The document summarizes the structure and function of the nervous system in three main divisions:
1) The central nervous system (CNS) consists of the brain and spinal cord. The peripheral nervous system (PNS) consists of the nerve network outside the CNS.
2) Neuroglia are supportive cells in the nervous system that form myelin sheaths and blood-brain barriers. The most numerous neuroglial cell is the astrocyte.
3) Neurons handle communication in the nervous system and are classified as sensory, motor, or interneurons. Impulse conduction in neurons involves changes in membrane potential and the firing of action potentials.
2. Divisions of the Nervous System
• Central nervous system (CNS)
• Consists of brain and spinal cord
• Peripheral nervous system
(PNS)
• Consists of nerve network
4. Neuroglia
• Supportive cells of the nervous system
– Oligodendrocytes: Form myelin sheath in the
CNS
– Ependymal cells: Line spinal cord and cavities of the
brain
– Microglia: Perform phagocytosis
– Astrocytes: Form blood–brain barrier
– Schwann cells: Form myelin sheath in peripheral
nervous system
11. Question
Which neurons detect sensations (such as touch or heat)
and relay information about the stimuli to the central
nervous system?
A. Efferent neurons
B. Interneurons
C. Afferent neurons
D. Schwann cells
12. Answer
Correct answer: C
Rationale:
• Efferent neurons are motor neurons and relay messages
from the brain to muscles and gland cells.
• Interneurons connect incoming sensory pathways with
outgoing motor pathways in the central nervous system.
• Schwann cells form the myelin sheath.
13. Impulse Conduction
• Caused by an electrical current
• Membrane potential: When ions with opposite
electrical charges are separated by a membrane
• Polarization: When a membrane has an excess of
positive ions on one side and an excess of negative ions
on the other
20. Question
What is another term for nerve impulse?
A. Repolarization
B. Resting potential
C. Depolarization
D. Action potential
21. Answer
Correct answer: D
Rationale:
• Repolarization is when the nerve cell restores its electrical
balance in preparation for a new stimulus.
• Resting potential is when the neuron is resting but has the
potential to react.
• Depolarization occurs as the membrane becomes more
positive.
30. Question
Bundles of axons within the white matter that serve as
routes of communication to and from the brain are called:
A. ganglions.
B. fascicles.
C. tracts.
D. plexuses.
31. Answer
Correct answer: C
Rationale:
• Ganglions are cell bodies of the dorsal neurons clustered
in a knot-like structure.
• Fascicles are bundles of nerve fibers contained within a
nerve.
• Plexuses are nerve networks outside the spinal cord.
32. Regions of the Brain
Cerebrum
Diencephalon
Cerebellum
Midbrain
Pons
Medulla oblongata
Brainstem
33. Meninges of the Brain
Dural sinus
Subdural space
Subarachnoid
space
Falx cerebri
Dura mater
Arachnoid mater
Pia mater
Skull
36. Question
Which statement about gray matter is true?
A. It is charged with thought, learning,
and reasoning.
B. It forms the interior of the brain.
C. It consists of myelinated fibers.
D. It secretes cerebrospinal fluid.
37. Answer
Correct answer: A
Rationale:
• Gray matter covers the surface of the brain,
and it consists of unmyelinated fibers.
• The choroid plexus secretes cerebrospinal
fluid.
42. Question
The cardiac and respiratory centers are found in the:
A. pons.
B. cerebellum.
C. diencephalon.
D. medulla oblongata.
43. Answer
Correct answer: D
Rationale:
• The pons conveys signals to and from other parts of the
brain.
• The cerebellum monitors body movement.
• The diencephalon houses the thalamus and
hypothalamus.
50. Question
The limbic system is charged with:
A. emotion and learning.
B. identifying the location of pain.
C. planning muscle movements to form words.
D. determining muscle movements to perform a task.
51. Answer
Correct answer: A
Rationale:
• The somatic sensory association area allows us to
pinpoint the location of pain.
• Broca’s area plans muscle movements to form words.
• Neurons in the motor association area determine the
movements required to perform a task.
