Ch. 11 - Nervous Tissue Lecture

Nervous system structures

• Brain

• cranial nerves (12 pairs)

• spinal cord

• spinal nerves (31 pairs)

3 parts of the nervous system

• Central Nervous system

• Peripheral Nervous system

• Enteric Nervous system

Central Nervous System (CNS)

• brain + spinal cord.

• Doesn’t include cranial and spinal nerves.

• Processes information and integrates/coordinates both sensory and motor commands

Peripheral Nervous System (PNS)

• Contains nerves (bundles of axons) ganglion (Collections of cell bodies, grey matter)

• Two subsections

  • Sensory (afferent) and motor (efferent)

Sensory PNS

• Brings information to CNS

• General sensory receptors (Pain, touch, pressure, temperature)

• Special sensory organs (Smell, taste, sight, sound)

Motor PNS

• receives motor commands from CNS

• 2 subsections of PNS:

  • Somatic (voluntary movement such as movement of skeletal muscle)

  • Autonomic (involuntary movement such as smooth and cardiac muscle)

Enteric Nervous System (ENS)

• focuses on the digestion of food. Known as the nervous system of the gut

Overview of nervous system function (steps)

1. Sensory receptors receive info, detect changes in internal/external environment

2. Information travels to CNS

3. Information gets processed in CNS

4. Development of motor commands in motor division of PNS

5. Effectors respond to motor commands

Nervous tissue cells

neurons and neuroglia

Neurons

• electrically excitable-action potentials

• unique functions of NS

Neuroglia

• Support, nourish, and protect neurons

• Divide throughout lifetime

Axon

• Carries info to other cells

• Regions:

  • Axon hillock

  • Axolemma

  • Axoplasm

  • Axon terminals

Axon terminals

• Branches at the end of the axon that contain tiny pouches, or sacs, called synaptic vesicles.

axoplasm

cytoplasm of axon

Axolemma

plasma membrane of axon

axon hillock

• Cone shaped region where axon joins cell body

• Where action potential begins if it decides to carry it out

Dendrites

• Receive stimuli from environment/other neurons

• highly branched = dendritic spines

Cell body (Soma)

• nucleus

• cytoplasm

  • includes lysosomes, mitochondria, Golgi complex

  • Nissl bodies = rough ER

Synapse

• Where neuron (presynaptic cell) communicates with another cell (postsynaptic cell)

• Neurotransmitters are most common method of communication

collateral branch

• single neuron branches to 1+ cell

anaxonic neuron

• small neurons

• lack axonal distinguishing features

• in brain + special sense organs

• functions poorly understood

bipolar neurons

• 2 distinct processes

  • 1 dendritic process

  • 1 axon.

  • These are small, rare

  • in special sense organs

Unipolar neurons

• Dendrites + axon are continuous (fused)

• Most neurons in PNS

• can be over 1 meter long

Multipolar neurons

• 2+ dendrites + 1 axon

• Most neurons in CNS

• make up all motor neurons to skeletal muscles

• may be long

sensory (afferent) neurons

• Receiving stimuli and sends information to CNS

• Mostly Unipolar

• Contain Sensory ganglia (collection of neuron cell bodies in PNS)

  • Somatic sensory neurons: outside world

  • Visceral sensory neurons: internal systems

• Different receptors on sensory neuronsi:

• interoceptors; internal organs

• proprioceptors; position/movement of skeletal muscles and joints

• exteroceptors: external environment

interneuron

1. In CNS

2. between sensory and motor neurons.

3. Receive info from PNS & CNS

4. responsible for higher functions (memory and learning)

Motor (efferent) neurons

• Send signals from CNS to muscles + glands (effectors)

• 2 types:

  • Somatic motor neurons → skeletal muscles

  • voluntary

  • cell body in CNS

• Visceral motor neuron → smooth, cardiac muscle, glands, adipose tissue

CNS neuroglia

• Support, protect, nourish neurons.

• smaller than neurons, more numerus

• ~1/2 volume of nervous system

• no action potential

• cell division in mature nervous system

myelinated axons in CNS

• axons w/ myelin sheaths (CNS white matter)

• internodes - myelin-wrapped areas

Nodes of Ranvier

gaps between internodes

unmyelinated axon in CNS

• axons without myelin sheaths

• CNS grey matter

• cell bodies, dendrites, unmyelinated axons

PNS neuroglia

2 types:

• Schwann cell: produce myelin in PNS (A in image).

