Human Nervous System Notes

Responding to the Environment - Humans

Human Nervous System

Need for the Nervous System

  • The nervous system is essential for:

    • Detecting changes in both the internal and external environments of the body.

    • Coordinating various bodily activities.

External Environment

  • The environment outside the body. The nervous system detects changes and responds to maintain a constant internal environment (homeostasis).

Internal Environment

  • The environment within the body, including:

    • Body temperature

    • Blood sugar level

    • Salt content

    • Blood pressure

Actions Coordinated by the Nervous System

  • Voluntary actions: Actions under conscious control (e.g., walking, talking).

  • Involuntary actions: Actions not under conscious control (e.g., heartbeat, sweating).

Terminology

  • Involuntary action:

    • Definition: Any action not under direct control of one's will.

    • Example: Dilation and constriction of blood vessels.

  • Voluntary action:

    • Definition: Any action under direct control of one's will.

    • Example: Walking to a friend's home.

Nervous Tissue

Components

  • The nervous system is composed of organs, which are made of tissues. Tissues are made of cells called neurons.

  • Types of neurons:

    • Sensory neurons

    • Connector neurons

    • Motor neurons

Terminology

  • Stimulus:

    • Definition: Any change in the environment (e.g., temperature change).

  • Receptors:

    • Definition: Nerve tissue or sense organs that detect stimuli (e.g., eyes, ears, nerve endings in fingertips).

  • Impulse:

    • Definition: A nerve message, which is converted from a stimulus and transmitted by neurons.

  • CNS:

    • Definition: Central Nervous System, comprising the brain and spinal cord.

  • Effector:

    • Definition: A muscle or gland that brings about a response to a stimulus (e.g., pancreas lowering blood sugar).

Structure of a Typical Neuron

  • Two main parts:

    • Cell body

    • Axon

Cell Body

  • Contains:

    • Dendrites: Branching cytoplasmic threads that carry nerve impulses toward the cell body.

    • Cytoplasm: Ground substance within the cell body.

    • Nissl granules (Nissl bodies): Involved in the manufacture of substances for nerve impulse transmission.

    • Nucleus

    • Mitochondria, ribosomes, and endoplasmic reticulum (ER)

Axon

  • A long outgrowth surrounded by the neurilemma.

  • Myelin sheath: Occurs below the neurilemma providing electrical insulation and speeding up nerve impulse transmission.

  • Nodes of Ranvier: Interruptions in the myelin sheath at intervals.

  • Terminal branches: Branched structures at the end of the axon that transmit nerve impulses away from the cell body.

Synapse

  • A tiny gap between neurons where the dendrite of one neuron lies next to the terminal branch of another.

  • Impulses are carried across the synapse by electrical impulses and special chemicals called neurotransmitters (synaptic contact).

  • Significance: Allows for communication between neurons.

Types of Neurons

  • Sensory neuron: Carries impulses from receptors to the brain and spinal cord; has a centrally situated cell body.

  • Motor neuron: Carries impulses from the central nervous system to muscles or glands (effectors); cell body on one end, axon terminals on the other.

  • Connector neuron (Interneuron): Carries impulses from sensory to motor neurons within the central nervous system.

The Human Nervous System

Divisions

  • Central Nervous System (CNS)

  • Peripheral Nervous System

  • All parts function together for coordination and homeostasis.

Central Nervous System (CNS)

  • Brain:

    • Upper, enlarged part of the spinal cord, protected by the cranium and meninges (dura mater, arachnoid, pia mater).

    • Composed of cerebrum, cerebellum, and brain stem.

    • Internally has ventricles filled with cerebrospinal fluid.

    • Gives rise to 12 pairs of cranial nerves.

  • Functions:

    • Receives and interprets sensations.

    • Controls higher thought processes.

    • Responsible for voluntary and involuntary actions.

    • Coordinates voluntary actions.

    • Maintains muscle tone, balance, and equilibrium.

Cerebrum

  • Largest part of the brain, made of grey matter (outside) and white matter (inside).

  • Divided into left and right hemispheres:

    • Right hemisphere controls the left side of the body.

    • Left hemisphere controls the right side of the body.

  • Hemispheres held together by nerve fibers, the largest being the corpus callosum.

  • Thalamus is found below the callosum.

  • Hypothalamus is found below the thalamus.

  • Functions:

    • Controls all voluntary actions.

    • Responsible for sensations (sight, taste, hearing).

    • Responsible for memory, judgment, and reasoning (higher thought processes).

Hypothalamus

  • Functions:

    • Body temperature

    • Blood pressure

    • Sleep

    • Emotions

    • Appetite

    • Thirst

Cerebellum

  • Located behind the pons Varolli and medulla oblongata; made of grey matter (outside) and white matter (inside).