52. Special Senses
Primary gustatory complex
Primary visual cortex
Visual association area
Olfactory association area
Auditory association area
Primary auditory complex
53. Sleep
• Stage 1: Drowsiness
• Stage 2: Light sleep
• Stage 3: Moderate to deep sleep
• Stage 4: Slow-wave sleep
• REM sleep: Dream sleep
56. Autonomic Nervous System
• Regulates activities that maintain homeostasis
• Sends impulses to cardiac and smooth muscle
• Also called visceral motor system
64. SYMPATHETIC PARASYMPATHETIC
Originates in thoracolumbar
region
Originates in craniosacral
region
Ganglia lie in chain alongside
spinal cord
Ganglia lie in or near target
organs
Has short preganglionic and
long postganglionic fiber
Has long preganglionic and
short postganglionic fiber
Employs mostly
norepinephrine as a
neurotransmitter (occasionally
uses acetylcholine)
Employs acetylcholine as a
neurotransmitter
Produces widespread,
generalized effects
Produces local effects
Sympathetic vs. Parasympathetic Division
65. Question
Which of the following is an action of the parasympathetic
division of the autonomic nervous system?
A. Increase heart rate
B. Constrict bronchial tubes
C. Stimulate sweat glands
D. Inhibit intestinal motility
67. Neurotransmitters
Cholinergic fibers
• Preganglionic fibers of
sympathetic and
parasympathetic divisions
• Postganglionic fibers of
parasympathetic division
• Secrete acetylcholine
Adrenergic fibers
• Include most
postganglionic
fibers of the sympathetic
division
• Secrete norepinephrine
68. Cholinergic Receptors
Nicotinic receptors
• Occur within the adrenal
medulla and the
neuromuscular junction
• Produce an excited
response
Muscarinic receptors
• Occur on glands, smooth
muscle, and cardiac
muscle cells of organs
innervated by cholinergic
fibers
• Produce a variable
response
69. Adrenergic Receptors
• Cells with alpha-adrenergic receptors are excited
by norepinephrine (NE).
• Cells with beta-adrenergic receptors are inhibited
by NE.
View animation on “Receptors of the sympathetic and
parasympathetic divisions”
72. Question
The effect produced by a neurotransmitter is ultimately
determined by:
A. the amount of neurotransmitter released.
B. the type of neurotransmitter released.
C. the type of receptor.
D. the number of receptors.
73. Answer
Correct answer: C
Rationale:
• The amount of neurotransmitter influences the strength
(not type) of a response.
• Although the effect varies according to the type of
neurotransmitter, the same neurotransmitter bound to a
different receptor will have a different response.
Therefore, it is the receptor that ultimately determines the
response.
• The number of receptors influences the strength of the
response.
Editor's Notes
The peripheral nervous system consists of everything outside of the brain and spinal cord.
Because the nervous system performs so many functions, it is helpful to further subdivide the peripheral nervous system, as shown here.
Neuroglia—also called glial cells—are the supportive cells of the nervous system. They perform various functions that enhance the performance of the nervous system.
Other than Schwann cells (which are found in the peripheral nervous system), all neuroglia reside in the central nervous system.
Some ependymal cells secrete CSF, whereas others have cilia to propel circulation of CSF.
Astrocytes—the most numerous of the glial cells—are pervasive throughout the brain.
A tiny “foot” exists at the end of each of the astrocyte’s star-like projections. Some of the feet latch on to a capillary; others connect with a neuron. This allows the astrocyte to funnel glucose from the bloodstream to the neuron.
The feet of the astrocytes join with endothelial cells lining the capillaries to create a membrane called the blood–brain barrier (BBB).
The BBB is a semipermeable membrane that exists throughout the brain. It allows small molecules (like oxygen, carbon dioxide, and water) to diffuse across to the brain but blocks larger molecules. This helps protect the brain from foreign substances. It also prevents most medications from reaching brain tissue.
Neurons handle the nervous system’s role of communication.
Sensory (afferent) neurons detect stimuli (such as touch, pressure, heat, cold, or chemicals) and transmit information about the stimuli to the central nervous system (CNS).
Interneurons (which are found only in the CNS) connect the incoming sensory pathways with the outgoing motor pathways.
Motor (efferent) neurons relay messages from the brain to the muscle or gland cells.
Neurons assume a variety of shapes and sizes; even so, neurons have three basic parts: a cell body and two extensions called an axon and dendrite.
The cell body (also called the soma) is the control center and contains the nucleus.
Dendrites receive signals from other neurons and conduct the information to the cell body. Some neurons have only one dendrite; others have thousands.
The axon carries nerve signals away from the body. Nerve cells have only one axon; the length of the axon ranges from a few millimeters to a meter.
The axons of many (but not all) neurons are encased in a myelin sheath, which insulates the axon. The myelin sheath (which consists mostly of lipids) is formed by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system.
Gaps in the myelin sheath, called nodes of Ranvier, occur at evenly spaced intervals.
