CH 12 Nervous Tissue Overview
Divisions of the Nervous System
Anatomical Divisions
Central Nervous System (CNS)
Brain
Spinal cord
Peripheral Nervous System (PNS)
Nerves (groups of axons in the PNS)
Ganglia (groups of cell bodies in the PNS)
Functional Divisions
Somatic Nervous System
Communicates with sense organs and voluntary muscles.
Functions include somatic sensory neurons (e.g., touch, pain) and somatic motor neurons (innervate skeletal muscle).
Autonomic Nervous System (ANS)
Communicates with internal organs and glands.
Controls visceral motor responses by autonomic ganglia.
Sympathetic Division (arousing)
Parasympathetic Division (calming)
Enteric Nervous System (ENS)
Involved in digestion.
Operates independently of the brain and spinal cord, responsible for autonomous digestive functions.
Gray Matter vs. White Matter
Gray Matter:
Composed of neuron cell bodies, dendrites, unmyelinated axons, and glia.
In the brain, gray matter is superficial; in the spinal cord, it is deep.
Functions as processing centers (e.g., initiation of reflexes in spinal cord).
White Matter:
Composed mainly of myelinated axons.
In the brain, white matter is deep; in the spinal cord, it is superficial.
Functions as transmission pathways (e.g., pathways for sensory and motor functions between the periphery and brain).
Parts of a Multipolar Neuron (Order of Polarity)
Dendrites:
Receive incoming signals from other neurons.
Cell Body (Soma):
Contains the nucleus and organelles; integrates incoming signals and generates action potentials.
Axon:
Transmits electrical signals away from the cell body.
Axon Terminals:
Form synapses with other neurons or effector cells, releasing neurotransmitters.
Types of Glial Cells & Functions
CNS Glial Cells:
Astrocytes:
Adjust blood supply to neurons, help facilitate material transfer from blood to the nervous system, and maintain the blood-brain barrier.
Oligodendrocytes:
Myelinate CNS axons, forming the myelin sheath for rapid signal transmission.
Microglial Cells:
Act as immune defense in the CNS; remove debris, microbes, and damaged tissue.
Ependymal Cells:
Line ventricular surfaces and produce cerebrospinal fluid (CSF).
PNS Glial Cells:
Schwann Cells:
Myelinate PNS axons, wrapping around single axons to form the myelin sheath.
Satellite Cells:
Surround neuronal cell bodies in PNS ganglia; support and regulate the neuron's environment.
Major Functions of the Nervous System
Sensation (Sensory Function):
Detects changes through sensory receptors; receives information about the environment (stimuli).
Integration (Integrative Function):
Processes and interprets sensory information, stores some aspects, and makes decisions about appropriate behaviors.
Response (Motor Function):
Responds to stimuli by initiating a motor output, such as muscle contractions or gland secretions.
Communication
Resting Membrane Potential (RMP) Components
Key Ions:
Na^+ (more abundant outside the cell)
K^+ (more abundant inside the cell)
Cl^-
Membr ane Proteins:
Sodium-potassium pump (Na^+/K^+ ATPase):
Actively transports 3 Na^+ out for every 2 K^+ into the cell, maintaining the concentration gradients.
Leakage Ion Channels:
Randomly open and close, allowing specific ions (especially K^+) to pass through based on concentration gradients.
Typical RMP Value:
Approximately -70 mV, established and maintained by the Na^+/K^+ pump and leak channels.
Action Potential & Membrane Changes
Definition: An action potential (AP) is an electrical impulse that travels along a neuron, characterized as a rapid, all-or-none change in the membrane potential.
Phases of Action Potential:
Resting State:
The membrane is at its resting potential of approximately -70 mV. Voltage-gated Na^+ and K^+ channels are largely closed.
Threshold:
A stimulus causes local depolarization. If the membrane potential reaches the threshold (typically -55 mV), voltage-gated Na^+ channels open rapidly, initiating the AP.
Depolarization:
Na^+ ions rush into the cell, making the inside of the membrane more positive, reaching a peak of about +30 mV.
Repolarization:
Voltage-gated Na^+ channels inactivate, and slower voltage-gated K^+ channels open. K^+ ions exit the cell, making the inside less positive and returning it toward a negative state.
Hyperpolarization:
K^+ channels remain open slightly longer, allowing excess K^+ outflow and causing the membrane potential to become temporarily more negative than the resting potential.
Return to Resting Membrane Potential:
The Na^+/K^+ pump and leak channels restore the original ion distribution and resting potential, preparing the neuron for the next action potential.
Refractory Period:
Absolute Refractory Period:
The period during which a neuron cannot respond to another stimulus, regardless of strength, because voltage-gated Na^+ channels are inactivated.
Relative Refractory Period:
The period when a stronger-than-normal stimulus is required to elicit another action potential, due to the hyperpolarized state and some Na^+ channels having reset.
Myelination and Signal Speed
Myelin Sheath:
Produced by Schwann cells (in PNS) and oligodendrocytes (in CNS).
An insulating layer around axons, critical for rapid communication.
Myelination significantly speeds up signal transmission through saltatory conduction, where action potentials jump between Nodes of Ranvier.
Factors Affecting Action Potential Speed:
Myelination: Myelinated neurons transmit signals faster than unmyelinated neurons.
Axon Diameter: Larger diameter axons have faster conduction speeds.
