Nervous System: Structure and Function

Basic Structure and Function of the Nervous System

• This topic covers the anatomical and functional divisions of the nervous system.
• It relates the structural differences between gray and white matter to neuron structure.
• It also lists the basic functions of the nervous system.

Basics of the Nervous System

• Basic functions include sensation, response, and integration.

Sensation

• It is processing information from outside or inside the body, known as a stimulus.
• Sensation interprets adjustments away from homeostasis.
• Taste and smell are chemical stimuli.
• Touch is a physical or mechanical stimulus.
• Sight is interpreting light stimuli.
• Hearing is the perception of sound, which is physical.

Response

• Responses are based on sensory stimuli.
• Actions include muscle contractions and reflexes.
• Glands can secrete or inhibit production.
• Voluntary responses are controlled by the somatic nervous system.
• Involuntary responses are controlled by the autonomic nervous system.

Integration

• Sensory information is processed with memories, past experiences, and emotional states.
• Integration leads to the generated response.
• The response depends on the stimulus context and individual experiences.

CNS and PNS

• The nervous system has two main regions:
  • The Central Nervous System (CNS), consists of the brain and spinal cord.
  • The Peripheral Nervous System (PNS), includes everything else.
• The cells are glial cells and neurons.

Terminology

• A nucleus is a localized collection of neuron cell bodies in the CNS.
• A ganglion is a cluster of neuron cell bodies in the PNS.
• A tract is a bundle of axons in the CNS.
• A nerve is a bundle of axons in the PNS.
• Optic Nerve becoming the Optic Tract is an example of the above

Types of Nervous System

• The somatic nervous system involves conscious perception and voluntary response.
• The autonomic nervous system involves involuntary control and homeostasis.
• The enteric nervous system pertains to the digestive system and is part of the PNS.
• The sensory system involves afferent signals from the periphery to the CNS.
• The motor system involves efferent signals to the effectors from the CNS.

Nervous Control

• Sensory receptors detect stimuli like Miesner, Pacinian, and nociceptors.
• The afferent pathway transmits signals to the CNS.
• The control center contains interneurons that analyze and respond to stimuli.
• The efferent pathway sends motor output to effectors.
• Effectors carry out decisions, such as muscles and glands.

Nervous Tissue Objectives

• Describing the basic structure of a neuron.
• Identifying the different types of neurons on the basis of polarity.
• Listing the glial cells of the CNS and describing their function.
• Listing the glial cells of the PNS and describing their function.

Neurons

• The cell body (soma) contains most organelles and processes.
• The axon connects.
• Dendrites receive signals and provide directional information flow.

Neurons - Gray Matter

• Gray matter is area with many cell bodies and dendrites.

Neurons - White Matter

• White matter is area with many axons.

Neurons - Myelin

• Myelin is a lipid-rich substance made from glial cells that insulates the axon.

Gray and White Matter

• Gray matter contains many cell bodies and dendrites.
• White matter contains many axons.
• Myelin causes the white color.

Nervous Tissue

• Includes neurons and glial cells.
• Information flows in one direction: dendrites to soma to axon.
• The cytoplasm in the axon is called axoplasm and it starts at the axon hillock.
• Myelin covers the axon, with breaks at the node of Ranvier.
• The area with myelin is called an axon segment.
• The axon terminal is the end of an axon, finding the synaptic end bulb or knob.

Properties of Neurons

• Excitability is responding to stimuli of Acetylcholine (Ach) and GABA.
• Conductivity is transmitting an electrical signal or charge.
• Secretion is producing and expelling neurotransmitters.

Types of Neurons

• Unipolar neurons have one process emerging from the cell and are sensory.
• Bipolar neurons have two processes, one from each end of the cell body, found in olfactory epithelium and the retina.
• Multipolar neurons include all neurons that are not unipolar or bipolar.

Glial Cells of CNS

• Four different types: Astrocytes, Oligodendrocytes, Microglia, and Ependymal cells.
• Astrocytes support and maintain chemical concentration, are star-shaped, and form the blood-brain barrier (BBB).
• Oligodendrocytes insulate axons and create myelin.
• Microglia are phagocytic and important for neuron connections.
• Ependymal cells produce CSF.

Glial Cells of PNS

• Satellite cells support similar to astrocytes.
• Schwann cells insulate axons and create myelin.

Myelin

• The myelin sheath is 100-1000 times longer than it is wide and wrapped with many layers.
• Myelin enables electrical signals.
• Glial cells make them.
• Demyelination causes electrical signaling to be slower.

Action Potential

• This section describes the components of the membrane that establish the resting membrane potential.
• Also describes the changes that result in an action potential when the membrane is responding.

