Download Chapter 21 - The Nervous System: Organization

Chapter 14 - The Nervous System: Organization
Vertebrates
Vertebrates have complex sense organs and exhibit complex behaviors.
These require a complex nervous system.
Vertebrates are segmented and this segmented arrangement can be seen in
their nervous systems.
Divisions of the Vertebrate Nervous System
The central nervous system (CNS) is the brain and spinal cord.
The peripheral nervous system (PNS) is the nerves and ganglia. (Ganglia
are clusters of nerve cell bodies outside the CNS.)
Peripheral Nervous System
Nerves
Nerves are bundles of neurons; either long dendrites and/or long axons.
There are no cell bodies in nerves. The cell bodies are in the ganglia
(PNS) or nuclei (in gray matter of the CNS).
Most nerves contain both kinds of neurons (sensory and motor). The
sensory neurons conduct information to the CNS, the motor neurons
conduct away from the CNS.
All of the neurons in some nerves conduct in the same direction. These
nerves contain either sensory or motor neurons.
Cranial Nerves and Spinal Nerves
Humans have 12 pairs of cranial nerves and 31 pairs of spinal nerves.
Cranial nerves are sensory, motor, or mixed, and all but the vagus are
involved with the head and neck region; the vagus nerve manages the
internal organs.
Spinal nerves are all mixed nerves. Their regular arrangement reflects the
segmentation of the human body.
Spinal nerves are connected to the spinal cord by two branches called
roots.
The dorsal root contains sensory neurons.
The dorsal root ganglion contains the cell
bodies of sensory neurons. Sensory neurons
therefore have long dendrites.
The ventral root contains motor neurons.
Motor neurons have short dendrites and long
axons.
Somatic Nervous System
The somatic nervous system provides conscious, voluntary control.
It includes all of the nerves that serve the skeletal muscles and the exterior
sense organs.
Reflex arcs
Reflexes are simple, stereotyped and repeatable motor actions (example:
movements) brought about by a specific sensory stimulus. The reflex is
involuntary but may involve the use of voluntary (skeletal) muscle and
nerves.
Reflexes are quick and produce behaviors that are typically beneficial. For
example, when you fall, reflex arcs immediately act to extend your arm so
that your arm prevents your head and body from hitting the ground.
Some reflexes involve the brain, others do not.
A whole series of responses may occur since some sensory neurons
stimulate several interneurons which, in turn send impulses to other parts
of the CNS. If you were to fall forward, interneurons would use
information from the ears to determine the direction of the fall and extend
the arms in a forward direction. If you were to fall toward the left side,
interneurons would select neurons that activate muscles to extend your
arm to the left side.
Example: The stretch reflex
The stretch reflex is involved in helping the body maintain its position
without having to consciously think about it.
When a muscle is stretched, stretch-sensitive receptors are stimulated. An
action potential is conducted to the spinal cord. The axon terminals
synapse with motor neurons leading right back to the muscles. This causes
the muscle to contract to its original position.
Autonomic Nervous System
This part of the nervous system sends signals to the heart, smooth muscle,
glands, and all internal organs.
It is generally without conscious control.
The autonomic nervous system uses two or more motor neurons:
The cell body of one of the motor neurons is
in the CNS. The cell body of the other one is
in a ganglion.
Sympathetic
The sympathetic nervous system prepares the body to deal with
emergency situations. This is often called the "fight or flight" response.
Stimulation from sympathetic nerves dilates the pupils, accelerates the
heartbeat, increases the breathing rate, and inhibits the digestive tract.
The neurotransmitter is norepinephrine (similar to epinephrine
[adrenaline], the heart stimulant).
Sympathetic nerves arise from the middle (thoracic-lumbar) portion of the
spinal cord.
Parasympathetic
When there is little stress, the parasympathetic system tends to slow down
the overall activity of the body.
It causes the pupils to contract, it promotes digestion, and it slows the rate
of heartbeat.
The neurotransmitter is acetylcholine.
The actual rate of stimulus to each organ is determined by the sum of
opposing signals from the sympathetic and parasympathetic systems.
Parasympathetic nerves arise from the brain and sacral (near the legs)
portion of the cord.
Evolution of Vertebrate Central Nervous Systems
The central nervous system evolved by adding on to what was there. The
oldest parts of the human nervous system deal with reflexes. Newer layers
are associated with memory, learning, and thinking.
Central Nervous System
The central nervous system is the brain and spinal cord.
