Ch. 12: Fundamentals of the Nervous System and Nervous Tissue

Special somatic senses

"localized"; somatic senses whose receptors are confined to relatively small areas rather than spread widely throughout the body most are confined to the head, including hearing, equilibrium (receptors in inner ear), and vision (receptors in the eye) 1. Hearing 2. Balance 3. Vision

General somatic senses

"widespread"; senses whose receptors are spread widely throughout the outer tube of the body includes the many senses experienced on the skin and in the body wall, such as touch, pain, pressure, vibration, and temperature

Neuroglia in the CNS

1. Astrocytes 2. Microglial cells 3. Ependymal cells 4. Oligodendrocytes Neuroglial cells of the CNS have branching processes and a central cell body. Can be distinguished from neurons by their smaller size and their darker-stainign nuclei Outnumber neurons in the CNS by about 10 to 1 & make up 1/2 the mass of the brain Glial cells can divide throughout life

2 main types of synapses

1. Axodendritic 2. Axosomatic

Peripheral nervous system (PNS)

1. Cranial nerves 2. Spinal nerves 3. Ganglia Links all regions of the body to the CNS

How does a synapse function?

1. Impulse travels down axon of presynaptic neuron 2. The impulse stimulates the synaptic vesicles to fuse with the presynaptic membrane 3. Fused area ruptures, causing the vesicles to release their NT molecules 4. NT molecules diffuse across the synaptic cleft and bind to the postsynaptic membrane 5. This binding changes the membrane charge on the postsynaptic neuron, influencing the membrane's ability to generate a nerve impulse

2 types of cells in nervous tissue

1. Neurons - excitable nerve cells that transmit electrical signals 2. Neuroglia - non excitable support cells that surround and wrap the neurons both types develop from same embryonic tissues: neural tube and neural crest

Formation of myelin sheath in PNS

1. Schwann cell envelops an axon 2. Schwann cell rotates around and around the axon, wrapping its plasma membrane loosely around it in successive layers 3. Schwann cell cytoplasm is forced from between the membranes. The tight membrane wrapping surrounding the axon form the myelin sheath

3 overlapping functions of the nervous system

1. Sensory input - Uses millions of sensory receptors to monitor changes inside and outside the body. Each of these changes is called a stimulus 2. Integration - processes and interprets the sensory input and makes decisions about what should be one at each moment 3. Motor output - dictates a response by activating effector organs, our muscles, or glands

4 main subdivision of the PNS

1. Somatic sensory - sensory innervation of outer tube 2. Visceral sensory - sensory innervation of viscera 3. Somatic motor (Somatic nervous system) - motor innervation of skeletal muscles *voluntary* 4. Visceral motor (Autonomic nervous system) - motor innervation of cardiac muscles, smooth muscles, and glands) *involuntary*

Special visceral senses

1. Taste 2. Smell sensory receptors in the tongue and nasal cavity, respectively

Nerves of the PNS

Afferent PNS fibers respond to sensory stimuli Efferent PNS fibers transmit motor stimuli from CNS to muscles and gland

Neurons

Basic structural units of the nervous system Highly specialized cells that conduct electrical signals from one part of the body via nerve impulses/action potentials - extreme longevity (>100 years) - not able to divide (can't replace themselves if destroyed) - high metabolic rate (needs abundant oxygen and glucose)

Neurofibrils

Bundles of intermediate filaments (neurofilaments) Form a network between the chromatophilic substance); Keep the cell from being pulled apart when subjected to tensile forces

Nerves

Cable-like organs in the PNS Consist of numerous axons wrapped in connective tissue Most nerves contain myelinated and non-myelinated sensory and motor axons Axon is surrounded by Schwann cells

Functional classification of neurons

Classification according to the direction the nerve impulse travels relative to the CNS 1. Sensory (PNS --> CNS) 2. Motor (CNS --> PNS) 3. Interneurons (all located in CNS)

Nerves of the CNS

Composed of interneurons that 1. Receive sensory info 2. Direct/transport info to specific CNS regions 3. Initiate appropriate motor responses

5 Essential Components of Reflex Arcs

Each activates the next 1. Receptor 2. Sensory neuron 3. Integration center 4. Motor neuron 5. Effector

Ependymal cells

Form a simple epithelium that lines the central cavity of the spinal cord and brain They provide a fairly permeable layer between the cerebrospinal fluid that fills this cavity and the tissue fluid that bathes the cells of the CNS. They bear cilia that help circulate cerebrospinal fluid

