Nervous System Structure and Function

Structural Classification of the Nervous System

• The central nervous system (CNS) is composed of the brain and spinal cord.
• The peripheral nervous system (PNS) includes nerves outside the brain and spinal cord, along with ganglia and nerves.

Functional Classification of the Peripheral Nervous System

• The sensory (afferent) division carries information to the CNS.
• The motor (efferent) division carries impulses away from the CNS.
  • The somatic nervous system is voluntary, controlling skeletal muscles.
  • The autonomic nervous system is involuntary, regulating glands, smooth muscles, and cardiac muscles.
    • The autonomic nervous system has two divisions: sympathetic and parasympathetic.

Functions of the Nervous System

• Sensory input: monitors changes inside and outside the body (stimuli).
• Integration: processes and interprets sensory input and decides if action is needed.
• Motor output: a response to integrated stimuli that activates muscles or glands.

Motor Neuron

• Dendrites receive signals.
• The soma contains the nucleus and other organelles.
• The axon hillock is where the signal travels down the axon.
• The myelin sheath insulates the axon.
• The axon transmits the signal.
• Synaptic terminals release neurotransmitters to other cells.

Sensory Neuron

• Dendrites receive signals from sensory receptors.
• The soma contains the nucleus and other organelles.
• The axon carries the signal to the CNS.
• Sensory endings are at the end of the dendrites that detect stimuli.

Nerve Impulse Propagation

• Impulses travel faster when fibers have a myelin sheath, due to insulated charge flow along the axon, rather than the entire length of the axon.
• Increasing the diameter of the axon decreases the internal resistance to ion flow, facilitating faster transmission of the nerve impulse.

Nervous Tissue

• The basic structural unit of the CNS is the nerve cell (neuron).
• Neurons are cells specialized for transmitting messages.
• Major regions of neurons include the cell body (nucleus and metabolic center), and processes (fibers extending from the cell body).
  • Nissl bodies (granules), contain RER
  • Nucleus- Contains the nucleolus
• Dendrites conduct impulses towards the cell body.
• Axons conduct impulses away from the cell body.
• A nerve is formed by a collection of axons.

Axons and Nerve Impulses

• Axons end in axonal terminals that contain vesicles with neurotransmitters.
• Axonal terminals are separated from the next neuron by a gap called the synaptic cleft.
• The synapse is the junction between nerves. Synaptic vesicles release neurotransmitters into the synaptic cleft. Target receptors are triggered by this neurotransmitter. This allows a signal to be passed between neurons.

Nerve Fiber Coverings

• Schwann cells produce myelin sheaths around axons. -Insulates and speeds up transmission.
• Nodes of Ranvier are gaps in the myelin sheath along the axon.

Neuron Cell Body Location

• Most neurons are found within the central nervous system.
• Gray matter contains cell bodies and unmyelinated fibers.
• Nuclei are clusters of cell bodies within the white matter of the CNS.
• Ganglia are collections of cell bodies outside of the CNS.

Sectional Organization of the Spinal Cord

• The spinal cord has a central canal surrounded by gray matter.

Functional Properties of Neurons

• Irritability: ability to respond to a stimulus.
• Conductivity: ability to transmit an impulse

Resting Membrane Potential (RMP)

• Resting membrane potential is the voltage (mV) across a neuron's cell membrane at rest.
• In nerves, the RMP is approximately -70mV.
• In muscles, the RMP is approximately -90mV.

Action Potential

• The action potential (nerve impulse) is a reversal of membrane polarity.
• The exchange of ions initiates the action potential.
• When the action potential begins, it propagates along the entire axon.
• Afterward, potassium ions rush out of the neuron, repolarizing the membrane.
• The sodium-potassium pump restores the original configuration of ions (3 Na+ pumped out, 2 K+ pumped in).

Functional Classification of Neurons

• Sensory (afferent) neurons carry impulses from receptors to the CNS.
• Motor (efferent) neurons carry impulses from the CNS to effectors (muscles, glands).
• Interneurons (association neurons) connect sensory and motor neurons within the CNS.

Nerve Impulse Propagation

• Nerve impulses travel faster in myelinated fibers, due to saltatory conduction (jumping between nodes of Ranvier).
• Increasing axon diameter decreases internal resistance to ion flow, facilitating faster AP propagation.

Autonomic Nervous System (ANS)

• The ANS controls involuntary functions like heart rate, digestion, and glandular secretions.
• The ANS has two divisions: sympathetic and parasympathetic.

Sympathetic Division

• The sympathetic division, often called the "fight-or-flight" system, is activated in stressful or emergency situations.
• It is often involved with increasing energy expenditure and preparing the body for action.
• It also regulates the activity of blood vessels, sweat glands, and bronchioles.

Parasympathetic Division

• The parasympathetic division, often called the "rest-and-digest" system, is activated under relaxed conditions,.
• It is the "brake" on expenditure of energy.

Neurotransmitters

• Acetylcholine (ACh) is a neurotransmitter involved in both somatic and autonomic nervous systems.
• Norepinephrine (NE) is a neurotransmitter released by most postganglionic sympathetic neurons.

Drug Effects on the Autonomic Nervous System

• Neostigmine, a synaptic cleft enzyme inhibitor, blocks acetylcholinesterase, leading to the accumulation of ACh.
• Tricyclic antidepressants prolong norepinephrine binding to postsynaptic receptors.
• Beta-blockers block norepinephrine binding to beta-1 receptors, reducing heart rate.

Antagonistic Control

• Sympathetic and parasympathetic divisions often have opposing effects on target organs (e.g., heart rate).
• The control mechanisms function to maintain balance and homeostasis.

Cooperative Effects

• The sympathetic and parasympathetic divisions work together to regulate some functions that require both increase and decrease (e.g., external genitalia).

Sympathetic ONLY

• Norepinephrine and epinephrine are released in response to sympathetic stimulation.
• Sweat glands are stimulated, blood vessels constrict, and renin is released.
• Thermoregulation mechanisms can also be activated, such as vasodilation of capillaries.

Central Nervous System Regulation of the Autonomic Nervous System

• The central nervous system (CNS) controls the autonomic nervous system.
  • The medulla oblongata regulates heart rate.
  • The pons regulates respiration.
  • The hypothalamus coordinates many autonomic functions.

Hypothalamus

• The hypothalamus coordinates autonomic functions, such as heart rate, blood pressure, temperature, endocrine functions, and behaviours.

Explore the structural and functional classifications of the nervous system. This quiz covers the central and peripheral nervous systems, as well as the sensory and motor divisions. Test your understanding of how the nervous system processes information and generates responses.