Lecture 14: Autonomic Nervous System & Somatic Nervous System

Nerves

Groups of axons in PNS

Tracts

Groups of axons in CNS

Nuclei

Groups of cell bodies in CNS

Ganglia

Groups of cell bodies in PNS

Autonomic system

Controls involuntary functions, maintains homeostasis, has afferent and efferent neurons, and efferent neurons innervate visceral organs

Somatic system

Controls voluntary functions, has afferent and efferent neurons, and efferent neurons innervate skeletal muscles

Somatic Motor Division

Neurons innervate skeletal muscles and lead to voluntary muscle contractions initiated consciously

Autonomic Motor Division

Neurons innervate smooth muscle cells, cardiac muscle cells, and glands, leading to involuntary actions initiated not consciously

Preganglionic neuron

Initial efferent neuron with cell body residing within CNS, all axon terminals release ACh

Postganglionic neuron

Cell body resides in autonomic ganglion in PNS, axons travel to target cells, release either ACh or norepinephrine, and trigger specific changes

Visceral reflex

A reflex specific to the autonomic division to which it belongs

Central nervous system

Coordinates and contributes to autonomic functions

General senses

Senses that include touch, pain, temperature, and proprioception

Special senses

Senses that include sight, hearing, taste, smell, and equilibrium

Sensory neurons

Neurons related to general senses and special senses

Motor neurons

Neurons that innervate skeletal muscles

Involuntary actions

Actions produced by the autonomic motor division

Voluntary muscle contractions

Contractions produced by the somatic motor division

Afferent neurons

Neurons that carry sensory information to the central nervous system

Efferent neurons

Neurons that carry motor commands from the central nervous system to effectors

Homeostasis

The maintenance of stable internal conditions by the autonomic system

Enteric division

Part of the autonomic nervous system that controls digestion

Target effector

The organ or tissue that responds to autonomic input

Somatic Nervous System

The axon of a single, myelinated somatic motor neuron extends from the CNS to the skeletal muscle fiber it innervates.

Parasympathetic Nervous System

Craniosacral division; most active during resting conditions; 'rest and digest' response.

Sympathetic Nervous System

Thoracolumbar division; most active during times of stress/exertion; 'fight or flight or freeze or fawn' response.

Sympathetic NS Effects

Prepares body for increased activity levels; dilates pupils, increases heart rate, increases breathing rate, decreases digestive activity.

Parasympathetic NS Effects

Conserves energy during relaxation; constricts pupils, decreases heart rate, decreases breathing rate, increases digestive activity.

ANS Motor Pathways

Most autonomic motor pathways consist of two motor neurons in series: a preganglionic neuron and a postganglionic neuron.

Sympathetic Division

Stimulation leads to increased alertness and metabolism to be ready for an emergency.

Parasympathetic Division

Stimulation slows down most body activities.

Sympathetic Preganglionic Axons

Short; secretes ACh.

Sympathetic Postganglionic Axons

Long; secretes NorE. or ACh.

Parasympathetic Preganglionic Axons

Long; secretes ACh.

Parasympathetic Postganglionic Axons

Short; secretes ACh.

Sympathetic Ganglia

Cell bodies of the sympathetic preganglionic neurons are part of the lateral gray horns of all thoracic segments and of the first two lumbar segments of the spinal cord.

Sympathetic Trunk Ganglia

Lie in a vertical row on either side of the vertebral column.

Prevertebral Ganglia

Lie anterior to the vertebral column and close to the large abdominal arteries.

Sympathetic Outflow

Cell bodies of preganglionic neurons are in the lateral horns of the gray matter in the 12 thoracic and first 2 or 3 lumbar segments.

Parasympathetic Outflow

Cell bodies of the preganglionic neurons are in the nuclei of four cranial nerves (III, VII, IX, and X) in the brain stem and in the lateral gray matter of sacral segments 2-4 of the spinal cord.

Autonomic Plexuses

The abdomen and pelvis contain major autonomic plexuses which are often named after the artery along which they are distributed.

Celiac Plexus

One of the major autonomic plexuses in the abdomen.

Superior Mesenteric Plexus

One of the major autonomic plexuses in the abdomen.

Inferior Mesenteric Plexus

One of the major autonomic plexuses in the abdomen.

Renal Plexus

One of the major autonomic plexuses in the abdomen.

Hypogastric Plexus

One of the major autonomic plexuses in the abdomen.

Cholinergic Neurons

Release the neurotransmitter acetylcholine.

Cholinergic Receptors

Include nicotinic receptors and muscarinic receptors.

Adrenergic neurons

Release norepinephrine (noradrenalin)

Sympathetic

Part of the autonomic nervous system that prepares the body for stressful or emergency situations.

Parasympathetic

Part of the autonomic nervous system that conserves energy and restores the body to a state of calm.