It is wrapped in 3 layers of membranes called meninges. Meningitis is an
infection of these coverings.
The brain contains fluid-filled ventricles that are continuous with the
central canal of the cord.
Divisions of the Brain
Generally, many body functions involve cells in several areas of the brain.
However, certain areas of the brain tend to be more important in some
functions while other areas dominate the control of other functions.
Some major parts of the brain are listed below.
Hindbrain: medulla oblongata, cerebellum,
pons
Midbrain
Forebrain: thalamus, hypothalamus,
cerebrum
Hindbrain
Medulla oblongata
The medulla controls vital functions such as breathing, heart rate, and
blood pressure.
It also contains reflexes such as vomiting, coughing, sneezing, hiccupping,
swallowing, and digestion.
Information that passes between the spinal cord and the rest of the brain
must pass through the medulla. In the medulla, sensory and motor axons
on the right side cross to the left side and axons on the left side cross to the
right side. As a result, stimuli passing through from the left side of the
body are sent to the right side of the brain and signals passing through
from the right side of the brain stimulate the left side of the body.
Cerebellum
The cerebellum coordinates and refines complex muscle movements.
Movement information that is initiated in higher brain centers (the cerebral
cortex) is compared to the actual position of the limbs. The cerebellum
then adjusts and refines the movement.
It is large in birds because flight requires considerable coordination.
Pons
The pons is involved in some of the same activities as the medulla. For
example, it assists the medulla in controlling breathing.
The pons functions as a connection between higher brain regions, the
cerebellum, and the spinal cord.
Midbrain
The midbrain receives some sensory information and sends it to the
appropriate part of the forebrain.
The midbrain originally functioned for reflexes associated with visual
input. It is the most prominent part of the brain in fishes and amphibians
and has major control of the body.
The midbrain of reptiles, birds, and mammals controls visual reflexes such
as the pupil response to light intensity but the forebrain of these
vertebrates processes the visual information (see diagram below).
The midbrain also controls some auditory reflexes and helps control
posture.
Brainstem
The medulla oblongata, pons, and midbrain look like the spinal cord and
appear to connect the rest of the brain to the spinal cord. They are
collectively referred to as the brainstem.
Forebrain
Thalamus
Like the midbrain of mammals, the thalamus serves as a relay area to the
cerebrum from other parts of the spinal cord and brain. For example, it
receives sensory input (except smell) and sends to appropriate areas of the
cerebral cortex.
The Thalamus contains part of the reticular formation (see below).
Reticular Formation
The reticular formation is a net of nerve cells extending from the thalamus
through the brain stem (midbrain, pons and medulla oblongata) to the
spinal cord.
It acts as a filter to incoming stimuli and discriminates important from
unimportant. Hundreds of millions of sensory receptors flood the brain;
the brain does not have the capacity to deal with even a small fraction of
this information, so much of it must be ignored.
Examples:
You may be unaware of conversation in a
crowded situation but the system alerts you
when you hear your name.
You can sleep in the presence of some kinds
of sounds but others will wake you.
The reticular activating system (RAS) is the part of the reticular
formation that maintains wakefulness.
Sleep centers are located in the reticular formation. Neurons in one sleep
center secrete serotonin, a chemical that inhibits the RAS and thus causes
drowsiness and sleep.
Another sleep center secretes factors that counteract serotonin and bring
about wakefulness.
Damage to these centers can lead to unconsciousness or coma.
Hypothalamus
The hypothalamus regulates the endocrine system by controlling the
secretions of the pituitary gland or by producing some of the hormones
that are secreted by the pituitary. These hormones affect the body or affect
other glands in the body. Their overall affect is to maintain homeostasis.
The hypothalamus also contains neurons associated with the limbic system
(below).
Limbic System
The limbic system contains neural pathways that connect portions of the
cortex, thalamus, hypothalamus, and basal nuclei (several areas deep
within the cerebrum).
It causes pleasant or unpleasant feelings about experiences (rage, pain,
pleasure, sorrow). This guides the individual into behavior that is likely to
increase survival.
Cerebrum
The cerebrum became greatly enlarged as evolution progressed from fish
to mammals. In reptiles, birds, and mammals, it receives sensory
information and coordinates motor responses.
Motor responses to the skeletal muscles originate in the cerebrum but are
refined and coordinated by the cerebellum.
In humans, the cerebrum is the largest part of the brain. Characteristics
such as thinking, intelligence, and emotion are controlled here.
