Biology, The nervous system

Raven Biology 13th edition, CH42

Nervous System Organization, All animals must be able to respond to environmental stimuli

animals use sensory receptors (detect stimulus), motor effectors (respond to it), and a nervous system (links the two)

Central Nervous System, CNS consists of

the brain and spinal cord

Types of Neurons, Vertebrates have three types of neurons

sensory neurons (afferent) carry impulses to CNS, motor neurons (efferent) carry impulses from CNS to effectors, and interneurons (association neurons) provide reflexes and associative functions

Peripheral Nervous System, PNS consists of

sensory and motor neurons; somatic NS stimulates skeletal muscles; autonomic NS stimulates smooth/cardiac muscles and glands

Peripheral Nervous System, Sympathetic and parasympathetic NS

counterbalance each other

Components of a Neuron, Neurons have the same basic structure

cell body (enlarged part with nucleus), dendrites (short extensions receiving stimuli), and axon (long extension conducting impulses away from cell body)

Supportive Cells, Neuroglia support neurons structurally and functionally

Schwann cells and oligodendrocytes produce myelin sheaths surrounding axons

Supportive Cells, In CNS, myelinated axons form white matter

dendrites/cell bodies form gray matter; in PNS, myelinated axons bundle to form nerves

Electrical Difference Across the Plasma Membrane, A potential difference exists across every cell's plasma membrane

negative pole is cytoplasmic side, positive pole is extracellular fluid side

Electrical Difference Across the Plasma Membrane, When a neuron is not stimulated it maintains a resting potential

ranges from -40 to -90 mV, average about -70 mV

Contributions to Resting Potential, Three factors contribute to resting potential

sodium-potassium pump (2 K+ in for every 3 Na+ out), ion leakage channels (more K+ diffuses out than Na+ in), and differential distribution of negatively charged proteins

Nerves Impulse Transmission, Sodium-potassium pump creates concentration gradient

K+ concentration much higher inside cell; membrane not permeable to negative ions; equilibrium potential balances diffusional and electrical forces

Uniqueness of Neurons, Uniqueness of neurons is not resting membrane potential

rather the sudden temporary disruptions in response to stimuli: graded potentials and action potentials

Overview of Potentials, Graded potentials are small consistent changes due to activation of gated ion channels

action potentials are transient disruptions triggered by threshold change that move along axon

Gated Channels, Chemically-gated or ligand-gated channels

ligands (hormones or neurotransmitters) induce opening and cause changes in cell membrane permeability

Depolarization, Depolarization makes membrane potential more positive

hyperpolarization makes it more negative; these small changes result in graded potentials

Depolarization, Summation is the ability of graded potentials to combine

size depends on stimulus strength or amount of ligand; graded potentials can reinforce or negate each other

Action Potentials, Result when depolarization reaches threshold potential (-55 mV)

caused by voltage-gated Na+ and K+ channels

Voltage-Gated Channels, Voltage-gated Na+ channels have activation and inactivation gates

at rest activation gate closed, inactivation gate open; transient Na+ influx causes depolarization

Voltage-Gated Channels, Voltage-gated K+ channels have single activation gate closed at rest

channel opens slowly; K+ efflux repolarizes the membrane

Phases of an Action Potential, Action potential has three phases

rising, falling, and undershoot; always separate all-or-none events; intensity coded by frequency not amplitude

Nerve Impulse Propagation, Each action potential reflects reversal in membrane polarity

positive charges from Na+ influx depolarize adjacent region to threshold; previous region repolarizes; signal does not go back toward cell body

Velocity of Conduction, Two ways to increase conduction velocity

large diameter axon (less resistance, found in invertebrates) or myelinated axon (action potential only at nodes of Ranvier, saltatory conduction)

Synapses, Specialized intercellular junctions with other neurons, muscle cells, or gland cells

presynaptic cell transmits action potential, postsynaptic cell receives it; two types: electrical and chemical

Electrical and Chemical Synapses, Electrical synapses involve direct cytoplasmic connections via gap junctions

rare in vertebrates

Chemical synapses, Have a synaptic cleft between the two cells

end of presynaptic cell contains synaptic vesicles packed with neurotransmitters

Chemical Synapses, Action potential triggers Ca2+ influx

synaptic vesicles fuse with cell membrane; neurotransmitter released by exocytosis diffuses and binds to ligand-gated receptors producing graded potentials

Neurotransmitters: Acetylcholine, Acetylcholine (ACh) crosses the synapse between motor neuron and muscle fiber at neuromuscular junction

binds to receptor causing ligand-gated channels to open, producing EPSP that stimulates muscle contraction

Neurotransmitters: Acetylcholine, Acetylcholinesterase (AChE) degrades ACh

causing muscle relaxation

Neurotransmitters: Amino Acids, Glutamate is major excitatory neurotransmitter in vertebrate CNS

glycine and GABA are inhibitory, opening Cl- channels producing IPSP (hyperpolarization)

Neurotransmitters: Biogenic Amines, Epinephrine and norepinephrine are responsible for "fight or flight" response

dopamine controls body movements; serotonin regulates sleep

Neurotransmitters: Neuropeptides, Substance P is released from sensory neurons activated by painful stimuli

intensity of pain perception depends on enkephalins and endorphins; nitric oxide gas causes smooth muscle relaxation

