Chapter 28. Nervous system 2018 Biology2 Chapter 28. Nervous system

NERVOUS SYSTEM STRUCTURE AND FUNCTION 28.1 Nervous systems receive sensory input, interpret it, and send out appropriate commands The nervous system has three interconnected functions Sensory input Integration Motor output

Peripheral nervous system (PNS) Central nervous system (CNS) Analysis and interpretation Formulation of appropriate responses SENSORY INPUT Sensory neuron INTEGRATION Sensor interneuron MOTOR OUTPUT Motor neuron Brain and spinal cord Effector Muscle, gland Peripheral nervous system (PNS) Central nervous system (CNS) Figure 28.1A

Heat Light touch Pain Cold Hair Epidermis Dermis Nerve Connective tissue movement Strong pressure

The nervous system can be divided into two main divisions The central nervous system (CNS) consists of the brain and, in vertebrates, the spinal cord The peripheral nervous system (PNS) is made up of nerves and ganglia that carry signals into and out of the CNS A cluster of neuron cell bodies

Three types of neurons correspond to the nervous system’s three main functions Sensory neurons convey signals from sensory receptors into the CNS Interneurons integrate data and relay signals Motor neurons convey signals to effectors

Reflex: rapid and involuntary response to a stimulus 1 Sensory receptor 2 Sensory neuron Brain Ganglion 3 Motor neuron Spinal cord Quadriceps muscles 4 Interneuron CNS Nerve Flexor muscles PNS Reflex: rapid and involuntary response to a stimulus

28.2 Neurons are the functional units of nervous systems Neurons are cells specialized to transmit nervous impulses They consist of a cell body dendrites (highly branched fibers) an axon (long fiber)

Supporting cells (glia) protect, insulate, and reinforce neurons The myelin sheath is the insulating material in vertebrates It is composed of a chain of Schwann cells linked by nodes of Ranvier (oligodendrocytes in CNS) It speeds up signal transmission Multiple sclerosis (MS) involves the destruction of myelin sheaths by the immune system

* Astrocyte Anchor neurons to their blood suply Regulate external environment of neurons (K+, neurotransmitter) * Ependymal cell: circulation of cerebrospinal fluid

Glia: astocyte, satellite cell, microglia Surround neurons and hold them in place Regulate the external environment of neurons Supply nutrients and oxygen to neurons Insulate the neuron from another Destroy pathogens and remove dead neurons

Signal direction Dendrites Cell body Cell body Node of Ranvier Myelin sheath Signal pathway Axon Schwann cell Nucleus Nucleus Nodes of Ranvier Schwann cell Myelin sheath Synaptic terminals

28.3 A neuron maintains a membrane potential across its membrane NERVE SIGNALS AND THEIR TRANSMISSION 28.3 A neuron maintains a membrane potential across its membrane The resting potential of a neuron’s plasma membrane is caused by the cell membrane’s ability to maintain a positive charge on its outer surface a negative charge on its inner (cytoplasmic) surface Voltmeter Plasma membrane Microelectrode outside cell –70 mV Microelectrode inside cell Axon Neuron

Resting potential is generated and maintained with help from sodium-potassium pumps These pump K+ into the cell and Na+ out of the cell OUTSIDE OF CELL K+ Na+ K+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ channel Na+ Plasma membrane K+ Na+ - K+ pump K+ channel Na+ K+ K+ K+ Protein K+ K+ K+ K+ K+ K+ INSIDE OF CELL K+

28.4 A nerve signal begins as a change in the membrane potential A stimulus alters the permeability of a portion of the plasma membrane Ions pass through the plasma membrane, changing the membrane’s voltage It causes a nerve signal to be generated Voltage gated Na+ channel Voltage gated K+ channel

An action potential is a nerve signal It is an electrical change in the plasma membrane voltage from the resting potential to a maximum level and back to the resting potential

Neuron interior Neuron interior 3 4 5 2 1 1 Na+ K+ Na+ K+ Additional Na+ channels open, K+ channels are closed; interior of cell becomes more positive. 4 Na+ channels close and inactivate. K+ channels open, and K+ rushes out; interior of cell more negative than outside. Na+ Action potential 3 4 2 The K+ channels close relatively slowly, causing a brief undershoot. Na+ Threshold potential 5 2 A stimulus opens some Na+ channels; if threshold is reached, action potential is triggered. 1 1 5 Resting potential Neuron interior Neuron interior 1 Resting state: voltage gated Na+ and K+ channels closed; resting potential is maintained. 1 Return to resting state.

