A&P Lecture Module 7- Nervous System and Nervous Tissue

Functions of Nervous Tissue

• Maintaining homeostasis. Regulate and coordinate activities to maintain balance.

• Receiving sensory input. Monitor internal, external stimuli.

• Integrating information. Brain and spinal cord process sensory input and initiate responses.

• Controlling muscles and glands. Stimulation of skeletal muscle and some smooth muscle to contract; secretions from glands.

• Establishing and maintaining mental activity. Consciousness, thinking, memory, emotion.

Central Nervous System Organs

• Brain

• Spinal cord

Peripheral Nervous System Organs

All of the nerves

Somatic nervous system

Voluntary

Automatic nervous system

Involuntary

Sympathetic nervous system

Arousing

Parasympathetic nervous system

calming

Sensory Nervous System

Detects stimuli and transmits info from receptors to CNS

Somatic sensory

Sensory input from the receptors of five senses and proprioceptors

Motor nervous system

Initiates and transmits info from CNS to effectors

Somatic motor

Motor output to skeletal muscle

Autonomic motor

Motor output to cardiac muscle, smooth muscle, and glands

Sensory division

Transmits action potentials from receptors toward CNS

Sensory Receptors

Can be neuron endings or specialized cells that detect external and internal stimuli; send input along nerves to brain or spinal cord

Motor division

Transmits action potentials from CNS to effectors (muscles, glands)

Parts of neuron

• Soma (cell body)

• Dendrites

• Axon

• Axon hillock

• Axon terminals

• Synaptic knobs (presynaptic terminals)

Soma

Cell body, contains nucleus

Dendrites

Short unmyelinated processes branching off cell body

Axons

Long process emanating from cell body; makes contact with other neurons, muscle cells, or glands

Axon hillock

Attachment to cell body-axon

Axon terminals

Ending of axon

Synaptic knobs (presynaptic terminals)

House synaptic vesicles containing neurotransmitters

Nucleus

a bunch of neuron cell bodies hang out together

Tract

Inside the brain and spinal cord (CNS) axons travel in bundles called

Nerve

a bundle of parallel axons in the PNS

Nerves have __________ wrappings

connective tissue

Connective tissue wrappings of nerves

• Epineurium

• Perineurium

• Endoneurium

Ganglia

Neuron cell bodies gather in little “stations”

Sensory nerves

Afferent fibers only – carries information only to the CNS

Motor nerves

Efferent fibers only – carries information only away from the CNS

Mixed nerves

carries information to and from CNS

• Most named nerves

Ganglion

cluster of neuron cell bodies in PNS

Excitability

responsiveness to a stimulus

Conductivity

ability to propagate electrical signal

Secretion

release of neurotransmitter in response to conductive activity

Extreme longevity

cell can live throughout person’s lifetime

Amitotic

After fetal development, mitotic activity is lost in most neurons

Multipolar

Many dendrites and an axon

Bipolar neuron

Dendrite and axon

Pseudo-unipolar neuron

Appears to have an axon and no dendrites

Anaxonic Neuron

Multiple dendrites but no axons

Synapse

place where a neuron connects to another neuron or an effector

Presynaptic neuron’s

axon terminal produces signal..

Postsynaptic neuron

receives signal

Most commonly with one of its dendrites

Electrical synapse

• Presynaptic and postsynaptic neurons bound together by gap junctions

• Fast

Glial cells

• Nonexcitable, support cells found in CNS and PNS

• Approximately same number as there are neurons

• Account for about half the volume of nervous system

Glial Cell General Characteristics

Mitosis

• Protect and nourish neurons

• Provide physical scaffolding for nervous tissue

• Guide migrating neurons during development

• Critical for normal function at neural synapses

Microglia

Phagocytic cells within CNS

Oligodendrocytes

Extensions form myelin sheaths of axons within CNS

Astrocyte

Foot processes cover the surface of neurons, blood vessels, and Pia mater. Provides structural support and regulates what moves from the blood into the tissue of the CNS

Ependymal Cells

cells lining ventricles of the brain and central canal of the spinal cord help move CSF

Ependymal cells secrete

cerebrospinal fluid

Neurolemmocytes/Schwann cells

wrap around portion of only one axon to form myelin sheath.

Satellite cells

surround neuron cell bodies in sensory and autonomic ganglia; provide support, nutrients, and protection from heavy-metal poisons.

