12: Efferent Division: Autonomic and Somatic Motor Control

Autonomic Nervous System (ANS)

• is the part of the efferent nervous system that controls involuntary functions of internal organs

• two branches: sympathetic (fight or flight) during stress and parasympathetic (rest and digest) during calm states

Functions of Sympathetic Branch

• Prepares body for acting in stressful situations

• increasing heart rate, dilating blood vessels to muscles, stimulating glucose production in the liver, reducing digestive activity by diverting blood away from gastrointestinal tract

• Can be localized and does not always trigger a full fight or flight response

• regulated by hypothalamus

• originate in thoracic and lumbar spinal cord

• short pre and long post

• taken back up into the varicosity or diffuse away

Function of Parasympathetic Branch

• promoting digestion and routine maintenance of the body during restful periods

• originate in brainstem and sacral spinal cord and ganglia near target organs

• long pre and short post

• broken down enzymatically or diffuse away

key homeostatic control centers

• pons, medulla, and hypothalamus

• receive sensory input from senses and receptors

• initiate autonomic, endocrine, and behavioral responses

hypothalamus

• temperature control, water balance, eating behavior

pons

• urinary bladder control in upper pons

• blood pressure control in the lower pons

medulla

• blood pressure control

• respiratory control

4 key homeostatic properties

• maintaining internal environment fitness

• tonic (up-down) regulation

• antagonistic control

• using chemical signals that have different effects in different tissues

All autonomic pathways are composed of two neurons in the series

• preganglionic neuron that originates in the CNS and projects to an autonomic ganglion

• postganglionic neuron that has its cell body in the ganglion and projects to the target tissue

ganglion

• cluster of nerve cell bodies outside the CNS

Neurotransmitters are

• released at the synaptic cleft between the preganglionic and postganglionic neurons

• postganglionic neuron then synapses with the target tissue

chemical signals

• Acetylcholine to nicotinic cholinergic receptors

• norepinephrine (NE) onto adrenergic receptors on target cells

Norepinephrine release at sympathetic neuroeffector junction steps:

• action potential arrives at the varicosity

• depolarization opens voltage-gated Ca2+ channels

• Ca2+ entry triggers exocytosis of NE-containing vesicles

• NE binds to adrenergic receptors on the target cell, activating a G-protein and eliciting a cellular response

• NE action ends when it diffuses away or is taken up by blood vessels

• NE is removed from the synapse by active transport back into the varicosity, where it can be recycled or metabolized

• NE is metabolized by monoamine oxidase (MAO) in mitochondria

Adrenal medulla

• specialized neuroendocrine tissue

• atop the kidneys

• primarily secretes epinephrine

• neural origin, functioning as a neurosecretory structure

• acts like a modified sympathetic ganglion

adrenal cortex

• true endocrine gland of epidermal origin that secretes steroid hormones

Direct Agonists/Antagonists

• bind to receptors to mimic or block neurotransmitter action

Indirect Agonists/Antagonists

• alter neurotransmitter secretion, uptake, or degradation

Somatic motor pathway

• controls skeletal muscles

• consists of a single neuron that originates in the CNS and extends its axon directly to the skeletal muscle

• always targets skeletal muscle

• always excitatory = always stimulate muscle contraction

Somatic motor neuron

• cell bodies are found in ventral horn of the spinal cord or in the brain

• has a single, long, myelinated axon that extends from the cell body to its skeletal muscle target

• axons can be very long