The Nervous System

Nervous System Three basic functions Sensation Integration Response gather information Integration process information use of multiple sources of information. Response coordinated action appropriate to environment

The central nervous system is divided into two parts: the brain and the spinal cord. The average adult human brain weighs 1.3 to 1.4 kg (approximately 3 pounds). The brain contains about 100 billion nerve cells (neurons) and trillions of "support cells" called glia. The spinal cord is about 43 cm long in adult women and 45 cm long in adult men and weighs about 35-40 grams. The vertebral column, the collection of bones (back bone) that houses the spinal cord, is about 70 cm long. Therefore, the spinal cord is much shorter than the vertebral column. The peripheral nervous system is divided into two major parts: the somatic nervous system and the autonomic nervous system. Somatic Nervous System The somatic nervous system consists of peripheral nerve fibers that send sensory information to the central nervous system AND motor nerve fibers that project to skeletal muscle. 2. Autonomic Nervous System The autonomic nervous system is divided into three parts: the sympathetic nervous system, the parasympathetic nervous system and the enteric nervous system. The autonomic nervous system controls smooth muscle of the viscera (internal organs) and glands The preganglionic neuron is located in either the brain or the spinal cord. This preganglionic neuron projects to an autonomic ganglion. The postganglionic neuron then projects to the target organ.. Notice that the somatic nervous system has only one neuron between the central nervous system and the target organ while the autonomic nervous system uses two neurons. The enteric nervous system is a third division of the autonomic nervous system that you do not hear much about. The enteric nervous system is a meshwork of nerve fibers that innervate the viscera (gastrointestinal tract, pancreas, gall bladder). In the Peripheral Nervous System, neurons can be functionally divided in 3 ways: 1 Sensory (afferent) - carry information INTO the central nervous system from sense organs. OR Motor (efferent) - carry information away from the central nervous system (for muscle control). 2 Cranial - connects the brain with the periphery. OR Spinal - connects the spinal cord with the periphery. 3 Somatic - connects the skin or muscle with the central nervous system. OR Visceral - connects the internal organs with the central nervous system.

EK3E2: Animals have nervous systems that detect external and internal signals, transmit and integrate information, and produce responses. a. The neuron is the basic structure of the nervous system that reflects function. 1. A typical neuron has a cell body, axon and dendrites. Many axons have a myelin sheath that acts as an electrical insulator. 2. The structure of the neuron allows for the detection, generation, transmission and integration of signal information. 3. Schwann cells, which form the myelin sheath, are separated by gaps of unsheathed axon over which the impulse travels as the signal propagates along the neuron.

Primary rat hippocampal neurons The neuron is the basic structure of the nervous system that reflects function. 1. A typical neuron has a cell body, axon and dendrites. Many axons have a myelin sheath that acts as an electrical insulator. 2. The structure of the neuron allows for the detection, generation, transmission and integration of signal information. 3. Schwann cells, which form the myelin sheath, are separated by gaps of unsheathed axon over which the impulse travels as the signal propagates along the neuron. Primary rat hippocampal neurons Rat Cortical Neuron Classification On the basis of function (direction of impulse transmission) there are sensory (afferent) and motor (efferent) neurons and association neurons (interneurons). Dendritic endings of sensory neurons are bare (pain receptors), or are associated with sensory receptors (Figure 7.3). On the basis of structure, there are unipolar, bipolar, and multipolar neurons; the terminology reveals the number of processes extending from the cell body. Motor and association neurons are multipolar; most sensory neurons are unipolar. The exceptions are sensory neurons in certain special sense organs (ear, eye), which are bipolar. Hippocampal Neuron

Neurons are similar to other cells in the body because: 1. Neurons are surrounded by a cell membrane. 2. Neurons have a nucleus that contains genes. 3. Neurons contain cytoplasm with organelles 4. Neurons carry out basic cellular processes such as protein synthesis and energy production. However, neurons differ from other cells in the body because: 1. Neurons have specialized extensions called dendrites and axons. Dendrites bring information to the cell body and axons take information away from the cell body. 2. Neurons communicate with each other through an electrochemical process. 3. Neurons contain some specialized structures (for example, synapses) and chemicals (for example, neurotransmitters).

Neuron Trivia Neurons are the oldest and longest cells in the body! You have many of the same neurons for your whole life. Although other cells die and are replaced, many neurons are never replaced when they die. In fact, you have fewer neurons when you are old compared to when you are young. On the other hand, data published in November 1998 show that in one area of the brain (the hippocampus), new neurons CAN grow in adult humans. Neurons can be quite large - in some neurons, such as corticospinal neurons (from motor cortex to spinal cord) or primary afferent neurons (neurons that extend from the skin into the spinal cord and up to the brain stem), can be several feet long!

