2. Neurons and Nervous Systems (Chapter 37 pages 692-713)
37.1 Definition of the Nervous System
37.2 Nervous Tissue (Review)
37.3 The Central Nervous System
37.4 The Peripheral Nervous System

3. The nervous system
is vital in complex animals, enabling them to seek food and mates and to avoid danger.
It ceaselessly monitors internal and external conditions and makes appropriate changes to maintain homeostasis.

4. Organization of the Nervous System
(Fig page 694)
1.Central nervous system (CNS)
2.Peripheral nervous system (PNS)

5. Organization of the Human Nervous System

6. Nervous tissue:
1.Neurons (nerve cells)
2.Neuroglia
Neurons vary in appearance depending on their function and location.
They consists of 3 major parts:
1.The cell body
2.The dendrites
3.The axon

7. Neurons and Neuroglia

8. 5. Ependymal cells: line the ventricles of the brain,
There are several different types of neuroglial cells in the CNS, each with a specific function:
1.Microglia: are phagocytic cells.
2.Astrocytes: most numerous type.
serve many roles in maintaining neuron health
& function.
3. Oligodnedrocytes: form the myelin sheath in the CNS.
4. Schwann cells: form the myelin sheath in the PNS; leaving
gaps called nodes of Ranvier.
5. Ependymal cells: line the ventricles of the brain,
they produce the CSF.
6. Satellite cells: surround neuron cell bodies in ganglia
participate in responses to injury
& inflammation.

9. Types of Neurons (Fig.37.4 page 695)
Neurons are classified according to their shape into;
1.Unipolar neurons
2.Multipolar neurons
Neurons are also classified according to their function into:
1.Motor or efferent neurons
2.Sensory or afferent neurons
3.Interneurons or association neurons

10. Neuron Anatomy

11. Composition of Nerves

12. Transmission of the Nerve Impulses:
The nerve impulse is an electrochemical phenomenon involving the movement of unequally distributed ions on either side of an axonal membrane (the plasma membrane of the axon).
Voltage is a measure of the electrical potential difference between 2 points.
An electrical potential difference across a membrane is called the membrane potential.

13. Types of Potentials in a neuron:
1.Resting Potential: (Fig "a" page 696)
Is when the axon is not conducting an impulse.
The voltmeter records a membrane potential equal to about –70 mV, indicating that the inside of the neuron is more negative than the outside.
The existence of this polarity can be correlated with a difference in ion distribution on either side of the axonal membrane.
The resting potential is due to 3 factors:
1. The action of the sodium-potassium pump.
2. The membrane is more permeable to K+ than to Na+.
3. There are also large, negatively charged proteins in the
cytoplasm of the axon.

14. 2. Action Potential:
(Fig "b" page 696, "c“, “d” & “e" page 697)
Involves rapid changes in polarity across a portion of an axonal membrane as the nerve impulse occurs.
Uses 2 types of gated ion channel in the axonal membrane:
1. A gated ion channel allows Na+ to pass through the
membrane.
2. A gated ion channel allows K+ to pass though the
Is generated only after the occurrence of a threshold value.

15. Threshold is the minimum change in polarity across the axonal membrane that is required to generate an action potential.
The action potential is an all-or-none event.
The action potential swing up from –70 mV to +35 mV.
Repolarization occurs as the action potential swings down from +35 mV to –70 mV.
An action potential only takes 2 milliseconds.

16. Resting and Action Potential of the Axonal Membrane

19. Propagation of Action Potential:
In nonmyelinated axon, the action potential travels down an axon one small section at a time, at a speed of about 1m/second.
In myelinated axon, the gated ion channels that produce an action potential are concentrated at the nodes of Ranvier. This mode of conduction, called saltatory conduction.
Speeds of 200m/second (about 450 miles/hr) have been recorded.

20. As soon as an action potential has moved on, the previous section undergoes a refractory period, during which the Na+ gates are unable to open.
Therefore, the action potential is a one-way direction of impulse.
The intensity of a signal traveling down a nerve fiber is determined by how many nerve impulses are generated within a given time span.

21. Saltatory Conduction

22. Transmission across a synapse:
(Fig.37.6 page 698)
Every axon end is tipped by a small swelling called an
axon terminal.
The region between axon terminal and the dendrite (or the cell body) of another neuron is called a synapse.
At a synapse, the membrane of the first neuron is called the presynaptic membrane, and the membrane of the next neuron is called postsynaptic membrane.
The small gap between the neurons is the synaptic cleft.