The end of the axon branches extensively, with each axon terminal ending in a synaptic knob. Within the synaptic knobs are vesicles containing a neurotransmitter.
In the peripheral nervous system, Schwann cells wrap themselves around the axon, laying down multiple layers of cell membrane. The inside layers form the myelin sheath; the nucleus and most of the cytoplasm of the Schwann cell are located in the outermost layer. The outer layer, called the neurilemma, is essential for an injured nerve to regenerate.
In the central nervous system, projections from one oligodendrocyte wrap themselves around the axons of nearby nerves. Because the nucleus of the cell is located away from the myelin sheath, there is no neurilemma. This prevents injured CNS neurons from regenerating.
Myelin helps speed impulse conduction. (Typically, unmyelinated nerve fibers perform functions in which speed isn’t essential, such as stimulating the secretion of stomach acid. Nerve fibers stimulating skeletal muscles, where speed is more important, are myelinated.)
Myelination begins during the 14th week of fetal development but is not complete until late adolescence.
Impulse transmission results from the flow of charged particles from one point to another.
In the body, whenever ions with opposite electrical charges are separated by a membrane, the potential exists for them to move toward one another. This is called membrane potential.
A membrane that exhibits membrane potential (an excess of positive ions on one side of the membrane and an excess of negative ions on the other side) is said to be polarized.
When a neuron is not conducting an electrical signal, the interior has a negative electrical charge and the exterior has a positive charge.
The outside of the cell is rich with sodium ions (Na+), whereas the inside contains an abundance of potassium ions (K+).
The interior also contains other large, negatively-charged proteins and nucleic acids. These additional particles give the cell’s interior its overall negative charge.
Because of the membrane’s permeability, a certain amount of sodium and potassium ions leaks across the membrane. However, the sodium-potassium pump constantly works to restore the ions to the appropriate side.
This state of being inactive and polarized is called resting potential. The neuron is resting, but it has the potential to react if a stimulus comes along.
A stimulus (such as chemicals, heat, or mechanical pressure) causes channels on the neuron’s membrane to open and Na+ from outside the membrane rushes into the cell.
The addition of the positively charged ions changes the charge of a region of the cell’s interior from negative to positive. As the membrane becomes more positive, it is said to depolarize.
If the depolarization goes above the threshold level, adjacent channels also open, which allows more Na+ to enter. This creates an action potential, which means that the neuron has become active as it conducts an impulse along the axon. (Another term for action potential is nerve impulse.)
The action potential continues down the axon as one segment stimulates the segment next to it.
The sudden influx of Na+ triggers other channels to open; this allows K+ to flow out of the cell.
Soon after K+ begins to exit, the Na+ channels shut to prevent any more Na+ from flowing into the cell. This repolarizes the cell; however, Na+ and K+ are now flip-flopped, with the outside containing more K+ and the inside containing more Na+.
As long as Na+ and K+ are on the wrong sides of the membrane, the neuron won’t respond to a new stimulus even though the membrane is polarized. This is called the refractory period.
The sodium-potassium pump returns Na+ to the outside and K+ to the inside. When this is completed, the nerve is again polarized and in resting potential until it receives another stimulus.
Myelin blocks the free movement of ions across the cell membrane; the only place ion exchange can occur is at the nodes of Ranvier.
Electrical changes occur at the nodes of Ranvier, creating an action potential. The current flows under the myelin sheath to the next node, where it triggers another action potential.
Because the action potentials only occur at the nodes, the impulse seems to “leap” from node to node. This type of signal conduction is called saltatory conduction.
As impulses move from one neuron to the next, they pass through a synapse:
When an action potential reaches a synaptic knob, the membrane depolarizes. Ion channels open and calcium ions enter the cell.
The infusion of calcium causes vesicles to bind to the cell wall and release their store of a neurotransmitter into the synapse.
The neurotransmitter binds to receptors on the postsynaptic membrane. Each neurotransmitter has a specific receptor. (For example, the neurotransmitter epinephrine can only bind to receptors specific to epinephrine.)
The specific neurotransmitter determines whether the impulse continues (called excitation) or whether it is stopped (called inhibition). If the impulse is inhibitory, K+ channels open and the impulse stops. If the neurotransmitter is excitatory—as shown here—Na+ channels open, the membrane becomes depolarized, and the impulse continues.
The receptor releases the neurotransmitter, after which it is reabsorbed by the synaptic knobs and recycled or destroyed by enzymes (as shown here).
The spinal cord is part of the central nervous system; it relays messages from the brain to the rest of the body.