Myelin Sheaths in the PNS

Formed by Schwann cells; develop during the fetal period and the first year or so of postnatal life Only thick, rapidly conducting axons are sheathed with myelin

Dendrites

Function: (1) receptive sites, providing an enlarged surface area for receiving signals from other neurons; (2) conduct electrical signals TOWARD the cell body Structure: processes that branch from the cell body like the limbs on a tree - most neurons have numerous dendrites - organelles in the cell body occur in dendrites, and chromatophilic substance extends into the basal part of each dendrite

Nerve impulse

Generated at the initial segment of the axon where it leaves the axon hillock Conducted along the axon to the terminal boutons NTs are released from vesicles @ terminal boutons

Oligodendrocytes

Have fewer branches than astrocytes Line up in small groups and wrap their cell processes around the thicker axons in the CNS Produce myelin sheaths in the CNS

What is the most common type of neuron in the body?

Interneurons --> reflects the vast amount of info processed in the human CNS

Chromatophilic substance (Nissi bodies)

Large clusters of rough ER and free ribosomes Renew the membranes of the neuron and the proteins of the cytosol Extend only into the basal part of dendrites and to the base of the axon hillock

Interneuorns

Lie between motor and sensory neurons. All are multipolar neurons confined entirely to the CNS. Link together into chains that form complex neuronal pathways. Make up 99.98% of the neurons in the body Show great diversity in size and in the branching patterns of their processes

Astrocytes

Most abundant glial cell in the CNS. Star shaped with many radiating processes that have bulbous ends Some of the bulbs cling to neurons (including the axon terminals) whereas others cling to capillaries Functions are numerous and not well understood 1. Regulate NT levels by increasing the rate of NT uptake in regions of high neuronal activity 2. Signal increased blood flow through capillaries in active regions of the brain 3. Control the ionic environment around neurons 4. Help synapses form in developing neural tissue 5. Produce molecules necessary for neural growth (BDTF) 6. Propagate calcium signals taht may be involved with memory Active role in neural activity

*CONFUSION ALERT* Neuron Nerve fiber Nerve

Neuron = nerve cell Nerve fiber = long axon Nerve = collection of axons in the PNS

Neuroglia/glial cells

Non-nervous supporting cells. 6 types - 4 in the CNS and 2 in the PNS CNS: astrocytes, microglial cells, ependymal cells, oligodendrocytes PNS: satellite cells, schwann cells Provide a supportive scaffolding for neurons Cover all nonsynaptic parts of the neurons, thereby insulating the neurons and keeping the electrical activities of adjacent neurons from interfering with each other

Structural link between the PNS and the CNS

Occurs in the gray matter of the CNS Simplest example of this neuronal integration is the reflex arc

Myelin Sheaths in the CNS

Oligodendrocytes form the myelin sheaths in the brain and spinal cord Each oligodendrocyte has multiple processes that coil around several different axons Gaps are present, but they are more widely spaced than those in the PNS

Neuroglia in the PNS

Satellite cells and Schwann cells - very similar cell types that differ mainly in location

Receptor of a reflex arc

Site where stimulus acts Located at the terminal end of the peripheral process of a sensory neuron

Microglial cells

Smallest and least abundant neuroglia of the CNS. Elongated cell bodies and cell processes with many projections Phagocytes (macrophages of the CNS) Migrate to and engulf invading microorganisms and injured or dead neurons Derived from blood cells called monocytes; the monocytes that become microglial cells migrate to the CNS during the embryonic and fetal periods

Non-myelinated axons in the PNS

Thin, slowly conducting axons lack a myelin sheath Found in portions of the autonomic nervous system and in some sensory fibers Schwann cells surround the axons but do not wrap around them in concentric layers of membrane A single Schwann cell can partly enclose >15 axons, each of which occupies a separate tubular recess in the surface of the Schwann cell

Non-myelinated

Thinnest axons in the CNS are non-myelinated They are covered by the many processes of glial cells (i.e. astrocytes) that are so abundant in the CNS

(T/F) Most neurons in the CNS function as both presynaptic and postsynaptic nueorns

True; they get info from some neurons and dispatch it to others

Visceral reflexes

activate smooth muscle, cardiac muscle, or glands

Nerve fiber

any long axon

Terminal arborization

area at the end of an axon where it branches profusely; end in knobs called terminal boutons

