The Nervous System

Sensory (afferent) division

consists of nerves that convey impulses to the central nervous system from sensory receptors located located on various parts of the body. Keeps the CNS constantly informed of events going on both inside and outside the body. (“To go toward.”)

Motor (efferent) division

carries impulses from the CNS to effector organs, the muscles and glands. These impulses activate muscles and glands.

Peripheral nervous system (PNS)

the part of the nervous system outside the CNS. consists mainly of the nerves that extend from the brain to the spinal cord. These contain cranial nerves which carry impulses to and from the brain.

Somatic nervous system (SNS)

allows us to consciously, or voluntarily, control our skeletal muscles. AKA the voluntary nervous system.

Central nervous system (CNS)

consists of the brain and spinal cord, which occupy the dorsal body cavity and act as the integrating and command centers if the nervous system. This interprets incoming sensory information and issues instructions based on past experience and current conditions.

Automatic nervous system (ANS)

regulates events that are automatic, or involuntary, such as activity of smooth and cardiac muscles and glands. Split into two parts: sympathetic and parasympathetic. AKA involuntary nervous system.

Occipital lobe

the visual area located in the posterior part of the head; where vision is detected

Frontal lobe

contains the central sulcus; the primary motor area is anterior to this

Parietal lobe

where the primary somatic sensory area is located. this is posterior to the central sulcus.

Corticospinal tract

AKA pyramidal tract. The axons of the motor neurons form this. is the major neuronal pathway providing voluntary motor function. This tract connects the cortex to the spinal cord to enable movement of the distal extremities.

Brocha’s area

a specialized cortical area that is very involved in our ability to speak. Found at the base of the precentral gyrus. Damage to this area causes inability to say words properly (you know what you want to say but you can’t vocalize the words).

Temporal lobe

the auditory area of the brain, bordering the lateral sulcus. the olfactory (scent) area is also found deep within this

Posterior association area

encompasses part of the posterior cortex. this area plays a role in recognizing patterns and faces, and blending several different inputs into an understanding of the whole situation.

Interbain (diencephalon)

sits atop the brain stem and is enclosed by the cerebral hemispheres. the major structures of this are the thalamus, hypothalamus, and epithalamus.

Basal nuclei

“islands” of grey matter. they are buried deep within the white mater of the cerebral hemispheres. help regulate voluntary motor activities by modifying instructions sent to the skeletal muscles by the primary motor cortex. Internal capsule passes between these and the thalamus.

Midbrain

a relatively small part of the brain stem. extends from the mammillary bodies to the pons inferiorly. the cerebral aqueduct travels through here. made up of cerebral penduncles and the corpora quadrigemina.

Medulla oblongata

the most inferior part of the brain stem. it merges into the spinal cord below without any obvious change in structure. an important fiber tract area. contains many nuclei that regulate vital visceral activities. controls heart rate, blood pressure, breathing, swallowing, and vomiting.

Hypothalamus

makes up the floor of the diencephalon. it is an important autonomic nervous system center because it plays a role in the regulation of body temperature, water balance, and metabolism. also the center for many drives and emotions; an important part if the limbic system

Reticular formation

extends the entire length of the brain stem and is a diffuse mass of grey matter. the neurons of this are involved in motor control of the visceral organs. contains the reticular activating system which plays a role in the consciousness and the awake/sleep cycle.

Thalamus

encloses the third ventricle. a relay station for sensory impulses passing upward to the sensory cortex. as impulses go through here, we have a crude recognition of whether the sensation we are about to have is pleasant or unpleasant.

Pituitary gland

hangs from the anterior floor of the hypothalamus by a slender stalk.

Sympathetic nervous system

referred to as the “fight or flight” system. activated when we are excited or we find ourselves in a sense of emergency. pounding heart, rapid breathing, cold/sweaty skin, a prickly scalp, and dilated eye are all signs of this. also dilates the bronchioles, causes glucose to be released to blood, constricts blood vessels, increases heart rate and blood pressure.

Parasympathetic nervous system

most active when the body is at rest and not threatened in any way. AKA the “resting-and-digesting” system. chiefly concerned with promoting normal digestion, eliminating feces and urine, and conserving body energy.

