Autonomic Nervous System and Higher-Order Functions (Chapter 16) – Study Notes

Autonomic Nervous System and Higher-Order Functions

Overview of the Nervous System

The nervous system is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS consists of the brain and spinal cord, while the PNS includes all nervous tissue outside the CNS. The PNS is further divided into sensory (afferent) and motor (efferent) divisions, which control different effectors in the body.

• CNS: Brain and spinal cord; responsible for processing and integrating information.

• PNS: Nerves and ganglia outside the CNS; transmits sensory and motor signals.

• Receptors: Specialized cells that detect changes in the environment (e.g., special sensory, visceral sensory, somatic sensory receptors).

• Effectors: Muscles or glands that respond to nervous system commands.

Example: Sensory information from the skin is sent to the CNS, which processes the input and sends a motor command to skeletal muscles via the PNS.

Functional Organization: Somatic vs. Autonomic Nervous Systems

The Somatic Nervous System (SNS) and the Autonomic Nervous System (ANS) are the two main functional divisions of the efferent (motor) system.

• SNS: Controls voluntary movements by innervating skeletal muscles.

• ANS: Regulates involuntary functions by controlling smooth muscle, cardiac muscle, glands, and adipose tissue.

Key Differences:

• Effectors: SNS targets skeletal muscle; ANS targets visceral effectors (smooth muscle, cardiac muscle, glands, adipocytes).

• Control: SNS is under conscious control; ANS is largely unconscious and involuntary.

• Neural Pathways: SNS uses a single motor neuron; ANS uses a two-neuron chain (preganglionic and postganglionic neurons).

Autonomic Nervous System: General Organization

The ANS uses a two-neuron pathway to reach its effectors:

• Preganglionic neuron: Cell body in the CNS; axon extends to an autonomic ganglion.

• Ganglion: Cluster of neuron cell bodies in the PNS where preganglionic neurons synapse with postganglionic neurons.

• Postganglionic neuron: Cell body in the ganglion; axon extends to the effector organ.

Example: A preganglionic neuron from the spinal cord synapses in a ganglion, and the postganglionic neuron innervates the heart.

Divisions of the Autonomic Nervous System

The ANS is divided into the Sympathetic and Parasympathetic divisions, each with distinct anatomical and functional characteristics.

Sympathetic Division (Thoracolumbar Division)

The sympathetic division prepares the body for 'fight or flight' responses.

• Origin: Preganglionic neurons arise from thoracic and lumbar segments (T1–L2) of the spinal cord.

• Ganglia: Sympathetic chain ganglia located near the spinal cord.

• Functions:

  • Increases heart rate and force of contraction

  • Dilates pupils and bronchioles

  • Inhibits digestive and urinary functions

  • Stimulates release of glucose and adrenaline

Example: During stress, the sympathetic division increases heart rate and redirects blood flow to muscles.

Parasympathetic Division (Craniosacral Division)

The parasympathetic division supports 'rest and digest' activities.

• Origin: Preganglionic neurons arise from the brainstem and sacral spinal cord (S2–S4).

• Ganglia: Located in or near target organs.

• Functions:

  • Decreases heart rate

  • Constricts pupils

  • Stimulates digestive gland secretion and nutrient absorption

  • Promotes urination and defecation

  • Associated with sexual arousal

Example: After eating, the parasympathetic division increases digestive activity and slows the heart rate.

Dual Innervation and Functional Significance

Most vital organs receive input from both sympathetic and parasympathetic divisions, a phenomenon known as dual innervation. These divisions often have opposing effects, allowing precise regulation of organ function.

• Opposing Effects: For example, sympathetic stimulation increases heart rate, while parasympathetic stimulation decreases it.

• Coordinated Control: Dual innervation enables the body to maintain homeostasis by balancing the activity of both divisions.

Autonomic Reflexes: Long and Short Reflexes

Autonomic reflexes help regulate involuntary functions through two main types of pathways:

• Long Reflexes: Involve sensory input to the CNS, integration, and motor output via autonomic ganglia to effectors.

• Short Reflexes: Bypass the CNS; sensory neurons synapse directly with autonomic ganglia, producing a quick, localized response.

Example: The enteric nervous system in the gut can control digestive activity through short reflexes without CNS involvement.

Higher-Order Functions: Memory, Consciousness, and Sleep

Higher-order functions of the nervous system include memory formation, levels of consciousness, and sleep regulation.

• Memory: The process of encoding, storing, and retrieving information. Involves short-term and long-term memory, with consolidation occurring in the hippocampus and cerebral cortex.

• Consciousness: A state of awareness of self and environment, maintained by ongoing CNS activity. Altered states include sleep, coma, and anesthesia.

• Sleep: Divided into rapid eye movement (REM) and non-REM (deep) sleep. REM sleep is associated with dreaming and brain activity similar to the awake state.

Example: During REM sleep, EEG patterns resemble those of an awake adult, and most dreaming occurs.

Reticular Activating System (RAS) and Neurotransmitters

The Reticular Activating System (RAS) is a network in the brainstem that regulates wakefulness and consciousness by stimulating the cerebral cortex. Neurotransmitters such as serotonin, norepinephrine, and acetylcholine play key roles in modulating brain function and mood.

• RAS: Maintains alertness and filters sensory input.

• Neurotransmitters: Chemical messengers that influence synaptic transmission and brain states. For example, serotonin reuptake inhibitors are used to treat depression and anxiety.

Example: Drugs like LSD can alter perception by affecting neurotransmitter systems in the brain.

Summary Table: Comparison of Somatic and Autonomic Nervous Systems

Feature	Somatic Nervous System (SNS)	Autonomic Nervous System (ANS)
Control	Voluntary	Involuntary
Effectors	Skeletal muscle	Smooth muscle, cardiac muscle, glands, adipocytes
Neural Pathway	One motor neuron	Two-neuron chain (preganglionic and postganglionic)
Neurotransmitter	Acetylcholine	Acetylcholine, norepinephrine
Conscious Control	Yes	No