23. Transmission across a synapse is carried out by molecules called neurotransmitters (NT), which are stored in synaptic vesicles.
NT molecules bind with specific receptor proteins located on the postsynaptic membrane.
Depending on the type of NT &/or the type of receptor, the response of the postsynaptic neuron can be toward excitation or toward inhibition.

24. Synapse Structure and Function

25. Neurotransmitters
More than 100 substances known or suspected to be NTs in both CNS and PNS.
Many of these can have opposing effect on different tissues.
Norepinephrine, dopamine, and serotonin, are NTs present in both CNS and PNS.
The effect of Acetylcholine on muscles varies.
In the CNS: Norepinephrine is important to dreaming, waking,
and mood.
Dopamine is involved in emotions, learning, and
attention.
Serotonin is involved in thermoregulation,
sleeping, emotions, and perception.

26. Endorphins are NTs that bind to natural opioid receptors in the brain
Endorphins are NTs that bind to natural opioid receptors in the brain. They are associated with the “runner’s high” of exercisers because they also produce a feeling of tranquility. Endorphins are produced by the brain not only when there is physical stress but also when emotional stress is present.
What will happen to an NT after it has been released into a synaptic cleft and has initiated a response?
The short existence of an NT at a synapse prevents continuous stimulation or inhibition of postsynaptic membranes.

27. Many drugs that affect the nervous system act by interfering with or potentiating the action of NTs.
Drugs can:
1. Enhance or block the release of an NT,
2. Mimic the action of an NT or,
3. Block the receptor, or
4. Interfere with the removal of an NT from a synaptic cleft.
Depression, a common mood disorder, appears to involve imbalances in norepinephrine and serotonin.
Some antidepressant drugs, such as fluoxetine (Prozac®), prevent the reuptake of serotonin, and others, including bupropion hydrochloride (Wellbutrin®), prevent the reuptake of both serotonin and norepinephrine. Blocking reuptake prolongs the effects of these 2 NTs in networks of neurons within the brain that are involved in the emotional state.

28. Synaptic Integration:(Fig.37.7 page 699)
A single neuron has many dendrites plus the cell body, and both can have synapses with many other neurons.
,000 synapses/single neuron is not uncommon.
A neuron is on the receiving end of many excitatory and inhibitory signals.
Neurons integrate the incoming signals, and do so specifically at the area of the neuron cell body where the axon emerges, called axon hillock.
Integration is the summing up of excitatory and inhibitory signals.

29. Synapse Integration

30. The Central Nervous System (CNS)
CNS consists of the spinal cord and the brain.
It has 3 specific functions:
1. Receiving sensory input
2. Performing integration
3. Generating motor output
CNS is protected by:
1.Bone
2.Meninges
3.Cerebrospinal fluid

31. The Spinal Cord:(Fig.37.12 b & c. page 707)
It is a bundle of nervous tissue enclosed in the vertebral column.
It extends from the base of the brain to the vertebrae just below the rib cage.
Divided into the following according to regions of the body:
1. Cervical nerves.
2. Thoracic nerves.
3. Lumbar nerves.
4. Sacral nerves.
Functions: 1. It is a center for many reflex actions.
2. It provides a means of communication between
the brain and the spinal nerves.

32. The spinal cord is composed of a central portion of gray matter and a peripheral region of white matter.
Gray matter is shaped like a butterfly, or the letter H,
with 2 dorsal (posterior) horns
and 2 ventral (anterior) horns
surrounding a central canal.
The gray matter contains portions of sensory neurons and motor neurons as well as short interneurons that connect these 2 types of neurons.

33. Myelinated long fibers of interneurons that run together in bundles called tracts give white matter its color.
These tracts connect the spinal cord to the brain.
These tracts are like a busy superhighway, by which information continuously passes between the brain and the rest of the body.
In the dorsal part of the cord the tracts are primarily ascending, taking information to the brain.
Ventrally, there are primarily descending tracts carrying information from the brain.

35. Because the tracts at one point cross over, the left side of the brain controls the right side of the body, and the right side of the brain controls the left side of the body.
There are about 100,000 miles of myelinated nerve fibers in the adult human brain.
If the spinal cord is severed as the result of an injury, paralysis will result, which can be:
1.Quadriplegia
2.Paraplegia
In amyotrophic lateral sclerosis (ALS) or Lou Gehrig’s disease, motor neurons in the brain and spinal cord degenerate and die, leaving patients weakened, then paralyzed, then unable to breathe properly.