Oligodendrocyte functions

1. Myelinated and insulates CNS axons

2. Allows faster action potential propagation along axons in CNS

Astrocyte functions

1. Helps form blood-brain barrier

2. Regulates interstitial fluid composition

3. Provides structure support and organization to the CNS

4. Assists with neuronal development

5. Alters synaptic activity

6. Replicates to occupy space of dying neurons

Ependymal Cell functions

1. Lines ventricles of brain and central canal of spinal cord

2. Assists in production and circulation of CSF

Microglial Cell Functions

1. Phagocytes substances in CNS

2. Protects CNS by engulfing infectious agents and harmful substances

Satellite Cell Functions

1. Electrically insulates PNS cell bodies

2. regulates nutrient and waste exchange for cell bodies in ganglia

Neurolemmocyte Functions

1. Myelinated and insulates PNS axons

2. Allows faster action potential propagation along an axon in PNS

Neurogenic atrophy Clinical View

Regeneration in the CNS limited. If a nerve stays compressed or dead for too long (weeks to months), the muscle loss can become permanent.

Myelination

process of wrapping an axon with myelin

Myelin

Several layers of membrane of glial cells

High lipid content means

glossy-white appearance and insulates axon

Neurolemmocytes in

PNS

CNS

Oligodendrocytes in

Myelin acts as

electrical insulation, allowing action potentials to travel faster by being regenerated mainly at the Nodes of Ranvier.

Gray matter

contains unmyelinated axons, cell bodies, dendrites.

Gray matter function

information is processed

White matter

contains myelinated axons; propagate action potentials. The highways connecting processing centers

White Matter and CNS

clusters of neuron cell bodies are nuclei; bundles of myelinated axons are nerve tracts

White matter and PNS

clusters of cell bodies are ganglia; bundles of axons with their connective tissue sheaths are nerves.

Multiple Sclerosis Clinical View

• Multiple sclerosis (MS) is an autoimmune disease that damages myelin in the central nervous system (CNS).

• The immune system attacks myelin produced by oligodendrocytes.

• Loss of myelin slows or blocks action potential conduction along axons.

Multiple Sclerosis Symptoms

Muscle weakness, numbness, vision problems, and impaired coordination.

Action potentials

Electrical signals produced by nervous system

Membrane potential

the result of ionic concentration differences across plasma membrane and permeability of membrane.

Ion distribution outside the cell

More Na+ and Cl-

more K⁺ and negatively charged proteins

Ion distribution in cell

Na+ and K+ have concentration gradient in ________ direction

Opposite

Why do ions need channels

• Large, negatively charged proteins stay inside the cell

• Cl⁻ tends to remain outside because the inside of the cell is already relatively negative

• Ions cannot freely cross the lipid membrane — they move through ion channels

• Two main types: leak channels and gated channels

K+ and Cl-

More ____ and ____ leak channels

Gated ion channels

• Voltage-gated ion channels open or close in response to specific, small voltage changes across the cell membrane.

At rest, membrane is negative on the inside relative to the outside.

When cell is stimulated, that relative charge changes and voltage-gated ion channels either open or close.

Ligand-gated ion channels

open or close in response to ligand such as neurotransmitter or hormone binding to receptor protein.

Touch receptors

Respond to mechanical stimulation of the skin; use mechanically gated ion channels.

Temperature receptors

Respond to temperature changes in the skin.

Opposite charges across the membrane, so membrane is

Polarized

Balanced

Resting does NOT mean ions stop moving. It means their electrical currents are

Positive charges accumulate ______ membrane

Outside

Membrane is more permeable to

K+

K+ diffuses ___ so membrane is ____

out, more negative

______ ________ ____ maintains concentration gradient

Sodium Potassium Pump

Depolarization

inside of cell becomes more positive; for example, from −70mV to −55mV – Threshold is reached

factors that lead to depolarization

Sodium ions

Calcium ions.

WHY DO SODIUM IONS LEAD TO DEPOLARIZATION

If gated sodium channels open, sodium diffuses into cell down its concentration gradient, and inside of cell becomes more positive.

Most common way neurons become depolarized

WHY DO CALCIUM IONS BECOME DEPOLARIZED

Calcium entry also causes depolarization, as seen in pacemaker cells (SA and AV nodes- heart)

Depolarization

Inside of cell becomes more positive

K+

Voltage-gated __ channels respond more slowly, so they remain mostly closed during the depolarization phase.

• As depolarization reaches its peak, channels open and moves out of the cell, initiating repolarization

Depolarization

Initiated when K+ channels open and K+ moves out the cell

Hyperpolarization

Inside of cell becomes more negative; for example, from −70mV to −90mV.

Factors leading to hyperpolarization

Potassium ions

Chlorine ions