Chapter 14 Nervous Tissue (lecture)

Structural division

between central and peripheral nervous systems

Functional division

between sensory and motor nervous systems

Central Nervous System

brain and spinal cord

Peripheral Nervous System

cranial nerves, spinal nerves, ganglia

Three general functions of sensory and motor nervous system (both with CNS and PNS components)

collect information (PNS), process and evaluate information (CNS), initiate response (PNS and CNS)

Two components of sensory (afferent) nervous system

somatic sensory (conscious): touch, pain, pressure, vibration, and proprioception

visceral sensory (generally not conscious of): blood vessels and viscera

Two components of motor (efferent) nervous system

Somatic motor (somatic nervous system SNS): voluntary, contraction of skeletal muscles

Autonomic motor (autonomic nervous system ANS): involuntary, regulate smooth and cardiac muscle, as well as glands

Two distinct types of cells within the nervous system

Neurons: electrically excitable cells that initiate, transmit, and receive nerve impulses

Glial cells: non-excitable cells that support and protect the neurons

Special characteristics of neurons

High metabolic rate

Extreme longevity

Non-mitotic

Excitable (respond to a stimulus)

Conductive (propagate electrical signal)

The cell body (soma) contains what organelles

Nucleus and nucleolus

mitochondria

chromatophilic substance (free ribosomes and rough endoplasmic reticulum)

neurons without an axon

anaxonic

The region where the axon connects to the cell body

axon hillock

Structures associated with axons

Axon collaterals (side branches off the main axon)

Terminal arborizations (fine extensions at the end of the axon and its collaterals)

Synaptic knobs (expanded regions at the tips of arborizations)

Perikaryon (cytoplasm within cell body)

Neurofilaments (intermediate filaments that form neurofibrils, which help support cell extensions (dendrites, axon)

Three structural classes of neurons

Unipolar (single, short process branches like a T from a cell body)

Bipolar (two processes, one dendrite and one axon)

Multipolar (many dendrites and a single axon; most common type of neuron

Three functional classes of neurons

sensory (afferent) neurons: transmit impulses about stimuli from sensory receptors to CNS

motor (efferent) neurons: transmit impulses from CNS to muscles or glands

Interneurons (association neurons): retrieve, process, and store information; communicate between sensory and motor neurons

Glial cells (neuroglia)

protect and nourish neurons

found in both cns and pns

smaller and more numerous than neurons

capable of mitosis

typically do not transmit impulses but do help with the functions of neurons

brain tumors are more likely to be derived from glial cells than neurons

four types of glial cells in cns

astrocytes

ependymal cells

microglial cells

oligodendrocytes

Functions of glial cells

help form the blood-brain barrier (BBB)

regulate tissue fluid composition

form a structural network

replace damaged neurons

assist neuronal development

help regulate synaptic transmission

change synapse numbers

ependymal cells

ciliated cuboidal epithelial cells that line ventricles of brain and central canal of spinal cord

help produce cerebral spinal fluid and help form choroid plexus

microglial cells

Microglia are small cells that move through CNS and engulf debris

oligodendrocytes

Associated with cns axons only

wrap themselves around the axons like electrical tape wrapped around a wire

produce myelin, which is an insulator of electrical activity

Two types of glial cells are found in the PNS

satellite cells (regulate fluid around neuron cell bodies in pns ganglia)

neurolemmocytes (schwann cells) form myelin around pns axons

myelination

the process of wrapping the axon with a myelin sheath to electrically insulate it

neurilemma

the thin, outer membrane of neurolemmocyte

saltatory conduction

myelinated

an impulse “jumps” from one neurofibril node to another

continuous conduction

unmyelinated axons

the impulse travels along the entire length of the axon membrane

Three types of connective tissue wrappings

endoneurium (around each individual axon)

perineurium (around each fascicle of axons)

epineurium (around the entire nerve)

synapses

specialized junctions between two neurons, or between a neuron and a muscle or gland cell

A typical synapse consists of

presynaptic neuron (has synaptic knobs at axon endings for sending signal)

synaptic cleft (narrow space between cells)

postsynaptic neuron (receives signal)

Three common types of synapses

axodendritic (between synaptic knobs and dendrites)

Axosomatic (between synaptic knobs and soma)

Axoaxonic (between synaptic knobs and axon)

Electrical synapses

ions rapidly flow from one cell to another

rare in the nervous system

involve gap junctions between joined cells

chemical synapses

calcium enters synaptic knob and triggers transmitter release

neurotransmitter diffuses across cleft and binds to receptors on postsynaptic cell

gated ion channels in postsynaptic cell are opens to that the cell’s voltage is changed

the neurotransmitter is then cleared (acetylcholine is broken down by acetylcholinesterase)

involves release of neurotransmitter by presynaptic cell upon arrival of impulse at end of axon

Four types of neural circuits

converging (impulses come together to a single postsynaptic neuron like saliva production)

Diverging (impulses spread from one presynaptic neuron to many postsynaptic neurons like a few neurons in the motor cortex of brain stimulating thousands of muscle fibers in a muscle)

Reverberating (cyclical stimulation of the circuit like breathing rhythm)

Parallel-after-discharge (several neurons process the information simultaneously in parallel, like higher-order thinking)