The Peripheral and Autonomic Nervous Systems

The PNS Links the central nervous system with the rest of the body All sensory information and motor commands are carried by axons of the PNS The sensory and motor axons are bundled together into peripheral nerves or nerves and clusters of cell bodies or ganglia.

The PNS includes cranial nerves and spinal nerves There are 12 pairs of cranial nerves with connect to the brain not to the spinal cord. There are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. Each pair monitors a region of the body surface known as a dermatome.

Nerve Plexuses A complex, interwoven network of nerves is called a nerve plexus. The three large plexuses are the cervical plexus, the brachial plexus, and the lumbosacral plexus. The latter can be divided into the lumbar plexus and sacral plexus.

Reflexes A reflex is an automatic involuntary motor response to a specific stimulus A reflex arc is the “wiring” of a single reflex.

There are five steps in a reflex arc 1. Arrival of a stimulus and activation of a receptor 2. Activation of a sensory neuron 3. Information processing 4. Activation of a motor neuron 5. Response by an effector

Monosynaptic reflex Simplest reflex arc in which a sensory neuron synapses directly on a motor neuron that acts as the processing center. The stretch reflex regulates skeletal muscle length and muscle tone. The patellar reflex is an example of a stretch reflex.

Polysynaptic reflexes Have at least one interneuron between the sensory afferent and the motor efferent, Take longer Produce more complicated responses The flexor reflex is a withdrawal reflex affecting the muscles of a limb

The brain can either facilitate or inhibit reflex motor patterns based in the spinal cord.

The Autonomic Nervous System The ANS coordinates cardiovasclar, respiratory, digestive, excretory and reproductive functions. Preganglionic neurons in the CNS send axon to synapse on ganglionic neurons in autonomic ganglia outside the CNS The axons of the postganglionic fibers innervate cardiac muscle, smooth muscles, glands, and adipose tissues.

Preganglionic fibers from the thoracic and lumbar segments form the sympathetic division (fight or flight) Preganglionic fibers leaving the brain and sacral segments form the parasympathetic division (the rest and digest)

The Sympathetic Division The sympathetic division consists of preganglionic neurons between segments T1 and L2, ganglionic neurons in ganglia near the vertebral column, and specialized neurons in the adrenal gland.

Preganglionic fibers entering the adrenal glands synapse within the adrenal medullae. During sympathetic activation these endocrine organs secrete epinephrine and norepinephrine into the bloodstream In a crisis, the entire division responds, producing increased alertness, a feeling of energy and euphoria, increased cardiovascular and respiratory activity and elevation in muscle tone.

Sympathetic symptoms. Stimulates sweat gland activity Reduces circulation to the skin Accelerates blood flow to muscles Releases stored lipids Accelerates heart rate Dilates respiratory passagewats Reduces blood flow to viscera Dilates pupils

The Parasympathetic Division The effects produced by the parasympathetic division center on relaxation, food processing and energy absorption. They are usually brief and restricted to specific sites. Include: increased digestive secretions, constrict pupils, decrease heart rate, increase smooth muscle activity digestive tract.

Relationship between the parasympathetic and sympathetic divisions The sympathetic division has widespread impact, reaching visceral and somatic structure throughout the body. The parasympathetic division innervates only visceral structures serviced by cranial nerves or lying within the abdominopelvic cavity.

Aging and the Nervous System Age related changes include: A reduction of brain size and weight A reduction of the number of neurons, Decreased blood flow to the brain Changes in synaptic organization of The brain Intracellular and extracellular changes in CNS neurons.