Effectors

glands or muscles (the response in these is caused by a motor output)

Functional classification of nerve fibers

sensory (afferent) division and motor (efferent) division

Oligodendrocytes

glia that wrap their flat extensions tightly around nerve fibers, producing fatty insulating coverings called myelin sheaths

Schwann cells

form the myelin sheaths around nerve fibers that are found in the PNS

Dendrites

neuron processes that convey incoming messages toward the cell body. neurons may have hundreds of these branching off of them

Neurotransmitters

inside of axon terminals. tiny vesicles/membranous sacs that contain chemicals. when impulses reach the axon terminals, they stimulate the release of these into extracellular space.

Ganglia

small collections of cell bodies; found in a few cities outside the CNS in the PNS

White matter

consists of dense collections of myelinated fibers

Interneurons

the third category of neurons. AKA association nerves. they connect the motor and sensory neurons in neural pathways. their cell bodies are located in the CNS

Multipolar neurons

classified as neurons that have several processes extending from the cell body. all motor and association neurons are considered these. the most common structural type.

Bipolar neurons

neurons with two processes: an axon and a dendrite. rare in adults, found only in some special sense organs like the eyes and the nose where they act in sensory processing as receptor cells

Electrical conditions of a resting neuron’s membrane

always polarized, this means that there are fewer positive ion sitting on the inner face of the neuron’s plasma membrane than there are on its outer face. the major positive cells inside the cell are potassium and the major positive ions outside the cell are sodium. As long as the inside remains more negative than the outside, the neurons will stay inactive.

Functional classification of neurons

sensory, motor, and association neurons (interneurons)

Action potential (nerve impulse)

a long distance signal in neurons.

initiation: if a stimulus is strong enough and the sodium influx is high enough, the local depolarization (graded potential) activates the neuron to start the signal.

generation: it is an all of nothing response. it is either propagated over the entire axon or it doesn’t happen at all.

Impulse conduction

fibers that have myelin sheaths conduct impulses much faster. repolarization occurs, a neuron cannot conduct another impulse.

Saltatory conduction

faster type of electrical impulse propagation. when the nerve impulse literally leaps from node to node. this occurs because no electrical current can flow across the axon membrane where there is fatty myelin insulation.

Transmission of the signal at synapses

a neurotransmitter chemical crosses the synapse to transmit the signal from one neuron to the next.

Synaptic cleft

each axon terminal is separated from the next neuron by this.

Reflex arcs

reflexes occur in these pathways. they involve both CNS and PNS structures

Somatic reflexes

include all reflexes that stimulate the skeletal muscles. EX: when you quickly pull your hand away from a hot object

Autonomic reflexes

regulate the activity of smooth muscles, the heart, and glands. these reflexes regulate such body functions as digestion, elimination, blood pressure, and sweating. EX: secretion of saliva and changes in size of eye pupils

Gyri

the entire surface of the cerebral hemispheres exhibits these elevated ridges of tissue. these are separated by sulci

Sulci

small grooves that separate the gyri

Cerebral cortex

Functions of its neurons: speech, memory, logical and emotional response, as well as consciousness, interpretation of sensation, and voluntary movement

Contains: primary somatic sensory area (Impulses traveling from the body’s sensory receptors are localized and interpreted in this area of the brain; allows you to detect pain, coldness, etc.)

Brain stem

contains the midbrain, pons, and medulla oblonganta. provides a pathway for ascending and descending tracts. has many small gray matter areas. the gray matter produces the rigidly programmed autonomic behaviors necessary for survival.

Reticular activating system (RAS)

plays a role in consciousness and the awake/sleep cycle. also acts as a filter for the flood of sensory inputs that stream up the spinal cord and brain stem daily. damage to this area can cause permanent unconsciousness.

Cerebellum

has two hemisphere and a convoluted surface. has an outer cortex made up of gray matter and an inner region made up of white matter.

provides precise timing for skeletal muscles activity and controls our balance and equilibrium, bc of this body movements are smooth and coordinated

Meninges

the three connective tissue membranes covering and protecting the CNS structures. the outermost later is the dura mater, this forms the periosteum and meningeal layer which forms the outermost coverings of the brain and becomes the dura mater of the spinal cord.

