Concept 34.1 Nervous Systems Consist of Neurons and Glia
Neurons and Nervous Systems
Concept 34.1 Nervous Systems Consist of Neurons and Glia
Nervous systems have two categories of cells:
Neurons,ornerve cells, are excitable—they generate and transmit electrical signals, called action potentials.
Glia, or glial cells, provide support and maintain extracellular environment.
Most neurons have four regions:
•Cell body—contains nucleus and organelles
•Dendrites— carries signals, called nerve impulses or action potentials, to the cell body
•Axon—generates action potentialsand conducts them awayfrom the cell body
•Axon terminal—synapse at tip of axon; releases neurotransmitters
Neurons pass information at synapses:
•The presynaptic neuron sends the message
•The postsynaptic neuron receives the message
Glial cells, or glia,outnumber neurons in the human brain.
•Glia do not transmit electrical signals but can release neurotransmitters.
•Glia also give support during development, supply nutrients, remove debris, and maintain extracellular environment.
•Important in neuroplasticity—synapse modification
Astrocytesare glia that contribute to the blood–brain barrier,which protects the brain.
The blood-brain barrier is permeable to fat-soluble compounds like alcohol and anesthetics.
Microglia provide the brain with immune defenses since antibodies cannot enter the brain.
Oligodendrocytes are glia that insulate axons in the brain and spinal cord.
Schwann cells insulate axons in nerves outside of these areas.
The glial membranes form a nonconductive sheath—myelin.
Myelin-coated axons are white matter and areas of cell bodies are gray matter.
Multiple sclerosis is a demyelinating disease.
Neurons are organized into neural networks.
Afferentneurons carry sensory information into the nervous system from sensorycells that convert stimuli into action potentials.
Efferent neurons carry commands to effectors such as muscles, glands—motor neurons are effectors that carry commands to muscles.
Interneurons store information and communicate between neurons.
Networks vary in complexity.
Nerve net—simple network of neurons
Ganglia—neurons organized into clusters, sometimes in pairs, in simple animals
Brain—the largest pair of ganglia, found in animals with complex behavior requiring more information-processing
Concept 34.2 Neurons Generate and Transmit Electrical Signals
Neurons generate changes in membrane potential—the difference in electrical charge across the membrane.
These changes generatenerve impulses, or action potentials.
An action potential is a rapid, large change in membrane potential that travels along an axon and causes release of chemical signals.
Voltage is a measure of the difference in electrical charge between two points.
Electrical current in solution is carried by ions. Major ions in neurons:
•Sodium (Na+)• Potassium (K+)
•Calcium (Ca2+)• Chloride (Cl–)
Different concentrations and charges inside and out produce the membrane potential.
Membrane potentials can be measured in all cells with electrodes.
Resting potential is the membrane potential of a resting, or inactive, neuron.
The resting potential of a membrane is between –60 and –70 millivolts (mV).
The inside of the cell is negative at rest. An action potential allows positive ions to flow in briefly, making the inside of the cell more positive.
Ion channels and ion transporters in the membrane create the resting and action potentials.
Sodium–potassium pump—moves Na+ ions from inside, exchanges for K+ from outside—establishes concentration gradients
The Na+–K+ pump is an antiporter,or sodium–potassium ATPase, as it requires ATP.
Potassium channels are open in the resting membrane and are highly permeable to K+ ions—allow leak currents
K+ ions diffuse out of the cell along the concentration gradient and leave behind negative charges within the cell.
K+ ions diffuse back into the cell because of the negative electrical potential.
These two forces acting on K+ are its electrochemical gradient.
The equilibrium potential is the membrane potential at which the net movement of an ion ceases.
The Nernst equation calculates the value of the equilibrium potential by measuring the concentrations of an ion on both sides of the membrane.
Some ion channels are “gated”—open and close under certain conditions:
•Voltage-gated channels respond to change in voltage across membrane
•Chemically-gated channels depend on molecules that bind or alter channel protein
•Mechanically-gated channels respond to force applied to membrane
Gating provides a means for neurons to change their membrane potentials in response to a stimulus.
