The Nervous System Chapter 44

Nervous System Organization All animals must be able to respond to environmental stimuli -Sensory receptors = Detect stimulus -Motor effectors = Respond to it -The nervous system links the two -Consists of neurons and supporting cells

Types of Neurons Vertebrates have 3 types of neurons -Sensory neurons to CNS(afferent neurons) -Motor neurons (efferent neurons) to effectors (muscles and glands) -Interneurons (association neurons) provide associative functions

A Neuron Neurons have the same basic structure -Cell body = Enlarged part containing nucleus -Dendrites = Short, cytoplasmic extensions that receive stimuli -Axon = Single, long extension that conducts impulses away from cell body

A Neuron’s Charge The inside of the cell is more negatively charged than the outside because of: 1. Sodium-potassium pump = Brings two K+ into cell for every three Na+ it pumps out 2. Ion leakage channels = Allow more K+ to diffuse out than Na+ to diffuse in When a neuron is not being stimulated, it maintains a resting Potential @ -70mv

Sodium-potassium pump Helps create a Neuron’s charge

Synapses Synapses are intercellular junctions Two basic types: electrical and chemical Electrical synapses involve direct cytoplasmic connections between the two cells formed by gap junctions -Relatively rare in vertebrates

Synapses Chemical synapses have a synaptic cleft between the two cells -End of presynaptic cell contains synaptic vesicles packed with neurotransmitters

Neurons Begin to Communicate @ Synapses Synapses are intercellular junctions -Presynaptic cell transmits action potential -Postsynaptic cell receives it

More Facts About the Synapse Action potential triggers influx of Ca2+ -Synaptic vesicles fuse with cell membrane -Neurotransmitter is released by exocytosis -Diffuses to other side of cleft and binds to chemical- or ligand-gated receptor proteins -Neurotransmitter action is terminated by enzymatic cleavage or cellular uptake

Neurotransmitters Acetylcholine (ACh) -Crosses the synapse between a motor neuron and a muscle fiber -Neuromuscular junction

Neurotransmitters Acetylcholine (ACh) -Binds to ligand-gated receptor in the postsynaptic membrane -Produces a depolarization called an excitatory postsynaptic potential (EPSP) -Stimulates muscle contraction -Acetylcholinesterase (AChE) degrades ACh -Causes muscle relaxation

Excitatory and Inhibitory Neurotransmitters Amino acids -Glutamate is the major excitatory neurotransmitter in the vertebrate CNS -Glycine and GABA (g-aminobutyric acid) are inhibitory neurotransmitters -Open ligand-gated channels for Cl– -Produce a hyperpolarization called an inhibitory postsynaptic potential (IPSP)

Neurotransmitters

Nerve Impulse Transmission Chemically-gated or ligand-gated channels -Ligands are hormones or neurotransmitters -Induce opening and cause changes in cell membrane permeability

Nerve Impulse Transmission Depolarization makes the membrane potential more positive, whereas a hyperpolarization makes it more negative -These small changes result in graded potentials -Can reinforce or negate each other Summation is the ability of graded potentials to combine

Synaptic Integration

Synaptic Integration Integration of EPSPs (depolarization) and ISPSs (hyperpolarization) occurs on the neuronal cell body -Small EPSPs add together to bring the membrane potential closer to the threshold -IPSPs subtract from the depolarizing effect of EPSPs -And will therefore deter the membrane potential from reaching threshold

Synaptic Integration There are two ways that the membrane can reach the threshold voltage -Spatial summation -Many different dendrites produce EPSPs -Temporal summation -One dendrite produces repeated EPSPs

Nerve Impulse Transmission

Nerve Impulse Transmission Action potentials result when depolarization reaches the threshold potential about -55mV Remember resting potential is -70mV so Positive charge has to come in to get it to be -55mV!!

Nerve Impulse Transmission When the threshold voltage is reached, sodium channels open rapidly -Transient influx of Na+ causes the membrane to depolarize In contrast, potassium channel opens slowly -Efflux of K+ repolarizes the membrane

The intensity of a stimulus is coded by the frequency, not amplitude, of action potentials

Propagation of an Action Potential -The action potential is caused by voltage-gated ion channels -Two different channels are used: -Voltage-gated Na+ channels -Voltage-gated K+ channels -Positive charges due to influx of Na+ can depolarize the adjacent region to threshold -And so the next region produces its own action potential -Meanwhile, the previous region repolarizes back to the resting membrane potential

Nervous System Organization Neurons are supported both structurally and functionally by cells called neuroglia -Schwann cells (PNS) and oligodendrocytes (CNS) produce myelin sheaths surrounding axons -In the CNS, myelinated axons form white matter -In the CNS dendrites/cell bodies form gray matter -In the PNS, myelinated axons are bundled to form nerves

Nerve Impulse Transmission Two ways to increase velocity of conduction: 1. Axon has a large diameter -Less resistance to current flow -Found primarily in invertebrates 2. Axon is myelinated -Action potential is only produced at the nodes of Ranvier -Impulse jumps from node to node -Saltatory conduction

Nerve Impulse Transmission Action potential is only produced at the nodes of Ranvier Insulation allows for charge, accumulated at a distance, to be detected by neighbor-Na+channels