Olfactory Bulbs- The anterior parts of the cerebral hemispheres are
called the olfactory bulbs. It receives input from the olfactory nerves
(smell). The olfactory bulbs of primitive vertebrates comprise a large
proportion of the cerebrum.
Cerebral Cortex- Over evolutionary time, gray matter developed over
the cerebrum. This is the cerebral cortex and it is an informationprocessing center. It increased in size more rapidly than the skull so that it
has become folded (convoluted) in order to fit in the skull.
The human cerebral cortex is thin (1.5-4 mm thick) and is highly folded to
increase its surface area.
Intelligence, emotion, creativity, learning, and memory are localized in the
cerebral cortex.
Lobes of the cerebral cortex
The cerebral cortex is divided into four lobes, each receives information
from particular senses and processes the information into higher levels of
consciousness.
Lobe
Frontal
Function
motor functions; permits conscious control of skeletal muscles;
contains the primary motor cortex
conscious thought
Parietal sensory areas from the skin; contains the primary sensory cortex
Occipital The primary visual cortex is located within the occipital lobe.
Temporal hearing and smell
Primary Sensory and Primary Motor Cortex- The primary sensory
cortex is a narrow band of cortex tissue that extends from one side of the
cortex near the ear over the top of the brain to the other side. Information
from sensory receptors in the skin arrive at this area. The motor cortex is a
band of cortex tissue directly anterior (in front) of the primary sensory
cortex. Signals that control the skeletal muscles originate in this area.
Corpus Callosum
The corpus callosum contains neurons that cross from one side of the brain
to the other, allowing each half to communicate with each other.
The corpus callosum of people with severe epilepsy is sometimes cut to
reduce the frequency and intensity of seizures. Researchers presented
some of these people with words such as cowboy. When viewing this
word, the first three letters (cow) are viewed in the left visual field of each
eye and is projected onto the right half of the brain. These people could
write the word cow with their left hand because their right brain controls
the left side of the body and it is aware of the word cow but not boy. Their
other hand could only write the word ''boy". Moreover, they could only
say "boy" because language is controlled by the left hemisphere. Although
they could see themselves write cow, they could only say boy.
Summary of Brain Structure
Brain
Structure
Function
Vital functions such as breathing, heart rate, and blood pressure
Medulla
oblongata
Reflexes such as vomiting, coughing, sneezing, hiccupping,
swallowing, and digestion
Neurons cross
Pons
Breathing, connects spinal cord, cerebellum and higher brain centers
Cerebellum
Motor coordination
Receives visual, auditory, and tactile information
Midbrain
In mammals, this information is sent to the thalamus and higher brain
centers. In lower vertebrates, the information is further processed in
the midbrain.
Relays sensory information to the cerebral cortex.
Thalamus
Contains part of the reticular formation (controls arousal).
Maintains homeostasis, regulates the endocrine system
Hypothalamus
Contains part of the Limbic system (controls emotion)
Cerebrum
Processes sensory information and produces signals that move the
skeletal muscles.
This is the outer layer of the cerebrum.
Cerebral Cortex Thinking, intelligence, and cognitive functions are located here.
Processing of sensory information and motor responses
Memory
The limbic system is involved in memory formation.
The hippocampus, a structure that is deep in the cerebrum and a part of
limbic system, is necessary to form new memories. People with a
damaged hippocampus cannot remember things since the time the damage
occurred but can remember from before.
Short-term memory is probably stored as electrical differences because
they can be removed by the application of an electrical shock.
Long-term memory is probably stored as new or different synapses.
Research on snails shows that learning is associated with an increased
number of synapses. Forgetting is associated with a decreased number.
Disuse can cause a synapse to wither and sever the connection between
two neurons. Intensively stimulated synapses form stronger connections,
grow, or sprout buds to form more connections.
Memory appears to be stored in sensory areas of the cerebrum.
The Spinal Cord
The vertebrae surround and protect the spinal cord.
Cerebrospinal fluid within the central canal functions to cushion the spinal
cord.
Many sensory - motor reflex connections are in the spinal cord.
Interneurons often lie between sensory and motor neurons.
White matter
White matter contains tracts that connect the brain and the spinal cord.
The white color is due to the myelin sheaths.
Gray matter
Gray matter looks gray because it is unmyelinated. It contains the short
interneurons that connect many sensory and motor neurons. Sensory
neurons enter the gray matter and the axons of motor neurons leave it.
The cell bodies of these motor neurons are located in the gray matter.