The membrane is depolarized when Na+ enters the cell and the inside of the neuron becomes less negative.
If gated K+ channels open and K+ leaves, the cell becomes more negative inside and the membrane is hyperpolarized.
Graded membrane potentials are changes from the resting potential.
Graded potentials are a means of integrating input—the membrane can respond proportionally to depolarization or hyperpolarization.
Voltage-gated Na+ and K+ channels are responsible for action potentials—sudden, large changes in membrane potential.
At rest most of these channels are closed.
Local depolarization by gated channels in dendrites produces a graded potential.
It spreads to the axon hillock, where Na+ voltage-gated channels are concentrated.
The membrane in the axon hillock may reach its threshold—5 to 10 mV above resting potential.
Many voltage-gated Na+ channels (activation gates) open quickly and Na+ rushes into the axon.
The influx of positive ions causes more depolarization, the membrane potential is briefly positive, and an action potential occurs.
The axon quickly returns to resting potential due to two things:
•Voltage-gated K+ channels open slowly and stay open longer—K+ moves out
•Voltage-gated Na+ channels (inactivation gates) close
Voltage-gated Na+ channels cannot open again during the refractory period—a few milliseconds.
An action potential is an all-or-none event—positive feedback to voltage-gated Na+ channels ensures the maximum action potential.
An action potential is self-regenerating because it spreads to adjacent membrane regions.
Axon diameter and myelination by glial cells increase the speed of action potentials in axons.
The nodes of Ranvier are regularly spaced gaps where the axon is not covered by myelin.
Action potentials are generated at the nodes and the positive current flows down the inside of the axon.
When positive current reaches the next node, the membrane is depolarized—another axon potential is generated.
Action potentials appear to jump from node to node, a form of propagation called saltatory conduction.
Concept 34.3 Neurons Communicate with Other Cells at Synapses
Neurons communicate with other neurons or target cells at synapses.
In a chemical synapse neurotransmitters from a presynaptic cell bind to receptors in a postsynaptic cell.
The synaptic cleft—about 25 nanometers wide—separates the cells.
In an electrical synapse, cells are joined through gap junctions.
Gap junctions are made of proteins (connexins) that create channels.
Ions flow through the channels—the action potential spreads through the cytoplasm.
These action potentials are fast but do not allow for complex integration of inputs.
The neuromuscular junction is a chemical synapse between motor neurons and skeletal muscle cells.
An action potential causes voltage-gated
Ca+ channels to open in the presynaptic membrane, allowing Ca+ to flow in.
The presynaptic neuron releases acetylcholine(ACh) from its axon terminals (boutons) when vesicles fuse with the membrane.
The postsynaptic membrane of the muscle cell is the motor end plate.
ACh diffuses across the cleft and binds to ACh receptors on the motor end plate.
These receptors allow Na+ and K+ to flow through, and the increase in Na+ depolarizes the membrane.
If it reaches threshold, more Na+ voltage-gated channels are activated and an action potential is generated.
The postsynaptic cell must sum the excitatory and inhibitory input.
Summation occurs at the axon hillock, the part of the cell body at the base of the axon.
Spatial summation adds up messages at different synaptic sites.
Temporal summation adds up potentials generated at the same site, over time.
Neurotransmitters are cleared from the cleft after release in order to stop their action in several ways:
•Diffusion
•Reuptake by adjacent cells
•Enzymes present in the cleft may destroy them
Example: Acetylcholinesterase acts on ACh.
There are many types of neurotransmitters, and each may have multiple receptor subtypes.
For example, ACh has two:
•Nicotinic receptors are ionotropic and mainly excitatory
•Muscarinicreceptors are metabotropic and mainly inhibitory
The action of a neurotransmitter depends on the receptor to which it binds.
