The Nervous System- Nervous Tissue Chapter 9

The Nervous system has 3 major functions: Sensory – monitors/detects stimuli of internal & external environment through presence of receptors (afferent neurons) Integration – interpretation of sensory information (information processing); complex/higher order functions (interneurons- most common) Motor – response to information processed through stimulation of effectors (efferent neurons) muscle contraction glandular secretion

General Organization Two Divisions of Nervous System Central nervous system (CNS) Brain and Spinal cord Peripheral nervous system (PNS) All other nervous tissue outside CNS Somatic nervous system (SNS) Special senses and skeletal muscles…voluntary Autonomic nervous system (ANS) Visceral organs (stomach & lungs), smooth muscle, cardiac muscle, glands…involuntary Sympathetic and parasympathetic divisions Enteric nervous system (ENS)= autonomic Brain of the gut (contractions and secretions)…involuntary

Histology of neural tissue Two types of neural cells in the nervous system: Neurons - For processing, transfer, and storage of information Neuroglia – For support, regulation & protection of neurons (generally smaller and more numerous)

Neuron structure (3 parts): Dendrites Cell Body Axon (& axon terminal)

Schwann cells surround all axons of neurons in the PNS creating a neurilemma around them. Neurilemma allows for potential regeneration of damaged axons creates myelin sheath around most axons of PNS; white color Myelin increases from birth to maturity (responses to stimuli get better)

Myelin will get laid down in segments (internodes) along the axon, leaving unmyelinated gaps known as “nodes of Ranvier” Regions of the nervous system containing groupings of myelinated axons make up the “white matter” “gray matter” is mainly comprised of groups of neuron cell bodies, dendrites, unmyelinated axons, & neuroglia (shaped like a butterfly) of Ranvier

Neuroglia (glial cells) CNS neuroglia: astrocytes oligodendrocytes microglia ependymal cells PNS neuroglia: Schwann cells (neurolemmocytes) satellite cells

Astrocytes create supportive framework for neurons create “blood-brain barrier” Play a role in learning and memory secrete chemicals for embryological neuron formation

Oligodendrocytes Produce & maintain myelin sheath around axons of neurons in the CNS.

Microglia phagocytize cellular wastes/debris & pathogens/microbes

Ependymal cells line ventricles of brain & central canal of spinal cord produce, monitor & help circulate CSF (cerebrospinal fluid)

Conduction across synapses In order for neural control to occur, “information” must not only be conducted along nerve cells, but must also be transferred from one nerve cell to another across a synapse 2 types of conduction: continuous (unmyelinated axons) saltatory (myelinated axons) *diameter, myelin, and temp of axon determine speed of nerve impulse

The Structure of a Typical Synapse One direction: presynaptic to postsynaptic. *A typical neuron in the CNS receives input from 1000-10,000 synapses

Acetlycholine (Ach)- excitatory or inhibitory *nicotine imitates Most synapses within the nervous system are chemical synapses, & involve the release of a neurotransmitter molecules/receptors 100 substances ? (50 known): Acetlycholine (Ach)- excitatory or inhibitory *nicotine imitates Glutamate Aspartate Gamma aminobutyric acid (GABA) *Barbiturates/depressants/alcohol/THC enhance Glycine excitatory inhibitory

Neurotransmitters (cont.) Norepinephrine (NE)- arousal from sleep, dreaming, moods Dopamine (DA)- emotional response, addictive behavior, pleasurable experience *Cocaine/Amphetamines/THC block uptake of dopamine (lasts longer) *caffeine increases production of dopamine Serotonin- Sensory perception, temp regulation, control of mood/appetite/onset of sleep *ecstasy increases Endorphins- Body’s natural painkillers * Opiates increase

Neurotransmitters in our food? Chocolate: Over 300 substances have been identified in small amounts. caffeine theobromine (another, less powerful stimulant) Phenylethylamine (a substance related to a family of stimulants called amphetamines) Anandamide (the neural receptors for anandamide are the same ones to which THC, the main active ingredient in cannabis)  you would have to eat well over 30 kilos of chocolate to experience effects comparable to one dose of cannabis!

Anatomical organization of neurons Neurons of the nervous system tend to group together into organized bundles The axons of neurons are bundled together to form nerves in the PNS & tracts/pathways in the CNS. Most axons are myelinated so these structures will be part of “white matter” The cell bodies of neurons are clustered together into ganglia in the PNS & nuclei/centers in the CNS. These are unmyelinated structures and will be part of “gray matter”

Neural Tissue Organization

Anatomical structure of Nerves Fig. 14.6