introduction to the nervous system - A + P 1

what structures does the central nervous system (CNS) include

• brain

• spinal cord

• both enclosed and protected by the cranium and vertebral column

what processes most of the information in the nervous system

central nervous system (CNS)

what structures does the peripheral nervous system (PNS) include

• nerves

• ganglia

what is the general function of the peripheral nervous system (PNS)

directs information to and from the CNS

what is a nerve (structural)

bundle of nerve fibers (axons) wrapped in fibrous connective tissue

how do nerves reach the rest of the body

nerves emerge from the CNS through foramina in the skull and vertebral column

what is the function of nerves

they carry signals to and from other organs of the body

what are ganglion

knot-like swelling in a nerve where cell bodies of neurons are concentrated

what are the two divisions of the PNS

• sensory division

• motor division

what are the four divisions stemming from the sensory and motor divisions

• visceral sensory division (sensory)

• somatic sensory division (sensory)

• visceral motor division (motor)

• somatic motor division (motor)

what are the final two divisions of the PNS that stem from the visceral motor division

• sympathetic division

• parasympathetic division

PNS - what does the sensory division do

carries sensory signals by way of afferent nerve fibers from sensory receptors (cells and organs that direct stimuli) to the CNS

PNS - what does the motor division do

carries motor signals by way of efferent nerve fibers from the CNS to effectors (cells and organs that carry out activities), mainly gland and muscle cells

PNS - sensory division - what does the visceral sensory division do

carries signals mainly from the viscera of the thoracic and abdominal cavities such as the heart, lungs, and stomach

PNS - sensory division - what does the somatic sensory division do

carries signals from receptors in the skin, muscles, bones, and joints

PNS - motor division - what does the visceral motor division (autonomic nervous system) do as a whole

• carries signals to glands, cardiac muscle, and smooth muscle

• no voluntary control over these effectors

• operates at an unconscious level

• responses of this system and its effectors are visceral reflects

PNS - motor division - visceral motor division - what does the sympathetic division do

• flight or fright

• prepares the body for action

• accelerates the heart rate and increases the respiratory airflow

• inhibits digestion

PNS - motor division - visceral motor division - what does the parasympathetic division do

• rest and digest

• ends to adapt the body to a state of rest

• reduces the heart rate and respiratory airflow

• stimulates digestion

PNS - motor division - what does the somatic motor division do

• carries signals to skeletal muscle

• produces muscle contractions that are under voluntary or involuntary control

• called somatic reflexes

what is the functional unit of the nervous system

neurons or nerve cells

what is the function of neurons or nerve cells

they carry out the communication process

what are the universal properties of the neurons/nerves

• excitability

• conductivity

• secretion

what does excitability mean for a neuron/nerve

• ability to respond to environmental changes (stimuli)

• highly developed in neurons

what does conductivity mean for a neuron/nerve

ability to respond to stimuli by producing electrical signals that reach other cells at distant locations

what does secretion mean for a neuron/nerve

when an electrical signal reaches the end of a nerve cell, the neuron secretes chemicals called neurocrines that cross the gap and stimulate the next cell

what are the functional classes of neurons

• sensory (afferent) neurons

• interneurons (association neurons)

• motor (efferent) neurons

what are sensory (afferent) neurons specialized to do

• detect stimuli such as light, heat, pressure, and chemicals

• transmits the information to the CNS

what is the anatomic location of sensory (afferent) neurons

• they originate in almost any organ but always end up in the spinal cord/brain

• afferent = signal travels from the periphery to the CNS

how are sensory (afferent) neurons the same as/different than receptors

• pain and smell receptors are neurons themselves

• in other cases (taste, hearing), the receptor is a separate cell that communicates directly with a sensory neuron

what is the anatomic location of interneurons

• they are association neurons that lie entirely within the CNS

• they make up ninety percent of all neurons

• they lie between and interconnect the incoming sensory pathways and the outgoing motor pathways of the CNS

what are the functions of interneurons (accessory neurons)

