The Human Nervous System Comprehensive Notes

The Human Nervous System

Overview

  • The human nervous system enables coordinated reactions to external and internal stimuli. It functions by:

    1. Gathering information using sense organs.

    2. Transmitting information to processing areas.

    3. Processing information.

    4. Formulating responses to stimuli.

    5. Sending information to effector organs to execute the response.

Stimuli and Responses

  • The body detects external and internal changes and responds accordingly.

  • A stimulus is anything that causes a response. (Plural: stimuli)

  • The nervous system receives and responds to stimuli.

  • Receptor cells are the cells that receive stimuli.

Components of the Nervous System

  • Central Nervous System (CNS): The brain and spinal cord.

  • Peripheral Nervous System (PNS): All nerves leaving the CNS.

  • The nervous system sends messages via electrical impulses along nerves.

  • The nervous system consists of neurons, which include:

    1. Sensory neurons (unipolar)

    2. Motor neurons (multipolar)

    3. Interneurons (connectors)

Divisions of the Nervous System

  • Central Nervous System (CNS):

    • Brain and spinal cord.

    • Integrated control center.

  • Peripheral Nervous System (PNS):

    • Peripheral nerves (cranial and spinal).

    • Communication between the CNS and the body.

  • Sensory (Afferent) Division:

    • Consists of sensory neurons.

    • Transports impulses from receptors to the CNS.

  • Motor (Efferent) Division:

    • Consists of motor neurons.

    • Transports impulses from the CNS to the effectors.

  • Autonomic Nervous System:

    • Controls involuntary reactions.

  • Somatic Nervous System:

    • Controls voluntary movements.

  • Sympathetic Division:

    • Mobilizes body systems.

    • "Fight or flight" response.

  • Parasympathetic Division:

    • Conserves energy.

    • "Rest and digest" response.

Neuron Classification by Function

  • Sensory Neuron:

    • Conducts impulses from receptors to the CNS.

  • Motor Neuron:

    • Conducts impulses from the CNS to the effectors.

  • Effectors: Muscles and glands that bring about a response.

  • Interneuron:

    • Connects sensory and motor neurons.

Neuron Classification by Structure

  • Unipolar Neuron:

    • One outgrowth from the cell body.

    • Typically sensory/afferent neurons.

  • Bipolar Neuron:

    • One dendrite and one axon.

  • Multipolar Neuron:

    • Many dendrites and one axon.

    • Typically motor/efferent neurons.

  • Note: Neurons always have only one axon but can have many dendrites.

Neuron Structure

  • Cell body

  • Dendrites

  • Axon

Cell Body

  • Contains a large nucleus with Nissl granules, which produce neurotransmitters.

  • Neurotransmitters are chemicals that transmit nerve impulses.

Dendrites

  • Conduct impulses toward the cell body.

  • Connect one neuron with the next.

Axon

  • Conducts impulses away from the cell body.

  • Ends in terminal knobs.

  • Covered with neurilemma.

  • Under the neurilemma is the myelin sheath.

  • The myelin sheath provides insulation and speeds up the transmission of impulses.

Synapse

  • The junction between two or more neurons, forming a small gap.

  • Impulses are transmitted from one neuron to the next across the synapse by neurotransmitters.

  • Neurons do not make direct contact.

Synaptic Transmission

  1. Impulse reaches the terminal knob at the end of the axon.

  2. Vesicles produce and secrete neurotransmitters.

  3. Neurotransmitters diffuse across the synapse.

  4. Neurotransmitters attach to the dendrite of the next neuron.

  5. A new electrical impulse travels along the dendrite.

Central Nervous System (CNS) Protection

  • Cranium and vertebral column.

  • Three meninges:

    • Pia mater: Delicate membrane lining the folds of the brain, containing blood vessels to supply food and oxygen.

    • Arachnoid mater: Thin and vascular.

    • Dura mater: Outer, tough, fibrous membrane lining the skull.

