Peripheral Nervous System and Motor Output

Exam 4

  • Exam 4, also called the final exam, will cover the current chapter and the muscle chapter. It will not be comprehensive.

  • This exam is the hardest of the semester, so preparation is key.

Neuron Review

  • A neuron consists of a cell body, axon, and presynaptic terminals that release neurotransmitters onto receptors of another cell at a synapse.

  • The receiving cell can be another neuron or a muscle cell.

Nerves vs. Tracts

  • Nerve: A bundle of axons in the peripheral nervous system (PNS).

  • Tract: A bundle of axons in the central nervous system (CNS).

  • Analogy: A nerve is like a rope made of many smaller strings (axons).

Ganglia vs. Nuclei

  • Ganglion: A group of cell bodies in the peripheral nervous system (PNS).

  • Nucleus: A group of cell bodies in the central nervous system (CNS).

Peripheral Nervous System Overview

  • The peripheral nervous system (PNS) consists of:

    • Receptors

    • Nerves (bundles of axons)

Sensory and Motor Nerves

  • Sensory Nerve: Carries information from receptors to the central nervous system (CNS).

    • This is called sensory input; information conducts to the central nervous system.

  • Motor Nerve: Carries information from the central nervous system (CNS) to effectors.

    • This is called motor output; information conducts away from the central nervous system.

  • Information travels via action potentials along axons (saltatory or continuous conduction).

  • Effectors: Structures that receive signals from the motor nerves (e.g. Muscles).

Divisions of the Peripheral Nervous System

  • Two divisions of the peripheral nervous system (PNS):

    • Somatic Nervous System

    • Autonomic Nervous System

Subdivisions of the Autonomic Nervous System

  • Three subdivisions of the autonomic nervous system:

    • Sympathetic Nervous System

    • Parasympathetic Nervous System

    • Enteric Nervous System (primarily controls the digestive system)

      • Will be discussed in the second semester, focusing on sympathetic and parasympathetic for now.

Sensory Receptors: Somatic vs. Autonomic

  • Sensory receptors are divided into:

    • Somatic Receptors

    • Autonomic Receptors

  • Somatic receptors can influence the autonomic nervous system, and vice versa.

Somatic Receptors

  • Located in the skin, muscles, joints, and nose.

  • Exteroceptors: Detect stimuli from the external environment.

    • Examples: Merkel's disc and Pacinian corpuscle (detect pressure).

    • Free nerve endings (dendrites) for pain detection.

    • Olfactory receptors for smell.

  • Proprioceptors: Muscle spindles and Golgi tendon apparatus (already covered).

Autonomic Receptors

  • Located in the viscera (internal organs).

  • Receive stimuli from the internal environment.

    • Examples: Blood vessels, liver, stomach, bladder, uterus, lungs.

  • Types of stimuli:

    • Pressure (e.g., blood pressure detected by baroreceptors).

    • Stretch (e.g., stomach mechanoreceptors).

    • Chemicals (e.g., H+ ions, sodium bicarbonate detected by chemoreceptors).

    • Osmolarity (concentration of fluids detected by osmoreceptors).

Receptor Function

  • Receptors convert stimuli into electrical signals (membrane potential changes).

  • This is translated into action potential frequency, which carries information.

  • Each receptor type has a specific stimulus that it detects.

Action Potential Frequency and Stimulus Intensity

  • A receptor is stimulated, causing a change in membrane potential. If threshold is reached, action potentials are generated at a certain frequency.

  • Increased stimulus strength leads to increased action potential frequency.

  • Decreased stimulus strength leads to decreased action potential frequency.

Sensory Adaptation

  • Sensory adaptation will not be covered.

Motor Output Overview

  • Appropriate motor output ensures effectors perform their specific jobs.

  • The central nervous system (CNS) communicates with effectors via the peripheral nervous system (PNS), using nerves (axons).

Cholinergic and Adrenergic Terminology

  • Cholinergic: Anything associated with acetylcholine.

    • A neuron that releases acetylcholine is a cholinergic neuron.

    • Receptors that bind acetylcholine are cholinergic receptors.

    • Two types of cholinergic receptors: muscarinic and nicotinic.

  • Adrenergic: Anything associated with epinephrine and norepinephrine (focus on norepinephrine).

    • A neuron that releases norepinephrine is an adrenergic neuron.

    • Receptors that bind norepinephrine are adrenergic receptors.

    • Two types of adrenergic receptors: alpha and beta.

Somatic Motor Output

  • Central nervous system (brain stem and spinal cord) controls effectors via motor nerves.

  • Effectors: Skeletal muscles.

  • Motor neurons of the somatic nervous system are myelinated, enabling saltatory conduction.

  • Appropriate output: Skeletal muscle contraction for movement.

  • Single motor neuron extends from the central nervous system to the effector.

  • Motor neurons release acetylcholine at the synapse with skeletal muscle cells.

  • The receptors on the skeletal muscle cells are nicotinic receptors.

  • Somatic motor output is voluntary; we have conscious control over skeletal muscles (though not always requiring constant thought).

Autonomic Motor Output

  • The central nervous system (brain stem and spinal cord) controls effectors via motor nerves.

  • Cervical region (C) has no autonomic motor output.

  • Effectors: Cardiac muscle, smooth muscle, visceral cells (glands, fat cells, liver cells).

  • Two motor neurons connect the central nervous system to the effectors:

    • Preganglionic Neuron: Myelinated, arises from the central nervous system.

    • Postganglionic Neuron: Not myelinated, synapses with the preganglionic neuron and extends to the effector.

  • Cell bodies of postganglionic neurons form ganglia of the autonomic nervous system.

  • Ganglia are groups of cell bodies in the PNS.