Preganglionic

Neurons that release acetylcholine to nicotinic receptors.

Postganglionic

Neurons that release acetylcholine to muscarinic receptors.

Norepinephrine

A neurotransmitter that binds to α- or β-adrenergic receptors.

Sympathetic Receptor Classes

Two types: Adrenergic receptors and Cholinergic receptors.

Adrenergic receptors

Bind to epinephrine and norepinephrine.

Alpha adrenergic

A major type of adrenergic receptor.

Beta adrenergic

A major type of adrenergic receptor with subtypes: Beta-1, Beta-2, and Beta-3.

Beta-1 receptors

Located in the plasma membrane of cardiac muscle cells, certain kidney cells, and adipose cells.

Beta-2 receptors

Located in the plasma membrane of smooth muscle cells lining airways of the respiratory tract and in the wall of the urinary bladder.

Beta-3 receptors

Primarily found in adipose cells and smooth muscle cells in walls of the digestive tract.

Muscarinic receptors

Type of cholinergic receptor found on sweat glands in skin.

Nicotinic receptors

Type of cholinergic receptor located in the plasma membrane of all postganglionic neurons.

Cholinergic vs. Adrenergic

Cholinergic neurons release acetylcholine while adrenergic neurons release norepinephrine.

Sensation

Conscious and subconscious awareness of changes in the external or internal environment.

Components of sensation

Stimulation of the sensory receptor → transduction of the stimulus → generation of nerve impulses → integration of sensory input.

Classification of Sensory Receptors

Divided into general senses (somatic and visceral) and special senses (smell, taste, vision, hearing, and equilibrium).

Types of Sensory Receptors

Includes free nerve endings (pain and thermoreceptors) and encapsulated nerve endings (like pacinian and Meissner's corpuscles).

Exteroceptors

Sensory receptors located at or near the body surface.

Interoceptors

Sensory receptors located inside the body, including blood vessels, viscera, and the nervous system.

Proprioceptors

Sensory receptors located in muscles, tendons, joints, and the inner ear.

Mechanoreceptors

Sensory receptors that respond to touch and pressure.

Thermoreceptors

Sensory receptors that respond to temperature changes.

Nociceptors

Sensory receptors that detect pain.

Photoreceptors

Sensory receptors that are specific to the retina.

Chemoreceptors

Sensory receptors that respond to smell, taste, O2, and CO2.

Osmoreceptors

A type of chemoreceptor that responds to osmotic pressure.

Baroreceptors

A type of mechanoreceptor that responds to pressure.

Sensory Receptor Adaptation

The decrease in potentials during a maintained, constant stimulus.

Rapidly adapting receptors

Receptors that detect pressure, touch, and smell.

Slowly adapting receptors

Receptors that detect pain, body position, and chemical composition of the blood.

Somatic Sensations

Sensory perceptions from receptors in the skin, muscles, tendons, and joints.

Tactile Sensations

Sensations that include touch, pressure, vibration, itch, and tickle.

Referred Pain

Pain felt in or just deep to the skin that overlies the stimulated organ or in a surface area far from the stimulated organ.

Proprioceptive Sensations

Sensations detected by proprioceptors, which include weight discrimination.

Somatic Sensory Pathways

Pathways that carry information from somatic sensory receptors to the primary somatosensory area in the cerebral cortex and to the cerebellum.

First-order neurons

Neurons that carry impulses from somatic receptors to the brainstem or spinal cord.

Second-order neurons

Neurons that carry impulses from the brainstem and spinal cord to the thalamus.

Third-order neurons

Neurons that carry impulses from the thalamus to the primary somatosensory area of the cortex on the same side.

Primary Somatosensory Area

The postcentral gyri located on both parietal lobes of the brain that receive sensory input from different parts of the body.

Lower motor neurons (LMNs)

Nerves that extend out of the brainstem and spinal cord, innervating skeletal muscles.

Somatic Motor Pathways

Pathways that provide input into lower motor neurons and are divided into distinct circuits.

Direct motor pathways

Pathways that deliver signals to LMNs from the cerebral cortex.

Indirect motor pathways

Pathways that deliver signals to LMNs from motor centers in the basal nuclei, cerebellum, and cerebral cortex.

Primary Motor Area

The area of the brain where each point of the body maps to a specific region.

Autonomic Nervous System

A part of the nervous system that controls involuntary bodily functions.

Target Effector Response

The reaction of a target organ or tissue to autonomic input based on the released signaling molecule.

Central Nervous System Contribution

The role of the brain and spinal cord in coordinating and regulating autonomic functions.

Stimulus-Response Motor Pathway

The pathway through which sensory stimuli are processed and lead to motor responses.

Sympathetic Preganglionic Neurons

Neurons that originate in the spinal cord and synapse in the sympathetic trunk ganglia.