  • Cerebrospinal Fluid:

    • Located in the sub-arachnoid space.

    • Acts as a shock absorber.

    • Helps maintain pressure.

    • Keeps nervous tissue moist.

    • Provides food and oxygen to neurons.

    • Removes waste products.

Brain Regions

  1. Cerebrum

  2. Cerebellum

  3. Mid-brain (Thalamus and Hypothalamus)

  4. Pons Varolii

  5. Medulla Oblongata

Brain Lobes

  • Frontal Lobe: Consciousness.

  • Parietal Lobe: Perception and sensation.

  • Occipital Lobe: Vision.

  • Temporal Lobe: Speech recognition.

Brain Structures and Functions

Cerebrum

  • Two hemispheres joined by a band of white fibers called the corpus callosum.

  • Grey matter (neuron cell bodies) on the outside.

  • White matter (myelinated axons) on the inside.

  • The left hemisphere controls activities on the right side of the body, and vice versa.

  • Functions:

    1. Controls all voluntary activities, including speech.

    2. Receives and interprets sensations (vision, taste, smell, hearing, and touch).

    3. Seat of higher mental activities (memory, creativity, reasoning, imagination).

Cerebellum

  • Two hemispheres joined by a thin membrane called the vermis.

  • Grey matter (neuron cell bodies) on the outside.

  • White matter (myelinated axons) on the inside, known as the arbor vitae.

  • Functions:

    1. Controls the coordination of voluntary movements (walking, running).

    2. Controls balance, muscle tone, and equilibrium.

Medulla Oblongata

  • Merges with the spinal cord at the foramen magnum.

  • Connects the spinal cord to higher brain regions.

  • Lower part has grey matter inside and white matter outside.

  • Functions:

    1. Controls vitally important reflexes such as breathing, heartbeat, blood pressure, swallowing, peristalsis, coughing, hiccups, vomiting, sneezing.

    2. Transmits nerve impulses to and from the brain.

    3. Nerve fibers cross over, meaning the left side of the brain controls the right side of the body and vice versa.

Spinal Cord

  • Lies within the vertebral canal (33 vertebrae).

  • Consists of 31 pairs of spinal nerves.

  • Protected by the vertebral column and the three meninges.

  • Structure:

    • Inner central canal contains cerebrospinal fluid, providing food and oxygen to nerve cells.

    • H-shaped grey matter on the inside.

    • White matter on the outside.

  • Functions:

    1. Pathway for impulses to and from the brain.

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

Ganglion

  • A cluster of neuron cell bodies located outside the central nervous system (CNS), primarily within the peripheral nervous system (PNS).

  • Acts as a gathering point where multiple nerve cells connect, forming a relay station for nerve impulses.

  • Location: Found in the PNS, which includes nerves and ganglia outside the brain and spinal cord.

  • Function: Act as synaptic relay stations, transmitting signals between neurons.

  • Types:

    • Sensory ganglia: Contain the cell bodies of sensory neurons, transmitting sensory information from the periphery to the CNS.

    • Autonomic ganglia: Involved in the autonomic nervous system, regulating involuntary bodily functions.

  • Example: The dorsal root ganglia are sensory ganglia located along the spinal cord.

Reflex Arc and Reflex Action

  • Reflex arc is the path an impulse travels along to bring about a response to a stimulus during a reflex action.

  • Reflex action is a fast, automatic response to a stimulus by an organ or receptor to prevent damage to the body.

  • A reflex arc involves 3 types of neurons and the spinal cord; the brain is not involved.

Reflex Arc Components

  1. Pin (stimulus)

  2. Receptor sets up an impulse

  3. Afferent neuron (sensory neuron)

  4. Interneuron

  5. Efferent neuron (motor neuron)

  6. Finger pulls away (response)

Action of Reflexes

  • Prevents the body from harm.

  • Gives a quick response without thinking.