CJ

Chapter 15: Chemical Control of Behavior

  • Introduction (refer to figure 15.1)

    • Point-to-point: restricted synaptic activation of target cells and signals of brief duration

    • Secretory neurons release hormones directly into the bloodstream to affect target cells

    • The ANS has networks of interconnected neurons that activate tissues all over the body

    • Diffuse modulatory systems extend their reach with divergent axonal projections that release NT into the area to be used by neurons in the area

  • The Secretory Hypothalamus

    • The hypothalamus integrates somatic and visceral responses per the needs of the brain

      • It is involved in homeostasis, a regulatory process within the body to maintain an optimal internal environment

        • Ex: Shivering when cold and sweating when hot

        • Homeostasis also regulates blood volume, pressure, salinity, acidity, and blood oxygen and glucose levels

      • It also controls the four f’s: fighting, fleeing, feeding, and fornication by releasing hormones

      • The hypothalamus controls allostasis, or the body’s response to abnormalities by briefly changing its internal environment

        • Ex: raising body temp, as a result of the flu

      • Each side of the hypothalamus (surrounding the third ventricle) is separated into the lateral, medial, and periventricular zones

        • The periventricular zone receives input from the lateral and medial zones, as well as the brain stem and telencephalon

          • There are three groups of cells within the zone:

            • The suprachiasmatic nucleus (SCN) receives information from the retina for the synchronization of circadian rhythms

            • The cell group that controls the ANS regulates the sympathetic and parasympathetic innervation of visceral organs

            • Neurosecretory neuron axons project to the pituitary gland; they command our attention

      • The pituitary gland acts as the “mouthpiece” by which the hypothalamus “speaks” with the rest of the body

        • It is separated into two lobes (anterior and posterior), which the hypothalamus controls in different ways

          • Magnocellular neurosecretory cells extend into the posterior lobe of the pituitary, where they release neurohormones oxytocin and vasopressin into capillaries

            • Oxytocin the “love hormone” is a peptide released during sexual or intimate behaviors and lactation

              • Positive feedback loop

            • Vasopressin or antidiuretic hormone (ADH) is also a peptide that regulates blood volume and salt concentration

              • When blood pressure is low, the kidneys secrete the enzyme renin that converts angiotensinogen, from the liver, into angiotensin II (a smaller peptide hormone)

              • Angiotensin II increases blood pressure in the kidneys but is also detected by the subfornical organ of the telencephalon

              • Axons from the subfornical organ project to the hypothalamus and activate vasopressin-containing neurosecretory cells and cells in the lateral hypothalamus to produce thirst

                • If there are low amounts of water in the body, the kidney retains water by limiting urination

          • The anterior lobe of the pituitary is a gland, whose cells synthesize and secrete hormones and regulate secretions from other body glands (FSH & LH for gonads, TSH for thyroid, ACTH for adrenal, and GH for mammary glands and all cells)

            • Parvocellular neurosecretory cells in the periventricular area secrete hypophysiotropic hormones into a network of blood vessels called the hypothalamo-pituitary portal circulation

              • These hormones travel to receptors for stimulation or inhibition of anterior pituitary hormone release

            • The adrenal cortex of the adrenal gland produces cortisol which is regulated via parvocellular neurosecretory cells in the hypothalamus

              • If a stimulus is deemed stressful, these neurons release corticotropin-releasing hormone (CRH) into the portal circulation, where it stimulates the release of adrenocorticotropic hormone (ACTH) from the posterior pituitary to the adrenal cortex to release cortisol

                • Negative feedback loop

              • Addison’s disease results from adrenal insufficiency, or not enough cortisol production (JFK had this)

              • Cushing’s disease results from elevated ACTH and cortisol, due to pituitary gland dysfunction

  • The Autonomic Nervous System

    • The autonomic nervous system balances excitation and inhibition to achieve widely coordinated and graded control via the periventricular zone of the hypothalamus

      • These actions are carried out automatically, without conscious, voluntary control

      • The sympathetic division of the ANS increases heart rate and blood pressure while depressing digestive reserves to mobilize glucose reserves

      • The parasympathetic division of the ANS slows the heart rate and blood pressure while increasing digestive function

    • Neural output of the CNS: somatic motor and autonomic nervous (sympathetic and parasympathetic) systems

      • Somatic: innervates and commands skeletal muscle fibers

        • The cell bodies are in the CNS in the ventral horn or brain stem

        • Monosynaptic pathway

      • Autonomic: innervates and commands tissue and organs

        • The cell bodies are in the autonomic ganglia (neurons = postganglionic neurons)

          • Preganglionic neurons drive postganglionic neurons, with cell bodies in the spinal cord and brain stem

            • The NT of the ANS preganglionic neurons is ACh

            • The NT of the sympathetic postganglionic neurons is NE

            • The NT of the parasympathetic postganglionic neurons is ACh

        • Disynaptic pathway

    • The sympathetic and parasympathetic divisions operate in parallel

      • Preganglionic axons of the sympathetic division emerge from the thoracic and lumbar segments of the spinal cord

        • They lie within the intermediolateral gray matter of the spinal cord

        • These axons go through the ventral roots to synapse on neurons in the ganglia of the sympathetic chain, which lies next to the spinal cord

      • Preganglionic axons of the parasympathetic division emerge from the brain stem and sacral segments of the spinal cord

        • They go up to or innervate target tissue

    • The ANS innervates glands, smooth muscle, and cardiac muscle. It also innervates/regulates:

