Medulla

The medulla is the inferior part of the brain stem. 

The medulla contains the cardiac, respiratory, vomiting and vasomotor centers and deals with autonomic functions such as breathing, heart rate and BP.

 

It contains:

1) nuclei (neurons) of the cranial nerves:

• IX = salivation
• X = PS efferent supply of thoracic & abdominal viscera (heart, lung, GI)

2) Reticular formation- a diffuse group of brainstem nuclei that modulate activity of the nervous system by filtering incoming stimuli and ignoring the irrelevant background stimuli.

3) Area postrema– vomiting center. Has no protection from the blood brain barrier.

Limbic System

The limbic system is involved in the generation of emotions, instincts and memories.  It receives instructions from the hypothalamus and projects instructions to the areas of the cerebral cortex responsible for cognition and intelligent decisions. The response from these cortex areas is sent back to the limbic system, hypothalamus and reticular formation, changing the activity of the descending autonomic efferent pathways and secreting pituitary hormones.

Hypothalamus

The hypothalamus is the major autonomic regulatory center. It’s main function is to regulate homeostasis through:

• water balance
• temperature
• feeding

It receives sensory info (osmo, baro, visual receptors etc) from many autonomic afferent fibers such as the vagus nerve and glossopharingeal nerve. The information is collected and sorted and an endocrine response is elicited. The info is then sent to the amygdala and other structures of the limbic system where a behavior response is elicited.

SNS vs PSNS

Sympathetic

• fight or flight
• stress & emergency
• widespread
• long lasting
• uses energy

PSNS

• rest, normal activity
• local acting
• short duration
• saves energy

Nervous System Effects on Pupil Size

Anisocoria is a pathological condition of enequal pupil size.  It can be caused byphysical damage, drugs or more seriously, cranial nerve palsy.  

Mydriasis is the widening of the pupil diameter.  It is usually accomodated with relaxation of ciliary muscle, which allows far vision and widening of palpebral fissure (widening of eyelids).  Mydriasis is caused by stimulation of the sympathetic nervous system.

Miosis is constriction of the pupil. This is caused by stimulation of the PS nervous system

Physiological Changes of SNS Stimulation

• ↑ BP
• ↑ HR
• ↑ force of heart contraction
• ↑ heart conduction velocity
• ↑ depth and rate of respiration
• blood flow shifts away from skin and gut to skeletal muscles and heart
• glycogenolysis
• lipolysis
• contraction of spleen (↑ hematocrit)
• mydriasis
• relaxation of ciliary muscle (allows far vision)
• widening of palpebral fissure (eyelids open wide)
• piloerection (goose bumps)
• inhibition of GI motility & secretion,  contraction of spinchters
• cold sweats

Most blood vessels are under tonic control exerted solely by constrictory sympathetic nerves. This means sympathetic stimulation = vasoconstriction and ↑ BP.

Somatic Nervous System

The somatic nervous system is an efferent nervous system. Controls voluntary movement of body parts.

Transmitter: Ach (NMJ)

Pathway: 1 neuron (1 ax0n) pathway from CNS

Nerve fibers: Large (type A, large myellinated)

ANS

The ANS is an efferent nervous system which is based on involuntary controls. 

Transmitters: ACH, NE or peptide cotransmitters

Pathway:  1. preganglionic (type B myelinated fibers) & postganglionic (type C unmyellinated) neurons

Nerve fibers: small

Efferent Autonomic Reflexes of Cranial Nerves

Efferent autonomic reflexes carry impulses from the CNS to the viscera via the autonomic system.

Vagal nerve (CN X) – decreases heart rate.  Can be triggered by either somatic or visceral sensory afferents.

Trigemino-vagal – oculocardiac and diving = pulse drop in response to pressure applied to eye or stimulation of facial skin.

Nerve Fibers of the PNS

Afferent – sensory nerve fibers bring impulses to the CNS = receptors.

Efferent– motor nerve fibers bring impulses to the periphery = skeletal muscles, smooth muscle, cardiac muscle, gland.