Tracts

bundles of axons within the white matter traveling to similar places

Cranial nerves

carry signals to and from the brain

Spinal nerves

carry signals to and from the spinal cord

Nervous tissue

cells are densely packed and tightly intertwined made up of (1) neurons and (2) neuroglia

Ganglia

clusters of neuronal cell bodies (outside of the CNS)

Neurons w/ larger diameters

conduct impulses faster than those with smaller diameters because of the basic law of physics: resistance to the passage of an electrical current decreases as the diameter of any "cable" increases

Motor neuron of a reflex arc

conducts efferent impulses from the integration center to an effector

Perineurium

connective tissue wrapping surrounding a nerve fascicle

Central nervous system (CNS)

consists of brain and spinal cord, which occupy the cranium and the vertebral canal, respectively integrating and command center of the nervous system - it receives incoming sensory signals, interprets them, and dictates motor responses based on past experiences, reflexes, and current conditions

Integration center of a reflex arc

consists of one or more synapses in the gray matter of the CNS. in simple reflex arcs, the integration center is a single synapse between a sensory neuron and a motor neuron in more complex reflexes, it can involve multiple synapses that send signals through long chains of interneurons to other portions of the CNS (i.e. the brain)

Somatic body region

consists of the structures external to the ventral body cavity - the structures of the outer tube (skin, skeletal musculature, bones)

White matter

contains no neuron cell bodies but millions of axons and neuroglia white color comes from myelin sheaths around many of the axons; these axons either ascend from the spinal cord to the brain or descend from the brain to the spinal cord, allowing these 2 regions of the CNS to communicate with each other white matter consists of axons running between different parts of the CNS

Connective tissue in nerves

contains the blood vessels that nourish the axons and Schwann cells

Autonomic nervous system

controls function of visceral organs --> regulates the contraction of smooth and cardiac muscle and secretion by the body's many glands made up of general visceral motor neurons involuntary nervous system divided into sympathetic "fight or flight" and parasympathetic "rest and digest"

Axons

each neuron has only 1 axon; arises from a cone-shaped region of the cell called the axon hillock thin processes of uniform diameter throughout their length impulse generators and conductors that transmit nerve impulses AWAY from their cell body do not contain organelles length and diameter of axon varies among the different neurons of the body

Processes

extend from the cell bodies of all neurons 2 types: dendrites and axons

Nodes of Ranvier

gaps in the myelin sheath that occur at regular intervals; occur since adjacent Schwann cells along a myelinated axon do not touch each other Nerve impulses don't travel along the myelin-covered regions of the axonal membrane --> they jump from the membrane of one gap to the next in a way that greatly speeds impulse conduction

Gray matter

gray colored inner zone that surrounds the hollow central cavity of the CNS; it is a butterfly shaped-region dorsal half contains cell bodies of interneurons; ventral half contains cell bodies of motor neurons gray matter is the site where neuron cell bodies are clustered has a mixture of neuron cell bodies, dendrites, short non-myelinated neurons, and neuroglia synapse occur in the gray matter

Nerve Fascicles

groups of axons bound into bundles

Unipolar neurons (pseudounipolar neurons)

have a short, single process that emerges from the cell body and divides like an inverted T into 2 long branches most unipolar neurons start out as bipolar neurons whose 2 processes fuse together near the cell body during development found in sensory ganglia in the PNS, where they function as sensory neurons has central process (extends to CNS) and peripheral process (extends to receptors)

Multipolar neurons

have more than two processes. usually have numerous dendrites and a single axon. some small neurons have no axon and rely strictly on dendrites for conducting signals make up 99% of neurons

Terminal boutons

knobs at the end of the branches of an axon contain synaptic vesicles and abundant mitochondria because secretion of NTs require a great deal of energy

Endoneurium

layer of delicate loose connective tissue surrounding the axon (covering the Schwann cells)

Motor neurons (efferent neurons)

make up the motor division of the PNS. They transmit impulses AWAY from the CNS to effector organs (muscles and glands) Multipolar; cell bodies located in the CNS Form junctions with effector cells, stimulating muscles to contract or glands to secrete

Sensory neurons (afferent neurons)

make up the sensory division of the PNS. They transmit impulses TOWARD the CNS from sensory receptors in the PNS. Most sensory neurons are pseudounipolar, and their cell bodies are in ganglia outside the CNS Peripheral process extends from a sensory receptor; Central process terminates in the CNS Some sensory neurons are bipolar in structure (in special sense organs)

Nervous system

master control and communications system of the body; its cells communicate through electrical signals, which are rapid and specific and usually produce almost immediate responses