The spinal cord extends from the base of the brain until about the first lumbar vertebra. A bundle of nerve roots called the cauda equina extends from the end of the spinal cord.
Thirty-one pairs of spinal nerves branch out from the spinal cord; they are divided into regions.
Nerves from the cervical region innervate the chest, head, neck, shoulders, arms, hands, and diaphragm.
Nerves from the thoracic region extend to the intercostal muscles of the ribcage, the abdominal muscles, and the back muscles.
The lumbar spinal nerves innervate the lower abdominal wall and parts of the thighs and legs.
Nerves from the sacral region extend to the thighs, buttocks, muscles, skin of the legs and feet, and anal and genital regions.
The spinal cord sits inside the stacked vertebrae.
A cross section shows two types of nervous tissue (white matter and gray matter).
Gray matter appears gray because of its lack of myelin; it contains mostly cell bodies of motor neurons and interneurons. The H-shaped mass is divided into two sets of horns: the posterior (dorsal) horns and the ventral (anterior) horns.
White matter appears white because of its abundance of myelin. It contains bundles of axons (called tracts) that carry impulses from one part of the nervous system to another.
The central canal carries cerebrospinal fluid through the spinal cord.
The epidural space lies between the outer covering of the spinal cord and the vertebrae; it contains a cushioning layer of fat, as well as blood vessels and connective tissue.
Spinal nerves travel through gaps between the vertebrae and attach to the spinal cord by way of the dorsal and the ventral roots.
The dorsal (posterior) nerve root contains fibers that carry sensory information into the spinal cord. It enters the dorsal horn of the spinal cord. Cell bodies of the dorsal neurons are clustered in a knot-like structure called a ganglion.
Fibers in the ventral (anterior) nerve roots exit from the ventral horn to carry motor information out of the spinal cord.
A spinal nerve is a single nerve resulting from the fusion of the dorsal and ventral nerve roots. It contains both sensory and motor fibers (meaning it can transmit impulses in two directions); it is called a mixed nerve.
The spinal cord is protected by the bony vertebrae and by three layers of fibrous connective tissue called the meninges. The three layers are the pia mater, arachnoid mater, and dura mater.
The pia mater (the innermost layer) is a transparent membrane that clings to the surface of the brain and spinal cord; it contains blood vessels. The arachnoid mater lies between the dura mater and pia mater; it is a delicate layer resembling a cobweb. The dura mater is the tough outer layer.
The subarachnoid space lies between the arachnoid mater and the pia mater; it is filled with cerebrospinal fluid.
Bundles of axons called tracts lie within the white matter; they serve as routes of communication to and from the brain. All nerve fibers in a single tract have a similar origination, destination, and function.
Ascending tracts convey sensory signals (such as pain) up the spinal cord to the brain. Ascending tracts are the dorsal column, spinocerebellar tract, and spinothalamic tract.
The dorsal column relays sensations of deep pressure, vibration, and those needed to create awareness of the body’s position (proprioception).
The spinocerebellar tract is responsible for proprioception.
The spinothalamic tract relays sensations of temperature, pressure, pain, and touch.
Descending tracts conduct motor impulses down the spinal cord to skeletal muscles. These include the corticospinal tracts and extrapyramidal tracts.
The corticospinal tracts (also called pyramidal tracts) are responsible for fine movements of hands, fingers, feet, and toes on the opposite side of the body.
The extrapyramidal tracts are a group of tracts associated with balance and muscle tone.
A nerve consists of many nerve fibers (axons) encased by connective tissue.
The number of nerve fibers contained in a single nerve varies from a few to a million.
Nerve fibers (and blood vessels) exist in bundles called fascicles; several fascicles are grouped together and wrapped in a dense connective tissue.
Most nerves contain both sensory and motor fibers and are called mixed nerves. These nerves can transmit signals in two directions.
A few nerves (such as the optic nerves) are sensory nerves and contain only sensory (afferent) fibers. They carry sensations toward the spinal cord.
Others are motor nerves and contain only motor (efferent) fibers and carry messages to muscles and glands.
The first cervical nerve exits the spinal cord between the skull and the axis; the other nerves pass through holes in the vertebra (intervertebral foramina).
Outside the spinal column, each spinal nerve forms branches; some of these branches form nerve networks called plexuses. The four major plexuses are the cervical plexus, the brachial plexus, the lumbar plexus, and the sacral plexus.
The cervical plexus (in green) contains nerves that supply the muscles and skin of the neck, tops of the shoulders, and part of the head. The phrenic nerve, which stimulates the diaphragm for breathing, is located here.
