Lecture 1

Introduction to the Nervous System

• The nervous system consists of:

  • Central nervous system (CNS): Brain and spinal cord

  • Peripheral nervous system (PNS): Nerves serving the neck, arms, trunk, legs, skeletal muscles, and internal organs

The Stretch Reflex

• Simplest stimulus-response paradigm in the human nervous system.

• Stretch reflex: Muscle contraction in response to stretching within the muscle.

• Patellar-tendon stretch reflex:

  • Tapping the patellar tendon (attached to the quadriceps muscle) stretches the quadriceps.

  • This activates nerve impulses in stretch receptors located in the quadriceps muscle.

  • Nerve impulses are sent to the spinal cord along a sensory (afferent) neuron and activate another nerve cell.

  • The activated nerve cell sends impulses back to the quadriceps muscle, causing it to contract, resulting in a jerk or swing of the foot.

The Withdrawal Reflex

• Another reflex to be examined.

Components of the Nervous System

• Central Nervous System (CNS):

  • Brain and spinal cord

  • Composed of:

    • Cerebral cortex

    • Cerebellum

    • Brain stem

    • Spinal cord (anatomically similar along its length)

• Peripheral Nervous System (PNS):

  • Peripheral nerves

  • Receptors (e.g., stretch receptors in muscles)

  • Axons connecting the spinal cord to the muscles

Cells of the Nervous System

• Neuron: A nerve cell (distinct from a nerve).

• Two main types of cells:

  • Neurons (~10% of CNS cell number, ~50% of volume - bigger than glia).

  • Glia

• Types of Neurons:

  • Afferent neurons: Carry information from the periphery to the CNS.

  • Efferent neurons: Carry information from the CNS to the periphery.

  • Interneurons: Carry information between neurons within the CNS.

• Afferent and Efferent Neurons:

  • Generally excitatory.

  • Afferent neurons synapse either directly onto efferent neurons or onto interneurons (excitatory contact).

  • Efferent neurons make contact onto muscle (receiving input from afferent neurons or interneurons).

  • Parts of afferent and efferent neurons are always in the PNS.

• Interneurons:

  • Can be excitatory or inhibitory.

  • Located entirely within the CNS.

  • Receive input from afferent neurons or other interneurons.

  • Synapse onto efferent neurons or other interneurons.

The Stretch Reflex (Cellular Level)

• Hammer tap stretches muscle → activates afferent neuron → travels to spinal cord and synapses with an efferent neuron → efferent neuron travels back to the periphery → quadriceps muscle contraction

Glia

• The "glue" of the nervous system.

• About 90% of cells in the nervous system, occupying ~50% of the volume.

• Include:

  • Oligodendrocytes

  • Schwann cells

  • Astrocytes

  • Microglia

• Oligodendrocytes and Schwann cells:

  • Produce myelin.

  • Oligodendrocytes: myelin in the CNS.

  • Schwann cells: myelin in the PNS.

Spinal Cord

• Interface for Reflexes:

  • Two Main Components:

    • Gray matter: Horn or butterfly shape

    • White matter: Surrounds the gray matter

• Afferent and Efferent Neurons:

  • Innervate muscles but are separated around the muscle.

  • Travel towards the CNS within mixed peripheral or spinal nerves.

  • Mixed peripheral nerves contain both afferent and efferent fibers.

  • Afferent neurons: Carry information from the periphery to the spinal cord via dorsal roots.

  • Efferent neurons: Carry information from the spinal cord to the periphery via ventral roots.

  • Interneurons: Carry information between neurons and are located entirely within the CNS.

Reflex Loop

• Circular in nature.

• Receptor activation → Afferent fiber activation (enters via dorsal root) → Efferent fiber activation (leaves via ventral root) → Muscle activation.

Myelin

• Produced by oligodendrocytes in the CNS.

• Coats axons, enabling faster nerve impulse transmission.

• Myelinated axons are in the white matter of the spinal cord.

• White matter appears white due to the presence of myelin.

• Gray matter contains cell bodies of interneurons and unmyelinated processes, giving it a non-white appearance.

Stretch Reflex (Detailed)

• Tendon tap stretches quadriceps muscle → stretch receptor activation → afferent neuron sends information to the spinal cord → afferent neuron travels with efferent neurons in the mixed peripheral nerve until near the spinal cord → afferent neuron enters via the dorsal root and contacts an efferent neuron in the ventral part of the cord → efferent neuron exits via the ventral root → efferent neuron travels via the mixed peripheral nerve to the quadriceps muscle, causing contraction.

Interneurons in Stretch Reflex

• Can be excitatory or inhibitory.

• Always located within the CNS.

• Inhibitory interneuron inhibits efferent neurons innervating the hamstrings muscle.

• This prevents hamstrings contraction which could interfere with the reflex response.

Neuron Structure and Polarity

• Neuron: A nerve cell.

• Components:

  • Dendrites: Receive information from the periphery or other cells.

  • Cell body (soma): Contains the nucleus.

  • Axon hillock: Initial segment of the axon; integrates information and generates nerve impulse.

  • Axon: Propagates nerve impulses from the initial segment to axon terminals.

  • Axon terminals/synaptic terminals: Contain neurotransmitter-filled synaptic vesicles.

• Information flow in the nervous system is unidirectional: Dendrites/cell bodies → axon → axon terminals.

• Synapse: Junction between two neurons.

• Presynaptic neuron: Neuron before the synapse.

• Postsynaptic neuron: Neuron after the synapse.

Types of Neurons

• Neurons have varying morphologies but contain the same basic parts (dendrites, cell body, axon, axon terminals).

• Afferent neuron in stretch reflex is a pseudo-unipolar cell.

Afferent Neuron of Stretch Reflex

• Sensory neuron carrying information from muscle stretch to the CNS.

• Pseudo-unipolar cell.

• Cell body is in the dorsal root ganglion.

• Peripheral axon: Extends from the cell body to the muscle.

• Central axon: Extends from the cell body into the CNS, contacting other neurons.

Neuron Structure

• Myelinated axon enables faster nerve impulse transmission.

Stretch Reflex - Afferent Neuron Details

• Afferent neuron:

  • Pseudo-unipolar cell.

  • Peripheral axon extends to muscle and receives stretch input from the quadriceps muscle.

  • Central axon extends to the CNS.

  • Makes two synapses/contacts onto other neurons:

    • Efferent neuron to the quadriceps muscle (direct monosynaptic contact).

    • Inhibitory interneuron.

• Afferent neuron makes direct monosynaptic contact onto the efferent neuron which innervates the quadriceps muscle. This excitatory efferent neuron is activated, resulting in contraction of the quadriceps.

• The afferent neuron synapses with an inhibitory interneuron (afferent neuron is excitatory and activates this interneuron).

• The inhibitory interneuron contacts an efferent neuron innervating the antagonistic muscle (hamstring).

• Contact onto the inhibitory interneuron inhibits the efferent neuron traveling to the hamstrings muscle, preventing contraction of the hamstrings muscle.

• When the quadriceps muscle is stretched, it's activated to contract, while the contraction of the hamstrings muscle is inhibited.

• Interneurons and the two efferent neurons to the quadriceps and hamstrings muscles are multipolar cells:

  • Dendrites emanate from the cell body.

  • Multipolar neurons have an axon whose axon terminals terminate in the muscle, affecting the muscle's activity.