Biology 2401 Anatomy and Physiology I notes - Nervous System Exam 5 Ch 9
Anatomy of the nervous system
the central NS
- composed of brain and spinal cord
- enclosed in bony cavity - brain in cranium, spinal cord in vertebral canal.
- enclosed in 3 connective tissue membranes called the meninges
Fig 9.22
- dura mater is tough fibrous outer layer,
- arachnoid is middle layer composed of thin cells with delicate web of
collagen and elastic fibers, suspend brain and spinal cord;
subarachnoid space is filled with cerebrospinal fluid,
(the cerebrospinal fluid absorbs vibrations, shock absorber)
- pia mater is a thin inner layer that is anchored firmly to nervous tissue.
- blood-brain barrier is formed by extensions of the astrocytes - regulates what
substances leave the blood vessels into the brain.
- circumventricular organ - areas in hypothalamus where blood-brain
barrier is lacking, allows brain to “sample” blood for certain substances
*Describe three ways that the brain is protected.
*List the connective tissues enclosing the brain and spinal cord. Which acts as a shock
absorber?
*What is the importance of the circumventricular organ?
Brain - 98% of all nervous tissue in body; 100 billion neurons with 100+ trillion synapses and
900 billion glial cells
- brain is 2 % of body mass but uses 20% of energy in body
- in CNS groups of neuron cell bodies (neuronal pools) called centers, or nuclei;
surface layer of neuron cell bodies called cortex; in PNS called ganglia;
gray matter, no myelin
- in CNS bundles of axons called tracts, in PNS called nerves; white matter due
to myelin; no synapses
- brain and spinal cord form hollow tube; 4 wider areas of passageway in brain
called ventricles which are connected and continuous with central canal of
spinal cord
9.30 – 9.31
- choroid plexus in each ventricle; capillary beds that produce cerebrospinal fluid
which circulates through hollow passageway and arachnoid; resorbed into
blood in veins in dura mater of brain
- cerebrospinal fluid floats brain and spinal cord and absorbs vibrations; transports
nutrients and waste products
Brain composed of 4 major divisions:
cerebrum
Fig 9.27 – 9.29
- left and right hemisphere, separated by longitudinal fissure; control opposite side
of body
- each hemisphere divided into distinct lobes: frontal, temporal, parietal, occipital
and insula
- corpus callosum is band of white matter that connects left and right hemispheres
- cerebrum is covered by cerebral cortex (75% of all neurons in body) and contains
deeper centers of gray matter (basal nuclei).
White matter is myelinated axons connecting cortex and centers and other brain
regions
- cerebral cortex folded into ridges (gyri) and grooves (sulci) - increases surface
area
- 2 lateral ventricles, one in each hemisphere
- cortex areas specialize for different functions
sensory areas receive stimuli from receptors, producing sensations (ex. cutaneous,
taste, smell, vision)
association areas analyze and interpret sensory information - perform
higher functions (ex. memory, reasoning, judgment, emotions, speech)
motor areas initiate skeletal muscle movements
- higher neural functions occur in the cerebrum: learning, reasoning, language,
memory, emotions
*What are the two major structures that form the cerebrum?
*What are the three primary functions of the cerebrum?
*What are the two cavities in the cerebrum called?
*What is the term for the surface gray matter of the cerebrum?
*What is the term for the deeper gray matter areas?
Diencephalon
9.32
- contains the third ventricle
- is an important link between brain stem and higher brain, beginning of
consciousness
- thalamus forms upper 2/3, receives and relays most sensory input, maintains
cerebral arousal; beginning of consciousness, general awareness of sensations.
- hypothalamus forms lower 1/3 - primary area for maintaining homeostasis
control center for autonomic nervous system, coordinates sympathetic and
parasympathetic nervous systems
centers for emotions and primary drives such as rage and pleasure, thirst,
hunger, sex
regulates body temperature; fluid and electrolyte balance
regulates endocrine system, linking endocrine and nervous systems
- limbic system is series of nuclei, cortical areas and tracts of cerebrum and
diencephalon - “emotional brain” - involved in determining emotional
state (fear, anger, pleasure, sorrow) and appropriate behavior
brain stem composed of midbrain, pons and medulla oblongata
- connects spinal cord and diencephalon
- composed of several nuclei and many tracts
midbrain
- superior brain stem, composed of tracts connecting higher and lower brain
- surrounds cerebral aqueduct that connects third and fourth ventricles
- centers control eye movements reflex, auditory reflex
pons
- composed of tracts that connect brain stem, cerebellum, spinal cord, cerebrum
- contains control centers for several functions: chewing, saliva secretion, some
respiration control
medulla oblongata
- continuous with spinal cord; contains fourth ventricle
- composed of tracts that connect brain with spinal cord
- contains several control centers that regulate vital functions: cardiac centers,
vasomotor, respiratory centers, swallowing, coughing, sneezing, vomiting
reticular formation is network of centers and tracts throughout brain stem, reticular activating
system - filters incoming stimuli and stimulates cerebrum to alert awareness
cerebellum
- composed of white matter tracts (arbor vitae) and gray matter cortex, connected
to brain stem via tracts
- reflex center for integrating body position and muscle movements, rapidly adjusts
muscle movements to maintain position and balance (equilibrium)
- coordinates and fine-tunes muscle movements (does not initiate movements)
Spinal cord
Fig 9.22 - 9.25
- hollow tube (central canal) with a shallow posterior medial sulcus and deeper
anterior median fissure
- extends from the brain to the lumbar region (Li-L2)
- 31 pairs of spinal nerves branch along its length, emerge through intervertebral
foramen between vertebra
- each spinal nerve has 2 roots
- a dorsal root that contains the axons of sensory nerves and a dorsal root ganglion
that contains the cell bodies of the sensory neurons.