Synaptic vesicles

membrane-bound sacs filled with neurotransmitters; found in the terminal bouton

Knee-jerk reflex

monosynaptic reflex; impact of a hammer on the patellar ligament stretches the quadriceps muscles of the thigh. This stretching initiates an impulse in a sensory neuron that directly activates a motor neuron in the spinal cord, which signals the quad to contract the contraction of the quadriceps counteracts the original stretching caused by the hammer

Axodendritic synapses

most synapses occur between the terminal boutons of one neuron and the dendrites of another

Visceral body regions

mostly contains the viscera within the ventral body cavity - the structures of the body's inner tube (digestive tube, lungs, heart, bladder, etc.)

Efferent division of the PNS

motor division; conducts impulses from the CNS to effectors (muscles and glands) consists of motor nerve fibers divided into somatic nervous system and autonomic nervous system

Effector of a reflex arc

muscle or gland cell that responds to efferent impulses by contracting or secreting

Axonal transport

neurofilaments, actin myofilaments, and microtubules provide structural strength and aid in transport of substance to and from the cell body as the axonal cytoplasm is continually recycled and renewed

Structural Classification of Neurons

neurons are grouped by the number of processes that extend from the cell body 1. Multipolar 2. Bipolar 3. Unipolar

Presynaptic neuron

neurons that conducts signals toward a synapse

Postsynaptic neuron

neurons that transmits signals away from the synapse

Axon collaterals

occasional branches of axons that extend from the axon at more or less right angles

Somatic nervous system

portion of the PNS that stimulates contraction of the skeletal muscles in the body; under voluntary control

Body sense

position and movement of body in space

Myelin sheaths

produced by oligodendrocytes in the CNS and by Schwann cells in the PNS segmented structures that are composed of the lipoprotein myelin surround the thicker, faster conducting axons of the body each segment of myelin consists of the plasma membrane of a glial cell rolled in concentric layers around the axon forms an insulating layer that (1) prevents the leakage of electrical current from the axon, (2) increases the speed of impulse conduction along the axon, and (3) makes impulse propagation more energy efficient

Reflexes

rapid, automatic motor responses to stimuli unlearned, unpremeditated, and involuntary

Somatic reflexes

result in contraction of skeletal muscle

Proprioception

sense that detects the amount of stretch in muscles, tendons, and joint capsules informs you of the position and movement of your body in space, giving you a "body sense"

Afferent division of the PNS

sensory division; conducts impulses from sensory receptors to the CNS somatic and visceral sensory nerve fibers

Reflex arcs

simple chains of neurons that cause our simplest, reflexive behaviros and reflect the basic strctural plan of the nervous system

Monosynaptic reflex

simplest of all reflexes just one synapse; there is no interneuron between the sensory neuron and the motor neurons i.e. "knee-jerk" reflex

Gliomas

since glial cells can divide, most tumors that originate in the brain (60%) are gliomas gliomas are tumors formed by uncontrolled proliferation of glial cells Difficult cancers to treat; 1-yr survival rate is <50%

Synapse

site at which neurons communicate; most synapses transmit info through chemical messengers some neurons in certain areas of the CNS transmit signals electrically through gap junctions signals pass across most synapses in one direction only --> synapses determine the direction of information flow through the nervous system

Synaptic cleft

space that separates the plasma membranes of two neurons

General visceral senses

stretch, pain, and temperature, widely felt in the digestive and urinary tracts, reproductive organs, and other viscera also includes hunger and nausea

Cortex

superficial layer of gray matter in the cerebrum and cerebellum of the brain

Schwann cells

surround all axons in the PNS and form myelin sheaths around many of these axons

Satellite cells

surround neuron cell bodies within ganglia function similar to astrocytes

Axosomatic synapses

synapses between axons and neuron cell bodies (rare)

Epineurium

tough fibrous sheath surrounding the whole nerve

Sensory neuron of a reflex arc

transmits the afferent impulses to the CNS

Bipolar neurons

two processes that extend from opposite sides of the cell body rare neurons that occur in some of the special sensory organs (inner ear, olfactory epithelium of nose, retina of eye), where they serve as sensory neurons

Cell body

vary widely in size; all consist of a single nucleus surrounded by cytoplasm and usual organelles focal point for the outgrowth of the of the neuron processes during embryonic development

Where are most neuron cell bodies located?

within the CNS *** Ganglia = neuronal cell bodies in the PNS