The brachial plexus (in orange) innervates the lower part of the shoulder and the arm. Key nerves in this region include the axillary nerve, the radial nerve, the ulna nerve, and the median nerve.
The lumbar plexus (in purple) supplies the thigh and leg. A key nerve in this region is the large femoral nerve.
The sacral plexus (in blue) is formed from fibers from nerves L4, L5, and S1 through S4. The sciatic nerve, the largest nerve in the body, arises here and runs down the back of the thigh.
Reflexes are a quick, involuntary, predictable response to a stimulus.
Reflexes use a neural circuit called a reflex arc; reflex arcs bypass regions of the brain where conscious decisions are made (which is why someone only becomes aware of a reflex after it has occurred).
Autonomic (visceral) reflexes involve secretion from glands or the contraction of smooth muscle (such as dilation of the pupil). These reflexes are governed by autonomic neurons.
Somatic reflexes involve the contraction of a skeletal muscle after being stimulated by a somatic motor neuron. Somatic reflexes protect the body against harm and maintain posture.
Somatic reflexes involve these steps:
Somatic receptors (located in the skin, a muscle, or a tendon) detect a sensation, such as the stretching of the thigh muscle when the patellar tendon is tapped.
Afferent (sensory) nerve fibers send a signal directly to the spinal cord.
The impulse immediately passes to a motor neuron.
The motor neuron initiates an impulse back to the muscle, causing it to contract, producing a slight kick in the lower leg.
The brain is divided into four major regions: the cerebrum, the diencephalon, the cerebellum, and the brainstem.
The cerebrum is the largest portion. Thick ridges called gyri mark its surface. Shallow grooves called sulci divide the gyri. Deep sulci are called fissures.
The diencephalon sits between the cerebrum and the midbrain.
The cerebellum is the second largest region. It contains more neurons than the rest of the brain combined.
The brainstem makes up the rest of the brain. It consists of the midbrain, pons, and medulla oblongata.
A deep groove called the longitudinal fissure divides the cerebrum into right and left cerebral hemispheres. A thick bundle of nerves called the corpus callosum runs along the bottom of the fissure and serves to connect the two hemispheres.
The brain contains gray and white matter.
Gray matter, which covers the cerebrum and cerebellum in a layer called the cortex, is charged with thought, learning, and reasoning. Surface folds allow more gray matter to be packed into the skull.
The bones of the skull help protect the brain from trauma.
Inside the skull, three layers of meninges (dura mater, arachnoid mater, and pia mater) cover the brain.
The dura mater consists of two layers: the outer layer (the periosteal layer) is attached to the inner surface of the skull; the inner meningeal layer forms the outer covering of the brain and continues as the dura mater of the spinal cord.
The dura mater separates to create spaces called dural sinuses. These sinuses collect blood that has passed through the brain and is on its way back to the heart.
A subdural space separates the dura from the arachnoid mater.
A subarachnoid space separates the arachnoid from the pia mater.
In some places, the dura mater extends inward and separates major portions of the brain. The falx cerebri dips into the longitudinal fissure to separate the right and left hemispheres. Elsewhere, the tentorium cerebella extends over the top of the cerebellum, separating it from the cerebrum.
The brain contains four chambers, called ventricles.
Two lateral ventricles arch through the cerebral hemispheres: one in the right hemisphere and one in the left.
Each of the lateral ventricles connects to a third ventricle.
A canal leads to the fourth ventricle. This space narrows to form the central canal, which extends through the spinal cord.
A clear, colorless fluid called cerebrospinal fluid (CSF) fills the ventricles and central canal; it also bathes the outside of the brain and spinal cord.
The choroid plexus in each lateral ventricle secretes CSF.
CSF constantly flows through the central nervous system; it provides nourishment (glucose and protein), removes metabolic wastes, and cushions the brain against minor trauma. The brain monitors the level of CO2 in CSF and triggers responses to help the body maintain homeostasis.
CSF production proceeds as follows:
The choroid plexus in each lateral ventricle secretes CSF.
The CSF flows into the third ventricle, where the choroid plexus in that ventricle adds more fluid.
It then flows into the fourth ventricle, where still more CSF is added by the choroid plexus located there
Some of the CSF flows through two tiny openings (foramina).
From there, the CSF flows through the subarachnoid space, up the back of the brain, down around the spinal cord, and up the front of the brain.
The CSF is reabsorbed into the venous bloodstream by projections of the arachnoid mater into the dural sinuses (called arachnoid villi).