- a ventral root that contains the axons of motor neurons (the cell bodies of
2
the motor neurons are in the spinal cord)
- the dorsal and ventral root merge to form the spinal nerve before it exits the
intervertebral canal
- the spinal cord ends at lumbar vertebra 2, but spinal nerves continue through the
vertebral canal through the sacrum; cauda equina is term for this extension.
- the spinal cord is composed of gray matter (cell bodies and dendrites of neurons)
and white matter (myelin covered axons).
- gray matter composes the central “butterfly-shaped” areas (posterior, lateral and
anterior gray horns and gray commissure)
- reflex arc occur here, spinal reflexes
- white matter surrounds the gray matter and is made of ascending and descending
tracts (posterior, lateral and anterior white columns)
- ascending tract carry sensory impulses to brain
- descending tracts carry impulses from brain to muscles and glands
- cervical and lumbar enlargements are areas that are slightly thicker because
more cells bodies are located here to supply nerves for the limbs.
*What parts of the neurons form gray matter? What part forms white matter?
*What type neurons are in the dorsal roots of the spinal nerves? ventral roots?
*What causes the cervical and lumbar enlargements?
peripheral nervous system
Fig.9.17
- composed of sensory receptors, sensory nerves and motor nerves
- nerves are groups of neuron axons wrapped in dense fibrous connective tissue
- endoneurium enclosing each neuron (fiber),
- perineurium enclosing groups of neurons called a fascicle
-epineurium enclosing a nerve
- each neuron is either a sensory afferent neuron that carries action potential
towards cns or motor efferent neuron that carries action potential away
from cns (only one or the other, not both)
- nerves can be sensory (containing only sensory neurons), motor (containing only
motor neurons) or mixed (containing sensory and motor neurons)
*What is the difference between a mixed nerve and a sensory nerve?
*What is the difference between a neuron and a nerve?
*Describe the fibrous connective tissue wrappings in a nerve.
Nerve pathways
Figs 9.17 and 9.18
- reflexes are simplest pathways, simplest integration; fast, predictable (particular stimulus
causes particular response) and automatic
- can involve only spinal cord (spinal reflex) or only brain (cranial reflex)
- more complex integration can involve many neuronal pools
- relays action potential from sensory cells to brain and spinal cord (sensory), and
cranial nerves - emerge from the brain; there are 12 pairs (left and right); some are
sensory (contain only sensory neurons), some are motor (contain only motor
neurons) and some are mixed (contain both sensory and motor neurons)
spinal nerves - emerge from the spinal cord and pass through the intervertebral foramen
- there are 31 pairs, each passing between adjacent vertebra
- each spinal nerve has a dorsal root where sensory neurons enter the spinal cord
and a ventral root where motor neurons exit the spinal cord
- these merge together before the nerve exits the intervertebral foramen, all spinal
nerves are mixed (both sensory and motor)
- spinal nerves named for the vertebral region (cervical 1-8, thoracic 1-12, lumbar
1-5, sacral 1-5, coccygeal 1)
- sensory neuron cell bodies are in the dorsal root ganglia; motor neuron cell
bodies are in the anterior gray horn
- spinal nerves form plexuses, networks of nerves that are bundled together. These
are axons only so no synapses occur here. Nerves then leave plexus to
body region
*What structures compose the peripheral nervous system?
*What is the difference between cranial nerves and spinal nerves?
*What are plexuses? *Why can integration (summation) not occur in white matter?
Somatic and autonomic nervous systems
- the motor (efferent) portion of the peripheral ns is composed of 2 systems, the somatic and the
autonomic ns.
- the autonomic ns is divided into the sympathetic and the parasympathetic systems
- the somatic n.s. always has only one neuron between the brain or spinal cord and the
effector cell
- the effector cells are always skeletal muscle cells
- these neurons use only 1 neurotransmitter (acetylcholine) and it is always excitatory
- the somatic (soma means body) ns allows the body to respond to external environment
- the autonomic (means self-governing) ns always has 2 neurons between the brain or spinal cord
and the effector cell.
- the preganglionic neuron has its cell body in the brain or spinal nerve and its synaptic
terminal in a ganglion
- the postganglionic neuron has its cell body in the ganglion and its synaptic terminal at the
effector cell
- the effector cells are smooth muscle, cardiac muscle, glands
- preganglionic neurons release only 1 neurotransmitter (acetylcholine) and it is always
excitatory
- postganglionic neuron release 1 of two different neurotransmitters (acetylcholine or
norepinephrine) which may be excitatory or inhibitory
- the autonomic ns is composed of the parasympathetic and the sympathetic divisions
- in the sympathetic ns the ganglia are closer to the spinal cord and the postganglionic
neurons diverge to several effectors
- in the parasympathetic ns the ganglia are in, or very near, the specific organ where the
effector cells are located
- in the parasympathetic ns the postganglionic neuron releases acetylcholine which may have an
excitatory or inhibitory affect, depending on the receptor
- in the sympathetic ns the postganglionic neuron releases norepinephrine which may have an
excitatory (usually) or an inhibitory affect
- the parasympathetic and the sympathetic ns innervate many of the same visceral organs
(some exceptions: adrenal gland, sweat glands only sympathetic.)
- the sympathetic ns prepares the body for an emergency, the “fight or flight response”
- the parasympathetic ns slows the body into an energy producing and conserving response
- sympathetic neuron from spinal nerves T1-L2, parasympathetic neurons from cranial nerves
and S2-4
*What is the value of having dual innervation to many organs?
*List several effects of the sympathetic and parasympathetic ns on the body. Table 9.7
*Compare the number of neurons between the brain or spinal cord and the effector cells
in the somatic, sympathetic and parasympathetic ns.
*List the type effector cells for each of the three.