The Nervous System Overview

The Nervous System

• The nervous system is a complex communication system, containing over 100 billion nerve cells (neurons) in the brain alone.
• It's divided into two main parts:
  • Central Nervous System (CNS): Composed of the brain and spinal cord. It coordinates incoming and outgoing information.
  • Peripheral Nervous System (PNS): Consists of nerves carrying information between the body and the CNS.

Endocrine vs. Nervous Control

• Prolonged Response (Endocrine): Hormone release in response to imbalances takes time. Hormones travel through the bloodstream to target cells, causing a prolonged response.
• Speed (Nervous): The nervous system responds rapidly (seconds or fractions of seconds) to quick changes in the environment.

PNS: The Somatic System

• The somatic nervous system controls voluntary actions.
• This system controls skeletal muscles, bones, and skin. It relays information about the external environment to the brain.
• We have conscious control over this system. We can sense environmental changes like heat, light, touch, taste, and smell and respond accordingly.

PNS: The Autonomic System

• The autonomic nervous system controls involuntary actions. This system handles the internal organs (e.g., stomach churning, digestion).
• It's not under conscious control.
• It's balanced by the sympathetic and parasympathetic systems like an on/off switch.

PNS: Autonomic - Sympathetic vs. Parasympathetic

• Sympathetic: Dominates during stress, danger, excitement, or strenuous activity. It increases the heart rate and blood pressure.
• Parasympathetic: Dominates during quiet or low-stress situations. It reduces sympathetic responses, promoting functions like digestion.

The Nerve Cell

• Neurons are the functional units of the nervous system.
• Three types of neurons:
  • Sensory (afferent): Detect and relay information from the environment to the CNS. Located in ganglia outside the spinal cord.
  • Interneurons (association neurons): Connect other neurons, primarily within the brain and spinal cord.
  • Motor (efferent): Relay information from the brain to muscles, organs, and glands to respond to stimuli.

Neuron Structure

• Dendrites: Receive information from other nerve cells.
• Cell body: Contains organelles and carries out metabolic processes.
• Axon: Carries signals away from the cell body to other neurons, muscles, or glands.
• Myelin sheath: Insulating layer on some axons, increasing transmission speed.
• Nodes of Ranvier: Gaps in the myelin sheath that speed up signal transmission.

Electrochemical Impulse

• Resting potential: The membrane maintains a charge difference (-70 mV) when not sending signals.
• Action potential: When a neuron is excited, the membrane potential briefly reverses (+40 mV) creating a fast-moving excitation. Sodium and potassium ions contribute to this electrochemical process.

Neuron Excitation and the Threshold Level

• Sensory neurons respond to environmental change (stimuli).
• The stimulus’s intensity must reach a threshold level, causing the depolarization of the membrane in order for the nerve signal to be sent.
• A stronger stimulus does not cause a stronger response; instead the response is sent by the same speed and intensity.

Signal Intensity

• The brain interprets signal intensity based on the frequency of impulses (how often signals are sent).
• More intense stimuli trigger signals more rapidly.

The Synapse

• The space between neurons.
• Information passes from one neuron to another through neurotransmitters released by the presynaptic neuron..
• Neurotransmitters bind to receptors on the postsynaptic neuron, initiating the next signal.

Synaptic Transmission

• Chemical neurotransmitters travel across the synaptic cleft, initiating a response by binding to postsynaptic neuron receptors.
• The speed of transmission is influenced by the number of synapses involved, with multiple synapses resulting in slower transmission.

Neurotransmitters

• Acetylcholine is a common neurotransmitter that opens sodium ion channels in the postsynaptic neuron, transmitting signals.
• After transmission, the enzyme cholinesterase breaks down acetylcholine.

Botulism

• Botulism toxin interferes with the release of acetylcholine. This paralysis the muscles and stops them from contracting.
• The effects of botulism toxin can be enduring (permanent).

Reflex Arc

• Used for quick responses to threatening stimuli.
• Consists of:
  • Receptor that detects environmental change
  • Sensory neuron relays signal to the spinal cord.
  • Interneuron in the spinal cord integrates the information.
  • Motor neuron relays the signal to activate an effector (muscle, gland) to make a change.