The brainstem consists of the midbrain, pons, and medulla oblongata.
The midbrain contains tracts that relay sensory and motor impulses; it also contains centers for auditory and visual reflexes, as well as clusters of neurons integral to muscle control.
The pons contains tracts that convey signals to and from different parts of the brain. Several cranial nerves arise from this area.
The medulla oblongata attaches the brain to the spinal cord. It relays sensory and motor signals between the brain and spinal cord; it also contains the cardiac center (which regulates heart rate), the vasomotor center (which controls blood vessel diameter and, in turn, affects blood pressure), and two respiratory centers (which regulate breathing).
The medulla also houses reflex centers for coughing, sneezing, swallowing, and vomiting. Several cranial nerves either begin or end in the medulla.
The cerebellum houses more neurons than the rest of the brain combined.
Connected to the cerebral cortex by approximately 40 million neurons, the cerebellum receives, and processes, messages from all over the brain.
The cerebellum monitors body movements and send messages for balance, coordination, and posture; it stores the information necessary for muscle groups to work together to perform smooth, efficient, and coordinated movements; and it evaluates sensory input, such as touch, spatial perception, and sound.
People with cerebellar dysfunction have a spastic gait, poor balance, jerky movements, and tremors. They also tend to have poor impulse control and overreact emotionally.
A region deep inside the brain, the diencephalon consists of several structures, with the chief ones being the thalamus and the hypothalamus.
The thalamus is shaped like two eggs; it resides on the top of the brainstem. It acts as a gateway for nearly every sensory impulse traveling to the cerebral cortex. The thalamus processes and filters these impulses, transmitting some, but not all, to the cerebral cortex.
The thalamus also relays messages regarding certain complex movements and is involved in memory and emotion.
The hypothalamus is tiny in size, but it influences nearly every organ of the body. The hypothalamus:
Controls the autonomic nervous system.
Contains centers for hunger, thirst, and temperature regulation.
Controls the pituitary gland (often called the “master gland”).
Is involved in multiple emotional responses, including fear, anger, pleasure, and aggression.
A set of interconnected nuclei called the reticular formation is scattered throughout the brainstem. Fibers extend from there to the cerebrum, the cerebellum, and the spinal cord.
One component of the reticular formation is the reticular activating system (RAS). The RAS receives sensory input from the eyes and ears, filters out insignificant signals (such as routine noise), and sends impulses to the cerebral cortex so the mind remains conscious and alert. Drugs that depress the RAS induce sleep.
The reticular formation also has tracts extending into the spinal cord that are involved in posture and equilibrium.
Other components of the reticular formation include the cardiac and vasomotor centers of the medulla oblongata, which are responsible for heart rate and blood pressure.
The cerebrum is the largest portion of the brain; it is divided into five lobes, each of which is named for bones of the skull that lie directly over them.
Frontal lobe: Governs voluntary movements, memory, emotion, social judgment, decision-making, reasoning, and aggression; is also the site for certain aspects of one’s personality
Parietal lobe: Concerned with receiving and interpreting bodily sensations (such as touch, temperature, pressure, and pain); also governs proprioception (the awareness of one’s body and body parts in space and in relation to each other)
Occipital lobe: Concerned with analyzing and interpreting visual information
Temporal lobe: Governs hearing, smell, learning, memory, emotional behavior, and visual recognition
Insula: Hidden behind the lateral sulcus; plays a role in many different functions, including perception, motor control, self-awareness, and cognitive functioning
The bulk of the cerebrum is white matter (which consists of bundles of myelinated nerve fibers, called tracts); tracts carry impulses from one part of the cerebrum to the other, or from the cerebrum to other parts of the brain or spinal cord.
Most tracts passing from one hemisphere to the other travel through the corpus callosum; this allows the brain’s two hemispheres to communicate with each other.
The surface of the cerebrum (the cerebral cortex) consists of a thin layer of gray matter; gray matter makes up about 40% of the brain’s mass.
Masses of gray matter (called basal nuclei, or basal ganglia) lie deep within the cerebrum and play a role in the control of movement.
Other tracts carry information back and forth between the brain and the spinal cord. These tracts are extensions of the ascending (sensory) spinothalamic tracts and the descending (motor) corticospinal tracts. The tracts cross in the brainstem, with the right side of the brain sending impulses to the left side of the body (and vice versa).
The limbic system is the seat of emotion and learning; it is formed by a complex set of structures that encircle the corpus callosum and thalamus.