28.5 The action potential propagates itself along the neuron Axon Action potential Axon segment 1 Na+ K+ Action potential 2 Na+ K+ K+ Action potential 3 Na+ K+

Saltatory conduction

28.6 Neurons communicate at synapses The synapse is a key element of nervous systems It is a junction or relay point between two neurons or between a neuron and an effector cell Synapses are either electrical or chemical Action potentials pass between cells at electrical synapses (through gap junctions) At chemical synapses, neurotransmitters cross the synaptic cleft to bind to receptors on the surface of the receiving cell (ionotropic receptor, metabotropic receptor) norepinephrin

1 2 3 4 5 6 SENDING NEURON Action potential arrives Axon of sending neuron Vesicles Synaptic knob SYNAPSE 2 3 Vesicle fuses with plasma membrane Neurotransmitter is released into synaptic cleft SYNAPTIC CLEFT 4 Receiving neuron Neuro- transmitter binds to receptor RECEIVING NEURON Neurotransmitter molecules Ion channels Neurotransmitter Neurotransmitter broken down and released Receptor Ions 5 Ion channel opens 6 Ion channel closes

28.7 Chemical synapses make complex information processing possible Excitatory neurotransmitters trigger action potentials in the receiving cell Inhibitory neurotransmitters decrease the cell’s ability to develop action potentials The summation of excitation and inhibition determines whether or not the cell will transmit a nerve signal

A neuron may receive input from hundreds of other neurons via thousands of synaptic knobs Dendrites Synaptic knobs Myelin sheath Receiving cell body Axon Synaptic knobs Figure 28.7

Summation of postsynaptic potentials Terminal branch of presynaptic neuron E1 E1 E1 E1 E2 E2 E2 E2 Postsynaptic neuron Axon hillock I I I I Threshold of axon of postsynaptic neuron Action potential Action potential Membrane potential (mV) Resting potential –70 E1 E1 E1 E1 E1 + E2 E1 I E1 + I (a) Subthreshold, no summation (b) Temporal summation (c) Spatial summation (d) Spatial summation of EPSP and IPSP Summation of postsynaptic potentials

28.8 A variety of small molecules function as neurotransmitters Most neurotransmitters are small, nitrogen-containing organic molecules Acetylcholine metabotropic receptor (heart, decrease in beat rate), ionotropic receptor, sarin:acetylcholinesterase inhibitor, botulinum toxin Biogenic amines (norepinephrine, serotonin, dopamine) Norepinephrine (tyr) is an excitatory NT in the autonomic nervous system, LSD and mescaline, serotoninedepression, Dopamine(Tyr) and seratonin (Trp) are released in the brain and affect sleep, mood, attention, learning. A lack of dopamine  Parkinson’s desease Amino acids (glutamate, glycinespinal, GABAbrain) important neurotransmitter in CNS, Peptides (substance P and endorphins) Metabotropic receptor. Substance P is involved in perception of pain, while endorphins in decreasing pain perception (morphine heroin) Dissolved gases (nitric oxide, CO)

28.11 Vertebrate nervous systems are highly centralized and cephalized CENTRAL NERVOUS SYSTEM (CNS) PERIPHERAL NERVOUS SYSTEM (PNS) Brain Cranial nerve Spinal cord Ganglia outside CNS Spinal nerves

Brain: master control center Homeostatic center Sensory center Center of emotion and intellect Motor command center Spinal cord

The brain and spinal cord contain fluid-filled spaces Cerebrospinal fluid Dorsal root ganglion (part of PNS) Gray matter Meninges BRAIN White matter Central canal Spinal nerve (part of PNS) Ventricles Central canal of spinal cord SPINAL CORD (cross section) Spinal cord

28.12 The peripheral nervous system of vertebrates is a functional hierarchy Motor system Autonomic nervous Sympathetic division Parasympathetic Enteric Digestive tract Pancreas Gall bladder

PNS The autonomic nervous system exerts involuntary control over the internal organs The motor nervous system exerts voluntary control over skeletal muscles

The autonomic nervous system consists of two major divisions 28.13 Opposing actions of sympathetic and parasympathetic neurons regulate the internal environment The autonomic nervous system consists of two major divisions The parasympathetic division: rest and digest The sympathetic division: arousal and energy generation

PARASYMPATHETIC DIVISION Brain Eye Constricts pupil Dilates pupil Salivary glands Stimulates saliva production Inhibits saliva production acetylcholine norepinephrine Lung Constricts bronchi Relaxes bronchi Accelerates heart Slows heart Heart Adrenal gland Stimulates epinephrine and norepi- nephrine release Liver Spinal cord Stomach Stimulates stomach, pancreas, and intestines Pancreas Stimulates glucose release Inhibits stomach, pancreas, and intestines Intestines Bladder Stimulates urination Inhibits urination Sacral segment Promotes erection of genitals Promotes ejacu- lation and vaginal contractions Genitals