Synapses can be fast or slow:
•Neurotransmitters binding to an ionotropicreceptor, or ion channel, cause a change in ion movement—response is fast and short-lived
•Metabotropic receptors induce signaling cascades in the postsynaptic cell that lead to changes in ion channels.
Cell responses are generally slower and longer-lived.
Concept 34.4 The Vertebrate Nervous System Has Many Interacting Components
Vertebrate nervous systems:
Brain, spinal cord, and peripheral nerves that extend throughout the body.
Central nervous system (CNS)—brain and spinal cord
Peripheral nervous system (PNS)—cranial and spinal nerves that extend or reside outside of the brain and spinal cord, and connect the CNS to all tissues
The afferent part of the PNS carries sensory information to the CNS.
The efferent part of the PNS carries information from the CNS to muscles and glands.
Efferent pathways can be divided into two divisions:
•The voluntary division, which executes conscious movements
•The involuntary, or autonomic, division, which controls physiological functions
Autonomic Nervous System (ANS)—the output of the CNS that controls involuntary functions
ANS has two divisions that work in opposition—one will increase a function and the other will decrease it
•Sympathetic division prepares the body for emergencies—“fight or flight”
•Parasympathetic division slows the heart, lowers blood pressure and increases digestion—“rest and digest”
Autonomic efferent pathways begin with preganglionic neurons with cell bodies in the CNS.
Axons of preganglionic neurons synapse on a second neuron outside the CNS in a collection of neurons called a ganglion.
The second neuron is postganglionic—its axon leaves the ganglion and synapses in the target organs.
Sympathetic postganglionic neurons are noradrenergic—use norepinephrine as their neurotransmitter.
Postganglionic neurons of the parasympathetic division are mostly cholinergic—release acetylcholine.
Target cells respond in opposite ways to acetylcholine and norepinephrine.
Example: Pacemaker cells in the heart.
Sympathetic and parasympathetic divisions have different anatomy.
The sacral region contains preganglionic neurons of the parasympathetic region.
The thoracic and lumbar regions contain sympathetic preganglionic neurons.
Anatomy of the spinal cord:
•Gray matter is in the center and contains cell bodies of spinal neurons
•White matter surrounds gray matter and contains axons that conduct information up and down the spinal cord
•Spinal nerves extend from the spinal cord
Each spinal nerve has two roots.
•One spinal root connects to the dorsal horn, the other to the ventral horn
•Afferent (sensory) axons enter through the dorsal root
•Efferent (motor) axons leave through the ventral root
Spinalreflex—afferent information converts to efferent activity without the brain.
The knee-jerk reflex is monosynaptic:
•Stretch receptors send axon potentials through dorsal horn to ventral horn via sensory axons
•At synapses with motor neurons in the ventral horn, action potentials are sent to leg muscles, causing contraction
Most spinal circuits are more complex—limb movement is controlled by antagonistic muscle sets.
•Flexors bend or flex the limb
•Extensors straighten or extend the limb
Coordination of relaxation and contraction is done by interneurons—they make inhibitory synapses in a polysynapticreflex
The embryonic neural tube develops into the hindbrain, midbrain, and forebrain.
The hindbrain becomes the medulla, the pons, and the cerebellum.
Together the pons, medulla, and midbrain are known as the brainstem.
All information between the spinal cord and the brain passes through the brainstem.
Many sensory axons have branches in the brainstem that form synapses with the reticular system—a network of neurons in the brainstem.
Low to mid-brainstem activity is involved with balance, coordination.
The embryonic forebrain develops the diencephalon and telencephalon.
The diencephalon consists of the:
•Thalamus—the final relay station for sensory information
•Hypothalamus—regulates physiological functions such as hunger and thirst
Structures in primitive regions of the telencephalon form the limbic system—responsible for basic physiological drives.
•Amygdala—involved in fear and fear memory
•Hippocampus—transfers short-term memory to long-term memory
The cerebrum is the dominant structure in mammals, with left and right cerebral hemispheres.