The limbic system links areas of the lower brainstem (which control automatic functions) with areas in the cerebral cortex (associated with higher mental functions).
Two key structures of the limbic system are the hippocampus and amygdala. The hippocampus is crucial for memory and learning. The amygdala is concerned with emotions; it stores, and can recall, emotions from past events.
Feelings of anger, fear, sexual feelings, sorrow, and pleasure result because of a functioning limbic system. Other parts of the cerebral cortex must also be engaged to ensure that those feelings are expressed in socially acceptable ways.
Sensory nerve fibers transmit signals up through the spinal cord to the thalamus, which forwards them to the postcentral gyrus.
The postcentral gyrus is the primary somatic sensory area of the brain. It receives impulses of heat, cold, and touch from receptors all over the body.
Because of decussation, the right postcentral gyrus receives signals from the left side of the body and the left gyrus receives signals from the right side.
The somatic sensory association area allows us to pinpoint the location of pain, identify a texture, and be aware of how our limbs are positioned.
The primary somatic motor area is the precentral gyrus.
Neurons in the motor association area determine which movements are required to perform a specific task and send the appropriate signals to the precentral gyrus. Neurons in the precentral gyrus then send impulses through the motor tracts in the brainstem and spinal cord to the skeletal muscles, and movement occurs.
Written words stimulate the primary visual cortex.
The angular gyrus translates the written words into a form that can be spoken.
Wernicke’s area formulates the words into phrases that comply with learned grammatical rules.
Broca’s area plans the muscle movements required of the larynx, tongue, cheeks, and lips to form the words; it then sends the appropriate impulses to the primary motor cortex.
The primary motor cortex sends impulses to the muscles necessary to pronounce the word.
Primary gustatory complex: Handles the interpretation and sensation of taste
Primary visual cortex: Responsible for sight; governs the recognition of size, color, light, motion, and dimension
Visual association area: Interprets the information acquired through the primary visual cortex, allowing us to recognize familiar objects
Primary auditory complex: Responsible for hearing
Auditory association area: Gives us the ability to recognize familiar sounds (including a person’s voice)
Olfactory association area: Interprets the sense of smell
Stage 1: People will awaken easily. Brain waves are active.
Stage 2: Brain waves show occasional spikes in amplitude, a reflection of the interaction of the thalamus and cerebral cortex.
Stage 3: Begins about 20 minutes after stage 1; muscles relax, the heart rate slows, and blood pressure drops.
Stage 4: Called slow-wave sleep because of the rhythmic brain waves; the muscles are very relaxed and the sleeper will be difficult to awaken.
Rapid eye movement (REM) sleep: About five times a night, a sleeper will backtrack from stage 3 to stage 2, at which time REM sleep begins. The eyes move rapidly back and forth, and dreaming occurs. This stage lasts from 10 minutes to an hour.
The left hemisphere is the more analytical side; it focuses on language and the types of reasoning used in math and science. The right hemisphere is more concerned with creativity and spatial ability.
The two hemispheres communicate via the corpus callosum, allowing for the smooth integration of information.
Twelve pairs of cranial nerves arise directly from the brain to relay messages to the rest of the body.
Each cranial nerve is identified by a name and a Roman numeral (beginning in the anterior portion of the brain).
Some cranial nerves contain only sensory fibers, some contain primarily motor, whereas others contain both.
The cranial nerves are: (I) olfactory; (II) optic; (III) oculomotor; (IV) trochlear; (V) trigeminal; (VI) abducens; (VII) facial; (VIII) vestibulocochlear; (IX) glossopharyngeal; (X) vagus; (XI) spinal accessory; and (XII) hypoglossal.
Activities include secretion of digestive enzymes, constriction and dilation of blood vessels for maintenance of blood pressure, and the secretion of hormones.
Activities occur without awareness or control (autonomously).
Also called visceral motor system because it targets organs.
The autonomic nervous system (ANS) asserts control through visceral reflexes (similar to somatic reflexes but affect an organ instead of a skeletal muscle).
All visceral reflexes follow similar steps; this illustration shows the visceral reflex arc responsible for the regulation of blood pressure:
Receptors detect a change in body conditions. (Here, pressure receptors in the carotid artery, called baroreceptors, detect a rise in blood pressure.)
Afferent neurons transmit information about this change to the central nervous system. (The glossopharyngeal nerve relays this information to the medulla oblongata.)
The brain processes the information and transmits a signal along an efferent nerve. (Vagus nerve sends a signal to the heart’s pacemaker to slow its rate.)
