ANS

Somatic Nervous System (SNS)

Consciously perceived or controlled processes; somatic sensory and somatic motor

Somatic sensory

Portion detects signals from special senses (vision, hearing, equilibrium, smell, taste) and from skin and proprioceptors

Somatic motor

Portion sends signals from CNS to skeletal muscles; voluntary movements involve cerebrum; reflexive movements involve brainstem and spinal cord

Autonomic Nervous System (ANS)

Also called autonomic motor or visceral motor system; processes regulated below conscious level; transmits signals from CNS to heart, smooth muscle, glands; responds to visceral sensory inputs; functions to maintain homeostasis

Lower motor neurons of somatic nervous system

A single lower motor neurons extends from CNS to skeletal muscles fibers; cell body within brainstem or spinal cord; myelinated axon with large diameter; releases ACh from synaptic knob to excite muscle fiber

Lower motor neurons of autonomic nervous system

ANS uses a chain of 2 motor neurons to reach effector; first is preganglionic neuron; second is ganglionic (postganglionic) neuron

Preganglionic neuron

First in chain of lower motor neurons of ANS; cell body within brainstem or spinal cord; thin, myelinated axon projects to autonomic ganglion in peripheral nervous system; releases ACh from synaptic knob to excite second motor neuron

ganglionic (postganglionic) neuron

Second in the lower motor neurons of ANS; cell body within autonomic ganglion; very thin, unmyelinated axon projects to cardiac muscle, smooth muscle, or gland; releases ACh or NE from synaptic knob to either excite or inhibit the effector

Allows for neuronal convergence and divergence

What are the advantages for the ANS to use a chain of two neurons instead of just one

Neuronal convergence

Multiple preganglionic neurons synapsing on one ganglionic neuron

Neuronal divergence

Branches of axon from one preganglionic neuron synapsing with numerous ganglionic neurons

Brain and spinal cord

ANS is regulated by parts of what

Hypothalamus

What is crucial for emotional ANS processes such as fight or flight and can be influenced by areas of cortex, thalamus, and limbic system

Brainstem

Mediates visceral reflexes; maintenance of blood pressure

Spinal cord

Governs some ANS reflexes; defecation and urination as controlled by the parasympathetic nervous system

Parasympathetic division

Rest and digest; for conserving energy, replenishing nutrients

Sympathetic division

Fight or flight; for exercise, excitement, emergency

Parasympathetic activity

Fairly localized; long preganglionic axons with limited branches influence only one or a few effectors at the same time

Sympathetic activity

Often facilitates mass activation; short preganglionic axons with many branches simultaneously influence several effectors; sympathetic stimulation of the release of adrenal medulla hormones facilitates this; sometimes however, a single effect can be engaged

Parasympathetic function

Maintain homeostasis at rest

Oculomotor, facial, glossopharngeal, and vagus nerves

Four cranial nerves that convey parasympathetic signals

Oculomotor Nerve (CN III)

Preganglionic axons from cell bodies in midbrain nuclei extend to ciliary ganglion without the orbit; postganglionic axons control ciliary muscles and pupil construction

Facial nerve (CN VII)

Preganglionic axons with cell bodies in pons extend to pterygopalatine and submandibular ganglia; postganglionic axons from pterygopalatine ganglion control lacrimical glands and small glands of nose and mouth; postganglionic axons from submandibular glands control submandubular and sublingual salivary glands

Glossopharnygeal nerve (CN IX)

Preganglionic axons from cell bodies in medulla extend to otic ganglion near ear; postganglionic axons control parotid salivary glands

Vagus nerve (CN X)

Preganglionic axons from cell bodies in medulla project extensively to a variety of ganglia in thorax and abdomen; postganglionic axons have a variety of functions including: slowing heart rate, constricting lung bronchioles, increasing digestive secretions, storing carbohydrates

Sympathetic function

Exercise and emergency

Sympathetic preganglionic neurons

Cell bodies housed in lateral horn of T1-L2; myelinated axons course through anterior roots of T1-L2 spinal nerves then branch off

Sympathetic trunks and ganglia

Left and right trunks just lateral to the vertebral column; trunk resembles a pearl necklace; “string” composed of axons, “pearls” composed of sympathetic trunk ganglia housing cell bodies

Acetylcholine (ACh) and norepinephrine (NE)

ANS uses what two transmitters; either can cause stimulation or inhibition depending on the postsynaptic receptor

Cholinergic neurons

Cells that release ACh; include all ANS preganglionic neurons; all parasympathetic ganglionic neurons; sympathetic ganglionic neurons innervating sweat glands and blood vessels in skeletal muscle; target cells have cholinergic receptors

Adrenergic neurons

Cells that release norepinephrine; most sympathetic ganglionic neurons; target cells have adrenergic receptors

Blood vessels usually in a partially constricted state due to sympathetic tone

In what instances does only one division controls the status of an effector

Dual innervation

Organ receives input from both the sympathetic and parasympathetic divisions; antagonistic effects

Antagonistic effects

Commonly the two divisions oppose each other; parasympathetic activity slows heart rate; sympathetic activity increases heart rate; parasympathetic activity increases GI tract mortality; sympathetic activity decreases mortality; parasympathetic active decreases pupil diameter; sympathetic activity dialates pupils