are in the thoracolumbar levels of the spinal cord (CNS)
• The axons of preganglionic neurons are short and synapse in sympathetic ganglion
• The axons of postganglionic neurons are long and extend into body

Sympathetic Chain Ganglia

• Sympathetic chain ganglia are on either side of the vertebral column.
• There are typically 21-23 pairs (usually 22).
• These ganglia are where preganglionic (neuron #2) axons synapse onto postganglionic (neuron #3) input zone.

Parasympathetic Nervous System: Exit from CNS and Position of Ganglia

• Cell bodies of preganglionic neurons are in cranial (brainstem) and sacral levels of the spinal cord.
• Axons of preganglionic neurons are long, and synapse in parasympathetic ganglia (in or near effector).
• Cell bodies of postganglionic neurons are in or near the effector organs.
• Axons of postganglionic neurons are short.

Summary of Sympathetic vs. Parasympathetic Pathways

• Sympathetic preganglionic neurons are located in the thoracolumbar spinal cord (T1 to L2), synapse in sympathetic chain or collateral ganglion, have relatively short fibers, and use acetylcholine.
• Parasympathetic preganglionic neurons are located in the craniosacral spinal cord, synapse in parasympathetic ganglion in or near the effector, have relatively long fibers, and use acetylcholine.
• Sympathetic postganglionic neurons have cell bodies in the sympathetic chain/collateral ganglion, relatively long fibers, and use noradrenaline (most).
• Parasympathetic postganglionic neurons have cell bodies in the parasympathetic ganglion in/near the effector, relatively short fibers, and use acetylcholine.