Drug Addiction Prolonged exposure to a stimulus may cause cells to lose the ability to respond to it -This process is called habituation -The cell decreases the number of receptors because there is an abundance of neurotransmitters

Drug Addiction Cocaine affects neurons in the brain’s “pleasure pathways” (limbic system) -Binds dopamine transporters and prevents the reuptake of dopamine -Dopamine survives longer in the synapse and fires pleasure pathways more and more -Prolonged exposure triggers the limbic system neurons to reduce receptor numbers -The cocaine user is now addicted

Drug Addiction Nicotine binds directly to a specific receptor on postsynaptic neurons of the brain -Brain adjusts to prolonged exposure by “turning down the volume” in two ways: 1. Making fewer nicotine receptors 2. Altering the sensitivity to stimulation by neurotransmitters

Nervous System Organization The CNS consists of the brain and spinal cord The Peripheral Nervous System (PNS) consists of sensory and motor neurons -Somatic NS stimulates skeletal muscles -Autonomic NS stimulates smooth and cardiac muscles, as well as glands -Sympathetic and parasympathetic NS -Counterbalance each other

Parasympathetic nervous CNS Brain and Spinal Cord Sensory Pathways Motor Pathways Sensory neurons registering external stimuli Sensory neurons registering external stimuli PNS Somatic nervous system (voluntary) Autonomic nervous system (involuntary) Sympathetic nervous system "fight or flight" Parasympathetic nervous system "rest and repose" central nervous system (CNS) peripheral nervous system (PNS)

Vertebrate Brains All vertebrate brains have three basic divisions: -Hindbrain or rhombencephalon -Midbrain or mesencephalon -Forebrain or prosencephalon In fishes, -Hindbrain = Largest portion -Midbrain = Processes visual information -Forebrain = Processes olfactory information

Vertebrate Brains The relative sizes of different brain regions have changed as vertebrates evolved Forebrain became the dominant feature...Neuronal correlate to: learning, association, emotions.

Compare Brain Diagrams

Vertebrate Brains Forebrain is composed of 2 elements: -Diencephalon -Thalamus: Integration and relay center -Hypothalamus: Participates in basic drives & emotions; controls pituitary gland -Telencephalon (“end brain”) -Devoted largely to associative activity -ALSO Called the CEREBRUM in mammals

Cerebral Cortex is the outer layer of the Cerebrum -Contains about 10% of all neurons in brain -Highly convoluted surface -Increases threefold the surface area of the human brain -Divided into three regions, each with a specific function

Cerebral Cortex 3 Regions Increase in brain size in mammals reflects the great enlargement of the cerebrum Cerebral cortex 1Primary motor cortex: Movement control 2Primary somatosensory cortex: Sensory control 3Association cortex: Higher mental functions Basal ganglia -Aggregates of neuron cell bodies -Form islands of grey matter within the cerebrum’s white matter

Sensory/Motor Homunculus

Complex Functions of the Brain Memory -Appears dispersed across the brain -Short-term memory is stored in the form of transient neural excitations -Long-term memory appears to involve structural changes in neural connections

Complex Functions of the Brain Alzheimer disease is a condition where memory and thought become dysfunctional -Two causes have been proposed 1. Nerve cells are killed from the outside in -External protein: b-amyloid 2. Nerve cells are killed from the inside out -Internal proteins: tau (t)

Spinal Cord The spinal cord is a cable of neurons extending from the brain down through the backbone -Enclosed and protected by the vertebral column and the meninges -It serves as the body’s “information highway” -Relays messages between the body and the brain

The Peripheral Nervous System The PNS consists of nerves and ganglia -Nerves are bundles of axons bound by connective tissue -Ganglia are aggregates of neuron cell bodies

The Peripheral Nervous System Sensory neurons: -Axons enter the dorsal surface of the spinal cord and form dorsal root of spinal nerve -Cell bodies are grouped outside the spinal cord in dorsal root ganglia Motor neurons: -Axons leave from the ventral surface and form ventral root of spinal nerve -Cell bodies are located in the spinal cord

Cutaneous Spinal Reflex (in Somatic Nervous Use “SAME DAVE” Acronym System) Use “SAME DAVE” Acronym

The Peripheral Nervous System PNS is divided into the: Somatic System & Autonomic System .

Somatic System In GENERAL, The Somatic System is responsible for skeletal muscle voluntary movements & reflexes. Quadriceps muscle (effector) Spinal cord Dorsal root ganglion Gray matter White Monosynaptic synapse Sensory neuro Nerve fiber Stretch receptor (muscle spindle) Skeletal Stimulus Response Motor neuron

The Autonomic Nervous System Composed of the sympathetic and parasympathetic divisions, plus the medulla oblongata In both, efferent motor pathway has 2 neurons -Preganglionic neuron: exits the CNS and synapses at an autonomic ganglion -Postganglionic neuron: exits the ganglion and regulates visceral effectors -Smooth or cardiac muscle or glands

The Autonomic Nervous System Efferent Ganglia outside of spinal cord General Model Seen In Both Sympathetic & Parasympathetic Division