Synaptic Integration, Integration of EPSPs and IPSPs occurs on neuronal cell body

small EPSPs add to bring membrane closer to threshold; IPSPs subtract from depolarizing effect

Synaptic Integration and Threshold Voltage, Two ways membrane can reach threshold voltage

spatial summation (many dendrites produce EPSPs) and temporal summation (one dendrite produces repeated EPSPs)

Drug Addiction, Habituation is prolonged exposure causing cells to lose ability to respond

cell decreases number of receptors due to excess neurotransmitters; more drug needed for same effect

Drug Addiction: Cocaine, Cocaine binds dopamine transporters and prevents reuptake

dopamine survives longer in synapse, firing pleasure pathways more and more

Drug Addiction: Nicotine, Nicotine binds directly to acetylcholine receptors on postsynaptic neurons

brain adjusts by making fewer receptors and altering activation patterns

Evolution of the Central Nervous System, Sponges are only major phylum without nerves

cnidarians have simplest nervous system (nerve net, no associative activity)

Evolution of the Central Nervous System, Free-living flatworms are simplest animals with associative activity

two nerve cords permit complex muscle control; subsequent changes are elaborations on flatworm characteristics

Vertebrate Brains, All vertebrate brains have three basic divisions

hindbrain (rhombencephalon), midbrain (mesencephalon), and forebrain (prosencephalon)

Vertebrate Brains, In fishes

hindbrain is largest portion; midbrain processes visual information; forebrain processes olfactory information

Forebrain, Forebrain is composed of diencephalon and telencephalon

diencephalon includes thalamus (integration/relay center) and hypothalamus (basic drives, controls pituitary)

Forebrain, Telencephalon ("end brain") is devoted largely to associative activity

called the cerebrum in mammals

Cerebrum, Increase in brain size in mammals reflects great enlargement of cerebrum

split into right and left hemispheres connected by corpus callosum; each hemisphere receives sensory input from opposite side

Cerebrum: Cerebral Cortex, Cerebral cortex is outer layer of cerebrum

contains about 10% of all neurons; highly convoluted surface increases threefold surface area of human brain

Cerebral Cortex, Primary motor cortex controls movement

primary somatosensory cortex controls sensation; association cortex handles higher mental functions

Other Brain Structures, Thalamus integrates visual, auditory, and somatosensory information

hypothalamus integrates visceral activities and controls pituitary gland

Other Brain Structures, Limbic system includes hypothalamus, hippocampus, and amygdala

responsible for emotional responses

Complex Functions of the Brain, Reticular-activating system controls consciousness and alertness

brain state monitored by electroencephalogram (EEG) which records electrical activity

Complex Functions of the Brain: Language, Left hemisphere is "dominant" for language

adept at sequential reasoning; right hemisphere adept at spatial reasoning and musical ability

Complex Functions of the Brain: Memory, Short-term memory stored as transient neural excitations

long-term memory involves structural changes in neural connections; hippocampus and amygdala involved in consolidation

Synaptic Plasticity, Cellular basis of learning and memory involves long-term changes in synaptic connection strength

examples: long-term potentiation (LTP) and long-term depression (LTD)

Spinal Cord, Cable of neurons extending from brain down through backbone

enclosed and protected by vertebral column and meninges

Composition of the Spinal Cord, Inner zone is gray matter (cell bodies of interneurons, motor neurons, neuroglia)

outer zone is white matter (sensory axons in dorsal columns, motor axons in ventral columns)

Role of the Spinal Cord, Serves as body's "information highway" relaying messages between body and brain

also functions in reflexes; knee-jerk reflex is monosynaptic; most reflexes involve a single interneuron

Composition of the Peripheral Nervous System, Consists of nerves (bundles of axons) and ganglia (aggregates of neuron cell bodies)

function is to receive info, convey to CNS, and carry responses to effectors

Neurons of the Peripheral Nervous System, Sensory neuron axons enter dorsal surface forming dorsal root

cell bodies in dorsal root ganglia

Neurons of the Peripheral Nervous System, Motor neuron axons leave ventral surface forming ventral root

cell bodies located in spinal cord

The Somatic Nervous System, Somatic motor neurons stimulate skeletal muscles to contract

in response to conscious command or reflex actions; antagonist muscle is inhibited by IPSPs

The Autonomic Nervous System, Composed of sympathetic and parasympathetic divisions plus medulla oblongata

efferent pathway has two neurons: preganglionic (exits CNS, synapses at ganglion) and postganglionic (regulates visceral effectors)

Divisions of the Autonomic Nervous System, Sympathetic division preganglionic neurons originate in thoracic and lumbar regions

most axons synapse in two parallel chains of ganglia outside spinal cord

Divisions of the Autonomic Nervous System, Parasympathetic division preganglionic neurons originate in brain and sacral regions

axons terminate in ganglia near or within internal organs

G Proteins, Mediate cell responses to autonomic signals

activate target cells

Cranial Nerves, Twelve pairs of cranial nerves arise from underside of brain

carry sensory neurons for special and general senses as well as somatic and autonomic motor neuron