L6 Human Physiology

🧠 Concepts Introduced

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Learning Objectives ➢ Describe the anatomy and functions of the autonomic and somatic nervous system ➢ Describe neurotransmitters and receptors associated with efferent division of the peripheral nervous system ➢ Understand how central nervous system regulates the autonomic branch of the nervous system

Organisation of the nervous system

Learning outcome #1 • Describe efferent divisions of the peripheral nervous system • Describe the anatomy and function of sympathetic and parasympathetic branches of autonomic nervous system • List the neurotransmitters and receptors in the autonomic nervous system

Efferent division of PNS

Autonomic nervous system

Parasympathetic and sympathetic branches

Action via ANS is by dual innervation (antagonistic control of internal organs)

Parasympathetic division is most active at rest

• “rest and digest” functions

Sympathetic division is most active in emergency or threatening situations

• “fight-or-flight’’ response

Homeostasis of autonomic nervous system

Parasympathetic: rest and digest

Sympathetic: flight-or-flight

Anatomy for autonomic

Preganglionic neuron – originates in CNS (spinal cord) and projects to autonomic ganglion outside CNS

Ganglion – sympathetic chain ganglion runs along vertebral column

Postganglionic neuron – cell body in ganglion and projects axons to target tissue/effector organ

Target tissue - cardiac muscle, smooth muscle or glands

Sympathetic vs Parasympathetic nervous system

Anatomical difference of two autonomic branches characterised by: • Origin • Length of fibres • Location of their ganglia

Sympathetic:

• Originate in thoracic and lumbar region of spinal cord
• Ganglia close to spinal cord
• Short preganglionic neurons
• Long postganglionic neurons to target tissues

Parasympathetic:

• Originate in brain stem (cranial nerves) and sacral region
• Ganglia on or near target organs
• Long preganglionic neurons
• Short postganglionic neurons

Pathway

Sympathetic and parasympathetic branches also differ in their neurotransmitter released from preganglionic & postganglionic neurons and receptors.

sympathetic acetycholine nicotinic receptor, norepinephrine / epinephrine adrenergic receptor parasympathetic acetycholine nicotinic receptor, acetycholine muscarinic receptor

between pre and post ganglion is acetylcholine noradrenaline is only between post ganglion and target organ

Response

Sympathetic Nervous System

Anatomy of Sympathetic

Sympathetic innervation of adrenal gland

(a) Anatomical Location

• Position: The adrenal glands (also known as suprarenal glands) are small, cap-like structures positioned superiorly on top of each kidney. • Dual Nature: Although located in close proximity to the urinary system, their primary function is endocrine (hormone-secreting) and neurological.

(b) Structure of the Adrenal Gland

A cross-section of the gland reveals two embryologically and functionally distinct layers:

1. Adrenal Cortex (Outer Layer): Described as a true endocrine gland. It produces and secretes steroid hormones (such as cortisol, aldosterone, and androgens) under chemical control (primarily by pituitary ACTH).
2. Adrenal Medulla (Inner Layer): Characterized as a modified sympathetic ganglion. Unlike typical endocrine tissues, the medulla is a specialized extension of the autonomic nervous system.

(c) Mechanism of Sympathetic Innervation

This section tracks the unique pathway by which neural impulses translate directly into blood-borne hormones: • 1. Spinal Cord (Origin): The pathway begins in the central nervous system within the lateral horn of the thoracic spinal cord. A preganglionic sympathetic neuron projects its axon out toward the adrenal gland. • 2. Direct Monosynaptic Pathway: In typical sympathetic pathways, a preganglionic neuron synapses inside a sympathetic chain ganglion with a postganglionic neuron. However, the pathway to the adrenal medulla is a notable exception: the preganglionic fiber travels directly to the adrenal medulla without synapsing anywhere else first. • 3. Neurotransmitter Release: At the terminal end within the adrenal medulla, the preganglionic neuron releases Acetylcholine (ACh) into the synaptic cleft. • 4. Chromaffin Cells (Modified Postganglionic Neurons): The target cells in the medulla are called chromaffin cells. Structurally, they are modified postganglionic sympathetic neurons that lost their axons and dendrites during development. • 5. Neurohormone Secretion: When activated by ACh, these chromaffin cells behave like endocrine cells. Instead of releasing neurotransmitters to act on a neighboring cell, they secrete epinephrine (adrenaline) and norepinephrine directly into a surrounding blood vessel. • 6. Systemic Response: Epinephrine enters systemic circulation as a neurohormone, traveling through the blood supply to quickly reach diverse target tissues throughout the body to initiate a widespread, coordinated "fight-or-flight" stress response.

Parasympathetic Nervous System

Anatomy of Parasympathetic

shorter post ganglionic neurons long pre ganglionic neurons

Autonomic neuroeffector junctions

Learning Outcome #2

• Describe the neuroeffector junctions in the autonomic nervous system
• Identify the brain areas that regulate the autonomic functions

Autonomic targets

• smooth muscle, cardiac muscle, exocrine glands, endocrine glands, lymphoid tissue, adipose tissue

Neuroeffector junction

• synapse between postganglionic autonomic neuron and target cells

Postganglionic axons

• Varicosities - instead of axon terminals, there are multiple branches and varicosities (swellings) along the axon over the surface of target cells

Autonomic varicosities

Norepinephrine (NE) release at a neuroeffector junction

1. Action potential arrives at the varicosity.

2. Depolarization opens voltage-gated Ca?+ channels,

3. Ca2+ entry triggers exocytosis of synaptic vesicles.

4. NE binds to adrenergic receptor on target.

5. Activity ceases when NE diffuses away from the synapse.

6. NE is transported back into the axon.

7. NE can be taken back into synaptic vesicles for re-release,

8. NE is metabolized by monoamine oxidase (MAO).

Autonomic control centers

Hypothalamus

• water balance
• temperature
• hunger

Pons

• respiration
• urinary

Medulla

• respiration
• cardiac

Integration of autonomic function

Coordination of homeostatic responses

• Autonomic
• Endocrine
• Behavioral

Autonomic vs Somatic System

Learning outcome #3 • Describe the somatic nervous system pathway • Explain the structure of neuromuscular junction (NMJ) • Describe neurotransmitter and receptor involved in signal transmission in NMJ

Anatomy of somatic nervous system

Single neuron

• CNS origin
• Myelinated axons

Terminus

• Branches
• Neuromuscular junction (NMJ)

Excitatory effect

• Muscle contraction

Neuromuscular junction

Terminal bouton

• axon terminals of motor neuron
• store and release acetylcholine (ACh)

Synaptic cleft

• space between axon terminal and sarcolemma (muscle membrane)
• Ach binds to nicotinic receptors

Motor end plate

• folding on the sarcolemma of the muscle
• contains nicotinic cholinergic receptors

Signal transmission at NMJ

Axon terminal (of presynaptic cell) – Action potential signals acetylcholine release

Motor end plate – folds in plasma membrane of skeletal muscle cells – Two acetylcholine (ACh) bind – Opens cation channel – Na+ influx – K+ efflux – Net Na+ influx depolarize membrane

Stimulates fiber contraction due to increased intracellular Ca2+ concentration