• receive signals from other neurons and carry out the integrative function of the nervous system

• process, store, and retrieve information, then make decisions about how the body responds to stimuli

what is the main function of motor (efferent) neurons

send signals predominantly to effectors (muscle and gland cells) that carry out the body’s responses to stimuli

where the does the name “motor (efferent) neurons” come from

• called motor neurons because most of them lead to muscle cells

• efferent = signal travels from the CNS to the periphery

what are varicosities

in the autonomic division, some motor neurons have enlarged regions along the axon called varicosities that store and release neurocrines

what are the structural classes of neurons

• multipolar

• bipolar

• pseudo-unipolar

• anaxonic

how are neurons classified

they are classified structurally according to the number of processes extending from the soma

multipolar neuron

• one axon and two or more (usually many) dendrites

• most common type of neuron

• includes most of the neurons in the brain and spinal cord

bipolar neuron

• one axon and one denrite

• for example, olfactory cells of the nasal cavity

unipolar neuron

• single process leading away from the soma that immediately branches into a proximal and distal axon

• for example, neurons that carry sensory signals to the spinal cord

anaxonic neuron

• multiple dendrites but no axon

• communicate through their dendrites but do not produce action potentials

• found in the brain and retina

general structure of neurons

• soma (cell body)

• dendrites

• axons

what does the “soma (cell body)” category of a neuron’s structure include

• nucleus

• nucleolus

what does the “dendrites” category of a neuron’s structure include

• dendrites

• dendritic spines

what does the “axons” category of a neuron’s structure include

• axolemma

• axoplasm

• axon collaterals

• axon terminals

• trigger zone (= axon hillock plus initial segment)

• terminal arborization

• synaptic knob

structure of a soma (cell body)

• single nucleus with a large nucleolus

• 5-135 um in diameter

what is the broad function of the soma (cell body)

control center of the neuron

what happens if a neuron is severed (regarding the soma)

if severed, sections separated from the soma will die because they lack the organelles (ribosomes, endoplasmic reticulum, Golgi apparatus) to make essential proteins

what happens when the axon of a somatic motor neuron is severed

the death of the distal portions of the nerve cause paralysis of the skeletal muscles that are innervated by the neuron

what happens when the axon of a somatic sensory neuron is severed

the person experiences loss of sensation (numbness, tingling) in the region previously innervated by the neuron

what is the general structure of dendrites

• soma gives rise to thick processes that branch into many thin dendrites

• they are named for their resemblance to the bare branches of a tree in winter

• some neurons have one dendrite, others have thousands

what is the general purpose of dendrites

• receive incoming information from neighboring cells and transfer it to an integrating region within the soma

• with more dendrites, the neuron receives more information from other cells to facilitate its decision-making

what is a key benefit to the shape that dendrites give to a neuron

• dendrites increase the surface area of a neuron which allows it to communicate with multiple other neurons

• dendrite surface area can be expanded even more by the presence of dendritic spines (thin spike, mushroom shaped knobs)

dendritic spines (function, characteristic regarding protein)

• they function as independent compartments

• they send signals back and forth with other neurons in the brain

• many dendritic spines contain polyribosomes and make their own proteins

what are changes in dendritic spines associated with

changes in dendritic spine morphology are associated with learning and memory, as well as with various pathogens

what is the general function of axons

• specialized for rapid conduction of nerve signals to points remote from the soma

• carry outgoing signals to target cells

what are axoplasm and axolemmas

• axoplasm is the cytoplasm of an axon

• axolemma is the membrane of an axon

what are axon collaterals

since most peripheral neurons have only a single axon which remains unbranched for most of its length, branches from the axon are called axon collaterals

axon size

• axons range in diameter from 1 to 20 um

• axons range in length from 2-3 mm to more than a meter long

• Note - if the soma of a somatic motor neuron were the size of a tennis ball, its dendrites would form a large bushy mass that could fill a 30 seat classroom and the axon would be a mile long and narrower than a garden hose