37. The Brain:(Fig.37.8 page 702)
Nerve impulses are the same in all neurons, so how is it that stimulation or our eyes causes us to see, and stimulation of our ears causes us to hear?
Essentially, the CNS carries out the function of integrating incoming data.
The brain allows us to perceive our environment, reason, and remember.
The exact processes by which the brain generates these higher functions remain largely mysterious.

38. The brain contains 4 cavities called ventricles which are:
1. The 2 largest are the lateral ventricles in the cerebrum.
2. The 3rd ventricle is surrounded by the diencephalon.
3. The 4th ventricle lies between the cerebellum and pons.
The ventricles are concerned with the production and circulation of Cerebrospinal fluid.
The brain is composed of:
The cerebrum.
The Hypothalamus.
The Thalamus.
The Pineal gland.
The Cerebellum.
The Brain Stem.

39. The Human Brain

40. The Human Brain

41. I. The cerebrum (Fig.37.8 page 702)
Is the largest, outermost portion of the brain in humans.
Is the last center to receive sensory input and carry out integration before commanding voluntary motor responses.
It communicates with and coordinates the activities of the other parts of the brain.
It is divided by the longitudinal fissure into 2 halves called left and right cerebral hemispheres.

42. Cerebral Hemispheres: (Fig.37.9 page 703)
Each hemisphere receives information from, and controls, the opposite side of the body.
The right hemisphere is associated with artistic and musical ability, emotion, spatial relationships, and pattern recognition.
The left hemisphere is more adept at mathematics, language, and analytical reasoning.
A bridge of tracts called corpus callosum connects the 2 hemispheres

43. Shallow grooves called sulci divide each hemisphere into paired lobes:
1.The frontal lobes
2.The parietal lobes
3.The temporal lobes
4.The occipital lobes

44. The Lobes of a Cerebral Hemisphere

45. The cerebral cortex:
Is a thin but highly convoluted outer layer of gray matter that covers the cerebral hemispheres.
Contains tens of billions neurons and is the region of the brain that accounts for sensation, voluntary movement, and all the thought processes required for learning, memory, language, and speech.
The cortex contains:
1. Motor areas
2. Sensory areas
3. Association areas

46. Basal nuclei:
Although the bulk of the cerebrum is composed of white matter (i.e. tracts), masses of gray matter are located deep within the white matter. These so-called basal nuclei (basal ganglia) integrate motor commands, ensuring that proper muscle groups are activated or inhibited.
Parkinson disease (PD), is a brain disorder characterized by tremors, speech difficulties, and difficulty standing and walking. PD results from a loss of cells in the basal nuclei that normally produce the NT dopamine.

47. II.Hypothalamus
Forms the floor of the third ventricle.
It is an integrating center that helps maintain homeostasis.
It controls the pituitary gland and serves as a link between
the nervous and endocrine systems.
III.Thalamus
Consists of 2 masses of gray matter.
Located in the sides and roof of the 3rd ventricle.
It receives all sensory input except smell.
It integrates this information and sends it on to the appropriate
portion of the cerebrum.

48. The thalamus is involved in arousal of the cerebrum, and it also participates in higher mental functions such as memory and emotions.
III.The pineal gland
IV.The cerebellum
Is the largest part of the hindbrain.
Maintains posture and balance.
Ensures that all of the muscles work together to produce smooth, coordinated voluntary movements, such as playing the piano or hitting a baseball.

49. The Human Brain

50. V. The Brain Stem contains:
a.The midbrain acts as a relay station for tracts passing
between the cerebrum and the spinal cord or
cerebellum.
The tracts cross in the brain stem so that the
right side of the body is controlled by the left
portion of the brain, and the left portion of the
body is controlled by the right portion of the
brain.

51. b.The pons contains bundle of axons that form a "bridge,”
traveling between the cerebellum and the rest of
the CNS.
It also works with the medulla oblongata to regulate
many basic body functions.
c.The medulla oblongata lies just superior to the spinal cord,
and it contains tracts that ascend or
descend between the spinal cord
and higher brain centers. It regulates
heartbeat, breathing, swallowing,
and blood pressure. It also contains
reflex centers for vomiting, coughing,
sneezing, and hiccupping.