Neuron Anatomy Soma/Cell Body: is the metabolic center of the neuron, contains the Nucleus and Mitochondrion. Dendrites: convey incoming messages to the cell body. Axon: generates nerve impulses and topically conduct them away from the cell body myelinated by either oligodendroglia in CNS or Schwann cells in PNS. Each neuron has only one axon. Axon Hillock: a cone like region from where an axon arises. Presynaptic terminals: The swollen, distal end of an axon; contains a neurotransmitter substance within synaptic vesicles. Also called synaptic ending or synaptic bouton. Synapse: Specialized junctions with other cells that are along the length or at end of an axon. a. perikayon - nerve cell body, contains nucleus and typical cell organelles *Nucleus - large, central in most, large amount of euchromatin (intense synthetic activity), Barr body (Dormant X chromosome of females). *rough endoplasmic reticulum (RER) - lots for synthesis of structural and transport proteins, Nissl bodies seen with light microscope are condensations of ths RER and free ribosomes. *Golgi apparatus - only found near nucleus in perikaryon. Expected, since intense synthetic activity of neurotransmitters and/or neurohormones. *Mitochondria - abundant for high energy requirements *Neurofilaments, microtubules - neurofilaments are intermediate filaments (10 nm) Microtubules - important in transport of materials (e.g. neurotransmitters) *Inclusions - pigment vesicles - function unknown. Lipofuscin deposits - residual bodies from autophagosome activity. Increase with age. b. dendrite - cell process, may be branched, forms receptive area for synaptic contacts from other neurons, has tiny rough projections or spines called gemmules that may be points of synaptic contact, dendrites from larger neurons may be lightly myelinated by oligodendroglia. Neurons may have more than one dendrite. Cytoplasm in these processes similar to that of perikaryon, but no golgi bodies. c. axon – a single, long, cell process extending away from perikaryon, may be branched, ends of branches form synapses with other neurons or muscle cells, may be myelinated by either oligodendroglia in CNS or Schwann cells in PNS. Each neuron has only one axon. *axon hillock (pyramid shaped region where axon originates from the perikaryon) *initial segment (unmyelinated intitial portion of axon) *remainder of axon (may be myelinated or unmyelinated, may be branched) *axons carry electrical impulses (action potentials) to synapses at end of axon. *accept for axon the hillock and the synaptic bouton, the axon cytoplasm (axoplasm) has few organelles, microtubules, or microfilaments. Not much synthetic activity in ths part of neuron. The synaptic bouton may have a number of mitochondria in its cytoplasm. d. synapse *Specialized junctions with other cells that are along the length or at end of an axon. *Act as transmission points for electrical impulses. *Synapse can transmit action potential, or can polarize or depolarize the postsynaptic cell. *Synapses at end of an axon or axon branches are swollen into a club shape, called boutons terminaux. *Those along length of axon result in varicosities (swellings) in the axon, called boutons en passage. e. General structure of synapse *terminal or presynaptic membrane - this is part of the neuron plasmalemma *synaptic gap is present - this is a space between the presynaptic membrane of the axon and the plasmalemma of the cell that receives the synaptic input *postsynaptic membrane - part of plasmalemma of a cell that receives input *high concentrations of small vesicles in bouton that contain neurotransmitter. *when action potential reaches synapse, these vesicles are exocytosed at the presynaptic membrane and their contents (neurotransmitter) are released into the synaptic gap. *neurotransmitter binds to receptors on postsynaptic membrane and propagates electrical impulse (action potential) or membrane charge change (polarization or depolarization) in post-synaptic cell.

Axons Take information away from the cell body Smooth Surface Generally only 1 axon per cell No ribosomes Can have myelin Branch further from the cell body Dendrites Bring information to the cell body Rough Surface (dendritic spines) Usually many dendrites per cell Have ribosomes No myelin insulation Branch near the cell body

Neuron Anatomy A)  axon; (B) myelin sheath; (C) nodes of Ranvier; (D) synapse; (E) dendrites F-soma G-Axon Hillock H-Axon Terminus

Neurons can also be classified by the direction that they send information: Sensory (or afferent) neurons: send information from sensory receptors (e.g., in skin, eyes, nose, tongue, ears) TOWARD the central nervous system. Motor (or efferent) neurons: send information AWAY from the central nervous system to muscles or glands. Interneurons: send information between sensory neurons and motor neurons. Most interneurons are located in the central nervous system.