Synaptic Transmission
Mechanisms of Neuron Communication:
Neurons communicate at synapses through neurotransmitters (NT).
Pre-synaptic Neuron: Initiates signal; releases NT into the synaptic cleft.
Post-synaptic Neuron: Receives NT and converts it into an electrical signal.
Steps in Chemical Synapse Functioning:
Action potential arrives at the presynaptic membrane.
Neurotransmitter is released from synaptic vesicles into the synaptic cleft.
NT diffuses across the synaptic cleft.
NT binds to receptors on the postsynaptic membrane, altering its potential.
Postsynaptic potential changes, potentially triggering an action potential at the postsynaptic axon hillock.
Neurotransmitters:
Chemical messengers that affect various body functions, including cognition, mood, and muscle control.
Examples: Acetylcholine (ACh), gamma-aminobutyric acid (GABA), serotonin, epinephrine.
Divisions of the Nervous System
Anatomical Divisions
Central Nervous System (CNS)
Brain
Spinal cord
Peripheral Nervous System (PNS)
Nerves (groups of axons in the PNS)
Ganglia (groups of cell bodies in the PNS)
Functional Divisions
Somatic Nervous System
Communicates with sense organs and voluntary muscles.
Functions include somatic sensory neurons (e.g., touch, pain) and somatic motor neurons (innervate skeletal muscle).
Autonomic Nervous System (ANS)
Communicates with internal organs and glands.
Controls visceral motor responses by autonomic ganglia.
Sympathetic Division (arousing)
Parasympathetic Division (calming)
Enteric Nervous System (ENS)
Involved in digestion.
Operates independently of the brain and spinal cord, responsible for autonomous digestive functions.
Gray Matter vs. White Matter
Gray Matter:
Composed of neuron cell bodies, dendrites, unmyelinated axons, and glia.
In the brain, gray matter is superficial; in the spinal cord, it is deep.
Functions as processing centers (e.g., initiation of reflexes in spinal cord).
White Matter:
Composed mainly of myelinated axons.
In the brain, white matter is deep; in the spinal cord, it is superficial.
Functions as transmission pathways (e.g., pathways for sensory and motor functions between the periphery and brain).
Parts of a Multipolar Neuron (Order of Polarity)
Dendrites:
Receive incoming signals from other neurons.
Cell Body (Soma):
Contains the nucleus and organelles; integrates incoming signals and generates action potentials.
Axon:
Transmits electrical signals away from the cell body.
Axon Terminals:
Form synapses with other neurons or effector cells, releasing neurotransmitters.
Types of Glial Cells & Functions
CNS Glial Cells:
Astrocytes:
Adjust blood supply to neurons, help facilitate material transfer from blood to the nervous system, and maintain the blood-brain barrier.
Oligodendrocytes:
Myelinate CNS axons, forming the myelin sheath for rapid signal transmission.
Microglial Cells:
Act as immune defense in the CNS; remove debris, microbes, and damaged tissue.
Ependymal Cells:
Line ventricular surfaces and produce cerebrospinal fluid (CSF).
PNS Glial Cells:
Schwann Cells:
Myelinate PNS axons, wrapping around single axons to form the myelin sheath.
Satellite Cells:
Surround neuronal cell bodies in PNS ganglia; support and regulate the neuron's environment.
Major Functions of the Nervous System
Sensation (Sensory Function):
Detects changes through sensory receptors; receives information about the environment (stimuli).
Integration (Integrative Function):
Processes and interprets sensory information, stores some aspects, and makes decisions about appropriate behaviors.
Response (Motor Function):
Responds to stimuli by initiating a motor output, such as muscle contractions or gland secretions.
Communication
Resting Membrane Potential (RMP) Components
Key Ions:
Na^+ (more abundant outside the cell)
K^+ (more abundant inside the cell)
Cl^-
Membrane Proteins:
Sodium-potassium pump (Na^+/K^+ ATPase):
Actively transports 3 Na^+ out for every 2 K^+ into the cell, maintaining the concentration gradients.
Leakage Ion Channels:
Randomly open and close, allowing specific ions (especially K^+) to pass through based on concentration gradients.
Typical RMP Value:
Approximately -70 mV, established and maintained by the Na^+/K^+ pump and leak channels.
Action Potential & Membrane Changes
Definition: An action potential (AP) is an electrical impulse that travels along a neuron, characterized as a rapid, all-or-none change in the membrane potential.
Phases of Action Potential:
Resting State:
The membrane is at its resting potential of approximately -70 mV. Voltage-gated Na^+ and K^+ channels are largely closed.
Threshold:
A stimulus causes local depolarization. If the membrane potential reaches the threshold (typically -55 mV), voltage-gated Na^+ channels open rapidly, initiating the AP.
Depolarization:
Na^+ ions rush into the cell, making the inside of the membrane more positive, reaching a peak of about +30 mV.
Repolarization:
Voltage-gated Na^+ channels inactivate, and slower voltage-gated K^+ channels open. K^+ ions exit the cell, making the inside less positive and returning it toward a negative state.
Hyperpolarization:
K^+ channels remain open slightly longer, allowing excess K^+ outflow and causing the membrane potential to become temporarily more negative than the resting potential.
Return to Resting Membrane Potential:
The Na^+/K^+$$ pump and leak channels restore the original ion distribution and resting potential, preparing the neuron for the next action potential.