• Satellite cells: surround peripheral cell bodies

  • regulate environment around neurons (B in image)

Myelinated axons in PNS

• Formed by myelinating Schwann cells

• A single Schwann cell wraps a single internode

Unmyelinated axons in PNS

• single nonmyelinating Schwann cell wraps around segments of a group of axons

• No nodes of ranvier

Axon injury and repair in CNS

• limited because…

  • many more axons involved

  • astrocytes make scar tissue that can block axon growth in damaged area

  • astrocytes release chemicals blocking axon regrowth

Membrane potential

• From unequal charge distribution across membrane

• Due to…

  • difference in membrane permeability

  • active transport mechanisms

Contributors to resting membrane potential

High Na+ concentration in extracellular fluid

High K+ concentration in cytosol

Maintaining Resting membrane potential

• Leak channels:

  • passive

  • allow K+ to leave, Na+ to enter

• Sodium-potassium pump:

  • active

  • ejects 3 Na+ ions for every 2 K+ ions that are brought back into the cell

chemically gated channels

• ligand-gated ion channels

• Open when they bind specific chemicals

  • Ex: neurotransmitters, hormones, ions

• Most abundant on dendrite and cell bodies

Voltage-gated channels

• Changes in membrane potential open the gate

• sodium channels w/ 2 gates - activation gate, inactivation gate

• Na+ and K+ channels along axon

• Ca2+ channels along axon terminal

Mechanically gated channels

• Open in response to physical distortion of membrane surface

  • (stretch, pressure, touch, vibration)

Graded potentials

• stimulus causes ligand/mechanically gated ion channel to open

  • @ resting potential, chemically gates channels are closed

• membrane expose to chemical that opens chemically gates Na channels

  • gates open, Na+ enters

  • shift from resting potential to + value

• Sodium ions entering move away from channels (attract to - charges along inner surface)

• local current

changes in membrane potential

When a chemical stimulus opens sodium ion channels, depolarization occurs (Na+ comes into cell) making the membrane potential more positive (excitatory response)

When a chemical stimulus opens a potassium channel, potassium leaves the cell, making membrane potential more negative (inhibitory response)

Pain

• degree of depolarization decreases with distance from stimulus

• The change of membrane potential is proportional to the stimulus size

Potentials and their channel type

• depends on leak channels

• Graded potentials depends on chemically gated channels

• Action potentials depend on voltage gated channels

• excitatory (depolarizing)

• inhibitory (hyperpolarizing)

Action potentials

• All-or-none

• irreversible

• once formed, always the same amplitude

• rapid (1 ms)

• reverse membrane potential, then restores it

• short and long distance communication

• runs along axon

steps of action potential generation

1. Depolarization to threshold

2. Activation of Na+ channels, rapid depolarization

3. Inactivation of Na+ channels and activation of K+ channels

4. Potassium on channels close, return to resting membrane potential

abolute refractory period

• the time following an action potential which a new action potential cannot be initiated (can't respond to another stimulus)

relative refractory period

• A period after firing when a neuron is returning to its normal polarized state and will fire again only if the incoming message is much stronger than usual

propogation of the action potential

• transmission of an action potential down an axon

• 2 types:

  • Continuous propagation: action potential appears to move step by step through the entire axon

    • Occurs in unmyelinated axons, much SLOWER

  • Saltatory propagation: in myelinated axons, depolarizes only at nodes

    • faster, speed varies w/ axon diameter

Synapses

• Information is transferred from neuron to neuron or from neuron to an effector cell.

2 types:

1. Chemical synapse

2. Electrical synapse

Chemical synapses

• Most abundant type of synapse

• neurotransmitter

• Cholinergic synapses - release acetylcholine

  • most common

steps of cholinergic synapse

1. Axon terminal depolarized by arriving action potential.

2. Depolarization opens voltage gated calcium channels

3. ACh diffuses across synaptic cleft, binds to chemically gated Na+ channel receptors on postsynaptic membrane

4. Effects on the postsynaptic membrane are temporary

Electrical synapses

• contain tunnels called gap junctions

• connect pre and postsynaptic neurons

• rare

postsynaptic potential

• graded potential in postsynaptic membrane in response to a neurotransmitter

• 1. Excitatory postsynaptic potential (EPSP):

  • graded depolarization, shifts membrane potential closer to threshold

• 2. Inhibitory postsynaptic potential (IPSP):

  • Graded hyperpolarization, shifts membrane potential farther away from threshold

autonomic nervous system

• (involuntary movement such as smooth and cardiac muscle)

• 2 Subsections

  • Sympathetic: Stress, “fight or flight”

  • Parasympathetic: Relaxed, “rest and digest”

Ependymal cells

• lines central canal (spinal cord) + ventricles (brain)

• produce, circulate, monitor cerebrospinal fluid (CSF)

Microglia

• engulf invading microbes

• clear cell debris + waste products

Astrocytes

• form blood-brain barrier

• maintain ion, nutrient, gas concentration

• take up excess neurotransmitters

Oligodendrocytes

• support fibers in CNS

• produce myelin

resting membrane potential

• potential of neuron to send signals

• -70 mV

• inside more negative than outside

contributors to resting membrane potential

• extracellular fluid (ECF) - high Na+, Cl-

• cytosol - high K+, Pr-

graded potential

• temporary local change

• decreases with distance

• from stimulus

action potential

• electrical event

• triggered by sufficiently large graded potential

synaptic activity

• presynaptic - releases neurotransmitters

• postsynaptic - bind neurotransmitter to receptors

• changes permeability

• produce graded potentials

summation

• integration of effects of graded potentials

• collective effects of both EPSPs and IPSPs

• net effect may be no change in membrane potential