Tentorium cerebelli

this, along with the falx cerebri, are folds that attach the brain to the cranial cavity and they separate the cerebellum from the cerebrum

Cerebrospinal fluid

watery “broth” similar to its makeup to blood plasma, from which it forms. continually formed from blood by the choroid plexuses. located in and around the brain and cord and forms a watery cushion that protects nervous tissue from trauma. in the brain, it is constantly moving and it circulates from two lateral ventricles into the third ventricle then into the fourth ventricle. most of it circulates into the subarachnoid space.

Blood-brain barrier

neurons are kept separated from bloodborne substances by this. composed of the least permeable capillaries in the whole body.

Stroke

AKA cerebrovascular accidents (CVAs). the their leading cause of deaths in the US. these occur when blood circulation to a brain area is blocked by a blood clot or a ruptured blood vessel and vital brain tissue dies. can cause brain damage like hemiolehgia and aphasias.

Gray matter of the spinal cord

looks like a butterfly. the two posterior projections are dorsal horns and the two anterior projections are the ventral horns. this surrounds the cetral canal of the cord which contains cerebrospinal fluid.

Dorsal root ganglion

dorsal roots are found in this enlarged area. if this is damaged, sensation from the body area will be lost. s nodule-like structures found on the posterior roots of each spinal nerve, which contain the soma (or cell bodies) of the afferent sensory nerves carrying sensory signals back to the central nervous system

The sequence of nerves that exit the spinal cord

8 cervical, 12 thoracic, 5 lumbar, 5 saccral, and 1 coccygeal

Structure of a nerve

each nerve fiber is surrounded by a delicate connective tissue which is the endoneurium. Groups of fibers are bound by a coarser connective tissue wrapping which is the perineurium which forms fascicles. lastly, the fascicles are bound together by the epineurium.

Ramus

where the spinal nerve spinal spinal nerves come out on the outside of the spine; connect through the back of the spine.

Olfactory nerve

(I) purely sensory; carries impulses for the sense of smell. EX: sniffing

Number of sensory fibers that are involved in hearing

8- this is the vestibulocochlear

Brachial plexus

contains the axillary nerve (deltoid), the radial nerve (triceps and extensor muscles), the median nerve (flexor muscles), the musculocutaneous nerve (flexor muscles and skin of lateral forearm), and the ulnar nerve (wrist and hand muscles)

Sacral plexus

contains sciatic nerve and the superior and inferior gluteal nerves (gluteus muscle of hip)

Wristdrop

inability to extend hand and wrist; caused by damage of the radial nerve

Sciatic nerve

largest nerve in the body; splits into the common fibular and tibial nerves; fibular is the lateral aspect of leg and foot and the tubular is the posterior aspect

Somatic vs autonomic

in somatic, the cell bodies of the motor neuron are inside the CNS and their axons spread all the way to the skeletal muscles; whereas, the autonomic has a chain of two motor neurons which are the preganglionic neuron and the ganglionic neuron. autonomic is split into sympathetic and parasympathetic.

Anatomy of the sympathetic division

its preganglionic neurons are in the gray matter of the spinal cord. the preganglionic axons leave the cord in the ventral root, enter the spinal nerve, and then pass through the ramus communications, to enter a sympathetic trunk ganglion. this trunk lies alongside the vertebral column on each side.

Parasympathetic division effects

increases smooth muscle mobility and amount of secretion by digestive glands, constricts the bronchioles of the lungs, relaxes the sphincters in the bladder, decreases heart rate, stimulates the glands by producing saliva, tears, and gastric juice, constricts pupils, causes erection due to vasodilation

Nervous system as we age

elderly people often faint due to the sympathetic nervous system becoming less efficient and not constricting the blood vessels as often (orthostatic hypotension). Asteriosclerosis and high blood pressure result in a decreased supply of oxygen to brain neurons, this leads to senility(forgetfulness, irritability, confusion, etc) The brain also shrinks due to the amount of neurons deteriorating.