Somatic Nervous System Anatomy

Somatic Nervous System

• Involved in muscle contractions
• Part of the peripheral nervous system

Peripheral Nervous System Components

• Enteric: Autonomic, provides information to smooth muscle, especially in the gut.
• Autonomic: Sympathetic and parasympathetic.
• Somatic: Voluntary control.

Somatic Nervous System Anatomy

Precentral Gyrus

• Anatomical location of the primary motor cortex.
• Contains motor neurons that initiate movement.
• Controls voluntary motor commands.

Postcentral Gyrus

• Contains the primary somatosensory cortex.
• Responsible for perceiving somatic sensations like pain, touch, and temperature.
• Referred to as the primary sensory cortex or somatosensory cortex.
• The central sulcus serves as a key landmark.

Basal Nuclei

• Subconscious control of muscle
• Important role in development

Cerebellum

• Balance
• Posture and patterns
• Important role in development

Motor Neuron Directions

• Upper motor neurons in the primary motor cortex send information to the spinal cord.
• The specific pathway taken by motor neurons depends on the message being sent.
• Some messages travel from the brain stem to the spinal cord.
• Some bypass the spinal cord and exit directly from the brain stem via lower motor neurons.
• The level at which an action occurs dictates whether the signal exits from the brain stem or spinal cord.
• Motor tracts are bundles of axons delivering messages to skeletal muscles through corticospinal tracts.

Feedback Loop

• Information from the primary motor cortex (efferent signal) instructs muscles to contract or activate.
• Information from the body travels to the spinal cord and brain via afferent signals, reaching the postcentral gyrus (sensory input).
• Afferent signals occur after efferent signals.

Efferent Signals

• Begin in the primary motor cortex (precentral gyrus).
• Travel through the midbrain.
• Continue through the pyramids of the medulla where decussation occurs (crossing over to the opposite side).
• Continue along the corticospinal (pyramidal) tract.
• Upper motor neurons synapse with lower motor neurons in the anterior horn of the spinal cord.
• Every motor tract exits from the anterior horn of the spinal cord unless the message originates in the brain stem.
• The right side of the brain controls the left side of the body, and vice versa.
• If the primary motor cortex receives a signal on one side, it will decussate to affect neurons on the opposite side.

Motor Neuron Pathway

• Begins in the precentral gyrus.
• Synapses with a lower motor neuron in the anterior horn of the spinal segment.
• Delivers an efferent signal to the muscle.

Afferent Signals

Upper vs. Lower Motor Neurons

• Upper motor neurons originate in the brain and transmit signals down the spinal cord to activate lower motor neurons.
• Lower motor neurons receive signals from upper motor neurons and connect directly to muscles, activating movement.

Neuron Types (Both Afferent)

• Dorsal column system: Fine touch and proprioception.
• Spinothalamic tract: Pain and temperature.

Dorsal Column Pathway

• Travels through the dorsal root ganglion.
• Ascends through the dorsal columns.
• Decussates in the medulla.
• Proceeds to the thalamus.
• Terminates in the postcentral gyrus.

Spinothalamic Tract Pathway

• The first-order neuron immediately decussates in the spinal cord.
• Ascends via the lateral spinothalamic tract.
• Passes through the medulla.
• Proceeds to the thalamus.
• Terminates in the postcentral gyrus.

Afferent vs. Efferent Signals

• Efferent signals always decussate in the medulla.
• Afferent signals can decussate in either the medulla or the spinal cord.
  • Dorsal column neurons decussate in the medulla.
  • Spinothalamic tract neurons decussate in the spinal cord.
• The location of decussation is determined by evolutionary adaptations, optimizing motor coordination and sensory perception.
• Afferent signals decussate in the spinal cord, allowing the brain to process sensory information before integrating it with motor responses.
• Efferent signals decussate in the medulla at the pyramidal decussation, allowing precise and voluntary control of the contralateral side of the body.

Key Information to Know

• What different types of information they translate.
• Where they go.
• Where they decussate.

Neuron Types

First Order

• Extends from the origin to the location of decussation in either the dorsal or spinothalamic column.

Second Order

• Extends from the decussation to the thalamus.

Third Order

• Extends from the thalamus to the postcentral gyrus.

Neurons and Nerves

• The CNS contains 100 billion neurons.
  • Neurons communicate through synapses on dendrites.
  • Primarily located in the brain.
• A nerve consists of:
  • A bundle of neurons.
  • Enclosed cable-like neurons.
  • Found in the PNS.
  • Three types: sensory, motor, and autonomic.
  • Axons are seen on nerves are seen in PNS, short in CNS

Myelinated vs. Unmyelinated Nerves

Myelinated Nerves

• Have a myelin sheath around the axon, which aids in sending neural impulses.
• Myelin formed by glial cells:
  • Schwann cells in the PNS.
  • Oligodendrocytes in the CNS.
• Myelin enables fast transmission due to saltatory conduction.
• Numerous sodium channels exist between myelin segments (nodes of Ranvier), facilitating signal propagation.

Differences

• No myelin= slower speed due to numerous voltage-gated channels
• Myelin facilitates faster signal skipping to the sodium channels
• Myelinated nerves are white in color and are larger in diameter
• Unmyelinated nerves are grey and are smaller in diameter

Homunculus Mapping

• Homunculus represents a "little person" and is based on the sense of touch.
• Areas like the hands, lips, and tongue have a heightened sense of touch compared to other body parts.
• The legs have smaller representation than the tongue in the cortical homunculus.

Action Potential

• Nerve connections depend on electrical currents, measured in voltage.
• Current flows through the axon.
• Axons always allow some level of leakage, even when myelinated.
• Myelin insulates.
• Leakage = space constant.

Space Constant

• The axon's choice to pass a signal through or leak it out depends on:
  • Insulation to prevent leaking.
  • Resistance encountered by currents flowing longitudinally through the axoplasm.
  • Lots of leakage causes large space constant.
  • Poor conduction causes large space constant.

Space Constant in Muscle

• In the longest nerve in the body, information could leak and fade away without the action potential.

Action Potentials

• Dramatic and explosive.
• Have a very large voltage burst.
• 100mV amplitude.
• Occur cyclically.

Working Axon Potentials

• The axon membrane is mostly impermeable.
  • Active transport via the sodium-potassium pump.
  • Pumps 3 sodium ions out for every 2 potassium ions in.
  • The outside always has more sodium.
  • The inside always has more potassium.

Potential Difference

• Different ionic concentrations create a potential difference.
• Sodium potential difference: +61mV.
• Potassium potential difference: -88mV.

Resting Membrane Potential

• Maintained by differences in ion concentrations inside and outside of cells.
• In neurons, the resting membrane potential is more permeable to K+
• Potassium pulls the membrane potential down.
• Resting membrane potential ranges from -30mV to -90mV.