The Nervous System Overview

The Nervous System

• The nervous system is a complex communication network containing over 100 billion nerve cells, primarily in the brain.
• It's divided into two main parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

Endocrine vs. Nervous Control

• Endocrine control (hormones) takes time to affect change. The imbalance must be detected, the appropriate gland stimulated then hormones released into the bloodstream for target cells to be affected, potentially over multiple seconds or minutes.
• Nervous control is fast, in mere seconds or fractions of a second, responding swiftly to environmental changes.

Organization of the Nervous System

• The CNS consists of the brain and spinal cord, coordinating all incoming and outgoing information.
• The PNS includes nerves connecting the body's various areas to the CNS.

PNS: Somatic System

• The somatic nervous system controls skeletal muscles, bones, and skin, receiving sensory information and sending it back to the brain. It also has voluntary control.
• Sensory receptors relay changes in the environment, like heat, light, touch, taste, and smell, enabling responses.

PNS: Autonomic System

• The autonomic nervous system regulates the body's internal organs automatically, without conscious control. This includes processes like digestion, stomach churning, and intestinal movement.
• It's split into two further systems, the sympathetic and parasympathetic nervous system.

PNS: Autonomic - Sympathetic vs. Parasympathetic

• Sympathetic: dominates in stressful or physically demanding situations. Increases heart rate, blood pressure, dilates pupils.
• Parasympathetic: dominates in calm, resting states, promoting digestion and other restorative processes. Conversely slows heart rate and blood pressure. Both systems usually work in balance.

The Nerve Cell

• Neurons are the basic functional units of the nervous system, composed of three main parts:
  • Dendrites receive signals from other cells.
  • The cell body contains the cell nucleus and other organelles.
  • The axon transmits signals away from the cell body.
• Sensory neurons relay information from the environment to the CNS.
• Interneurons connect neurons within the CNS.
• Motor neurons relay information from the CNS to muscles or glands.

Neurons: Structure & Function

• Dendrites receive information from other nerve cells.
• The cell body maintains the neuron's life.
• Axons transmit the signals or impulses.
• Myelin sheath is an insulating layer around the axon of some neurons that speeds up transmission.
• Nodes of Ranvier are gaps in the myelin sheath.

Electrochemical Impulse

• The nerve signal is electrochemical, generated by the flow of ions (charged atoms) across the cell membrane. The movement of positive and negative ions is what causes the charge difference across the membrane.
• At rest, the membrane's charge difference is called the resting potential (−70 mV).
• Excitation or stimulation can cause a charge difference reversal (action potential), which propagates signals along an axon. Potassium and sodium ions are central to this, pumped in or out by a sodium-potassium pump.
• The refractory period is a brief time after an action potential when a neuron cannot fire again.

What Causes Neuron Excitation

• A stimulus needs to reach a threshold level to generate an action potential.
• Threshold prevents small, unimportant changes from triggering nerve impulses.
• Intensity of a stimulus determines the frequency of action potentials.

Threshold Level

• All-or-none response means a neuron either fires or doesn't, with no variations in intensity once the threshold is reached. The intensity of the stimulus is communicated through the frequency of the action potential.

How Body Interprets Signal Intensity/Frequency

• The brain interprets the intensity of a stimulus by the frequency of action potentials.
• Higher frequencies convey stronger stimuli.

The Synapse

• The synapse is the gap between two nerve cells or a nerve cell and a muscle.
• Chemical neurotransmitters transmit signals across the synapse.

Synaptic Transmission

• Neurotransmitters in vesicles are released into the synapse.
• Neurotransmitters bind to receptors on the receiving neuron.
• Once the signal is transmitted, the enzyme cholinesterase breaks the neurotransmitter down/recycles it.

Neurotransmitters

• Acetylcholine example of a neurotransmitter.

Botulism

• Botulism toxin, produced by bacteria, interferes with neurotransmitter function by paralyzing nerves, preventing muscle contractions.
• Recovery from botulism can take a long time due to the nerve regeneration process.
• Botulism toxin can be used medically in tiny doses to treat muscle spasms or in cosmetics for example.

Reflex Arc

• The reflex arc is a rapid, automatic response, bypassing the brain.
• It includes the receptor, sensory neuron, interneuron, motor neuron, and effector.
• It's critical in responding to danger or potential harm when rapid action is essential.

Explore the intricate structure and function of the nervous system, including the distinctions between the Central and Peripheral Nervous Systems. Understand the rapid responses of the nervous system compared to the slower endocrine system and learn about the somatic system's role in controlling voluntary movements.