52. The most common neurological disease of young adults is multiple sclerosis (MS).
It typically affects myelinated nerves in the cerebellum, brain stem, basal ganglia, and optic nerve.
MS is considered an autoimmune disease, in which the patient’s own W.B.Cs attack the myelin, oligodendrocytes, and eventually, neurons in the CNS.
Common symptoms are: Fatigue,
Vision problems,
Weakness,
Numbness, and
Tingling.

53. The Reticular Activating System.(Fig.37.10 page 705)
The reticular activating system (RAS) contains the reticular formation, a complex network of nuclei (masses of gray matter) and nerve fibers that extend the length of the brain stem. The reticular formation receives sensory signals that it sends to higher centers, and motor signals that it sends to the spinal cord.
The RAS arouses the cerebrum via the thalamus and causes the person to be alert.
A severe injury to the RAS can cause a person to be comatose, from which recovery may be impossible.

54. The Reticular Activating System

55. The Limbic system (Fig.37.11 page 705)
Is a complex group of brain structures that lie just under the cortex, near the thalamus.
It includes the hypothalamus, hippocampus, amygdala, olfactory bulb, and other nearby structures.
It blends higher mental functions and primitive emotions into a united whole.
Within the limbic system the hippocampus and amygdala are essential for learning and memory.
The hippocampus makes prefrontal area aware of past experiences stored in sensory association areas.
The amygdala can cause these experiences to have emotional overtones.

56. The Limbic System

57. Learning and Memory:
Memory is the ability to hold a thought in mind or recall events from the past.
Learning takes place when we retain and use memories.
Types of memory:
1.Short-term memory: the prefrontal area in the frontal lobe
is active during this type of memory.
2.Long-term memory: the hippocampus gathers this type of
memories and are stored as bits and
pieces throughout the sensory
association areas, and makes them
available to the frontal lobe.

58. Long-term memory is a mixture of :
a.Semantic memory
b.Episodic memory
c.Skill memoryI
Several diseases of the brain can affect memory.
Alzheimer disease is the most common cause of dementia, or a loss of reasoning, memory, and other higher brain functions, especially in people over age 65.

59. Peripheral Nervous System(PNS)
PNS lies outside the CNS and contains nerves (bundles of axons).
The cell bodies of neurons are found in the CNS and in ganglia.
Ganglia are collections of cell bodies outside the CNS.
Composed of:
1. 12 pairs of cranial nerves attached to the brain.
Some of these are sensory nerves, others are motor, and
still others are mixed nerves.
They are largely concerned with the head, neck, and facial
regions of the body.
The vagus nerve has branches not only to the pharynx and
larynx, but also to most of the internal organs.

61. 2. 31 pair of spinal nerves.
The paired spinal nerves emerge from the spinal cord via 2
short branches, or roots:
a. The dorsal root contains the axons of sensory neurons,
which conduct impulses to the spinal cord from sensory
receptors.
Cell body of a sensory neuron is in the dorsal root ganglion.
b. The ventral root contains the axons of motor neurons,
which conduct impulses away from the spinal cord to
effectors.
These 2 roots join to form a spinal nerve.
All spinal nerves are mixed nerves that contain many sensory and motor fibers.

63. Cranial and Spinal Nerves

64. The PNS has 2 divisions: I. Somatic system
The nerves serve the skin, joints, and skeletal muscles.
The active NT is acetylcholine.
Involuntary responses to stimuli, called reflexes.

65. The Reflex Arc (Fig.37.13 page 708)
Enable the body to react swiftly to stimuli that could disrupt homeostasis.
Involuntary reflexes allow us to respond rapidly to external stimuli.
Might involve only the spinal cord, or the spinal cord and the brain.
Examples: Flying objects cause eyes to blink.
Sharp pins cause hands to jerk away, even
without having to think about it.

66. A Reflex Arc Showing the Path of a Spinal Reflex

67. II.Autonomic System (Fig.37.14 page 710 & Table 37.1 page 709)
Regulates the activity of cardiac and smooth muscle and glands.
Is divided into:
1.Sympathetic division.
2.Parasympathetic division.

68. Neurons and Ganglia

69. Both of these divisions:
1.Function automatically and usually in an involuntary manner.
2.Innervate all internal organs.
3.Use 2 neurons and one ganglion for each impulse:
a.The 1st neuron has a cell body within the CNS and a
preganglionic fiber.
b.The 2nd neuron has a cell body within the ganglion and a
postganglionic fiber.
Reflex actions, such as those that regulate the blood pressure
and breathing rate, are especially important to the maintenance of homeostasis.

70. Autonomic System Structure and Function