A&P 3/5

Central Nervous System (CNS)

• Components: Includes the brain and spinal cord.

• Function: Involved in sending signals to various parts of the body, including the heart, affecting heart rate either by speeding it up or slowing it down.

Autonomic Nervous System (ANS)

• Divisions: Comprised of two parts:

  • Sympathetic Nervous System

  • Parasympathetic Nervous System

Sympathetic Nervous System

• Function: Responsible for the 'fight or flight' response.

• Reactions:

  • Fight

  • Flight

  • Freeze

  • Submit

  • Fawning

Examples of fight or flight reactions can include increased heart rate due to adrenaline release, while the freeze response is an instance where the body becomes immobilized in a dangerous situation (e.g., dogs may roll over to show submission).

Adrenaline Release

• The sympathetic system activates adrenaline (also known as epinephrine or norepinephrine - denoted as NE in medical contexts) to increase heart rate.

• The sympathetic nervous system pathways consist of:

  • Short preganglionic neurons

  • Long postganglionic neurons

Parasympathetic Nervous System

• Function: Primarily conserves energy, promoting 'rest and digest' activities.

• Signal Mechanism: Releases acetylcholine (ACh) to decrease heart rate.

• Structures include:

  • Long preganglionic neurons

  • Short postganglionic neurons

Neuron Structure and Function

• Preganglionic Neurons: Neurons that synapse before the ganglia.

  • Parasympathetic affords a long preganglionic neuron (remembered as a longer word).

  • Sympathetic provides a short preganglionic neuron.

• Postganglionic Neurons: Neurons that synapse after the ganglia.

Synaptic Transmission

• The area where neurons synapse is known as the ganglion.

• Within this neural setup, neuron types are designated:

  • Preganglionic Neurons: Located before ganglia.

  • Postganglionic Neurons: Located after ganglia.

• Synaptic Knob: The terminal end of neurons that release neurotransmitters.

Neurotransmitter Release

• Cholinergic Neurons/Fibers: Neurons that release acetylcholine (ACh).

  • Majority (3 out of 4) of the neurons in the autonomic pathways release ACh.

• Adrenergic Neurons/Fibers: Neurons that release adrenaline (epinephrine or norepinephrine).

  • Only 1 postganglionic neuron in the sympathetic division functions in this capacity.

Cholinergic and Adrenergic Receptors

• Cholinergic Receptors: Receptors that bind to acetylcholine.

  • Two Types:

  • Nicotinic: Found on postganglionic neurons; always excitatory; binds and activates directly.

  • Muscarinic: Found on target organs; functions can be either excitatory or inhibitory.

• Adrenergic Receptors: Receptors that bind to adrenaline.

  • Two Types:

  • Alpha: Generally excitatory.

  • Beta: Generally inhibitory.

Practical Implications

• Medical Relevance: Understanding adrenergic receptors is important, particularly when studying conditions like heart disease, where medications such as beta-blockers modulate heart function by affecting beta receptors.

• Exam Preparation: Utilize diagrams to memorize and visualize the connections, receptors, and neurotransmitters involved to effectively answer questions about the autonomic nervous system function. When identifying fibers, focus on whether they release ACh (cholinergic) or adrenaline (adrenergic) to discern how to approach any question regarding the sympathetic or parasympathetic systems.