b. Action potentials propagate impulses along neurons. EK3E2: Animals have nervous systems that detect external and internal signals, transmit and integrate information, and produce responses. b. Action potentials propagate impulses along neurons. 1. Membranes of neurons are polarized by the establishment of electrical potentials across the membranes. 2. In response to a stimulus, Na+ and K+ gated channels sequentially open and cause the membrane to become locally depolarized. 3. Na+/K+ pumps, powered by ATP, work to maintain membrane potential. A nerve impulse is an electrochemical event (initiated by various stimuli) that causes a change in neuron plasma membrane permeability, allowing sodium ions (Na+) to enter the cell (depolarization). Once begun, the action potential, or nerve impulse, continues over the entire surface of the cell. Electrical conditions of the resting state are restored by the diffusion of potassium ions (K+) out of the cell (repolarization). Ion concentrations of the resting state are restored by the sodium-potassium pump

How neurons conduct impulses: Membrane potential (as seen in muscle cells) K+ diffuses out of neurons faster than Na+ diffuses in, Na-K pump moves 3Na+ back out for 2K+ back in Cl-, phosphate, protein anions balance cations “Resting potential” ~ -70mV Nerve Impulse

Rate of conduction Saltatory Conduction Unmyelinated axons Action Potential occurs only in nodes, "leaps" from node to node Ion attraction and diffusion stimulate new A.P. in each node Saltatory conduction much faster than continuous conduction in unmyelinated fibers. up to 100 m/sec Unmyelinated axons Action potential propagates at 2-3 meters per second (m/sec)

c. Transmission of information between neurons occurs across synapses. EK3E2: Animals have nervous systems that detect external and internal signals, transmit and integrate information, and produce responses. c. Transmission of information between neurons occurs across synapses. 1. In most animals, transmission across synapses involves chemical messengers called neurotransmitters. Acetylcholine Epinephrine Norepinephrine Dopamine Serotonin GABA 2. Transmission of information along neurons and synapses results in a response. 3. The response can be stimulatory or inhibitory. A neuron influences other neurons or effector cells by releasing neurotransmitters, chemicals that diffuse across the synaptic cleft and attach to membrane receptors on the postsynaptic cell. The result is opening of specific ion channels and activation or inhibition, depending on the neurotransmitter released and the target cell

How does impulse get from cell to cell? Chemical synapses, 2 Axon (& A.P.) ends short of next cell, Synaptic knob (axon end) releases chemical transmitter Electrical synapses “Gap junctions,” tiny holes connect cytoplasm of adjacent cells A.P. continuous from cell to cell

Neurotransmission When Action Potential reaches synaptic knob, Synaptic vesicles unite with membrane, Release neurotransmitter, Neurotransmitter diffuses across synaptic cleft, Neurotransmitter binds to receptors in postsynaptic membrane, Chemically gated channels open Chemical Signals Mouse Party

Synapse Neurotransmitter can't remain in cleft (would continue to stimulate uncontrollably) ACh removed by acetylcholinesterase (AChE) Acetate & choline reabsorbed by axon end, resynthesized to ACh Other neurotransmitters taken back by axon or diffuse away

Neurotransmitters may be … Excitatory depolarize postsynaptic membrane Excitatory Postsynaptic Potential (EPSP) Inhibitory hyperpolarize postsynaptic membrane Inhibitory Postsynaptic Potential (IPSP)

Reward Pathway Reflex Arc

d. Different regions of the vertebrate brain have different functions. EK3E2: Animals have nervous systems that detect external and internal signals, transmit and integrate information, and produce responses. d. Different regions of the vertebrate brain have different functions.

LO 3.43 The student is able to construct an explanation, based on scientific theories and models, about how nervous systems detect external and internal signals, transmit and integrate information, and produce responses. LO 3.44 The student is able to describe how nervous systems detect external and internal signals. LO 3.45 The student is able to describe how nervous systems transmit information. LO 3.46 The student is able to describe how the vertebrate brain integrates information to produce a response. LO 3.47 The student is able to create a visual representation of complex nervous systems to describe/explain how these systems detect external and internal signals, transmit and integrate information, and produce responses. LO 3.48 The student is able to create a visual representation to describe how nervous systems detect external and internal signals. LO 3.49 The student is able to create a visual representation to describe how nervous systems transmit information. LO 3.50 The student is able to create a visual representation to describe how the vertebrate brain integrates information to produce a response.

Resources Action Potential Tutorial A&P Text, 2 Essential Study partner Links Parts of a Neuron Review Anatomy Drill Body Smart Nervous System Campbell’s Activity Quiz Human A&P Lessons Cerebral Commando Synaptic Transmission Tutorial Drugs, Brains & Behavior Parts of the Brain, 2 Video Quiz - Making Brain Cells Video Quiz - ALS Lost Nerve Power Video Quiz - Wild Young Brains Video Quiz - Vanishing Brain Video Quiz - Meth and the Brain Video Quiz - Paralysis Push