      • secretory glands (salivary, sweat, tear, and various mucus-producing glands

      • heart and blood vessels to control blood pressure and flow

      • bronchi of the lungs to meet the oxygen demands of the body

      • digestive and metabolic functions of the liver, gastrointestinal tract, and pancreas

      • functions of the kidney, urinary bladder, large intestine, and rectum

      • essential to the sexual responses of the genitals and reproductive organs

      • interacts with the immune system

    • The two divisions function inversely: when one is high, the other is low

      • The sympathetic division frenetically mobilizes the body for a short-term emergency

      • The parasympathetic division works calmly for the long-term good

    • Some tissue are innervated by both divisions (heart, lungs) while others are innervated by just the sympathetic (blood vessels of the skin and sweat glands) or the parasympathetic (lacrimal/tear glands)

      • Erection is triggered by parasympathetic division, while orgasm and ejaculation are triggered by the sympathetic division

    • The enteric division lies within the lining of the esophagus, stomach, intestines, pancreas, and gallbladder

      • The myenteric (Aurebach’s) plexus and submucous (Meissner’s) plexus control many of the physiological processes involved in the transport and digestion of food

      • It has 500 million neurons (the same amount as the spinal cord)

      • Sensory neurons monitor tension and stretch of the gastrointestinal walls, the chemical status of the stomach and intestinal contents, and hormone levels in the blood

      • It works autonomously and with input from the brain via axons of the ANS divisions

    • The hypothalamus is the main regulator of the autonomic preganglionic neurons

      • The nucleus of the solitary tract, located in the medulla and connected with the hypothalamus, integrates sensory information from the internal organs and coordinates output to the autonomic brain stem nuclei

    • The ANS is relatively simple compared to the CNS

      • Neurons of the peripheral parts of the ANS are outside the blood-brain barrier, so all drugs that enter the bloodstream have direct access to them

    • The preganglionic neurons of both divisions release ACh

      • ACh binds to nicotinic ACh ionotropic receptors which evokes EPSPs that trigger an action potential in the postganglionic cell

      • ACh also binds to muscarinic ACh metabotropic (G protein-coupled) receptors that cause the opening and closing of ion channels for slow EPSPs and IPSPs

      • Some terminals also release neuroactive peptides such as neuropeptide Y and vasoactive intestinal polypeptide

        • These interact with G protein-coupled receptors to produce small EPSPs that last for several minutes

    • The postganglionic neurons release either ACh (parasympathetic) or NE (sympathetic)

      • Parasympathetic ACh has a local effect on its targets and acts through mAChRs

        • Drugs that promote the muscarinic actions of ACh or inhibit the actions of NE are parasympathomimetic (propranolol)

      • Sympathetic NE spreads far, even into the blood where it can circulate widely

        • Drugs that promote the action of NE or inhibit the muscarinic actions of ACh are sympathomimetic (atropine)

    • The adrenal medulla releases epinephrine due to stimuli causing the hypothalamus to activate the adrenal medulla via nerve impulses

  • The Diffuse Modulatory System of the Brain

    • Common principles

      • a small set of neurons (several thousand)

      • arise from the central core of the brain, most of them from the brain stem

      • each neuron can influence many others across the brain

      • synapses release transmitter molecules into the extracellular fluid so they can diffuse to many neurons

    • Locus coeruleus (blue spot) neurons in the pons use NE

      • Axons innervate the cerebral cortex, thalamus, hypothalamus, olfactory bulb, cerebellum, midbrain, and spinal cord

      • Functions: regulation of attention, arousal, sleep-wake cycles, learning and memory, anxiety & pain, mood, and brain metabolism

      • Activation: new, unexpected, nonpainful sensory stimuli

    • The nine raphe nuclei neurons contain 5-HT

      • Axons innervate the cerebral cortex, thalamus, hypothalamus, cerebellum, basal ganglia, temporal lobe, and spinal cord

      • Functions: together with the noradrenergic system, comprise the ascending reticular activating system

        • Particularly involved in sleep-wake cycles and mood

      • Most active during wakefulness, when aroused, and when active

    • The substantial nigra and ventral tegmental area use DA

      • Substantia nigra

        • Axons project to the striatum (caudate nucleus and putamen)

        • Facilitates the initiation of voluntary movements (degeneration causes Parkinson’s disease)

      • Ventral tegmental area

        • Innervates circumscribed regions of the telencephalon (frontal cortex and parts of the limbic system)

          • Mesocorticolimbic dopamine system: dopaminergic projection from the midbrain

          • Involved in the “reward” pathway

    • The basal forebrain complex and brainstem complex use ACh

      • Basal forebrain complex

        • Nuclei at the core of the telencephalon, medial and ventral to basal ganglia

          • Medial septal nuclei innervate the hippocampus

          • The basal nucleus of Meynert innervates the neocortex

        • Function: mostly unknown, participates in learning and memory, as well as sleep-wake cycles

      • Brainstem (pontomesencephalotegmental) complex

        • In the pons and midbrain tegmentum

        • Function: regulates excitability of thalamic sensory relay nuclei

    • Many abused psychoactive drugs act directly on the modulatory systems (NE, DA, and 5-HT)

      • Hallucinogens

        • LSD may act on 5-HT receptors to inhibit the firing of raphe neurons

      • Stimulants

        • Cocaine and amphetamine both exert their effects at synapses made by DA and NE systems

          • They are both sympathomimetic

          • These drugs block catecholamine uptake to prolong the signal of DA and NE