Synapses

• Connections between electrically active cells which can be chemical or electrical.

Neurotransmitter Systems

• Cholinergic systems use nicotinic and muscarinic receptors with Acetylcholine.
• Amino acids includes glutamatergic, GABAergic, and glycinergic systems.
• Biogenic amines (catecholamines) lack carboxyl groups.
• Neuropeptides are chains of amino acids.

Neurotransmitters

• Acetylcholine (Ach) is excitatory. It causes Na+ to enter and cause depolarization (EPSP).
• GABA is inhibitory. It causes CI- to enter and reduce depolarization (IPSP).

Electrical Charge

• More Na+ and Cl- are outside the cell, while K+ is inside.
• Electrochemical exclusion means the pore is charge specific.
• Size exclusion means the pore's diameter is specific to an ion size.
• Nonspecific channels are selective for charge, but not size (Na+, K+, Ca++).
• Some channels are gated. Only opening for certain events.

Receptor Classes

• Ionotropic receptors have a channel that opens.
• Metabotropic receptors open other channels.

Electrically Active Membranes

• Membrane potential regulates what goes in and out with charged particles.
• Lots of anions are inside and lots of cations are outside.
• The difference of -70mV is the resting membrane potential.
• Concentration gradient and electrical potential.

Ligand Gated Channels

• Ligands open the channel which is found around dendrites.
• Ligands bind with ions crossing, causing membrane charge changes.
• Can be excitatory or inhibitory.
• Voltage becomes less negative.

Mechanically Gated Channel

• Physical changes to the cell membrane, mostly found around the dendrites.
• Touch channels are pressure to skin to allow channels to open.
• Temperature channels are an example.

Voltage Gated Channel

• Electrical charges in the membrane, in the axon hillock and nodes of Ranvier.
• Channels open.

Leakage Channel

• Randomly gated with resting transmembrane voltage.

Sodium-Potassium Pump

• This pump helps to maintain concentration gradient.
• It uses energy (ATP) to pump Na+ out of the cell.
• It uses energy (ATP) to pump K+ into a cell.
• Is greatly needed after an action potential.

The Action Potential

• Na+ rushes in, causing depolarization which cause the membrane potential to hit +30mV.
• Voltage gated channels open, with K+ leaving which leads to repolarization and hyperpolarization.

Stages of an Action Potential

• Resting voltage is -70mV/-55mV.
• External stimulus causes depolarization.
• Voltage rises as Na+ goes in.
• Voltage falls as the cell repolarizes and returns to rest.
• The Na+ activation gate is at -55mV and an inactivation gate at +30mV.
• The K+ gate opens and closes at -50mV, which makes it slower.

Action Potential: Flow of Signal

• The refractory period is when an action potential cannot start while another is in progress.
• The absolute refractory period is when an action potential cannot start because of the inactivation gate.
• The relative refractory period is when getting back to rest a stimulus can make Na+ rush into the cell. However K+ is still leaving the cell, so another action potential needs a larger stimulus.

Action Potential Propagation

• Propagation initiates at the initial segment.
• Depolarization spreads, Na+ channels open, and Na+ moves along inside of the membrane.
• Continuous conduction occurs in unmyelinated axons and is slow.
• Saltatory conduction occurs in myelinated axons where the action potential jumps.
• Depolarization spreads faster down wide axons due to lower resistance.

Communication Between Neurons

• Explanation of the differences between the types of graded potentials.
• Categorization of the major neurotransmitters by chemical type and effect.

Communication Between Neurons

• Local potential is a change in voltage in the receptive region that may not lead to an action potential, because it is incremental.
• Excitatory postsynaptic potential means the membrane moves toward the threshold.
• Inhibitory postsynaptic potential means the membrane moves away from the threshold.

Summation

• Small changes add together to lead the neuron to threshold.
• Spatial summation means multiple inputs to one neuron.
• Temporal summation means multiple action potentials coming from a single cell.

Function of Nervous Tissue

• A thermoreceptor in hand interprets the hot/cold signal, changing electrical state along a graded potential.
• If graded potential is strong enough to reach threshold, it will act as an action potential which travels down the axons and across synapses
• The signal goes to the thalamus (sensory relay) which then relays it to the cerebral cortex to be perceived
• The cerebral cortex sends a signal telling the muscles what to do in response to the hot stimuli in conjunction with your emotional state

Hot Water Example

• The cerebral cortex sends the signal down to the upper motor neuron which synapses with the lower motor neuron in the spinal cord
• The signal goes through a lower motor neuron and to the nerve and neuromuscular junction of the muscle
• The muscle then gets the action potential so it can contract

Review Questions

• The types of neurons.
• The different types of channels.
• What synapses are.