Ana chapter 14.3

The defining characteristic of the somatic nervous system:
- It controls skeletal muscles.
Somatic senses:
- Inform the nervous system about the external environment.
- Responses are through voluntary muscle movement.
The term “voluntary”:
- Suggests a conscious decision to make a movement.
- Some voluntary muscle actions occur without conscious control (e.g., breathing can shift to unconscious control).
Example of dual control of muscles:
- Breathing muscles are utilized for both unconscious control (breathing) and voluntary control (speech).

Sensory stimuli processing:
- Registered through receptor cells and relayed to the CNS along ascending pathways.
- Initial processing progresses through the cerebral cortex:
- Sensory perception → associative processing → integration in multimodal areas.
Role of sensory cortical areas:
- Located in the occipital, temporal, and parietal lobes.
Role of motor functions:
- Largely controlled by the frontal lobe.
Prefrontal areas:
- Most anterior regions of the frontal lobe:
- Important for executive functions leading to goal-directed behaviors.
- Include higher cognitive processes such as working memory to organize information.
- Responsible for aspects of attention (inhibiting distractions).
Phineas Gage case study:
- In 1848, Gage survived an accident involving a metal spike through his prefrontal cortex, resulting in drastic personality changes:
- From hardworking and amiable to irritable and lazy.
- Despite this, he learned to adapt and function as a stagecoach driver, indicating potential brain recovery.

Function of prefrontal cortex in motor initiation:
- Initiates actual movements through connections to secondary motor cortices:
- Includes premotor cortex and supplemental motor area.
Premotor cortex:
- More lateral, aids in controlling movements for posture during action.
Supplemental motor area:
- More medial and superior, involved in:
- Planning and coordinating sequential movements based on experience.
Frontal eye fields:
- Responsible for eye movements in response to visual stimuli, connected to the superior colliculus.
Broca’s area:
- Controls movements for speech production.
- Named after a French surgeon who studied patients unable to produce speech despite understanding it, indicating a damaged Broca’s area.

Location and structure:
- Located in the precentral gyrus of the frontal lobe.
Historical context:
- Based on Walter Penfield’s research where he stimulated the cortex to map out movement responses.
Motor homunculus:
- A topographical representation of the body in the primary motor cortex:
- Neurons for lower body musculature are in the medial wall of the precentral gyrus.
- Muscles performing fine movements (like fingers) occupy more cortical space than those for coarse movements (like back muscles).

Neurons in the primary motor cortex (Betz cells) descend into brainstem and spinal cord to control musculature through motor neurons.
The two primary descending pathways:
- Corticobulbar tract: Controls facial movements.
- Corticospinal tract: Controls limb movements, can be categorized based on decussation.
- Ipsilateral fibers in cortico-bulbar (same side) versus contralateral fibers in corticospinal (crossed).
Betz Cells:
- Large cortical neurons synapsing with lower motor neurons in the brainstem or spinal cord.
Pathway details:
- Corticospinal tract follows a specific route from cortex to spinal nerves:
- Moves through the internal capsule and pyramids in the medulla.
- Pyramidal decussation: Critical crossover point in the medulla.
Lateral corticospinal tract:
- Crosses at the pyramidal decussation,
- Controls appendicular muscles (arms and legs).
Anterior corticospinal tract:
- Regulates axial musculature (trunk), does not decussate until reaching the spinal cord.

Extrapyramidal system:
- Pathways outside the direct corticospinal tract, often influencing more automatic movements.
Pathways include:
- Tectospinal tract: Postural movements related to visual stimuli.
- Reticulospinal tract: Influences trunk and limb muscles for posture and locomotion.
- Vestibulospinal tract: Regulates balance and muscle tone through vestibular system input.

Importance of cerebellum: Compares motor commands from the cerebrum with proprioceptive feedback.
Explains corrections during movement:
- E.g., adjusting stride when walking in resistance (like water).
- Corrective commands sent via the rubrospinal tract.
Cerebellar dysfunction:
- Alcohol consumption affects the cerebellum’s ability to coordinate and assess body movements, leading to lack of coordination.

Lower motor neurons in the ventral horn innervate skeletal muscles:
- Large multipolar neurons that extend long axons to peripheral muscles.
- Axons branch to control multiple muscle fibers, forming a motor unit.
- Variation in motor units provides precision of control in muscle movement.
Neuromuscular junction:
- Where axon terminals of motor neurons synapse with muscle fibers, initiating contraction via acetylcholine release.
- Impacts on contraction strength depend on frequency of neuronal impulses.

Reflexes described as simple, fast motor responses:
- Include spinal and cranial reflexes, independent from higher brain functions.
Example - withdrawal reflex:
- Contract biceps brachii while relaxing triceps brachii via spinal cord connections.
- Muscle spindle reflex helps maintain muscle length and is exemplified by the knee jerk reflex.
Corneal reflex:
- Blinked initiated by sensory input through the cornea via the trigeminal nerve and motor response through the facial nerve.