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.