Nervous Tissue Overview

Nervous Tissue Overview

• Nervous tissue is a specialized type of biological tissue that carries information throughout the body.
• Its purpose is to maintain homeostasis by controlling conditions in the body.

Purpose of the Chapter

• Understand how the nervous system maintains stable internal environments for health.
• Learn about the different parts of the nervous system.
• Learn about different types of nerve cells.

Nervous System Overview

• The nervous system has two main divisions; Central Nervous System (CNS) and Peripheral Nervous System (PNS).
• The CNS includes the brain and spinal cord.
• Sensory neurons are specialized cells that respond to stimuli by the body and transmit signals to the CNS.
• Interneurons are located in the brain and spinal cord; process and integrate information gathered by sensory neurons,
• Motor neurons will send signals away from the CNS to muscles to respond to the stimulus.

Nervous System Organization

• The PNS consists of cranial and spinal nerves.
• Autonomic nervous system regulates involuntary actions such as heart rate.
• Somatic nervous system controls voluntary actions such as movement.
• Enteric plexuses in the gut regulate digestive processes.
• Sensory receptors in skin detect external stimuli, such as temperature.

Nervous System Functions

• Sensory: Detecting changes using sensory receptors.
• Integrative: Processing incoming sensory information, analyzing it, storing aspects of it, and making decisions for appropriate behaviors.
• Motor: Responding to stimuli by actions via effectors.

Nervous System Histology

• Neurons are electrically excitable cells that transmit nerve impulses.
• Neuroglia support neurons; maintain the body's chemical environment, multiply, and divide.

Neuron Structure

• Electrically excitable
• Cell body containing nerve impulses
• Dendrites receive signals
• Axon transmits signals
• Action potentials are nerve impulses

Neuron Classification (Structure)

• Multipolar: many processes extending from the cell body
• Bipolar: two processes extending from the cell body
• Pseudounipolar: one process extending from the cell body that divides into two branches

Neuron Classification (Function)

• Sensory (afferent): carry information from sensory receptors to the CNS
• Motor (efferent): carry information from the CNS to effectors (muscles or glands)
• Interneurons: connect neurons within the CNS

Neuroglia (CNS)

• Astrocytes: support neurons, maintain chemical balance
• Oligodendrocytes: produce myelin
• Microglia: phagocytes (immune function)
• Ependymal cells: line cavities, circulate cerebrospinal fluid (CSF)

Neuroglia (PNS)

• Satellite cells: support neurons
• Schwann cells: produce myelin

Myelination of Neurons

• Myelin sheath is a fatty layer produced by Schwann cells in the PNS and oligodendrocytes in the CNS.
• Increases the speed of nerve impulse conduction.
• Nodes of Ranvier are gaps in the myelin sheath where action potentials are generated.

Gray Matter vs White Matter

• Gray matter contains neuron cell bodies and dendrites.
• White matter primarily contains axons and myelin.

Electrical Signals in Neurons

• Communication between neurons occurs using actions potentials and graded potentials.
• Action potentials travel long distances, graded potentials short distances.
• Graded potentials change the membrane potential from the resting potential.

Ion Channels in Neurons

• Leakage channels continuously open and close; maintain the resting membrane potential.
• Ligand-gated channels open in response to neurotransmitters.
• Mechanically gated channels open in response to mechanical stimuli (pressure, vibration)
• Voltage-gated channels open in response to changes in membrane potential.

Resting Membrane Potential

• The membrane of a non-conducting neuron is negatively charged inside and positively charged outside.
• This is established by unequal ion distribution, selective permeability of the membrane to specific ions, and active transport using Na+/K+ pumps.

Graded Potentials

• Small changes in the membrane potential, caused by various stimuli.
• Can be depolarizing (making the inside less negative) or hyperpolarizing (making the inside more negative).
• Do not propagate down the axon as action potentials do.

Summation of Postsynaptic Potentials

• A series of graded potentials add up to determine whether an action potential will be relayed.
• Summation can be spatial or temporal, meaning several inputs together or continuous inputs, respectively.

Action Potentials

• Rapid changes in membrane potential; occur when the graded potentials reach a threshold.
• Two Phases: Depolarization begins as sodium ions enter the axon, repolarization occurs as potassium ions leave the axon.

Propagation of Action Potentials

• Action potentials are propagated along the axon for communication.
• The signal is generated at the axon hillock.
• Continues down the axon because of sodium and potassium channels on the membrane.

Factors Affecting Propagation Speed

• Axon diameter: Larger diameter axons conduct APs faster.
• Myelination: Myelin sheath increases speed.
• Temperature: Higher temperatures speed up APs.

Signal Transmission At Synapses

• Chemical synapses are one-way transfer of information from a presynaptic neuron to a postsynaptic neuron.
• Electrical synapses transmit info bi-directionally via gap junctions.

Postsynaptic Potentials

• Excitatory postsynaptic potentials (EPSPs): depolarizing.
• Inhibitory postsynaptic potentials (IPSPs): hyperpolarizing.

Neurotransmitter Receptors

• Two structures for receptors: Ionotropic and Metabotropic.
• Ionotropic have a binding site and an ion channel, whereas metabotropic have a binding site and a G-protein which controls the opening/closing/activation of separate ion channels.

Removal of Neurotransmitters

• Diffusion: Neurotransmitter diffuses away from the synaptic cleft.
• Degradation: enzymes break down the neurotransmitter.
• Uptake into cells: neurotransmitter is transported back into presynaptic neuron or other cells in the area.

Summation

• Summation can be spatial (Multiple inputs at once) or temporal (Inputs one after another in quick succession).
• The sum of ESPS's and IPSP's will determine if an action potential will be generated.

Action Potentials and Synapses

• Synapses are the junction between neurons.
• Action potentials are propagated.
• Multiple synapses can summate to generate an action potential.

Neuronal Regeneration and Repair

• Neurons have limited ability to regenerate.
• The process is dependent on whether the cell body and nerves around it are intact.

Neurogenesis

• Neurogenesis (CNS regeneration) is limited and there is no ability to replace old neurons as quickly as other cells.

Neuronal Disorders

• Multiple sclerosis is an autoimmune disease that progressively destroys the myelin sheath.
• Depression is several types of disorders where common feelings are lack of interest in activities, sadness, helplessness and possibly suicidal thoughts.

Other Neural Disorders

• Epilepsy is a neurological disorder characterized by seizures.
• Excitotoxicity is a neuronal death caused by overstimulation, usually due to high levels of neurotransmitter in the CNS. Note: Neuroglia play an important role in both excitatory and inhibitory responses and thus this phenomenon.

This quiz focuses on the structure and function of nervous tissue and its role in the nervous system. You will learn about the central and peripheral nervous systems, the types of nerve cells involved, and how they work together to maintain homeostasis in the body. Test your understanding of the key concepts related to nervous tissue.