Cerebral cortex—a sheet of gray matter covering each hemisphere, folded into convolutions
Regions of the cerebral cortex have specific functions.
Association cortex is made up of areas that integrate or associate sensory information or memories.
Each cerebral hemisphere consists of four lobes:
•Temporal lobe
•Frontal lobe
•Parietal lobe
•Occipital lobe
Temporal lobe:
Receives and processes auditory and visual information
Association areas of the temporal lobe involve:
•Identification
•Object naming
•Recognition
Agnosia: Disorder of the temporal lobe
Frontal Lobe:
Primary motor cortex is anterior to the parietal lobe and controls muscles in specific body areas.
Association areas involve:
•Feeling, planning
•Personality
Parietal lobe:
Primary somatosensory motor cortex—behind the primary motor cortex
•Receives touch and pressure information
The entire body surface is mapped, more area for fine discriminations in touch.
Occipital lobe:
Receives and processes visual information
Association areas involve:
•Making sense of the visual world
•Translating visual experience into language
Concept 34.5 Specific Brain Areas Underlie the Complex Abilities of Humans
The lateralization of language functions shows that 97 percent occurs in the left brain hemisphere.
An aphasia is a deficit in the ability to use or understand words; occurs after damage to the left hemisphere.
Language areas:
Broca’s area—in frontal lobe; damage results in slow or lost speech; still can read and understand language
Wernicke’s area—in temporal lobe. Damage results in inability to speak sensibly; written or spoken language not understood. Still can produce speech
Learning—modification of behavior by experience.
Memory—what the nervous system retains.
Long-term potentiation (LTP)describes how synapses become more responsive to repeated stimuli.
Associative learning occurs when two unrelated stimuli become linked to a response.
A conditioned reflex is a type of associative learning.
Example: Salivary reflex in Pavlov’s dog
Complex, or observational learning in humans has a pattern of three elements:
•We pay attention to another’s behavior
•We retain a memory of what we observe
•We try to copy or use that information
Declarative memory is of people, places, and things that can be recalled and described.
Procedural memory is howto perform a motor task and cannot be described.
Types of memory:
•Immediate—events happening now
•Short-term—lasts 10 to 15 minutes
•Long-term—lasts from days to a lifetime
Memories can be associated with specific brain regions and neuronal properties.
Memories are transferred from short- to long-term.
Hippocampal or limbic system damage may prevent this transfer.
Example: H.M. was unable to transfer memories to long-term storage after removal of the hippocampus.
Sleep research uses electroencephalogram (EEG):
Measures neuronal activity and records changes in electrical potential in entire brain regions
In birds and mammals, there are two main sleep states:
•Rapid eye movement (REM) sleep
•Non-REM sleep
When awake, nuclei in the brainstem are active and cells depolarize often.
Neurons in the thalamus and cortex are near threshold and sensitive to input— reflects a wakeful state.
At sleep onset, activity slows in the brainstem—less neurotransmitter is released, cells are less excitable.
Information processing slows and consciousness is lost—the state of non-REM sleep
Cells in non-REM sleep fire in bursts, called slow-wave sleep.
During non-REM and REM transition:
•Brainstem nuclei become active again and firing bursts cease
•Cortex can process information as cells at threshold can depolarize
•Sensory and motor pathways are still inhibited; without this feedback the cortex may produce bizarre dreams.
After a period of REM sleep, we return to non-REM sleep.
Repeated cycles occur in humans and other mammals.
Hypotheses for sleep patterns include immune function, maintenance of neural connections, and for learning and memory.
Consciousness refers to being aware of yourself, your environment, and events occurring around you.
Conscious experience requires a perception of self, using integration of information from the physical and social environment, with information from past experience.
Answer to Opening Question
Charles Whitman’s brain tumor pressed on the hypothalamus and parts of the limbic system, including the amygdala.
When neurons in the amygdala are activated, intense emotions such as fear and rage may be felt.
These strong emotions may have led to his actions.