Nervous System Overview

The Nervous System

• The nervous system is a complex communication system, containing over 100 billion nerve cells, primarily in the brain.
• The nervous system has two main divisions: Central Nervous System (CNS) and Peripheral Nervous System (PNS).
• The CNS comprises the brain and spinal cord, coordinating incoming and outgoing information.
• The PNS consists of nerves connecting the body's regions to the CNS.

Endocrine vs. Nervous Control

• Endocrine control is slower and sustained, relying on hormones traveling via the bloodstream to target cells.
• Nervous control is rapid, responding to changes within seconds.
• Nervous pathways are rapid and affect immediate changes in the body.

Organization of the PNS

• The PNS is further divided into somatic and autonomic systems.
• The somatic nervous system governs skeletal muscles, bones and skin, relaying external environment information to the brain. It controls voluntary movements.
• The autonomic nervous system manages internal organ functions, like digestion, without conscious control. This system is further divided into sympathetic and parasympathetic branches.

Autonomic Nervous System

• The sympathetic division is active during stress, danger, excitement, and strenuous activity. It increases heart rate, blood pressure, and dilates pupils.
• The parasympathetic division is active during quiet, low-stress situations. It slows heart rate and promotes processes like digestion. These systems work in opposition to control body functions.

The Neuron

• Neurons are the fundamental units of the nervous system, and are categorized into three main types.
• Sensory neurons (afferent) receive environmental information and relay it to the CNS.
• Interneurons link neurons within the brain and spinal cord.
• Motor neurons (efferent) relay information from the brain to effectors (muscles, organs).

The Nerve Cell Structure

• The cell body houses the cellular organelles and conducts metabolic processes.
• Dendrites receive information from other nerve cells.
• The axon transmits information away from the cell body. The myelin sheath (composed of glial cells) surrounds many axons and speeds nerve impulses.

Electrochemical Impulse

• When a nerve isn't sending information, it's at rest. There's a charge difference across the membrane called the resting potential (-70mV).
• Ions are distributed unevenly across the membrane (mostly sodium outside and potassium inside): The sodium-potassium pump maintains this balance.
• Excitation causes the membrane to become more permeable to sodium, and the membrane potential reverses (becomes +40mV), creating an action potential.
• The signal moves down the neuron in a wave of depolarization, which is carried by electrochemical impulses.
• During the refractory period the neuron re-polarizes, preparing for the next impulse. The refractory period delays the firing of the next impulse.

Threshold Level

• A stimulus needs to reach a certain strength (threshold) to trigger an action potential. The intensity of the stimulus (strength of the signal) doesn't affect the size of the action potential. Instead, the frequency dictates the intensity of the encoded message.
• When the threshold is reached, neurons fire at the same intensity and speed. These impulses follow an all-or-none rule.

Signal Intensity and the Brain

• The brain interprets signal intensity by the frequency of the nerve impulses. More frequent impulses indicate a stronger stimulus.

The Synapse

• The synapse is the space between nerve endings. Information transfer across this gap involves chemical neurotransmitters.
• Neurotransmitters are released in response to an action potential.
• Neurotransmitters bind to receptors on the postsynaptic neuron, initiating a process that can result in an action potential.

Neurotransmitters and Botulism

• Acetylcholine is a common neurotransmitter; for example, it triggers muscle contraction and plays a role in nerve impulse transmission across synapses.
• Botulism toxin interferes with neurotransmitter release, leading to muscle paralysis.

The Reflex Arc

• The reflex arc is a rapid, automatic response to an environmental stimulus.
• It involves a receptor, a sensory neuron, an interneuron, a motor neuron, and an effector.
• Signal processing happens in the spinal cord, allowing a quick response.
• The brain also receives the information (and can be aware of it) and the pathway that is used to transmit the information to the brain is slower.

Explore the intricate workings of the nervous system, including its divisions and functions. Understand the distinctions between nervous and endocrine control, and learn about the organization of the peripheral nervous system. This quiz covers essential concepts and structures crucial for understanding human physiology.