The effector organ receives the message and responds. (The heart rate slows and the blood pressure drops.)
In the somatic pathway:
The neuron’s cell body lies within the central nervous system (either the brain or spinal cord).
A single myelinated axon extends from the brainstem or spinal cord to a skeletal muscle.
At the target muscle, the neurotransmitter acetylcholine (ACh) is released to cause muscle contraction.
In the autonomic pathway:
A myelinated preganglionic neuron extends from the brainstem or spinal cord to a ganglion.
In the ganglion, it synapses with a postganglionic neuron and the neurotransmitter acetylcholine (ACh) is released.
The axon of the unmyelinated postganglionic neuron extends to the target organ. The neurotransmitter released varies: parasympathetic fibers release ACh, whereas sympathetic fibers release norepinephrine (NE).
The sympathetic division is called on in times of stress, anger, or fear—“fight or flight” reaction.
Note: Chapter 10 of the textbook contains a chart comparing the actions of the sympathetic and parasympathetic divisions.
Both the sympathetic and parasympathetic divisions work at the same time, providing a background level of activity called autonomic tone. The balance between sympathetic and parasympathetic activity constantly changes depending on the body’s needs.
Sympathetic preganglionic neurons begin within the spinal cord.
From the cell bodies, myelinated fibers reach to sympathetic ganglia, most of which exist in chains along both sides of the spinal cord. The preganglionic neurons are short.
Not all preganglionic neurons synapse in the first ganglion they encounter. Some travel up or down the chain to synapse with other ganglia at different levels. Others pass through the first ganglion to synapse with another ganglion a short distance away.
Unmyelinated postganglionic fibers leave the ganglia and extend to the target organs. Postganglionic fibers tend to be long.
Each preganglionic neuron branches and synapses with multiple postganglionic neurons. This allows one preganglionic neuron to excite multiple postganglionic neurons simultaneously, which is why the sympathetic division can ignite such an immediate, widespread response, such as what occurs during “fight or flight.”
Sympathetic preganglionic fibers pass through the outer cortex of the adrenal gland and terminate in the center (the adrenal medulla).
When stimulated, the adrenal medulla secretes a mixture of epinephrine and norepinephrine into the bloodstream. Both of these hormones can bind to the receptors of sympathetic effectors, which helps prolong the sympathetic response.
Neurons of the parasympathetic division arise from the brain and sacral region of the spinal cord (which is why this division of the autonomic nervous system is also called the craniosacral division).
Parasympathetic fibers leave the brainstem by joining either the oculomotor nerve (cranial nerve III), facial nerve (cranial nerve VII), glossopharyngeal nerve (cranial nerve IX), or vagus nerve (cranial nerve X); the vagus nerve carries about 90% of all parasympathetic preganglionic fibers.
The ganglia of the parasympathetic division reside in or near the target organ: preganglionic fibers are long and postganglionic fibers are short.
Because the ganglia are more widely dispersed, the parasympathetic division produces a more localized response than that of the sympathetic division.
The two divisions of the autonomic nervous system (ANS) tend to exert opposite effects, although there are exceptions (e.g., the sympathetic division dilates blood vessels leading to skeletal muscles and constricts blood vessels leading to most organs).
Two factors determine the effect of each division: the neurotransmitter released and the type of receptor on the target cells.
The ANS uses two different neurotransmitters: acetylcholine (ACh) and norepinephrine (NE). Fibers that secrete acetylcholine are called cholinergic fibers. Fibers that secrete NE are called adrenergic fibers.
The effects of the sympathetic division last longer than those of the parasympathetic division because acetylcholine is disposed of more quickly after release than is norepinephrine.
Neurotransmitters bind to receptors on the effector cells of target organs: acetylcholine (ACh) binds to cholinergic receptors, and norepinephrine (NE) binds to adrenergic receptors.
Cholinergic receptors may be one of two types: nicotinic or muscarinic. The different types of receptors determine the effect produced by a neurotransmitter.
The variable response produced by muscarinic receptors allows ACh to stimulate intestinal smooth muscle whereby inhibiting cardiac muscle.
The binding of NE to alpha-adrenergic receptors in blood vessels causes the blood vessels to constrict. The binding of NE to beta-adrenergic receptors in the heart and skeletal muscles causes the vessels to dilate.
Both alpha and beta receptors contain subtypes that can cause exceptions to these principles.
This chart summarizes the neurotransmitters and receptors of the sympathetic division of the ANS.
This chart summarizes the neurotransmitters and receptors of the parasympathetic division of the ANS.
