What is this chapter on? Summary
This chapter is about the autonomic nervous system (ANS), the involuntary branch of the nervous system that regulates internal organs, smooth muscle, glands, and blood vessels to maintain homeostasis. It explains how the sympathetic (“fight or flight”) and parasympathetic (“rest and digest”) pathways work through two‑neuron chains, neurotransmitters, and reflexes to balance body functions.
What is the ANS
- The ANS is the involuntary branch of the nervous system that regulates internal organs, smooth muscle, cardiac muscle, and glands.
- It’s automatic — you don’t consciously control it.
What does the ANS do?
- Maintains homeostasis by balancing body functions.
- Controls things like heart rate, blood pressure, digestion, urination, pupil size, glandular secretion, and sexual function.
- Coordinates with the endocrine system and behavior to adapt to stress, rest, exercise, and emotion.
Where is the ANS?
- Preganglionic neurons: cell bodies in the CNS (brainstem or spinal cord).
- Ganglia: clusters of neurons outside the CNS where preganglionic neurons synapse with postganglionic neurons.
- Postganglionic neurons: extend into the peripheral nervous system (PNS), projecting to target tissues (organs, vessels, glands).
- Targets: smooth muscle, cardiac muscle, glands, viscera throughout the body.
How does the ANS work?
- Uses a two-neuron chain:
1.Preganglionic neuron (CNS → ganglion) releases ACh onto nicotinic receptors.
2.Postganglionic neuron (ganglion → target tissue) releases:
- Sympathetic: mostly norepinephrine (NE) → adrenergic receptors.
- Parasympathetic: mostly ACh → muscarinic receptors.
Sympathetic system
“fight or flight” → increases HR, BP, dilates pupils, mobilizes energy.
Parasympathetic system
“rest and digest” → slows HR, promotes digestion, urination, glandular secretion.
When does the ANS work?
- Always active — both sympathetic and parasympathetic systems have tonic activity at rest.
- Balance shifts depending on situation:
- Stress/exercise → sympathetic dominates.
- Rest/relaxation → parasympathetic dominates.
- Works instantly in response to sensory input (e.g., standing up quickly → baroreflex adjusts BP so you don’t faint)
How is the ANS organized?
It has two branches—sympathetic (fight or flight) and parasympathetic (rest and digest)—and each uses a two‑neuron chain to reach organs (preganglionic → ganglion → postganglionic → target).
Afferent = Sensory input to the CNS
- Definition: Signals traveling from the body (organs, tissues) to the brain/spinal cord.
- Purpose: Tell the CNS what’s happening inside—pressure, stretch, chemical status, pain, irritation.
Efferent = motor output from the CNS
- Definition: Commands traveling from the CNS to the body to change function.
- Purpose: Adjust organ activity—speed the heart, dilate pupils, contract vessels, secrete enzymes.
What is the sympathetic afferent?
It is a visceral sensory fiber that carries information from internal organs to the CNS and travels alongside sympathetic pathways (often entering the spinal cord via thoracic/lumbar roots).
It carries:
- Mechanical signals: stretch/pressure in blood vessels or organs.
- Chemical signals: oxygen, CO₂, metabolites.
- Pain signals: ischemia, distension, inflammation—often “referred pain.”
Why it matters: These inputs are the “eyes and ears” of autonomic reflexes. Without them, the CNS wouldn’t know when to adjust heart rate, vessel tone, or digestion.
Sympathetic Afferents Trigger …
Autonomic Reflexes
- Such as the Baroreflex and the Chemoreflex
Shape the fight or flight readiness
Work in tandem with the parasympathetic afferents
Baroreflex
Afferents from blood pressure sensors (baroreceptors) → brainstem → sympathetic efferents adjust vessel tone and heart rate to keep BP stable when you stand up
Chemoreflex
Afferents reporting low oxygen/high CO₂ → increase sympathetic output to boost ventilation and perfusion.
Sympathetic afferents (sensory)
- Role: Report internal states (pressure, pain, chemistry) to CNS.
- Path: Organ → sensory fiber → dorsal root → spinal cord/brainstem.
- Effect: Inform reflex centers; modulate autonomic responses
Sympathetic Efferents (Motor)
- Role: Implement changes (increase HR, constrict vessels, dilate pupils).
- Path: CNS preganglionic neuron → ganglion → postganglionic neuron → organ.
- Transmitters: ACh in ganglia; NE at most targets; ACh at sweat glands.
- Effect: Execute “fight or flight” and moment‑to‑moment vascular tone.
Pathway of the sympathetic afferents and efferents
- Afferents enter the spinal cord (often thoracolumbar segments) and project to brainstem centers like the NTS and medullary cardiovascular centers.
- Efferents exit the intermediolateral horn of the spinal cord to sympathetic chain ganglia, then to widespread targets (heart, vessels, sweat glands, pupils
Orthostatic challenge (standing up fast)
- Afferent: Baroreceptors sense a drop in arterial pressure → send signals to CNS.
- Efferent: Sympathetic output increases—heart beats faster/stronger, vessels constrict → BP stabilizes
Cold Exposure
- Afferent: Skin thermoreceptors report cold → CNS integrates.
- Efferent: Sympathetic vasoconstriction reduces heat loss; piloerection; metabolic mobilization.
Gut distension after a meal
- Afferent: Stretch/chemo afferents report fullness.
- Efferent: Parasympathetic increases motility/secretions; sympathetic reduces during intense exercise.
Sympathetic Efferent Pathway
Preganglionic neurons begin in the intermediolateral horn of the thoracic (and upper lumbar) spinal cord. (These are the “command neurons” for sympathetic output)
They exit the spinal cord via the ventral root and the Neurotransmitter they release is acetylcholine (ACh).
Synapse in sympathetic chain ganglion (he sympathetic chain runs parallel to the spinal cord like a ladder of ganglia.)
- Preganglionic axons synapse here with postganglionic neurons.
Nicotinic receptor (always excitatory) on the postganglionic neuron
Note: This setup allows divergence: one preganglionic neuron can activate many postganglionic neurons → widespread effect.
Postganglionic axons project out to the target tissues (heart, blood vessels, glands, smooth muscle).
Neurotransmitter released = usually noradrenaline or norepinephrine, NE
Receptors on target = adrenergic receptors (α and β types).
**Exception: sweat glands use ACh on muscarinic receptors
Sympathetic Efferents Effect on ‘‘Fight or Flight’’
- ↑ Heart rate & blood pressure.
- Pupillary dilation.
- Mobilize energy stores (glucose, fatty acids).
- ↓ GI and urinary functions.
- Diffuse, widespread effects due to divergence
Sympathetic Afferent Pathway
Organ → sympathetic afferent fiber → dorsal root ganglion → spinal cord → brainstem centers.
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Nervous System28
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Nervous System 273
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Endocrine System 197
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Endocrine System 271
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Endocrine Pancreas51
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CNS Lecture 150
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Introduction to the Senses37
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Optics of the Eye51
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Visual Processing79
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Growth54
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Cranial Nerves12
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Hearing and Equilibrium41
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Color Vision49
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Somatic Senses75
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Smell and Taste29
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Hypothalamic Control of Feeding63
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Circadian Rhythm30
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Critical Motor Control Part 139
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Muscles 1 & 285
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ANS54
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CPG35
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Motor Control and Movement - Cortical Motor Areas56