  • Functions:

    • Coordinates voluntary movements.

    • Maintains muscle tone, balance, and equilibrium.

Brain Stem

  • Composed of pons Varolli, mid-brain, and medulla oblongata.

Medulla Oblongata

  • Connects to the spinal cord; made of white matter (outside) and grey matter (inside); contains cerebrospinal fluid in the central canal.

  • Functions:

    • Transmits impulses between the spinal cord and brain.

    • Responsible for involuntary actions (breathing, heartbeat, salivation, swallowing) via reflex centers.

  • Impulses cross over in the medulla oblongata, so the right side of the brain controls the left side of the body and vice versa.

Brain Scans

  • MRI (Magnetic Resonance Imaging):

    • Uses magnets and radio waves, no x-rays.

    • Patient lies still inside a long cylinder; noisy examination lasting about 30 minutes.

  • CAT scan (Computerized Axial Tomography):

    • Specialized x-rays; the patient lies on a couch in a circular opening while an x-ray tube rotates.

Spinal Cord

  • Continuation of the brain/medulla oblongata, located within the vertebral column which protects it.

  • Also protected by the meninges.

  • Spinal nerves enter and leave through openings between vertebrae.

  • Made of H-shaped grey matter (centrally located) surrounded by white matter.

  • The spinal canal is found in the grey matter and contains cerebrospinal fluid.

  • Sensory neurons enter via the dorsal root, making synaptic contact with connector neurons, which then contact motor neurons.

  • Motor neurons leave via the ventral root.

  • The dorsal root ganglion is a swelling in the dorsal root accommodating cell bodies.

  • Connector neurons are located between sensory and motor neurons.

  • Functions:

    • Transmits impulses between receptors, brain, and effectors.

    • Reflex center for actions like blinking, sneezing, coughing, and knee-jerk.

Reflex Arc and Reflex Action

Reflex Action

  • A rapid, automatic response to a stimulus.

Reflex Arc

  • The path an impulse travels during a reflex action; the functional unit of the nervous system.

Example of Reflex Arc

  1. Heat receptors in finger detect heat (stimulus).

  2. Stimulus converted to nerve impulse.

  3. Sensory neuron carries the impulse to the spinal cord via the dorsal root.

  4. Sensory neuron contacts connector neuron.

  5. Connector neuron contacts motor neuron.

  6. Motor neuron carries impulse to muscles (effector) via the ventral root.

  7. Muscles contract, and hand moves away from the flame (reflex action).

  8. A second impulse is sent to the brain to register pain and awareness.

  • Some reflex actions are controlled by the brain (e.g., sneezing, coughing).

Components Involved in Reflex Action

  • Spinal cord (CNS)

  • Spinal nerves with sensory, connector, and motor neurons (peripheral nervous system)

  • Receptor

  • Effector

Significance of Reflex Action

  • Protects delicate tissues (e.g., moving hand away from heat to prevent burning).

Terminology

  • Reflex action:

    • Definition: A rapid, automatic response to a stimulus received by a receptor.

    • Example: Pulling your toe away from a sharp object.

  • Reflex arc:

    • Definition: The path taken by the impulse during a reflex action.

    • Example: The quick removal of your toe from a sharp object.

The Peripheral Nervous System

Composition

  • All nerves outside the central nervous system, including cranial and spinal nerves.

Divisions

  • Somatic Nervous System

  • Autonomic Nervous System

Somatic Nervous System

  • Reacts to changes in the external environment.

  • Made up of:

    • Free nerve endings and sense organs

    • Sensory nerves

    • Motor nerves

    • Muscles and glands (effectors)

Autonomic Nervous System

  • Controls the internal environment, responsible for actions like heart rate and sweating.

  • Made up of:

    • Centers in the brain and spinal cord

    • Nerve cells in smooth muscles, glands, and internal organs

    • Sympathetic and parasympathetic nervous systems

  • Sympathetic and parasympathetic nerves work antagonistically (in opposition).

    • Example: Sympathetic nerves increase heart rate, while parasympathetic nerves decrease it.

Functions of the Autonomic Nervous System

  • Controls subconscious activities (e.g., heartbeat, dilation and constriction of blood vessels).

  • Main function of the peripheral nervous system: To connect the central nervous system (CNS) to the limbs and organs.

Terminology Summary

  • Involuntary actions: Actions not under conscious control.

  • Voluntary reactions: Actions under conscious control.

  • Stimuli: Changes in the environment.

  • Receptors: Nerve endings or sense organs detecting environmental changes.

  • CNS: Central nervous system (brain and spinal cord).

  • Effectors: Organs or glands that respond to stimuli.

  • Synapse: Microscopic gap between two neurons.

  • Reflex action: Rapid, automatic response to a stimulus.

  • Reflex arc: Path taken by an impulse during a reflex action.